Ecological Paradox in the Reuse of Internal Combustion Engines from Scrapped Vehicles for Electric Power Generation—Circular Economy Potential Versus Emission Certification Barriers

Abstract

Concepts such as reuse, repurposing, upcycling, remanufacturing, and re-powering can be applied to the reuse of combustion engines from passenger cars and trucks in stationary or mobile machines, such as power generators. Technical, economic, environmental, and research analyses indicate that such solutions may be justified; however, their implementation is limited by homologation and emission regulations. In most countries, there are no specific rules governing emissions from power generator engines, while in the European Union, such engines are categorized as mobile generators (portable or trailer-mounted) subject to Stage V (Reg. 2016/1628/EU), stationary generators (permanently installed) subject to the MCP Directive (2015/2193/EU), and emergency generators (limited operation) partially exempt from MCP but requiring registration. Consequently, engines recovered from road vehicles do not meet formal or technical emission compliance requirements for power generators and can only be used under conditional approval for research, experimental, or temporary purposes. This reveals a paradox of modern environmental policy: although reusing functional engines from dismantled vehicles could embody the principles of a circular economy, restrictive emission standards (Stage V, MCP, NSPS) effectively prevent such technological recycling. Addressing this issue requires legislative action and the development of simplified testing methods for used engines in new applications. This article is the first to systematically demonstrate that current Stage V, MCP and NSPS emission frameworks create a regulatory paradox that prevents the circular-economy reuse of functional automotive engines, and it proposes a dedicated secondary type-approval pathway enabling their legal and environmentally controlled application in power generators.

1. Introduction

According to the long-term projections of Moriarty and Hennery [1], the global vehicle fleet is expected to increase steadily, reaching approximately 1700 million units by 2030. This continuous growth of the automotive sector is accompanied by a parallel rise in the number of vehicles withdrawn from service. Current estimates indicate that about 40 million vehicles are scrapped worldwide each year [2], with regional contributions including approximately 5 million units in China [3], 4 million in the United States and Canada [4], and 6.34 million in the European Union (EU) [5]. It is estimated that about 75% [6] of vehicle materials can be recovered for reuse, including metals, tires [7], and batteries [8]. The main drivers of vehicle scrapping include economic downturns and declining demand for new cars, which lead to scrappage schemes [9]. High repair and maintenance costs for older vehicles further accelerate withdrawal from service [10]. Aging engines contribute to higher environmental pollution [4,10]. Technological progress results in newer vehicles offering improved functionality, safety, and efficiency [11,12]. Government policies also encourage vehicle scrapping [13]. Consumer behavior, including recognition of the benefits of scrapping programs, reinforces this trend [14]. Most scrapped vehicles exhibit body damage from accidents or corrosion, while power units are often still operational. The main recycling methods for internal combustion engines (ICEs) involve material recovery through mechanical processes. Crushing, screening, and sorting are applied to recover metals such as copper, steel, and aluminum from ICEs [15] and plastics [16,17]. Chemical recycling methods are also applied [18,19]. Pyrolysis can convert plastic components into high-quality oil suitable for diesel engines [20].

One proposed use of ICEs from scrapped vehicles is their integration into electric power generators. The application of second-hand engines in emergency generators appears to be a highly effective solution, since such units are operated infrequently and for short periods while providing reliable support for household energy needs [21]. When installed in systems designed for cogeneration, they can additionally supply both electricity and heat [22]. This approach is particularly valuable in regions affected by armed conflict or severe climatic conditions, where access to electricity and heat is unpredictable and often limited. Power generators can supply critical household devices, including refrigerators, heating systems, lighting, and even security installations [23]. They can also reduce losses associated with power outages, such as food spoilage or damage to water installations [23].

In the context of reusing ICEs from vehicles in stationary and mobile machines, several approaches can be distinguished within the framework of the circular economy [24,25]. Reuse involves the direct application of a functional engine in another machine without major modifications, for example, as a drive for a generator. Repurposing refers to adapting an engine to a new function different from its original purpose, such as converting a vehicle power unit into an energy source for agricultural or industrial machinery. An additional form of functional adaptation, relevant especially for generator applications, is the conversion of an engine to operate on alternative fuels such as liquefied petroleum gas (LPG) [26] or compressed natural gas (CNG) [27], which extends its usability beyond the original diesel or gasoline configuration and can significantly reduce operating emissions. Upcycling encompasses the modernization or creative transformation of an engine in a way that enhances its functional or operational value, for instance, by improving performance parameters or integrating advanced control systems [28]. Remanufacturing denotes a comprehensive restoration process that brings the engine to a condition comparable to a new one, including complete disassembly, reconditioning, replacement of worn components, and quality testing, thereby ensuring reliable reuse. Finally, re-powering involves replacing an old or damaged engine with a more efficient and lower-emission unit, which extends the operational lifespan of stationary and mobile machines while reducing both investment and environmental costs.

Repurposing ICEs from scrapped vehicles for energy generation is a viable and beneficial approach. It leverages the availability of engines, supports recycling efforts, and enhances energy efficiency through advanced waste heat recovery technologies. This approach not only addresses environmental concerns but also provides economic benefits, particularly in resource-constrained regions.

In the following sections of the article, a detailed review of the key pillars of the concept of reusing combustion engines from vehicles in stationary and mobile machines is presented, considering technical, economic, and scientific-research aspects.

Since the aforementioned aspects highlight the advantages of reusing ICEs from vehicles in stationary and mobile machines, the aim of this article is to assess the extent to which homologation regulations in the EU and in other industrialized countries permit the use of used automotive engines in non-road machines, e.g., energy generators. Despite a growing body of literature on end-of-life vehicle (ELV) management, circular-economy strategies and non-road emission legislation, no previous study has systematically examined why functional automotive engines cannot be legally repurposed for use in generators or other stationary and mobile machines, even when their technical condition would permit continued operation. This gap arises not only from the absence of integrative analyses comparing vehicle and machinery emission frameworks, but also from the lack of policy-oriented evaluations that address the regulatory paradox effectively preventing circular-economy reuse. The present study fills this gap by providing the first comprehensive assessment of these overlapping regulatory systems and by proposing a dedicated secondary type-approval pathway that enables the controlled, environmentally compliant reuse of automotive engines in generator applications.

2. A Review of Aspects Related to the Reuse of ICEs from Scrapped Vehicles

2.1. Technical Aspect

The inherent durability and structural redundancy of ICEs from older passenger cars and heavy-duty trucks make them highly suitable for stationary applications. The longevity of such engines largely depends on the wear resistance of key components, including crankshafts, piston rings, and cylinder liners, which maintain mechanical integrity under prolonged operation [29]. Regular maintenance and proper lubrication, such as the use of optimized low-viscosity oils, can further enhance fuel efficiency without compromising engine durability or increasing maintenance intervals [30]. The structural integrity of critical components such as crankshafts has been verified through advanced design methods, including finite element analysis, which revealed safety factors significantly exceeding design limits, thereby ensuring high reliability under stationary operating conditions [31]. Additionally, fatigue analysis and predictive wear assessment underline the importance of maintaining structural redundancy to sustain performance over extended periods [32]. The robust construction of ICEs from older vehicles allows them to perform reliably under constant loads and variable environmental conditions, which is essential for stationary machine operation [33]. Furthermore, the application of advanced materials and surface treatments, such as thermal barrier coatings, enhances resistance to thermal stress and mechanical degradation, improving the operational lifespan of these engines in demanding environments [34].

Stationary generators are designed to operate continuously at a constant speed and load, making their working conditions significantly less demanding than those of vehicle generators [35,36]. This steady-state operation allows for high efficiency, reduced fuel consumption, and extended maintenance intervals, as control systems can maintain optimal performance parameters under stable conditions [37]. In contrast, engines used in vehicles must function under dynamic and highly variable loads, facing rapid changes in speed, thermal cycling, and fluctuating power demands [38]. These challenges require advanced control strategies, durable materials, and sophisticated engineering solutions to ensure performance and longevity in mobile environments [39]. Overall, stationary generators demonstrate superior efficiency and operational stability due to their constant working conditions, while vehicle generators must balance durability and responsiveness under more complex and variable operating regimes.

Older diesel engines exhibit notable fuel versatility, enabling operation on a range of alternative fuels [40,41] such as vegetable oils [42,43] and second-generation biofuels [44,45], which address the growing need for renewable, cost-effective, and environmentally sustainable energy sources [46]. Pure vegetable oils, being biodegradable and derived from diverse crops, can be used directly in diesel engines with minimal modification, although issues like high viscosity and carbon deposits require mitigation through transesterification to produce biodiesel with improved combustion properties [47]. Engine performance using biodiesel is generally comparable to that of conventional diesel, with only slight increases in specific fuel consumption, though long-term use of straight vegetable oil may necessitate engine adjustments [48]. In terms of emissions, biodiesel significantly reduces pollutants such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides, and particulate matter (PM), while fuel blends incorporating gasoline can further enhance emission characteristics [49]. Additionally, second-generation biofuels—such as biodiesel–diesel blends (e.g., B20) and synthetic fuels like Fischer–Tropsch diesel or dimethyl ether (DME)—offer compatibility with older engines while maintaining durability and improving fuel efficiency [50]. Overall, these alternatives not only extend the operational life of existing diesel engines but also contribute to lower emissions and reduced reliance on fossil fuels.

To better illustrate the technical feasibility of engine reuse, the condition of key subsystems must be evaluated in relation to cumulative operating hours, thermal cycles, lubrication history, fuel quality, and maintenance records—factors that are generally unknown for engines recovered from end-of-life vehicles. As prior studies show, wear in cylinder liners, piston rings, bearings, injectors, and turbochargers progresses non-linearly and may remain undetectable without full disassembly or advanced diagnostics. Therefore, the identification of components that can be directly reused, those that require remanufacturing, and those that must be scrapped is inherently uncertain. To clarify these distinctions for the reader,

Table 1. Damage types disqualifying engines from reuse/regeneration.

Component	Type of Damage	Reason for Disqualification
Cylinder head	Cracks, burnt valves, corrosion	Loss of structural integrity and sealing
Combustion chamber	Deposits, knocking-related damage	Local overheating and secondary component failure
Pistons and cylinder walls	Melting, scoring, severe wear	Irreparable deformation and loss of tolerances
Crankshaft and bearings	Metal-to-metal contact, fatigue	Catastrophic bearing or journal failure
Connecting rods	Fatigue cracks, deformation	Structural failure risk
Cylinder liners	Cavitation erosion–corrosion	Damage beyond repair
Turbocharger	Thermal overload, bearing failure	High failure probability, short remaining lifespan
Valves	Cracks, notches, edge burning	Leakage and loss of operating efficiency
General engine parts	Maintenance-related failures	Multiple unpredictable defects

summarizes typical degradation patterns and lists the most probable reuse pathways for major ICE components.

Damage to critical components of ICEs can permanently disqualify them from reuse or regeneration, as it often indicates structural degradation beyond repair. Cracks in cylinder heads, burnt valves, and corrosion around coolant passages or valve seats are among the most common failures leading to irreversible loss of sealing and mechanical integrity [51]. Similarly, deposits of non-volatile combustion residues in the combustion chamber and ring grooves can cause localized overheating and knocking phenomena, which in turn damage crankshaft bearings and piston crowns [52]. High thermal loads may also result in the melting or fusion of piston head material, representing an irreparable defect [52], while excessive wear of cylinder liners and piston rings—typically caused by lubrication failures—produces deep scoring or deformation that exceeds the permissible limits for re-machining [51,52,53].

Within the crank–connecting-rod mechanism, hydrodynamic lubrication breakdown can lead to metal-to-metal contact, causing catastrophic damage to bearings and crank journals [53]. Connecting rods are particularly vulnerable to fatigue-related cracking or deformation due to long-term cyclic loading, poor material quality, or thermal–mechanical overstress [54]. Cylinder liners may also suffer erosion–corrosion damage resulting from coolant cavitation, and such deterioration frequently renders refurbishment impossible [55]. Turbochargers subjected to high-temperature overload or oil starvation show accelerated wear and failure of turbine shafts and bearings, significantly reducing their suitability for reuse [56]. Valve components are similarly prone to cracks, notches, and edge burning, which compromise sealing and airflow and often make reconditioning economically unjustifiable [57,58]. More generally, improper maintenance throughout the engine’s service life can produce a wide range of mechanical failures—including fretting on camshaft bearing surfaces or localized melting of cylinder head castings—that ultimately exclude the engine from safe and reliable repurposing [51].

These damages, due to their severity and impact on engine performance and safety, often disqualify the affected parts from being reused or regenerated.

One potential solution proposed by the authors for reliably assessing the condition of engines recovered from end-of-life vehicles is the introduction of an extended operational verification test. After initial inspections—such as visual assessment, oil analysis, compression measurement, and a preliminary exhaust emission check—the engine would undergo a multi-hour continuous endurance run designed to replicate stationary operating conditions. Such a test, lasting several to more than a dozen hours, provides an opportunity for hidden defects to manifest, including lubrication system instabilities, overheating tendencies, injector malfunctions, abnormal vibrations, and thermal–mechanical stress responses that may not be detectable during short diagnostic procedures. If the engine completes this endurance test without critical failures, excessive emissions, or signs of abnormal wear, the remanufacturer or distributor could, in principle, offer a limited operational warranty. This approach would provide a measurable, transparent verification standard, helping to minimize the risks associated with reusing high-mileage engines and increasing confidence among end users and regulatory bodies.

These technical considerations provide the foundation for understanding engine reuse, yet the practical viability of such solutions is also strongly influenced by the economic conditions surrounding recovery, remanufacturing, and deployment.

2.2. Economic Aspect

Reusing engines retired from vehicles presents clear economic advantages by significantly reducing investment costs compared to purchasing new factory-built units, as the remanufacturing process utilizes existing components and materials that are far less expensive than new ones. The reuse of ELV parts, including engines, generates measurable financial benefits for both dismantlers and consumers, with studies indicating potential savings of up to 12,739 EUR and 51,281 EUR, respectively [59]. However, challenges remain due to uncertainty in the quality and condition of retired engines, which complicates life cycle cost estimation and process selection; non-linear models considering component degradation have been proposed to optimize these evaluations [60].

The economic aspects of maintaining low operating and servicing costs in older engines are primarily influenced by the availability of spare parts, the experience of mechanics, and the use of advanced diagnostic tools. The accessibility and affordability of spare parts play a crucial role in reducing overall maintenance expenses, as reflected in economic models estimating service costs based on part prices and labor [61]. Skilled and experienced mechanics significantly contribute to lowering maintenance costs by ensuring efficient, high-quality repairs grounded in long-standing practical knowledge [62,63]. Maintenance costs, a major component of operational expenses, can be minimized through targeted maintenance strategies that replace only worn components rather than adhering to fixed replacement intervals [62,64]. Advanced diagnostic tools, including digital twins and multi-level monitoring models, enable early detection of engine degradation and predictive maintenance planning, further reducing unnecessary repairs and associated costs [65,66]. Economic models that integrate these factors allow for accurate forecasting of maintenance expenditures and can be adapted for future engine types with minimal modifications [61].

Adapting used ICEs from vehicles for generator applications offers a cost-effective and rapidly deployable energy solution for resource-constrained areas, particularly in developing regions or during emergencies such as power outages and natural disasters [67]. The reuse of retired engines significantly lowers initial investment costs compared to new units, while conversions to operate on alternative fuels such as biogas or LPG further reduce fuel consumption, carbon dioxide (CO₂) emissions, and operating expenses [68,69]. Studies show that biogas-powered ICE generators can achieve short payback periods—often under three years—especially when used in cogeneration systems that maximize energy efficiency [67]. These engines also provide fuel flexibility, operating on biogas [70], LPG [71], or natural gas [72,73], which are cheaper [74] and more accessible in many developing regions, while alternative fuels substantially decrease pollutant emissions.

Although a reused automotive engine can be acquired at a significantly lower initial cost compared to a new Stage V or EPA Tier 4 generator unit, additional expenditures related to adaptation and compliance may offset this advantage. Typical market estimates indicate that retrofitting an ELV-derived engine for stationary operation may require the installation of an independent cooling circuit, a modified exhaust system, dedicated control electronics, safety devices and generator coupling. The integration of the power unit into a complete generator system, including the mounting frame, vibration isolation, acoustic enclosure, wiring harness, fuel system and output protection, also generates additional costs. Developing relatively universal interfaces for these subsystems may not represent a substantially greater challenge than in the case of mounting new engines, yet the most significant economic barrier remains regulatory certification, particularly emission testing required for MCP and Stage V compliance.

Publicly available information from accredited test laboratories and industry reports indicates that the cost of a complete emission test campaign for a non-road Stage V engine family, including NRSC or NRTC testing, test cell preparation and reporting, typically falls in the range of 10,000 to 30,000 EUR and may exceed 50,000 EUR when durability testing and multiple variants are included. In contrast, MCP compliance for a stationary generator does not require type approval of the engine itself, but it does require on-site emission measurements of the installation. Commercial offers for MCP stack testing usually fall between 3000 and 10,000 EUR per unit, with an additional 1000 to 3000 EUR for permitting and consultancy services. For small operators, these costs can effectively eliminate the economic advantage of using a low-cost second-hand engine, especially in the 10 to 40 kW power range, where the purchase price of a new factory-certified generator set can be comparable to or lower than the total cost of retrofitting, testing and permitting a reused engine.

When these factors are combined, the total cost of repurposing an ELV engine can approach or exceed the retail price of a new factory-certified generator. This economic imbalance constitutes one of the key barriers preventing the commercial reuse of automotive engines in stationary power applications. At the same time, in many regions of the world, the absence of mandatory emission certification or the presence of significantly simplified permitting procedures makes the use of second-hand engines economically competitive, which explains their widespread adoption in low-regulation markets. Therefore, developing new, simpler and more affordable certification standards that support circular economy applications may be worth considering.

While economic feasibility is a key driver of reuse practices, the broader justification for repurposing ICEs increasingly depends on their environmental performance and alignment with circular-economy objectives.

2.3. Environmental Aspect

The environmental aspect of reusing ICEs is closely linked to the principles of the circular economy, emphasizing the extension of product life cycles and the reduction in waste through component repurposing and remanufacturing [68,75]. Instead of scrapping, reused and remanufactured engines conserve materials and energy, significantly lowering the environmental footprint associated with producing new units [75].

The secondary use of ICEs reduces emissions associated with the production of new units. It decreases the demand for energy and raw materials, leading to a lower environmental impact and a more sustainable product life cycle.

Additionally, older engines can be modified to operate on alternative fuels such as biogas or biofuel blends [76]. This approach significantly reduces the emission of harmful substances and contributes to improved air quality.

Beyond direct emission reductions and material conservation, the environmental benefits of reusing ICE components are supported by broader circular economy indicators. Studies show that part reuse within ELVs generates substantial CO₂ savings, with the highest reductions observed for electric ELVs and the lowest for petrol ELVs [59]. At the systemic level, the circularity index for ICE vehicles remains very low—0.01 in the Great Britain market—indicating significant unrealized potential for extending component life cycles. Implementing low-carbon strategies, including wider adoption of reuse and remanufacturing practices, could raise this index to 0.5, illustrating the magnitude of possible improvement in material circularity and environmental performance [77].

Remanufacturing policies further amplify these benefits. In China, financial incentives for engine remanufacturing combined with a 15% improvement in energy efficiency have been shown to support both economic growth and emission reductions [78]. Although each remanufactured engine displaces only 0.42 of a newly produced unit—highlighting the need for realistic assumptions in Life Cycle Assessment (LCA) modeling—remanufacturing remains substantially less carbon-intensive than producing new components [78]. For internal combustion engines, remanufacturing can reduce Global Warming Potential by up to 79% and, in the case of diesel engines, lower energy consumption by 66% while decreasing ozone depletion potential by as much as 97% [79,80]. These reductions stem from the disassembly, cleaning, repair and reassembly of existing components, which avoids the extraction and processing of primary materials [81].

Environmental gains can be further enhanced through design strategies such as Design for Upgradability (DfU), which enables the replacement or upgrading of degraded components without requiring full engine replacement, thereby extending service life and reducing the overall environmental burden [82]. Incorporating recycled materials into new ICE production also contributes to resource conservation; current ICE vehicles contain approximately 27% recycled materials, substantially reducing the demand for virgin raw materials [83]. However, despite these advantages, material dissipation remains a challenge: long-term studies indicate that recycling processes lead to losses of 22% of steel, 21% of nickel and up to 63% of chromium over periods of 50 years [84]. While reusing engine components more effectively extends service life, its influence on mitigating long-term material losses remains comparable to that of full recycling [84].

Innovative examples of repurposing further demonstrate the environmental potential of circular strategies. Recent work by Dillitzer et al. in 2025 [68] highlights the conversion of alternators recovered from end-of-life vehicles into functional electric motors to support low-cost mobility solutions in developing regions. Field validation campaigns in Africa and Germany show that repurposed alternators can provide a reliable and environmentally sustainable alternative to newly manufactured electric drives [68]. Such initiatives illustrate the broader environmental value of transforming ICE components into new functional products, thereby extending their lifecycle while reducing the need for resource-intensive manufacturing. The environmental benefits and constraints associated with ICE reuse highlight the need for further scientific insight, particularly to quantify impacts, validate technical assumptions, and support evidence-based policymaking.

2.4. Scientific and Research Aspect

Experimental research models based on repurposed ICEs can serve as valuable platforms for scientific testing and technological development. Such generator units enable controlled experimentation with alternative fuels [85], exhaust aftertreatment systems [86,87], and engine control methods under stable operating conditions [88]. Their adaptability makes them ideal for studying combustion processes, emissions behavior, and optimization strategies aimed at improving engine performance and reducing environmental impact.

Energy efficiency analysis using these repurposed engines allows for comprehensive comparisons of performance, fuel consumption, and operating costs relative to modern, purpose-built power units. By evaluating thermal efficiency and load response under stationary conditions, researchers can quantify potential economic and ecological benefits. This approach also helps identify optimal configurations for hybrid or decentralized power systems, particularly in applications requiring reliable and low-cost energy generation.

Durability and material studies conducted on engines operating in stationary setups provide insights into wear mechanisms and material degradation processes that differ from those observed in mobile applications [89]. Continuous and uniform loading conditions enable long-term assessment of thermal fatigue, lubrication efficiency, and corrosion resistance. The results of such studies contribute to improved understanding of lifecycle performance, guiding both maintenance strategies and the design of future engine generations with enhanced reliability and sustainability. These research findings indicate that technical and environmental opportunities for reuse must be framed within a broader circular-economy context, which defines how recovered engines can be retained in productive use.

2.5. Analysis of Risks Associated with Using Engines from ELV in Electricity Generators

A key risk associated with repurposing internal combustion engines from scrapped vehicles for generator applications is the lack of compliance with current emission standards and the resulting homologation problems.

Another important concern is the loss of functionality of exhaust aftertreatment systems, which are designed specifically for transient automotive operation. After dismantling, diesel particulate filters often lose their ability to regenerate [90], selective catalytic reduction systems operate ineffectively at steady loads and low exhaust temperatures [91], and exhaust gas recirculation systems may become clogged or fail to operate properly [92,93]. The absence of the original control unit further prevents accurate emission control [94]. These effects can cause emissions to increase by an order of magnitude, preventing any form of certified operation in generator applications.

The technical condition of engines obtained from ELVs is another major risk factor due to the presence of hidden defects and unknown degradation mechanisms. Such engines may exhibit wear of pistons, rings and cylinder liners resulting from poor fuel quality [95], injector wear caused by contaminants [96], loss of compression and increased blow-by [97], injector malfunction leading to higher emissions [98], and high sensitivity to fuel quality in modern common-rail systems [99]. Research also highlights the need for engine-health assessment systems [97] and advanced diagnostic algorithms [100]. These issues create a high likelihood of unexpected failures, repair costs and limited durability when used in steady-state generator applications.

Additional risks arise from lubrication and cooling system challenges. Generators impose continuous-load operating conditions that require stable thermal management and lubricants with properties different from those used in automotive driving cycles. If not properly adapted, these systems may experience overheating, accelerated wear or even seizure [101]. Closely related is the problem of inappropriate engine-control mapping. Automotive engine control units (ECUs) are calibrated for dynamic driving, not stationary power production. Traditional ECU calibration relies on time-consuming trial-and-error methods requiring extensive dynamometer testing [102,103], and these calibrations are optimized for transient vehicle operation. Stationary calibration requires substantially different parameters, taking into account fixed load points and the absence of vehicle movement [104,105]. Without proper recalibration, the engine may exhibit unstable operation, knock in spark-ignition (SI) engines, excessive smoke formation in compression-ignition (CI) engines or mechanical failures.

There are also mechanical and electrical safety risks. Improperly modified fuel systems, electrical harnesses and safety circuits may lead to fire hazards, fuel leaks or short circuits [106,107]. Noise and vibration present further concerns. Automotive engines are designed to operate with the benefit of vehicle chassis noise insulation, while generator installations lack this protection, resulting in excessive noise levels and difficulties meeting environmental noise restrictions [108].

Another limitation is the lack of technical documentation and operational history. Vehicle recycling processes do not require the transfer of service records or diagnostic data, which makes it impossible to evaluate past overheating, repairs or failures [109]. This increases uncertainty and risk during repurposing.

Further risks relate to reliability and maintenance. Reused engines may show reduced reliability and higher maintenance needs due to accumulated wear, leading to frequent failures and increased operational costs [110]. Although waste-heat-recovery systems can improve overall efficiency, they introduce mechanical complexity and may not compensate for fundamental inefficiencies in older engines [110,111]. Compatibility issues are also relevant, as older ICEs may not integrate well with modern energy-recovery technologies, limiting overall performance [112]. Their operational behavior can be inconsistent when used in continuous-duty applications for which they were not originally designed [68].

Economic and environmental risks also emerge in the context of remanufacturing. While remanufacturing ICEs can reduce energy and raw material consumption compared to producing new engines, establishing the required infrastructure for environmentally sound and efficient remanufacturing can be costly [113]. Moreover, proper resource management for ELVs is essential to prevent environmental degradation. Handling hazardous substances and ensuring that remanufacturing processes do not generate further pollution remain major challenges for the recycling industry [114,115]. Taken together, these technical, economic, environmental, and systemic considerations demonstrate both the potential and the limitations of ICE reuse, creating a necessary basis for analyzing the regulatory frameworks that ultimately determine whether such reuse is legally possible.

2.6. Comparison of Carbon Footprint: New Low-Emission Engines vs. Reused Engines Recovered from ELVs

The carbon footprint of internal combustion engines can be assessed using a LCA framework, which includes the production, use and end-of-life phases. For most internal combustion engines, the use phase dominates total life-cycle emissions because it generates continuous CO₂ emissions during operation [116,117]. For diesel engines, the carbon footprint has been estimated at approximately 0.36 kg CO₂eq per kilometer, with operational emissions being the largest contributor [116]. However, the production phase also generates a substantial environmental burden, as manufacturing processes for steel and cast iron involve significant emissions of CO₂, methane and carbon monoxide [118,119]. Reductions in these production-related emissions can be achieved by combining blast furnace technology with electric arc furnaces, which can lower CO₂ output by up to 20% [118,119]. Technological advances in internal combustion engines, such as reducing gearbox drag, improving fuel economy and expanding the use of biofuels, also contribute to lowering their life-cycle carbon footprint [120,121]. In particular, second-generation biofuels that do not compete with food production increase the environmental sustainability of internal combustion engines [121]. The key environmental impact categories associated with these systems include climate change, fossil resource use, freshwater ecotoxicity, acidification and particulate matter emissions [116].

When comparing the carbon footprint of new low-emission engines with that of older engines recovered through recycling, several environmental and technical considerations arise. New engines are designed to comply with stringent emission regulations and therefore achieve substantial reductions in CO₂, NO_x and particulate matter emissions [122,123,124]. These reductions are made possible through the widespread integration of advanced after-treatment technologies, including selective catalytic reduction (SCR) and diesel particulate filters (DPF) [122,124]. Although such engines achieve significantly lower operational emissions, their production requires large amounts of energy and raw materials, leading to major embedded emissions within the manufacturing phase [125,126]. Ongoing improvements in engine architecture, combustion control and after-treatment systems further reduce fuel consumption and operational emissions, enhancing their overall environmental performance [127].

Engines recovered from ELVs and subjected to remanufacturing present a different environmental profile. The remanufacturing process can reduce primary energy demand by as much as 82.55% and decrease global warming potential by 73.33% relative to the production of new components [128]. These benefits stem from avoiding energy-intensive material extraction and component manufacturing. Nevertheless, older engines, even after remanufacturing, typically produce higher emissions of CO, NO_x and particulate matter because their underlying designs and control strategies do not satisfy the requirements of modern emission standards [129,130]. Despite these limitations, reusing and remanufacturing engines preserves natural resources and reduces environmental pressures associated with mineral extraction and metal processing [131,132,133].

In assessing the environmental consequences of each approach, it is essential to weigh the significant production-related emissions associated with new engines against the higher operational emissions characteristic of older recycled engines. New engines offer clearly superior performance in terms of operational efficiency and pollutant control, but this comes at the expense of high embedded emissions from manufacturing [125,126,128]. Recycled engines substantially reduce the environmental impacts of production but may not achieve the low operational emissions required by current regulatory frameworks [128]. A complete life-cycle assessment is therefore necessary to fully understand the net environmental effect of both options [117]. Policymaking also plays an important role, as government incentives and vehicle retirement schemes often promote the replacement of older engines with cleaner alternatives, although the true environmental benefit depends on balancing the emissions associated with producing new equipment against the reductions achieved during operation [126,134,135].

In conclusion, the difference in carbon footprint between new low-emission engines and recycled older engines is complex and depends on the relative contributions of production and operational emissions. New engines deliver substantial reductions during use, while remanufactured engines significantly reduce environmental impacts associated with production. The optimal choice requires a balanced consideration of environmental and economic factors and should ideally be guided by comprehensive LCA results supported by appropriate public policies. Taken together, these considerations demonstrate that the feasibility of reusing internal combustion engines is shaped not only by technical, economic, and environmental factors, but also by increasingly complex regulatory requirements, which necessitate a detailed examination in the following section. These regulatory inconsistencies reveal the need for a more precise examination of how existing legal frameworks interact with the practical realities of engine reuse, which is further explored in the subsequent section.

3. Review of Homologation Regulations for Passenger Cars, Trucks, and Electric Power Generators—Their Occurrence in Selected Countries Worldwide

Globally, various homologation systems regulate exhaust emissions across a wide range of ICE applications. These include light- [136] and heavy-duty [137] vehicles, motorcycles [138], three-wheeled vehicles [139], non-road machinery (NRMM) [140,141], as well as stationary engines [142], locomotives [143,144], and marine engines [145]. Each of these categories is subject to specific regulations depending on operating conditions, fuel type, rated power, and emission characteristics. For light-duty vehicles (passenger cars and light commercial vehicles), the regulations define permissible limits for nitrogen oxides (NO_x), CO, HC, and PM. In the European Union, emission limits for light-duty vehicles are defined by the EURO 1–6 standards, while compliance is verified using dedicated test procedures such as the Worldwide harmonized Light vehicles Test Cycle (WLTC), the Worldwide harmonized Light vehicles Test Procedure (WLTP), and the Real Driving Emissions (RDE) test. In the United States, light-duty vehicle emissions are regulated under the Environmental Protection Agency (EPA) Tier 1, Tier 2 and Tier 3 standards. For heavy-duty vehicles such as trucks, buses and industrial machines, engine-based test cycles are used, including the World Harmonized Stationary Cycle (WHSC) and the World Harmonized Transient Cycle (WHTC) in Europe, whereas in the United States compliance with the EPA 2007 and EPA 2010 standards is assessed using the corresponding Federal Test Procedures for heavy-duty engines. These regulations account for the higher power output of heavy-duty engines and their real-world operating conditions, including variable loads and speeds. A distinct category is represented by NRMM, which includes excavators, power generators, forklifts, and agricultural equipment. In the EU, emission limits for this group are governed by the Stage I–V standards, while in the United States, the equivalent are the EPA Tier 1–4 standards, both of which progressively tighten allowable emission thresholds. Other specialized applications include locomotive and marine engines, which are regulated under international frameworks such as the IMO MARPOL Annex VI convention (for marine transport) and the EPA Locomotive Standards (for the U.S. railway sector). Finally, stationary engines, used in facilities such as power plants, generators, and pumping stations, are regulated by the Medium Combustion Plant (MCP) Directive in the EU and by New Source Performance Standards (NSPS) in the U.S. These frameworks focus on reducing continuous emission levels in fixed installations. Despite regional differences in regulatory scope and methodology, all these systems share a common objective: to reduce the environmental and health impacts of exhaust emissions through progressively stricter emission limits and the implementation of advanced exhaust after-treatment technologies.

The Stage V regulation (Regulation (EU) 2016/1628) [146] applies to engines installed in NRMM, that is, equipment which is mobile, transportable, or designed to be moved between work locations. This category includes excavators, loaders, forklifts, lawn mowers, and also mobile power generators—that is, units mounted on trailers, platforms, or wheels and used for temporary power supply, for example, on construction sites, at outdoor events, or during emergencies. In such cases, the generator must comply with Stage V emission standards, as it is legally classified as non-road mobile machinery.

In contrast, stationary generators, i.e., those permanently installed in fixed locations such as buildings, data centers, hospitals, or industrial facilities, are not subject to Stage V regulations. Instead, they fall under the provisions of the MCP Directive 2015/2193/EU [147], which governs emissions from combustion plants with a rated thermal input between 1 and 50 MW. The MCP Directive sets emission limit values for NO_x, CO, PM, and sulfur dioxide (SO₂), depending on the type of fuel used (diesel, gas, or biofuel) and the plant’s nominal capacity.

In practice, when a Stage V–certified engine is used in a stationary generator, its emissions are typically lower than the MCP Directive limits, since Stage V requirements are more stringent, particularly regarding PM and nitrogen oxides. However, a Stage V certificate cannot automatically serve as proof of compliance with MCP, because the two frameworks are based on different testing conditions—Stage V applies to dynamic (mobile) operation cycles, while MCP covers steady-state (stationary) operation. Therefore, a separate conformity declaration or manufacturer’s approval is required to confirm compliance of the stationary installation with MCP limits.

Manufacturers often use Stage V engines in stationary generator sets due to their cleaner performance and advanced emission control technologies. These units are then registered under MCP classification or accompanied by technical documentation demonstrating compliance with the relevant emission limit values. This approach ensures legal operation of Stage V engines in stationary applications, provided that appropriate documentation is in place.

For emergency standby generators, which operate only intermittently (e.g., less than 500 h per year), the regulations allow for partial exemption from full MCP requirements, though such units must still be reported to the competent environmental authority.

In summary, not all generator sets must comply with Stage V. This requirement applies only to mobile generators, whereas stationary units fall under the MCP Directive. In practice, using a Stage V engine in a stationary generator usually ensures MCP compliance, though formal technical verification is still necessary.

This distinction can be summarized as follows:

• Mobile generator (portable or trailer-mounted)—subject to Stage V (Reg. 2016/1628/EU) [146];
• Stationary generator (permanently installed)—subject to the MCP Directive (2015/2193/EU) [147];
• Emergency generator (limited operation)—partially exempt from MCP but must be registered.

In practical terms, within the EU, power generators are divided into two main regulatory categories: Stage V for mobile equipment and MCP for stationary installations. Both frameworks share the same environmental goal—reducing atmospheric emissions—but differ in their scope, certification process, and testing methodology.

Although the regulatory frameworks presented in this section describe the emission standards, certification pathways and test procedures applicable to road vehicles and non-road machinery, these frameworks also create specific compliance barriers that directly limit the reuse of engines recovered from ELVs. Automotive engines are type-approved under Euro 1–6, EPA Tier, or equivalent road-vehicle legislation using driving cycles such as WLTC, WLTP and RDE, while generators and non-road machines must comply with entirely different regulations—Stage V, MCP, Non-Road Steady Cycle (NRSC)/Non-Road Transient Cycle (NRTC), or NSPS—which evaluate emissions under steady-state or transient engine bench conditions. Because these regulatory regimes are not interoperable, a road-vehicle engine cannot transfer its existing type-approval to a stationary or non-road application. Furthermore, once removed from the vehicle, such engines typically cannot maintain their certified emission performance, as aftertreatment systems (DPF, SCR, EGR) are not validated, calibrated, or legally recognized for use outside the original vehicle configuration. As a result, used engines lack a legally acceptable certification pathway, cannot demonstrate conformity with Stage V or MCP limit values, and cannot be registered as compliant stationary or mobile non-road emission sources. These regulatory discontinuities constitute the fundamental compliance barrier preventing the legal reuse of automotive engines in generator applications, even when their technical condition would allow continued operation.

4. Engines from Scrapped Vehicles in the EU and Their Application in Electric Power Generators in the Context of Machine Homologation Focused on Exhaust Emission Limits

Engines recovered from scrapped passenger cars or trucks generally do not meet the exhaust emission requirements established for any of the three categories of power generators in the EU: mobile, stationary, or emergency.

For mobile (portable) generators, which are subject to Stage V requirements (Regulation (EU) 2016/1628) [146], automotive engines—even those compliant with Euro 6 or Euro VI—do not fulfill the Stage V standard. This is because Stage V applies exclusively to NRMM and is based on different test cycles and emission measurement procedures than on-road standards. Moreover, once removed from the vehicle, an engine loses its type approval, and any reuse in another application (such as a generator set) would require a new certification, which such used automotive engines do not possess. Consequently, the use of a road vehicle engine in a mobile generator is not legally permitted.

For stationary generators governed by the MCP Directive (2015/2193/EU) [147], compliance with emission limit values for NO_x, CO, PM and SO₂ is required. Automotive engines (Euro 4–6) are type-approved using transient driving cycles that include variable loads, speeds and accelerations (WLTC, RDE), whereas engines used in non-road machinery and generators are evaluated under steady-state operating conditions. Although DPF, SCR and exhaust gas recirculation (EGR) systems can operate effectively at sufficiently high and stable exhaust temperatures, their performance in stationary applications is often compromised: generators frequently operate at constant low to medium load, which may prevent DPF regeneration, reduce SCR conversion efficiency and limit EGR effectiveness. Moreover, once removed from the vehicle, these systems typically lose their certified configuration, making it impossible to obtain the conformity declaration required for MCP registration.

In the case of emergency (periodic) generators, operating less than 500 h per year, the MCP Directive provides for partial exemption from full emission limits; however, such installations must still be reported to the competent environmental authority. Their operation is permitted only if the engine remains in good technical condition and does not produce excessive emissions. In practice, this means that the use of an automotive engine from dismantled vehicles could be accepted only in temporary, experimental, or non-commercial applications.

A summary of the emission compliance requirements and the applicability of road vehicle engines for various types of generator sets is presented in

Table 2. Compliance of automotive engines with emission requirements for different types of power generators.

Generator Application	Applicable Emission Standard	Compliance of Used Automotive Engine	Remarks
Mobile (Stage V)	Stage V (Reg. 2016/1628/EU) [146]	No	No NRMM type approval, different test cycles
Stationary (MCP)	MCP Directive 2015/2193/EU [147]	No	No conformity declaration or certified emission data
Emergency (<500 h/year)	Partial exemption under MCP	Conditional	Possible only after notification and technical assessment

.

In conclusion, engines recovered from road vehicles do not satisfy the formal or technical emission compliance requirements defined for power generators under EU regulations. Their use is only conditionally permissible, limited to research, experimental, or temporary applications, provided that the installation is duly reported and approved by the relevant environmental authorities. While the European regulatory landscape illustrates the structural barriers to legal reuse, a broader international comparison is required to determine whether similar limitations exist globally, which Section 5 addresses in detail.

5. Limitations on the Applicability of Engines from Scrapped Vehicles in Power Generators—A Global Review

The regulatory analysis presented in this section refers specifically to the commercial use of ELV engines in power generator units intended for sale.

In most developed countries, the use of ICEs recovered from decommissioned vehicles in power generators is not permitted without obtaining new emission certification or complying with separate regulations for stationary and mobile sources (Figure 1).

In EU, mobile generators fall under Stage V Regulation (Regulation (EU) 2016/1628—Non-Road Mobile Machinery Emission Standard) [146], stationary generators are governed by the MCP Directive (2015/2193/EU) [147], and emergency generators are subject to the MCP regime with specific operational hour exemptions.

Similar regulations apply in EFTA/EEA (European Free Trade Association/European Economic Area) countries such as Norway, Iceland, and Liechtenstein, where legislation is harmonized with the EU, as well as in the United Kingdom, which enforces NRMM Stage V and the Environmental Permitting Regulations for stationary sources.

Switzerland applies NRMM-equivalent emission standards aligned with the EU.

In the United States, mobile generators are regulated under EPA Tier 4f (Environmental Protection Agency—Final Tier 4 Nonroad Standard), while stationary units are covered by NSPS (New Source Performance Standards, 40 CFR Part 60 Subparts IIII/JJJJ) and NESHAP (National Emission Standards for Hazardous Air Pollutants), supplemented by state-level regulations such as those issued by CARB (California Air Resources Board).

In Canada, federal Non-Road Compression-Ignition Engine Regulations apply alongside provincial environmental rules.

Australia and New Zealand enforce state or territorial environmental permitting systems, while in Japan, non-road and stationary engine standards are issued by METI (Ministry of Economy, Trade and Industry) and MLIT (Ministry of Land, Infrastructure, Transport and Tourism).

South Korea regulates non-road and stationary engines under standards issued by the KMOE/ME (Korean Ministry of Environment).

Singapore, Hong Kong, and Taiwan maintain stringent environmental regimes where the use of on-road engines in power generators without re-certification is generally prohibited.

In the United Arab Emirates (UAE), Saudi Arabia, and Qatar, environmental permits are required, often referencing EU or U.S. emission standards, while Israel imposes specific emission limits and certification procedures for stationary generators.

In Latin American countries such as Brazil, Chile, Colombia, and Mexico, new emission regulations are increasingly harmonized with EU and U.S. frameworks, requiring certification for non-road and stationary engines. Consequently, even Euro 6/Euro VI vehicle engines cannot be automatically used in power generators, since Stage V, MCP, and NSPS differ in scope and emission assessment methodology. Therefore, separate certification and/or registration of the emission source is required before such an installation can be legally operated.

India has established comprehensive emission standards for non-road diesel engines, including those used in construction, agricultural machinery, and electric power generators. The Bharat Stage (Construction Equipment Vehicles (CEV)/Tractor, Earth Moving and Material Handling Equipment (TREM)) IV standards came into force in October 2020, with the implementation of Bharat Stage V planned for April 2024, aligning closely with the European Union’s Stage V regulations. No clear and authoritative sources were found that confirm the Bharat Stage V implementation was completed in full and according to the original schedule for all engine categories. These standards limit emissions of CO, HC + NO_x, NO_x, PM, and particle number (PN) based on NRSC/NRTC test cycles. In parallel, India maintains dedicated emission limits for diesel generator sets up to 800 kW, first introduced in 2004 and tightened in 2014, specifying maximum allowable values for CO, NO_x + HC, PM, and smoke opacity (g/kWh). Larger stationary generators above 800 kW are regulated separately under national environmental legislation. Overall, India’s regulatory framework for non-road engines and generators is among the most advanced in Asia, gradually converging with EU and US Tier 4 Final emission norms.

In Turkey, emissions standards for non-road engines were harmonized with the EU regulations, with Stage V requirements becoming effective in October 2022.

In China, non-road emission standards were adopted up to Stage IV and later tightened; the new China IV non-road standards came into force in December 2022 for engines up to 560 kW.

While Russia has adopted some European emission standards for mobile nonroad engines, progress has been delayed, as some of the Eurasian Economic Union (EAEU) are Armenia, Belarus, Kazakhstan, Kyrgyzstan, and Russia countries have been slow in updating emission standards from Stage 0. Russia therefore had to return to Stage 0 in order to adhere to Union protocols. The Russian government planned to introduce Stage III in January 2014 in Russia, but the date has not been officially confirmed. A Russian GOST standard was developed in 2012 but it was not officially published.

Although the regulatory review presented in this section spans multiple regions and legislative systems, a clear pattern emerges across all jurisdictions: existing emission frameworks classify engines strictly according to their original application (on-road, non-road, or stationary), and none of these systems provide a legal pathway for cross-category reuse. As a result, an engine originally certified for on-road use cannot be transferred to a generator or other stationary or mobile machinery without undergoing full re-certification—an expensive and practically inaccessible procedure for individual users or small manufacturers. This global regulatory architecture, despite differing in detail from country to country, reveals the same structural barrier: emission legislation is designed to control pollutants within isolated sectors, not to support circular-economy practices. Consequently, even technically functional engines recovered from end-of-life vehicles cannot be legally repurposed, leading to systematic waste of usable components and preventing material, energy, and CO₂ savings that remanufacturing could otherwise deliver. This shared regulatory gap underscores the need for a dedicated, internationally recognized secondary approval pathway that would enable safe, controlled, and environmentally compliant reuse of automotive engines outside their original application category.

In contrast, several low- and middle-income countries in Africa, South America, and parts of Southeast Asia do not impose explicit restrictions on the reuse of automotive engines in generator sets; however, this permissiveness results not from a supportive regulatory pathway but from the absence of emission legislation for such applications. In these markets, engines recovered from ELVs are widely used in power generators, often without emission control systems, quality oversight, or environmental monitoring. While this practice enables cheap access to electricity, it also leads to significantly higher local air pollution and lacks any formal mechanism to ensure environmental compliance or operational safety.

Taken together, these findings reveal a global regulatory paradox: highly regulated regions prohibit engine reuse because of rigid category-specific emission rules, whereas less regulated regions allow reuse by default, but without safeguards. This shared gap across both ends of the regulatory spectrum highlights the need for a dedicated, internationally recognized secondary approval pathway that would enable safe, controlled, and environmentally compliant reuse of automotive engines recovered from ELVs. Taken together, the global overview of emission standards shows that the reuse of automotive engines is constrained by systemic regulatory fragmentation, highlighting the importance of identifying common patterns and underlying causes, which are analyzed in the next section.

6. Comparison of Homologation Procedures for Engines Intended for Vehicles and Power Generators (Discrepancies Preventing Direct Comparison of Emission Limit Values)

The comparison of exhaust emission limits between engines intended for road vehicles (subject to Euro I–VI standards) and those used in power generators (regulated under the Stage V Regulation or the MCP Directive) is a complex issue due to fundamental differences in design, purpose, and emission assessment methods.

The Euro standards were developed for vehicles operating in road traffic and are primarily aimed at reducing emissions under real driving conditions, with particular emphasis on improving urban air quality. In contrast, the Stage V Regulation (2016/1628/EU) [146] and the MCP Directive (2015/2193/EU) [147] govern emissions from stationary and NRMM, focusing on the total pollutant load emitted per unit of generated energy.

A key difficulty in directly comparing these categories lies in the different reference units and test cycles applied. In the Euro standards for passenger cars, emissions are expressed in grams per kilometer (g/km), whereas for heavy-duty vehicles and for non-road and stationary applications they are expressed in grams per kilowatt-hour (g/kWh). These two approaches refer to fundamentally different physical quantities—the distance traveled by a vehicle is not directly comparable to the mechanical energy output of a generator. Converting emission data from g/km to g/kWh requires assumptions about average speed, engine load, and efficiency, which introduces considerable uncertainty and may lead to misleading conclusions.

Further complications arise from differences in testing procedures. Light-duty vehicles are tested using dynamic driving cycles such as the New European Driving Cycle (NEDC) or Worldwide Harmonized Light Vehicles Test Procedure (WLTP), while heavy-duty engines follow the European Transient Cycle (ETC) or WHTC [148]. In contrast, non-road and stationary engines are tested under steady-state NRSC or simplified dynamic NRTC. These differ significantly in duration, load profile, and speed variation, meaning that an engine compliant with Euro standards can substantially exceed the permitted limits when assessed according to Stage V procedures.

Another major difference lies in the range of regulated pollutants and the measurement techniques used. Both Euro and Stage V frameworks cover CO, NO_x, HC, and PM, but Stage V also includes a limit for PN and imposes stricter requirements on nitrogen oxides. The MCP Directive further regulates sulphur oxides (SO₂) and total dust, which are not measured under the Euro testing framework.

Fuel types and aftertreatment system calibrations also differ. Euro standards assume compliance with EN 590 diesel and EN 228 gasoline specifications, whereas Stage V and MCP regulations allow for a wider range of fuels, including biodiesel and heavy fuel oils. This variability affects emission characteristics and the efficiency of aftertreatment systems such as EGR, SCR, and DPF.

Another distinction arises from operational profiles. Road vehicle engines operate over a wide range of loads and speeds, while power generators generally run at constant or near-constant operating points, often under continuous load. As a result, Stage V and MCP requirements for NO_x and PM emissions per unit of energy are more stringent, assuming long-term, stable operation.

Differences also exist in lifetime and durability requirements. For road vehicles, emission durability is verified for mileage ranging from 160,000 to 700,000 km depending on category, whereas Stage V defines lifetime in operating hours (typically 3000–10,000 h). Under the MCP Directive, compliance is verified through continuous or periodic stack monitoring during operation rather than through type-approval testing.

Certification procedures are also distinct. Euro standards rely on vehicle type-approval (UNECE R83, R49), Stage V applies to engine type-approval independent of the vehicle, and the MCP Directive requires an environmental permit supported by emission monitoring and reporting. Therefore, even an engine compliant with Euro 6 or Euro VI standards cannot automatically be considered compliant with Stage V or MCP requirements without additional certification or validation.

Partial exemptions exist for emergency generators, which fall under the MCP regime only to a limited extent. Such units may operate for short durations (typically below 500 h per year), but they still require registration with the relevant environmental authority.

In summary, differences in reference units, testing methods, operational assumptions, regulated pollutants, and certification procedures mean that engines salvaged from vehicles—even those compliant with the strictest Euro VI limits—cannot be regarded as equivalent to Stage V or MCP-compliant engines. Their use in generator applications requires additional certification or an individual conformity assessment to ensure compliance with the applicable regulatory framework. The identified barriers underscore the need for a structured framework capable of reconciling reuse practices with emission compliance, leading to the development of a conceptual regulatory model presented in Section 7.

7. Proposed Homologation Procedure for ELV Engines Intended for Power Generator Applications

The adaptation of ICEs recovered from ELVs for use in power generators requires a dedicated homologation and validation procedure to ensure compliance with environmental, safety, and operational standards applicable to NRMM or stationary combustion plants (MCP). The proposed multi-stage procedure integrates both technical and formal aspects, enabling a transparent and standardized approach to the reuse of automotive engines in power generation applications.

The first stage involves identification and preliminary assessment. At this point, all essential engine data should be recorded, including serial number, manufacturer, displacement, rated power, fuel type, emission level (Euro I–VI), and year of production. It is also necessary to verify the availability of the original type-approval documentation for the vehicle from which the engine was removed. A visual inspection should confirm structural integrity and the absence of leaks, corrosion, or deformation. Furthermore, the intended application—mobile, stationary, or emergency—must be clearly defined, as it determines the applicable legal framework, either under the Stage V Regulation or the MCP Directive.

The second stage focuses on technical adaptation and safety compliance. The installation must be modified to suit generator operation, ensuring proper coupling with the alternator, vibration isolation, and adequate cooling. The fuel, lubrication, and exhaust systems should be adapted for stationary operation, for example, through the addition of fuel filters, mufflers, or heat shields. The complete unit must be equipped with overspeed protection, grounding, and emergency shutdown systems compliant with ISO 8528-10 [149] and relevant national electrical safety standards.

Once the system is adapted, a preliminary emission screening should be carried out under steady-state conditions using the NRSC procedure or an equivalent method. During this stage, emissions of CO, NO_x, HC, PM, and, where applicable, PN should be measured under rated-load conditions. The results must then be compared with the emission limits defined in Stage V or MCP regulations to determine whether exhaust after-treatment—such as DOC, DPF, or SCR systems—is necessary. If compliance cannot be achieved, appropriate retrofitting of emission control devices should be implemented.

The next phase involves formal emission validation. Type-test emission measurements must be performed according to Regulation (EU) 2016/1628 [146] for non-road mobile machinery or Directive 2015/2193/EU [147] for stationary combustion plants. All tests should be conducted by an accredited laboratory (ISO/IEC 17025) [150], and a comprehensive test report must be prepared, providing emission results in grams per kilowatt-hour (g/kWh), along with measurement uncertainty and testing conditions.

Following successful testing, environmental conformity and registration must be ensured. The validation report should be submitted to the competent environmental authority—such as a national ministry or inspection body—to obtain a Certificate of Conformity for Secondary Use (CCSU). This certificate confirms that the adapted engine meets the required emission and noise limits. Depending on the configuration, the generator must then be registered either as a NRMM if it is portable or trailer-mounted, or as a MCP if it is permanently installed.

Once in operation, the generator must be maintained under a defined emission monitoring and maintenance schedule. Emission verification should be performed at least every 1000 operating hours or annually, and an operational logbook must be kept to record fuel consumption, operating hours, and maintenance activities. After-treatment components should be periodically cleaned or regenerated following the manufacturer’s or retrofit supplier’s instructions to ensure long-term compliance and efficiency.

Finally, periodic inspection and re-certification are required every three to five years or after any major modification, such as an engine overhaul, fuel type change, or relocation. Emission validation should be repeated if the output power or configuration changes by more than ±5%.

In conclusion, the proposed procedure ensures that reused automotive engines can be safely and sustainably integrated into power generator applications while maintaining conformity with current environmental and safety regulations. It provides a coherent homologation pathway bridging vehicle type-approval systems (UNECE R83/R49) with non-road and stationary certification frameworks (Stage V and MCP). By following this process, it becomes possible to facilitate the sustainable reuse of ELV engines while upholding both legal compliance and environmental protection standards. Although the proposed framework establishes the theoretical basis for enabling compliant engine reuse, its practical implications must be evaluated through case analyses and application scenarios, which are explored in Section 8.

8. Policy Paradox in Engine Reuse: Local Emission Limits Versus Global Environmental Gains

From an ecological perspective, this issue requires consideration of the overall environmental balance throughout the entire product life cycle (LCA). Reusing ICEs from dismantled vehicles makes it possible to reduce the consumption of raw materials, energy, and emissions associated with the production of new power units. In this context, functional reuse represents an important element of the circular economy, contributing to the reduction in the carbon footprint during the manufacturing phase.

On the other hand, existing emission limits—such as Stage V, MCP, and NSPS—are designed to reduce direct emissions of toxic compounds (NO_x, PM, HC, CO) that have a local impact on air quality and human health. Their ecological significance is therefore short-term and local, whereas the environmental benefits of component reuse are long-term and global.

From a scientific standpoint, neither of these strategies can be considered universally superior—the priority depends on the specific environmental context. In regions with high air pollution, maintaining strict emission limits is essential for public health. Conversely, in countries with well-developed emission control infrastructure and access to advanced exhaust after-treatment technologies, greater environmental benefits may result from the reuse of functional engines, especially in low-load or limited-use applications such as emergency generators.

From a systemic perspective, the optimal solution would be to introduce secondary homologation procedures (Secondary Type Approval) that would allow for the legal and controlled reuse of engines recovered from dismantled vehicles, while maintaining environmental requirements at an acceptable level. The case analyses demonstrate both the feasibility and the limitations of the proposed pathway, creating the foundation for a broader discussion on policy impacts and implementation challenges addressed in Section 9.

9. Proposed Guidelines for New Homologation Tests That Should Be Simple, Low-Cost, and Accessible—Supporting the Applicability of the Concept

From the perspective of implementing a circular economy, a key element is the development of dedicated homologation procedures for engines recovered from dismantled vehicles. The proposed tests should be simple, inexpensive, and widely accessible, allowing their execution not only by large research centers but also by smaller entities involved in adapting power units. The aim of such procedures would not be to fully replicate the costly homologation tests applied to new engines, but rather to verify the basic emission and operational parameters under real generator operating conditions.

The use of simplified testing methods, based on a limited number of measurement points (e.g., selected load and speed conditions) combined with the analysis of key exhaust components (CO, NO_x, PM), could provide a sufficient assessment of environmental compliance at minimal cost. Such an approach would enable a rational balance between ecological and economic objectives, supporting the development of the technological recycling sector without compromising environmental protection standards. The policy considerations outlined above indicate the need for strategic recommendations that reconcile environmental objectives with circular-economy practices, which are formulated in Section 10.

10. Academic Justification for Reviewing Legal Regulations and Guidelines

A review of legal regulations and guidelines is a rigorous academic undertaking of high scholarly value, particularly in the context of EU law. Such work combines legal analysis, comparative assessment, and the evaluation of regulatory impact on economic, administrative, and social systems. It requires methodological precision, systematic organization of sources, and interpretation in light of EU and national legal principles.

Its academic merit lies in the identification and analysis of legal instruments—including regulations, directives, harmonized standards, and common specifications—that form the foundation of the EU internal market and ensure compliance [151]. Comparative analysis further enhances this approach, revealing differences in how Member States implement regulations and highlighting areas for improvement [152].

An review also examines regulatory frameworks and compliance, clarifying how EU policies influence corporate responsibility, environmental protection, and consumer rights [153,154]. For example, the EU’s management of PFAS substances demonstrates the complexity of harmonized risk assessments required to safeguard public health and the environment [155].

Additionally, the role of judicial review remains central to maintaining the rule of law and limiting administrative discretion within EU institutions [156,157]. Finally, such analyses often lead to policy recommendations, identifying inconsistencies and proposing improvements for more effective implementation [158,159,160].

In summary, reviews of legal frameworks represent a recognized academic contribution, combining analytical rigor with practical relevance. They support a deeper understanding of the legal landscape, promote better regulatory design, and strengthen evidence-based policymaking—while also guiding the direction of engineering work or prompting the withdrawal from certain research paths when regulatory developments render them impractical or noncompliant [141,161,162]. These recommendations provide a coherent foundation for future regulatory development, paving the way for the concluding synthesis presented in Section 11.

11. Differences Between SI and CI Engines in the Context of Recycling Through Reuse

A To analyze the differences between SI engines and CI engines in the context of recycling and reuse, we can consider several aspects such as durability, fuel efficiency, emissions, and adaptability to alternative fuels.

From a recycling and reuse perspective, SI and CI engines exhibit substantial differences arising from their combustion systems, structural design and aftertreatment requirements. Modern SI engines increasingly employ direct gasoline injection (GDI) systems operating at injection pressures of 150–350 bar and, consequently, higher in-cylinder pressure peaks than earlier port-fuel-injection designs. However, even with GDI, SI engines still operate with homogeneous or quasi-homogeneous charge combustion and substantially lower compression ratios and peak cylinder pressures compared with CI engines [163]. As a result, the mechanical loading of the piston assembly, connecting rods, crankshaft and bearings remains notably lower than in CI engines, which reduces long-term material fatigue and generally facilitates component-level remanufacturing. In SI engines, the piston–connecting-rod assembly and crankshaft bearings account for a significant share of total mechanical losses—between 19.2% and 36.9% according to Romero et al. in 2023 [163]—and the lower load levels reduce fatigue stress on these components, improving their longevity. Although connecting rods in SI engines are subjected to alternating compressive and tensile stresses that may lead to fatigue failure over time [164,165,166], the comparatively lower mechanical and pressure loads than in compression-ignition engines mitigate these stresses and lower the probability of fatigue-induced damage, thereby improving remanufacturing feasibility. The fatigue strength and durability of components such as the piston pin and connecting rod depend strongly on material properties and geometric design, and studies show that design and material optimization can significantly extend component life [166,167,168]. Because fatigue behavior results from the combined action of thermal and mechanical loads, the reduced mechanical loading in SI engines also diminishes the thermo-mechanical stress acting on the crankshaft, bearings and other rotating parts [169,170]. As a result, components in SI engines are less prone to severe wear or deformation, simplifying inspection, refurbishment and reuse. Overall, the lower mechanical loading characteristic of SI engines translates into reduced material fatigue and greater durability of key rotating components, which in turn increases the remanufacturing and long-term reuse potential of these engines [163,164,166].

CI engines are generally more fuel-efficient than SI engines due to higher compression ratios and leaner combustion [171,172], which makes them suitable for applications requiring high torque and fuel economy, including heavy-duty vehicles and power generation [173,174]. They also benefit from advanced electronic controls and high-pressure fuel-injection systems that enhance performance and reliability [175]. SI engines, although less fuel-efficient, offer a higher power-to-weight ratio that suits smaller vehicles and applications where weight is a key factor [174,175]; they are easier to start and often exhibit lower exhaust emissions, which is advantageous under strict environmental constraints [176]. In terms of emissions, CI engines typically emit higher levels of nitrogen oxides (NO_x) and particulate matter [171,173,177], necessitating the use of after-treatment systems such as catalytic converters and diesel particulate filters [173], while alternative fuels such as biodiesel can help reduce their environmental impact [174,177]. SI engines generally produce lower NOx and particulate emissions [174,176], and apply methods such as exhaust gas recirculation (EGR) to further reduce emissions and improve efficiency [178,179]. Regarding fuel adaptability, CI engines can operate on biodiesel, dimethyl ether and various alcohol fuels [171,177], and dual-fuel systems combining diesel with high-octane fuels can further reduce emissions while maintaining efficiency [171,172]. SI engines, in turn, can be converted to operate on natural gas, which offers a high antiknock index and may improve thermal efficiency [180], and the use of biofuels can reduce soot particle emissions, albeit sometimes at the cost of increased specific fuel consumption [174]. From a recycling and reuse perspective, the high efficiency and durability of CI engines make them well-suited for long-term operation and secondary use in industrial environments, and their compatibility with various alternative fuels supports reuse across different applications. SI engines, with lower emissions and suitability for lighter-duty applications, can be reused effectively in urban settings where environmental requirements are more stringent, and their conversion to natural-gas operation remains a widely practiced reuse pathway. In conclusion, both CI and SI engines present distinct advantages and limitations in the context of recycling and reuse: CI engines offer superior efficiency and fuel adaptability, while SI engines provide lower emissions and greater suitability for lightweight or urban applications.

12. Conclusions

This review examined whether internal combustion engines recovered from end-of-life vehicles can be feasibly reused in stationary power generation, addressing the central question of how technical condition, environmental performance, economic viability, and regulatory compliance collectively shape this potential. The analysis showed that while reused engines offer clear environmental and resource-efficiency benefits—particularly through reduced material consumption and lower embodied carbon compared to manufacturing new units—they face substantial barriers, including uncertain component conditions, incompatibility of automotive after-treatment systems with stationary operation, high retrofitting and certification costs, and strict emission regulations in many jurisdictions. By systematically comparing international regulatory frameworks, the study highlights a critical gap: the absence of harmonized global rules for repurposed engines, which creates uncertainty for operators and limits the adoption of circular-economy solutions. The main contribution of this work lies in consolidating fragmented technical, economic, and legislative information into an integrated assessment that clarifies why large-scale reuse of ELV engines remains limited despite its sustainability potential. The study is constrained by the lack of quantitative field data on long-term performance and emissions of repurposed engines, as well as by the variability of national regulations, which limit generalizability. Future research should focus on developing standardized diagnostic procedures for evaluating used engines, creating simplified and low-cost certification pathways tailored to circular-economy applications, and generating empirical LCA and durability datasets to support evidence-based policymaking.