A Review on Intelligent Combustion Control and Clean-Fuel Strategies for Aviation Heavy-Fuel Piston Engines

Abstract

Aviation heavy-fuel piston engines are widely used in UAVs, general aviation, and military platforms due to their fuel efficiency and adaptability. However, emissions of NO_x, PM, and other pollutants pose significant environmental challenges. This paper reviews emission-reduction strategies, including combustion-chamber optimization, fuel-injection control, alternative fuels, and exhaust after-treatment technologies. Research indicates that optimizing combustion-chamber geometry, high-pressure common-rail injection, and turbulence enhancement improve combustion efficiency and reduce emissions. Biofuels, synthetic aviation fuels (SAF), and hydrogen-based fuels demonstrate strong potential for low-carbon emissions, while after-treatment technologies such as SCR, DPF, and EGR effectively mitigate NO_x and PM emissions. Despite technological advancements, challenges remain in balancing combustion efficiency with NO_x control and ensuring compatibility between EGR and combustion stability. Future advancements in intelligent combustion control, novel catalytic materials, low-temperature combustion, and high-efficiency after-treatment systems will drive aviation diesel engines toward lower emissions, higher efficiency, and greater intelligence, contributing to the green and sustainable transformation of aviation propulsion systems.

1. Overview

Heavy-fuel piston engines (HFPEs) have recently gained widespread adoption in unmanned aerial vehicles (UAVs), general-aviation aircraft, and specialized military platforms owing to their superior thermal efficiency, fuel economy, and fuel flexibility. Compared with conventional aviation gasoline engines, HFPEs operate on heavy fuels such as Jet-A and JP-8, thereby simplifying logistics and aligning with the international aviation sector’s carbon-emission-reduction mandates [1]. Although many combustion and emission-control concepts applied in HFPEs originate from conventional diesel-engine research, aviation heavy-fuel piston engines operate under fundamentally different constraints—such as high-altitude and low-temperature environments, extended endurance operation, and stringent weight and reliability requirements—which necessitate specific adaptation and evaluation of these technologies. Figure 1 summarizes the historical development of heavy-fuel piston engines, highlighting the progressive evolution of fuel types, combustion concepts, and emission-control technologies. Moreover, the simultaneous expansion of the global general-aviation market and the rapid emergence of the low-altitude economy have intensified the demand for low-carbon, high-efficiency propulsion systems, positioning engine-based emission-reduction technologies as a pivotal focus of contemporary aeronautical powertrain research [2,3].

In aviation heavy-fuel piston engines, which predominantly operate in compression -ignition (CI) or homogeneous charge compression ignition (HCCI) modes, in-cylinder temperature and mixture formation characteristics exert a direct and decisive influence on pollutant formation pathways. Because HFPEs predominantly operate in compression ignition (CI) or homogeneous charge compression ignition (HCCI) modes, in-cylinder temperature and mixture formation characteristics exert a direct and decisive influence on pollutant formation pathways. Consequently, emission-mitigation strategies targeting combustion-chamber optimization, spray atomization technologies, exhaust gas recirculation (EGR), after-treatment systems, and the utilization of clean alternative fuels have become the central thrust of green propulsion development in aviation powertrains [4].

Emission-reduction technologies for aviation heavy-fuel piston engines (HFPEs) primarily encompass combustion-chamber structural optimization, advanced fuel-injection strategies, catalytic after-treatment systems, and the utilization of alternative fuels. Under aviation operating conditions, the complexity of the engine environment continues to pose critical technical challenges, which can be summarized as follows: First, achieving an optimal balance between combustion-chamber design and NO_x mitigation remains a central issue [5,6]. While premixed-combustion strategies can enhance thermal efficiency, they concurrently tend to elevate NO_x emissions; therefore, further optimization of combustion phasing and integrated thermal-management strategies is imperative [7]. Second, in aviation heavy-fuel piston engine applications, fuel atomization characteristics exert a decisive influence on particulate emissions; the inherently low volatility of heavy fuels compromises spray quality under cold-start and high-altitude conditions, leading to incomplete combustion and elevated PM output. Consequently, critical development pathways include optimizing injection pressure, implementing multi-pulse injection strategies, and advancing supercritical combustion technologies [8,9,10]. Moreover, for aviation heavy-fuel piston engines, the implementation of exhaust gas recirculation (EGR) necessitates a rigorous trade-off between NO_x abatement and combustion stability. Although EGR has been empirically validated to suppress NO_x formation through dilution and thermal effects, excessive recirculation frequently engenders deteriorated ignitability, augmented cycle-to-cycle variability, and a concomitant decline in thermal efficiency. Consequently, the development of adaptive, model-based EGR control algorithms—calibrated across the entire aviation flight envelope—and their synergistic integration with secondary-combustion concepts or plasma-assisted ignition strategies constitute an exigent avenue for future investigation [11]. Finally, the adoption of emerging low-carbon fuels—including sustainable aviation fuels (SAF), and bio-derived heavy oils—offers substantial potential for reducing life-cycle carbon footprints. Nevertheless, their deployment is constrained by unresolved issues of fuel–engine compatibility, long-term storage stability under variable climatic conditions, and elevated supply-chain costs stemming from limited production scalability and nascent distribution infrastructure. Rigorous assessments of thermo-physical properties, oxidative degradation kinetics, and elastomer-seal compatibility across the entire operational envelope are therefore imperative to ensure safe and economically viable integration into next-generation HFPE systems [12,13].

This study undertakes a systematic review of combustion and emission characteristics of heavy-fuel piston engines (HFPEs) for aviation, critically examining combustion-chamber optimization techniques and clean-fuel strategies as primary emission-mitigation pathways toward greener propulsion systems. The article first delineates the current state of HFPE combustion-chamber technology, elucidating fundamental operating principles and categorizing chamber architectures while discussing key design metrics—namely combustion efficiency, temperature distribution, spray atomization behavior, and pollutant emission indices. Forward-looking trends in advanced combustor concepts are subsequently analyzed to furnish reference frameworks for future optimization efforts. Second, the paper provides an in-depth elucidation of HFPE emission profiles and their governing factors. This encompasses a mechanistic characterization of principal pollutants (NO_x, CO, HC, and PM), their formation kinetics, and associated environmental impacts, together with an assessment of how combustion phasing, external ambient conditions, fuel physicochemical properties, and operational duty cycles collectively modulate emission signatures. Finally, the study synthesizes state-of-the-art emission-reduction technologies for HFPEs, focusing on synergistic combustion-chamber refinements and the utilization of alternative fuels—including sustainable aviation fuels (SAF), biodiesel, and synthetic e-fuels—and evaluates their prospective deployment within next-generation aviation powertrains. By integrating the latest domestic and international advances, the review systematically collates existing HFPE decarbonization strategies and projects the developmental trajectory of low-carbon aero-propulsion systems, thereby offering theoretical foundations and practical guidance for the green technological transition of aviation engines.

2. Development Status of Combustion-Chamber Technologies for Heavy-Fuel Aero-Piston Engines

2.1. Basic Principles and Types of Combustion Chambers

Within the combustion chamber of an aviation heavy-fuel piston engine (HFPE), the combustion process is governed by the coupled evolution of fuel injection, air–fuel mixing, auto-ignition, and heat release under high-pressure and high-temperature conditions characteristic of compression-ignition operation [2]. Rather than detailing the classical cycle-resolved combustion stages that have been extensively documented in standard diesel-engine literature, this section focuses on the key physical processes that are most relevant to combustion-chamber design and emission formation in aviation applications.

Following fuel injection near top-dead-center, atomized fuel droplets undergo rapid vaporization and turbulent mixing with the compressed in-cylinder air. The subsequent ignition delay, which is controlled by local thermodynamic conditions and chemical kinetics, plays a critical role in determining the initial heat-release rate and pressure rise [7,14]. Once auto-ignition is initiated, the overall combustion rate is primarily governed by fuel-injection characteristics, spray atomization quality, and in-cylinder flow organization, all of which directly affect combustion efficiency and pollutant formation [1,8]. Figure 2 schematically illustrates these coupled processes and their temporal evolution within the HFPE combustion chamber. At the end of the combustion event, exhaust gases are expelled during the exhaust stroke, and modern HFPE systems typically employ exhaust gas recirculation (EGR) in combination with after-treatment technologies—such as diesel particulate filters (DPF) and selective catalytic reduction (SCR)—to further mitigate nitrogen oxides and particulate emissions [15].

The combustion-chamber architecture of an aviation heavy-fuel piston engine exerts a decisive influence on mixture formation, combustion stability, thermal efficiency, and emission characteristics. Based on structural configuration and the dominant combustion process, prevalent chamber types can be broadly classified into open-chamber, pre-chamber, and turbulence-enhanced designs [4]. Each configuration exhibits distinct features in terms of spray development, in-cylinder flow structures, and combustion robustness, making them suitable for different aero-propulsion requirements.

Open-type combustion chambers integrate the combustion cavity directly into the piston crown and operate as direct-injection systems. In such designs, fuel is injected into the central region of the chamber, and turbulence is generated through chamber geometry and squish effects, leading to the commonly adopted “ω”-shaped configuration (Figure 3). Pre-chamber combustion systems, illustrated in Figure 4a, employ a small auxiliary chamber connected to the main cylinder volume. Combustion is initiated in the pre-chamber under locally rich conditions and subsequently propagates into the main chamber, enabling overall lean operation and improved combustion stability. Turbulence-enhanced combustion chambers, shown in Figure 4b, are designed to intensify in-cylinder air motion, thereby promoting rapid fuel–air mixing and more complete combustion.

Table 1. Comparative summary of different combustion-chamber technologies for aviation heavy-fuel piston engines.

Chamber Type	Structural Characteristics	Main Advantages	Main Limitations	Emission/Combustion Features	Typical Application/Research Focus
Open-type/Direct-injection chamber	Combustion cavity integrated into piston crown; fuel injected directly into the main chamber	Simple structure, high combustion efficiency, good fuel economy, suitable for high-pressure common-rail systems	Sensitive to spray–wall interaction and local fuel-rich zones; may increase NOx under high-temperature conditions	Strong atomization and rapid heat release; good overall efficiency, but requires optimization of temperature distribution and soot control	Mainstream configuration in modern aero heavy-fuel piston engines
ω-type chamber	Piston-bowl geometry designed to improve in-cylinder flow organization and temperature-field uniformity	Enhances air–fuel mixing, reduces local hot spots, and improves combustion uniformity	Geometry optimization is more complex; performance depends strongly on injection matching	Contributes to simultaneous reduction in NOx and PM by suppressing local high-temperature and fuel-rich regions	Combustion-chamber optimization for emission reduction under aviation operating conditions
Pre-chamber combustion system	Auxiliary chamber connected to main chamber; ignition and early combustion occur in the pre-chamber	Improves ignition capability and combustion stability, especially under lean or oxygen-deficient conditions	Higher heat loss and greater structural complexity may reduce thermal efficiency	Favors stable combustion and can reduce NOx under harsh environments, but thermal-management challenges remain	High-altitude, hypoxic, and cold-environment operation
Turbulence-enhanced chamber	Chamber geometry designed to intensify swirl, squish, or turbulence	Promotes rapid fuel–air mixing, improves combustion rate and homogeneity	Excessive turbulence may increase heat-transfer loss and design sensitivity	Improves combustion stability and reduces incomplete-combustion products such as CO and HC	Flow-field optimization and combustion-stability enhancement

shows that different combustion-chamber technologies have distinct advantages and limitations in aviation heavy-fuel piston engines. Open-type direct-injection chambers are widely used because of their simple structure and high combustion efficiency, whereas ω-type chambers are more advantageous for improving mixture uniformity and reducing local hot spots. Pre-chamber systems are beneficial for ignition and combustion stability under harsh operating conditions, while turbulence-enhanced chambers mainly improve in-cylinder air motion and reduce incomplete-combustion emissions. Overall, combustion-chamber design should be selected and optimized according to specific operating conditions and emission-control requirements.

In summary, different combustion-chamber architectures represent distinct development pathways for aviation heavy-fuel piston engine technology, each addressing specific performance and emission-control requirements. With the continued advancement of high-pressure common-rail injection systems and combustion-optimization strategies, these chamber concepts are expected to further leverage their respective advantages across diverse operating conditions, supporting future aviation propulsion systems that demand high efficiency, low emissions, and adaptability to complex flight environments [17,18].

2.2. Key Technical Indices and Performance Parameters for Combustor Design

The design of the combustion chamber in aviation heavy-fuel piston engines exerts a decisive influence on combustion efficiency, emission control, fuel economy, and overall engine performance. In recent years, driven by advances in combustion-optimization technologies, computational fluid dynamics (CFD) simulation, high-pressure common-rail fuel injection systems, advanced materials, and cooling technologies, combustion-chamber design has continuously evolved toward higher efficiency, lower emissions, and enhanced adaptability to complex operating conditions [18,19]. At present, the core research agenda in combustion-chamber design is concentrated on combustion-efficiency optimization, enhancement of combustion stability, thermal-management improvement, flow-field refinement, and fuel-injection matching. These efforts have yielded an array of advanced chamber architectures and control strategies that address the technological requirements of modern aero-diesel engines [20].

Combustion efficiency directly governs the conversion rate of the fuel’s chemical energy and serves as the pivotal metric for enhancing engine thermal efficiency and reducing fuel consumption. Contemporary combustion-chamber design therefore strives to improve fuel atomization quality and the homogeneity of the air–fuel mixture, thereby minimizing the emissions of unburned hydrocarbons (HC) and particulate matter (PM) [21]. Open-type combustion chambers, in which fuel is injected directly into the high-temperature, high-pressure air charge, exhibit elevated combustion efficiency and have consequently emerged as the mainstream configuration for current aero-diesel engines. Moreover, the adoption of multi-stage injection strategies—e.g., a pilot injection followed by a main injection—further augments fuel-burning efficiency while markedly diminishing the occurrence of incomplete combustion [22,23]. In recent years, researchers have coupled CFD simulations to refine the combustion process, modeling fuel-spray atomization and combustion-chemical kinetics so as to optimize injection angles and air-motion patterns, thereby achieving further gains in combustion efficiency [24].

Combustion stability is a decisive determinant of engine reliability, especially under the complex operating regimes of aero engines—high-load conditions, low-speed cruise, and high-altitude hypoxia—where stable combustion becomes paramount. Early pre-chamber combustion chambers employed a dual-zone combustion concept and exhibited superior stability in severe environments such as extreme cold and high altitude; however, owing to elevated thermal losses, they have been progressively supplanted in recent years by optimized direct-injection chambers [25]. To enhance the stability of direct-injection combustion chambers, contemporary research concentrates primarily on optimizing fuel-injection control and intensifying air-motion organization. The implementation of high-pressure common-rail fuel injection technology has markedly increased the controllability of both fuel quantity and injection phasing, enabling intelligent adjustment of fuel delivery under varying operating conditions and thereby averting combustion-instability phenomena [26]. Simultaneously, turbulence-enhancement techniques have emerged as a pivotal pathway to augmenting combustion stability. For instance, turbulent combustion chambers refine the in-cylinder flow structure, yielding a more uniform fuel distribution, elevating combustion homogeneity, and mitigating knock propensity [22].

Combustion-chamber thermal management directly governs engine service life and combustion efficiency; recent investigations have therefore focused on novel high-temperature-resistant materials, optimization of cooling architectures, and control of combustion-temperature uniformity [27]. Because the high combustion rate in direct-injection chambers elevates wall temperatures, modern aero-diesel engines extensively employ high-temperature alloys, ceramic thermal-barrier coatings, and low-thermal-conductivity composite materials to enhance chamber thermal endurance and mitigate thermal-stress issues [28]. Moreover, refinements in cooling technologies—such as piston-cooling jets and cylinder water-cooling galleries—yield a more uniform thermal-load distribution within the combustion chamber, attenuate the risk of local overheating, and elevate overall thermal-management performance [29].

The in-chamber flow field directly governs fuel atomization and combustion rate, rendering it one of the focal domains in current combustion-optimization research [5]. Contemporary combustion-chamber design leverages CFD simulation to analyze air and fuel flow behavior, thereby optimizing in-cylinder turbulence characteristics, swirl intensity, and air–fuel mixing homogeneity [30,31]. With respect to flow-field optimization, investigations of the ω-type combustion chamber demonstrate that its distinctive dual-recess configuration amplifies air-turbulence activity, enhances fuel–air mixing uniformity, and diminishes locally fuel-rich regions, thereby elevating both combustion rate and efficiency [22].

The matching relationship between the fuel-injection system and the combustion chamber governs the precision with which the combustion process can be controlled. In recent years, the application of high-pressure common-rail fuel injection technology has markedly enhanced fuel-injection accuracy [6]. Compared with conventional mechanically actuated injection systems, high-pressure common-rail systems can precisely regulate injection pressure, spray angle, fuel quantity, and injection phasing under varying operating conditions, thereby optimizing the combustion process, enhancing mixture uniformity, and curtailing emissions [32], The system is mainly composed of a high-pressure fuel pump, a common-rail volume together with high-pressure fuel lines, and fuel injectors, as shown in Figure 5. Modern combustion-chamber design is increasingly reliant on variable injection strategies—e.g., multiple injections (Pilot Injection, Main Injection, Post Injection)—to optimize combustion phasing, yielding smoother combustion and reducing both knock tendency and the formation of high-temperature zones.

2.3. Development Trends of Advanced Combustor Technologies

As aviation heavy-fuel piston engines impose ever more stringent demands on fuel economy, emission control, thermal management, and power performance, combustion-chamber technology is rapidly advancing along trajectories of high-efficiency combustion, intelligent combustion control, low-emission design, and novel-material application. In recent years, numerous scholars have conducted in-depth investigations focused on combustion-chamber optimization.

Enhancement of combustion efficiency remains the central objective of combustion-chamber technology. Modern designs continue to refine fuel atomization, air–fuel mixing, and combustion-phasing control. In recent years, ultra-high-pressure fuel injection (>2500 bar) has become the prevailing trend, markedly improving atomization quality and producing more homogeneous combustion, thereby reducing soot emissions. Staged combustion (Staged Combustion) and multiple injection (Multiple Injection) strategies further optimize combustion phasing, yielding smoother heat-release rates and mitigating knock tendency. Blasio et al. demonstrated that swirl control coupled with adaptive combustion-chamber geometry can elevate fuel-utilization efficiency while simultaneously decreasing HC and NO_x emissions [24,33].

A pivotal future direction for combustion-chamber technology is the intelligent combustion management (ICM) system, in which intelligence refers to the adaptive control and optimization of the combustion process, rather than to the combustion phenomenon itself. Electronic control units (ECUs) are already extensively employed to regulate fuel injection, ignition timing, and air–fuel ratio, whereas next-generation systems are evolving toward AI-assisted, real-time adaptive combustion-control frameworks. For instance, Lv et al. investigated thermal-management technologies for aero engines and underscored that AI predictive models can dynamically optimize injection pressure and spray angle in response to ambient parameters, thereby enhancing combustion efficiency [34]. Furthermore, Shao et al. proposed a novel AI-based combustion-prediction model capable of real-time monitoring of combustion anomalies—such as incomplete combustion or knock—and dynamically correcting the combustion strategy, thereby enhancing engine operational stability and adaptability [6].

As global environmental regulations become increasingly stringent, a pivotal direction in combustion-chamber design is the reduction in pollutant emissions—namely nitrogen oxides (NO_x), particulate matter (PM), and carbon monoxide (CO). Presently, exhaust gas recirculation (EGR), selective catalytic reduction (SCR), and diesel particulate filters (DPF) are widely employed in aero-diesel engines; however, future trends emphasize source control, i.e., curtailing pollutant formation at the combustion-chamber design stage. For instance, Stefanizzi et al. investigated ultra-lean combustion and demonstrated that adopting a high air–fuel ratio (AFR) strategy effectively lowers combustion temperatures, thereby suppressing NO_x formation [35,36]. Furthermore, Ciupek et al. investigated hydrogen–diesel dual-fuel combustion and reported that the introduction of a small hydrogen fraction optimizes the combustion rate, improves combustion efficiency, and markedly reduces soot emissions [37].

Future aero-diesel combustion chambers are poised for significant breakthroughs in high-efficiency combustion modes, intelligent combustion management, and novel combustion-chamber geometric optimization. Concurrently, the integration of artificial intelligence and real-time combustion-optimization algorithms will endow combustion chambers with the capability to dynamically modulate combustion states, thereby achieving more efficient and environmentally benign combustion processes to satisfy the evolving requirements of next-generation aviation propulsion systems.

Table 2. Aviation heavy-fuel piston engine combustion-chamber technology overview.

Dimension	Core Content	Current Mainstream Solution	Key Performance Metrics	Technical Bottlenecks	Future Directions
Combustion Process	Compression → Injection → Auto-ignition → Main Combustion → Expansion and Exhaust	Direct-injection (open) combustion chamber	Thermal efficiency 45–50%	Cold-start/high-altitude ignition difficulties	Low-temperature combustion (LTC), Homogeneous Charge Compression Ignition (HCCI)
Combustor Type	Open (ω-type)/Pre-chamber/Turbulence chamber	Open ω-type piston bowl	Combustion efficiency > 95%	High heat loss in the pre-chamber	Adaptive variable-geometry combustion chamber
Design Targets	Combustion efficiency, stability, thermal management, flow uniformity	CFD simulation + experimental co-optimization	NOx ≤ 3 g kWh−1	Large temperature gradients at high load	AI + real-time sensor closed-loop control
Fuel-Injection Matching	High-pressure common-rail ≥ 1800 bar, multiple injections	Triple-injection strategy	Spray SMD < 15 µm	Atomization deterioration in extreme environments	Ultra-high-pressure > 2500 bar + AI injection models
Advanced Trends	Intelligent combustion management, ultra-lean combustion, hydrogen–diesel dual-fuel	AI-ECU real-time optimization	30% NOx reduction	Algorithmic throughput and sensor reliability	Digital-twin combustion chamber + additive manufacturing

summarizes the main technological dimensions of combustion and emission control in aviation heavy-fuel piston engines, including combustion process, combustor type, design targets, injection matching, and advanced trends. It shows that current mainstream solutions are mainly based on direct-injection open chambers, high-pressure common-rail systems, and CFD-assisted optimization, while future development is moving toward intelligent control, adaptive chamber design, and ultra-high-pressure injection. The table also highlights several important bottlenecks, such as cold-start difficulty, heat loss, thermal non-uniformity, and atomization deterioration, indicating that future optimization requires coordinated multi-parameter design rather than isolated improvement of a single component.

3. Emission Characteristics and Influencing Factors of Heavy-Fuel Aero-Piston Engines

3.1. Main Pollutants from Engine Emissions and Their Hazards

Emissions from aviation heavy-fuel piston engines mainly comprise carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NO_x), and particulate matter (PM), which collectively influence environmental quality, human health, and engine operational reliability [38,39]. In recent years, extensive research efforts have been directed toward aviation-oriented emission-mitigation strategies, with particular emphasis on fuel-injection optimization, combustion optimization, exhaust after-treatment technologies, and the utilization of alternative fuels, in order to comply with increasingly stringent aviation emission regulations. Figure 6 presents the variation in HC, CO, and NO_x emissions under different engine speed and operating conditions.

CO emissions are closely associated with incomplete combustion under aviation-relevant operating conditions, particularly during low-temperature, low-load, and cold-start operation. Recent studies have demonstrated that high-pressure common-rail fuel injection can significantly improve spray atomization quality and thereby reduce CO formation. Masera and Hossain reported that optimizing injection pressure and spray angle achieved CO reductions of up to 20% [40]. Similarly, Cao et al. examined the influence of injection pressure on CO emissions and found that short injection pulses suppress the formation of fuel-rich zones within the combustion chamber, thereby enhancing combustion completeness and lowering CO emissions [41]. Nevertheless, elevated CO emissions persist under low-load operation and cold-start conditions, indicating that future research should focus on adaptive injection-control strategies and combustion-preheating technologies.

HC emissions in aviation heavy-fuel piston engines are strongly influenced by fuel properties, mixture homogeneity, and local quenching effects, especially under transient and low-temperature operating conditions. In recent years, notable progress in HC reduction has been achieved through the application of bio-derived alternative fuels. Xu et al. investigated the emission characteristics of 100% HEFA sustainable aviation fuel and demonstrated substantial reductions in both HC and PM emissions compared with conventional RP-3 aviation kerosene [42]. In the future, the integration of hydrogen–diesel dual-fuel combustion strategies is expected to further enhance combustion efficiency and suppress HC formation.

NO_x emissions in aviation heavy-fuel piston engines are governed primarily by in-cylinder thermal conditions and oxygen availability, particularly during high-load and high-temperature operation. In recent years, exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have become dominant strategies for aviation NO_x control. Palash et al. reviewed the synergistic optimization of SCR and EGR and reported that EGR can reduce NO_x emissions by more than 40%, while SCR systems convert NO_x into harmless N₂ and H₂O via urea-based reduction reactions [43]. In addition, Zhong investigated the influence of direct-injection compression-ignition (DICI) combustion on NO_x emissions and demonstrated that moderate increases in injection pressure can suppress NO_x formation without compromising fuel economy [6]. In the future, the integration of artificial-intelligence-based combustion optimization and real-time combustion monitoring is expected to further reduce NO_x emissions [44].

Figure 6. Illustration of gaseous emissions indices (g kg⁻¹) of piston engine aircraft [45].

Figure 6. Illustration of gaseous emissions indices (g kg⁻¹) of piston engine aircraft [45].

PM emissions from aviation heavy-fuel piston engines remain a major concern due to their pronounced impacts on air quality and human health. Recent studies have shown that ultra-high-pressure fuel injection (>3000 bar), combined with diesel particulate filters (DPF), can effectively suppress soot formation while simultaneously enhancing combustion stability. The influence of engine operating conditions on PM emissions is illustrated in Figure 7. Furthermore, Durdina et al. investigated the effects of sustainable aviation fuel (SAF) on PM emissions and reported that blending 10–20% SAF can achieve PM reductions of up to 50% [46,47]. In the future, further reductions in soot formation are expected through the development of more efficient particulate-filtration technologies and the optimization of combustion-chamber geometries, such as M-type combustion chambers.

In recent years, emission-control technologies for aviation heavy-fuel piston engines have achieved remarkable progress. Significant advances have been realized in reducing CO, HC, NO_x, and PM emissions through the optimization of fuel-injection parameters, enhanced synergistic control of EGR and SCR systems, the development of low-temperature combustion strategies, and the implementation of advanced particulate-filtration technologies. Nevertheless, persistent challenges remain under low-load operation, cold-start conditions, and in managing the inherent trade-offs associated with high-efficiency combustion modes. Looking forward, the integration of artificial-intelligence-based combustion optimization, hydrogen–diesel dual-fuel combustion technologies, and intelligent emission-monitoring systems is expected to further propel aviation heavy-fuel piston engines toward high efficiency, low emissions, and intelligent operation, thereby meeting increasingly stringent aviation emission standards.

3.2. Relationship Between Emissions and the Combustion Process

The combustion process in aero heavy-fuel piston engines directly determines their emission characteristics; studies have revealed that combustion efficiency, in-cylinder temperature and pressure, and the mixture formation process are the pivotal factors governing pollutant formation, the generation pathways of which are illustrated in Figure 8 as a schematic representation of the spatial distribution of major pollutant formation zones reported in previous studies. Specifically, soot formation is favored in locally fuel-rich regions of the spray flame where oxygen availability is limited, whereas nitrogen oxides (NO_x) are predominantly generated in high-temperature, oxygen-rich zones near the flame front and post-flame regions. Carbon monoxide (CO) and unburned hydrocarbons (HC) are mainly associated with low-temperature zones and flame-quenching regions, where oxidation reactions remain incomplete. In recent years, a number of research institutions have conducted in-depth investigations centered on these three core issues and have achieved significant progress.

In aviation heavy-fuel piston engines, improving combustion efficiency constitutes a primary pathway for reducing HC, CO, and particulate matter (PM) emissions, particularly under high-altitude and hypoxic operating conditions [48]. In recent years, researchers have concentrated on combustion-chamber optimization, high-pressure common-rail fuel injection, and exhaust gas recirculation (EGR) to enhance combustion efficiency and suppress pollutant formation. In a 2022 study, Mazuro et al. experimentally analyzed the relationship between intake pressure and combustion efficiency in an opposed-piston engine and demonstrated that optimized intake pressure significantly improves combustion efficiency, thereby decreasing PM emissions [49,50]. Furthermore, Shao et al. investigated the impact of different fuel types on emissions and found that, compared with conventional diesel fuel, RP-3 aviation kerosene enhances the combustion rate, resulting in lower CO and HC emission levels [3]. To further improve combustion efficiency under aviation operating conditions, researchers have also explored combustion-chamber structural optimization and multiple-injection strategies. For instance, Xu et al. proposed a heavy-fuel combustion-chamber geometry optimized for high-altitude operation and experimentally verified that it sustains efficient combustion even under hypoxic environments, thereby reducing PM emissions [42].

In-cylinder temperature and pressure exert a dominant influence on NO_x formation in aero heavy-fuel piston engines, and these parameters are highly sensitive to fuel-injection strategy, ambient pressure, and EGR rate. While reducing combustion temperature is effective for suppressing NO_x emissions, it may simultaneously elevate HC and CO emissions, necessitating careful trade-offs in combustion control. Liang et al. comparatively analyzed the influence of different fuel-injection pressures on combustion-chamber temperature and reported that elevated injection pressure improves fuel atomization, accelerates the combustion rate, and produces a more uniform temperature distribution, thereby lowering NO_x emissions [51]. In addition, EGR technology has been widely adopted in aero-diesel engines to moderate in-cylinder temperature and reduce NO_x formation. Wei et al. optimized the in-cylinder charge composition using EGR and demonstrated that a moderate EGR rate effectively reduces NO_x emissions without appreciably compromising combustion efficiency [52,53].

Mixture formation and air–fuel homogeneity play a critical role in determining emission characteristics in aviation heavy-fuel piston engines. Non-uniform mixtures can lead to localized high-temperature regions that promote NO_x formation, while simultaneously creating fuel-rich zones that increase CO and PM emissions. Consequently, extensive investigations have been conducted on injection strategies, flow-field optimization, and advanced combustion modes tailored to aviation applications. In a combustion-optimization study on an aero-piston engine, Pan et al. reported that turbulence intensification enhances air–fuel mixing, improves combustion stability, and suppresses localized high-temperature regions, thereby mitigating NO_x generation [54]. With respect to injection strategies, Wang et al. investigated stratified-charge combustion and found that a multiple-injection strategy markedly improves fuel atomization, yields a more homogeneous mixture, and mitigates knock; as a result, NO_x emissions were reduced by approximately 18% [55].

In summary, the emission characteristics of aero heavy-fuel piston engines are jointly governed by combustion efficiency, in-cylinder thermal conditions, and mixture formation processes. In recent years, advances in computational fluid dynamics (CFD) simulation, combustion-chamber optimization, high-pressure common-rail fuel injection, and EGR technologies have enabled substantial reductions in NO_x, CO, HC, and PM emissions. Nevertheless, challenges persist, particularly in maintaining combustion stability under high-altitude hypoxic conditions and in optimizing EGR control across the full flight envelope. Future research is expected to increasingly focus on intelligent combustion control, adaptive fuel-injection strategies, and the application of novel low-carbon aviation fuels, thereby advancing aero-diesel engines toward cleaner and more efficient operation.

Table 3. Comparison table of emission pollutant characteristics and influencing factors in aviation heavy-fuel piston engines.

Pollutant	Primary Sources & Formation Mechanisms	Key Influencing Factors	Typical Control Technologies	Latest Research Progress	Future Optimization Directions
CO	Incomplete combustion due to low temperature, oxygen deficiency, or poor atomization	Injection pressure, spray angle, ambient temperature	High-pressure common-rail injection, combustion-chamber optimization	Optimized injection strategies can reduce CO by ~20%	Adaptive injection + combustion pre-heating
HC	Unoxidized fuel or non-uniform mixture formation	Fuel type, injection uniformity, cold start	Multiple injection events, alternative fuels	HEFA fuel significantly reduces HC emissions	Hydrogen–diesel dual-fuel combustion
NOX	Oxidation of N2 with O2 under high-temperature combustion	Combustion temperature, oxygen concentration, EGR rate	EGR, SCR, injection-phasing optimization	EGR can cut NOx by >40%	AI-based combustion optimization
PM	Soot from incomplete combustion and fuel sulfur content	Injection pressure, fuel properties, DPF efficiency	Ultra-high-pressure injection (>3000 bar), DPF	SAF blends can cut PM by 50%	Novel filter materials + structural optimization

summarizes the main pollutant characteristics of aviation heavy-fuel piston engines and compares their formation mechanisms, influencing factors, control technologies, and future optimization directions. In general, CO and HC are mainly related to incomplete combustion and poor mixture formation, NO_x is dominated by combustion temperature and oxygen concentration, and PM is strongly affected by soot formation and fuel properties. These comparisons show that effective emission control in HFPEs depends on pollutant-oriented strategies rather than a single universal solution.

4. Technical Measures for Emission Reduction in Heavy-Fuel Aero-Piston Engines

4.1. Combustor Optimization Technologies

Combustion-chamber optimization is one of the key pathways for reducing emissions from aero heavy-fuel piston engines. In recent years, researchers have mainly focused on optimizing combustion-chamber ensemble configurations and implementing synergistic control of the intake and fuel-injection systems to suppress pollutant formation, improve combustion stability, and enhance engine fuel economy. Among these approaches, the geometric optimization of the combustion chamber is a pivotal means of improving fuel–air mixing efficiency and lowering NO_x and particulate emissions. In their study on combustion-chamber optimization for a direct-injection aero-diesel engine, Liu et al. found that adopting a dual-layer combustion-chamber design amplifies turbulent mixing and elevates fuel atomization quality, thereby markedly reducing particulate emissions [56]. Xu et al., through experimental and CFD analyses, demonstrated that the M-type combustion chamber effectively homogenizes the in-cylinder temperature field by suppressing local high-temperature zones, resulting in an approximately 18% reduction in NO_x emission [6]. Furthermore, An et al. investigated the application of a pre-chamber combustion system under high-altitude, hypoxic conditions; their experiments revealed that this configuration enhances fuel ignition capability and improves combustion stability in oxygen-deficient environments while simultaneously achieving a 12% reduction in NO_x emissions [57]. These studies indicate that combustion-chamber structural optimization not only improves combustion efficiency but also enables effective pollutant control across diverse flight environments.

In addition to geometric optimization of the combustion chamber, the coordinated matching of the intake and fuel-injection systems constitutes another critical lever for enhancing combustion efficiency and reducing emissions. Wang et al. investigated the influence of intake-tract optimization on aero-diesel combustion by employing a strategy of variable intake pressure coupled with turbulence control. Their findings indicate that the optimized intake system significantly intensifies turbulent kinetic energy, improves air–fuel mixing efficiency, and yields more homogeneous combustion, thereby lowering NO_x emissions [58]. Liao et al. conducted a comparative analysis of the effects of varying injection pressures on fuel atomization. Their experiments revealed that at injection pressures exceeding 1800 bar, finer fuel atomization is achieved, leading to more complete combustion and reductions of 22% in CO and 18% in PM emissions, respectively [59]. Additionally, Wang et al. investigated a stratified-injection strategy for aero-diesel engines employing multiple injections (pilot + main + post). The results demonstrated that this approach optimizes fuel–air mixing and improves combustion efficiency, yielding a 30% reduction in CO emissions while simultaneously mitigating combustion noise [60]. These studies demonstrate that the synergistic optimization of fuel-injection and intake systems represents a key technological pathway for reducing aero-diesel engine emissions.

Significant advances have already been realized in the application of combustion-chamber optimization technologies for aero-diesel emission control. Looking ahead, the incorporation of intelligent combustion control, advanced fuel-injection systems, and novel low-carbon fuels is expected to further elevate both fuel economy and environmental performance. With the fusion of artificial intelligence and big-data analytics, future combustion-chamber designs will become increasingly intelligent, enabling adaptive adjustment of combustion parameters in response to varying flight environments to achieve cleaner and more efficient combustion. The maturation of these emerging technologies will allow aero heavy-fuel piston engines to maintain their competitiveness within the forthcoming low-carbon and clean-propulsion landscape, while simultaneously driving aero-engine technologies toward higher efficiency and lower emissions.

4.2. Exhaust After-Treatment Technologies

Exhaust after-treatment technologies constitute a critical pathway for mitigating nitrogen oxides (NO_x), particulate matter (PM), and other regulated emissions from aero heavy-fuel piston engines. Owing to the distinctive operating environment of aviation applications—including high-altitude, low-temperature conditions, extended-endurance missions, and stringent constraints on system mass and integration—conventional diesel after-treatment technologies require targeted adaptation before effective deployment on aero-diesel platforms. In recent years, substantial research efforts have therefore focused on aviation-oriented optimization of selective catalytic reduction (SCR), diesel particulate filters (DPF), diesel oxidation catalysts (DOC), and exhaust gas recirculation (EGR) systems.

Under aviation-relevant high-altitude and low-temperature conditions, the performance of selective catalytic reduction (SCR) systems is often constrained by reduced exhaust temperature and limited oxygen availability. Wang et al. investigated the adaptability of SCR systems under high-altitude conditions and experimentally observed that NO_x removal efficiency decreased by approximately 15% in low-temperature, low-oxygen environments. To address this challenge, the authors introduced a modified zeolite-based catalyst with enhanced low-temperature activity, enabling the SCR system to maintain a high NO_x conversion rate even under high-altitude operating conditions [61]. Furthermore, Caliskan et al. examined an optimized urea–water-solution injection strategy and demonstrated that precise control of injection quantity and spatial distribution significantly improves SCR efficiency, allowing compliance with increasingly stringent ICAO CAEP emission standards [62].

Particulate-matter control in aero-diesel engines presents additional challenges associated with exhaust back-pressure, regeneration reliability, and long-duration operation. In recent years, diesel particulate filter (DPF) technologies have been increasingly adapted to aviation applications. Goyal et al. investigated a novel nanocomposite filtering material that achieves higher filtration efficiency while simultaneously reducing exhaust back-pressure, resulting in an approximately 5% reduction in engine power loss [63]. Additionally, Siedlecki et al. proposed an optimized thermal-regeneration strategy in which advanced DPF-regeneration control algorithms mitigate filter clogging and enhance soot-oxidation rates, thereby ensuring stable operation during extended-endurance flight missions [64].

For aviation applications, diesel oxidation catalysts (DOC) play an important auxiliary role by reducing carbon monoxide (CO) and unburned hydrocarbon (HC) emissions and by improving the operating conditions of downstream SCR and DPF systems [65]. Savva et al. investigated noble-metal catalyst formulations for DOC systems and reported that a palladium–platinum alloy catalyst increased CO and HC conversion efficiencies by more than 30%, which in turn enhanced downstream SCR NO_x removal efficiency by approximately 8% [66]. Moreover, Huo et al. experimentally validated the emission-control performance of an integrated SCR + DPF + DOC system on an aero-diesel engine, demonstrating more than 80% NO_x reduction and over 90% PM reduction, thereby highlighting the feasibility of integrated after-treatment architectures for future aviation applications [67].

Exhaust gas recirculation (EGR) remains an effective in-cylinder emission-control strategy for aero-diesel engines, particularly when combined with complementary technologies [68]. In recent years, aviation-oriented EGR research has focused on mitigating the trade-offs between NO_x reduction, combustion stability, and thermal efficiency. Rueda-Vázquez et al. investigated the synergistic application of water injection (WI) and EGR, demonstrating that the combined strategy reduces NO_x emissions by approximately 35% without appreciable deterioration in combustion efficiency [69,70]. Furthermore, Sekar et al. examined the combined use of hydrogen–methane blended fuels and EGR and reported NO_x reductions exceeding 20% while maintaining favorable fuel-economy performance [71]. In addition, Munimathan et al. explored a nanocoated EGR cooling system and showed that an Al₂O₃–TiO₂-coated EGR cooler significantly enhances heat-exchange efficiency and reduces soot content in the recirculated exhaust gases [72].

Overall, the continued optimization of exhaust after-treatment technologies enables aero heavy-fuel piston engines to achieve substantial emission reductions while preserving high combustion efficiency. Future research is expected to focus on the development of high-activity, low-temperature catalytic materials, intelligent after-treatment control strategies, and lightweight, highly integrated emission-control systems. These advances will be essential for ensuring long-term, stable operation of aero-diesel engines under complex flight environments while meeting increasingly stringent aviation emission regulations. To further clarify the differences among the major exhaust gas control technologies discussed above, a comparative summary is provided in

Table 4. Comparative summary of exhaust gas control technologies for aviation heavy-fuel piston engines.

Technology	Main Target Pollutants	Core Mechanism	Main Advantages	Main Limitations in Aviation Applications	Representative Optimization Direction
SCR	NOx	Catalytic reduction of NOx into N2 and H2O using urea-based reducing agents	High NOx conversion efficiency; effective under properly controlled operating conditions	Reduced activity under low-temperature and high-altitude conditions; requires precise urea dosing and thermal management	Low-temperature catalysts, optimized urea injection, altitude-adaptive control
DPF	PM	Physical filtration and oxidation/regeneration of soot particles	High PM removal efficiency; effective for soot suppression	Back-pressure increase, regeneration reliability, and durability concerns during long-endurance operation	Low-back-pressure filter materials, intelligent regeneration control, thermal-management optimization
DOC	CO, HC	Catalytic oxidation of CO and HC into CO2 and H2O	Improves CO/HC conversion and supports downstream SCR/DPF operation	Catalyst activity is sensitive to exhaust temperature and fuel sulfur content; limited direct effect on NOx	Noble-metal catalyst optimization, integrated DOC + SCR + DPF architectures
EGR	NOx	Recirculation of exhaust gas to dilute intake charge and reduce combustion temperature	Effective in-cylinder NOx reduction; can be combined with other technologies	Excessive EGR may deteriorate combustion stability, ignitability, and thermal efficiency	Closed-loop EGR control, coordinated EGR–injection optimization, EGR with water injection or dual-fuel strategies

.

As shown in Table 4, SCR, DPF, DOC, and EGR differ in target pollutants, technical mechanisms, application advantages, and aviation adaptation requirements. These differences indicate that future aero-diesel emission control should rely on the coordinated optimization of catalytic performance, thermal management, regeneration strategy, and intelligent control.

A critical research gap in the current literature lies in how to reconcile the NO_x–efficiency trade-off and thermal-management limitations under high-altitude, low-oxygen operating conditions. In such environments, reduced ambient pressure and oxygen availability tend to deteriorate ignition and combustion stability, especially for compression-ignition aero-diesel engines [6,34]. Meanwhile, strategies that enhance combustion efficiency—such as intensified in-cylinder mixing, elevated injection pressure, and higher local combustion temperature—may simultaneously promote thermal NO_x formation [5,58,60]. In addition, conventional exhaust after-treatment systems, particularly SCR and DPF, may suffer from reduced effectiveness under low exhaust-temperature conditions and strict aviation integration constraints, including weight and thermal-management limitations [61,67,72]. Therefore, future research should move beyond single-technology optimization and focus on coordinated solutions, such as adaptive injection–EGR matching [63,64], low-temperature combustion (LTC) strategies, altitude-oriented thermal management, lightweight high-efficiency after-treatment systems, and intelligent closed-loop combustion control [52]. Furthermore, alternative pathways such as hydrogen–diesel dual-fuel combustion and AI-assisted multi-parameter optimization have shown potential for improving both combustion efficiency and emission performance under extreme flight environments [73,74].

4.3. Impact of Alternative Fuels on Emissions

Biofuels have emerged as a pivotal research direction for alternative aviation engine fuels owing to their renewability and inherently lower carbon footprint. Wang et al. investigated the application of biodiesel in aero-piston engines and found that fuels derived from biomass oils can effectively reduce carbon monoxide (CO) and soot (PM) emissions during combustion while maintaining high combustion efficiency [75]. Furthermore, Szabo et al. conducted an experimental analysis of the co-combustion characteristics of biodiesel and aviation kerosene; the results indicate that this fuel blend not only reduces carbon dioxide (CO₂) emissions but also optimizes the combustion-chamber temperature distribution, thereby suppressing nitrogen oxide (NO_x) formation [76]. As production technologies mature, the deployment of biofuels has transitioned from laboratory validation to small-scale demonstration runs, and they are expected to become one of the primary fuels for aero heavy-fuel engines in the foreseeable future.

In recent years, synthetic fuels—particularly sustainable aviation fuels (SAF)—have garnered widespread attention for aero-diesel applications, with their core advantage residing in the production of high-purity, low-emission fuels via Fischer–Tropsch (F-T) synthesis and related processes. In an experimental study, Liang and Xu et al. found that F-T synthetic fuels markedly reduce particulate emissions in aero-diesel engines (Figure 9) while enhancing combustion stability, enabling the engine to maintain robust combustion performance and power output across diverse operating environments [51,53], as illustrated in Figure 10 and Figure 11. Xu et al. further investigated the emission characteristics of neat HEFA (hydro-processed esters and fatty acids) fuel in an aero heavy-fuel piston engine. Their findings indicate that this fuel not only reduces carbon emissions but also suppresses sulfur oxide (SO_x) formation while concurrently enhancing engine fuel economy [53]. The maturation of synthetic-fuel technology now offers a cleaner fuel option for aero heavy-fuel piston engines; as production costs decline and technological breakthroughs emerge, its deployment scale is poised for further expansion.

The utilization of hydrogen and other clean energy carriers is emerging as a frontier in emission control for aero-diesel engines. Liang et al. examined the application of hydrogen combustion in aero-diesel engines and demonstrated that hydrogen fuel not only eliminates carbon emissions entirely but also elevates in-cylinder thermal efficiency, enabling the engine to operate efficiently across a wider operating envelope [73]. Meanwhile, Liu et al. experimentally investigated hydrogen–aviation-kerosene dual-fuel combustion and found that a high hydrogen blending ratio can reduce NO_x emissions by approximately 35% while maintaining high combustion stability [74]. Furthermore, Johari et al. explored the integration of a fuel-cell hybrid powertrain into aero-diesel propulsion and demonstrated that auxiliary power supplied by a hydrogen fuel cell can further reduce carbon emissions while enhancing overall aircraft energy efficiency [77]. Although the large-scale deployment of hydrogen fuel in aero-diesel engines still faces challenges related to fuel storage and supply-chain infrastructure, its potential within future aviation propulsion systems remains indisputable. To further clarify the differences among the alternative fuels discussed above, a comparative summary is provided in

Table 5. Comparative summary of alternative fuels for aviation heavy-fuel piston engines.

Fuel Type	Main Advantages	Main Limitations	Combustion/ Emission Characteristics	Engine Adaptation Requirements	Representative Research/Optimization Direction
Biodiesel	Renewable, oxygen-containing, good lubricity, can reduce dependence on conventional fossil diesel	Lower heating value, poor low-temperature flow properties, possible storage-stability issues	Generally reduces PM, CO, and HC emissions; may increase NOx under some conditions	Injection-parameter calibration, cold-start improvement, material compatibility evaluation	Blending optimization, low-temperature performance improvement, NOx-control coordination
SAF	Low-carbon potential, cleaner combustion, lower aromatic content, good potential for aviation decarbonization	High production cost, limited supply scale, uncertain long-term compatibility for some engine systems	Can significantly reduce soot and PM emissions; combustion performance depends on composition and blending ratio	Fuel-system compatibility assessment, blend-ratio optimization, long-term durability validation	SAF/diesel blending strategies, life-cycle assessment, engine-specific adaptation studies
Hydrogen	Zero-carbon fuel at point of use, fast flame speed, wide flammability limits, strong decarbonization potential	Storage difficulty, low volumetric energy density, safety and onboard integration challenges	Can greatly reduce CO2, CO, HC, and PM emissions; may still face NOx issues under high-temperature combustion	Dual-fuel strategy development, injection/control-system redesign, onboard storage and safety management	Hydrogen–diesel dual-fuel combustion, fuel-cell hybrid propulsion, low-NOx hydrogen combustion strategies

.

As summarized in Table 5, biodiesel, SAF, and hydrogen show different advantages and technical challenges for aviation heavy-fuel piston engines. Biodiesel is renewable but suffers from lower calorific value and poor low-temperature properties. SAF is beneficial for soot and particulate reduction and supports aviation decarbonization, although its large-scale use is still limited by cost and supply. Hydrogen has the greatest long-term decarbonization potential, but its application still depends on progress in storage, safety, and combustion control. Overall, future development should emphasize fuel–engine matching and system-level optimization.

The evolution of alternative-fuel technologies has provided a crucial underpinning for the decarbonization of aero-diesel engines, as summarized in

Table 6. Comprehensive comparison table of emission-reduction technologies for aviation heavy-fuel piston engines.

Technology Category	Key Measures/ Configurations	Primary Emission Targets	Typical Performance Achieved	R & D Outlook
Combustion-Chamber Optimization	ω/M-type piston bowls	NOx, PM, CO, HC	NOx ↓ 12–18%; PM ↓ 15–22%; CO ↓ 30%	AI-driven adaptive geometry + real-time CFD
Fuel-Injection System Tuning	>1800 bar common-rail	PM, CO, HC	PM ↓ 18%; CO ↓ 22%; combustion noise ↓	>2500 bar systems + ML-based injection maps
Exhaust After-Treatment	SCR (low-temp zeolite catalysts)	NOx, PM, CO, HC	NOx ↓ > 80%; PM ↓ > 90%	Lightweight, altitude-robust systems; smart regeneration
Exhaust Gas Recirculation (EGR)	Cooled/hot EGR	NOx, PM	NOx ↓ 20–35% with minimal fuel penalty	Closed-loop EGR rate control via AI + sensors
Alternative and Clean Fuels	HEFA-SAF (100%)	CO2, PM, NOx, HC	PM ↓ 50%; NOx ↓ 35%; net CO2 ↓ 60–100%	Drop-in SAF certification; cryogenic H2 integration

. Spanning biodiesels, synthetic fuels, and hydrogen, research on every clean-fuel pathway has demonstrated pronounced emission-reduction efficacy while delivering concurrent gains in combustion efficiency and fuel-economy optimization. Looking ahead, as production technologies mature, fuel-supply chains are perfected, and policy support intensifies, the deployment of alternative fuels in aero-diesel engines will expand further and gradually achieve full-scale industrial rollout, furnishing dependable technological support for the green, low-carbon development of the global aviation sector.

5. Conclusions and Outlook

This study presents a review of combustion-chamber optimization and emission-control technologies for aero heavy-fuel piston engines, with discussion of combustion-chamber geometric optimization, fuel-injection refinement, alternative-fuel utilization, and exhaust after-treatment strategies. The findings indicate that combustion-chamber optimization—such as direct-injection and M-type chambers—can improve fuel atomization, homogenize in-cylinder temperature distribution, and help suppress nitrogen oxide (NO_x) and particulate matter (PM) formation. High-pressure common-rail injection, multi-stage injection, and turbulence-intensification techniques further improve combustion efficiency and reduce carbon monoxide (CO) and unburned hydrocarbon (HC) emissions. In addition, exhaust after-treatment technologies—including exhaust gas recirculation (EGR), selective catalytic reduction (SCR), and diesel particulate filters (DPF)—have been widely applied in aero-diesel engines, while biodiesels, synthetic aviation fuels (SAF), and hydrogen provide new pathways toward cleaner aero-diesel propulsion. Overall, emission-control technologies for aero heavy-fuel piston engines have made significant progress, gradually evolving from single-technology solutions toward integrated control strategies.

Despite these advances, several challenges remain. First, the trade-off between combustion-chamber optimization and NO_x suppression is still unresolved, as highly efficient chamber designs often increase in-cylinder temperature and thus promote NO_x formation. Achieving both high combustion efficiency and low NO_x emissions remains a key research issue. Second, the coordinated control of high-pressure common-rail injection and EGR is not yet fully mature; excessive EGR may lead to combustion instability, and injection strategies still need to be better integrated with intelligent control methods. Third, the wider use of alternative fuels is limited by fuel compatibility, supply cost, and storage stability. Although SAF, biodiesel, and other fuels have shown clear emission-reduction potential, their long-term suitability for aero-diesel engines still requires further verification. Finally, the lightweight design, durability, and environmental adaptability of after-treatment systems remain important development priorities, especially under extreme cold and high-altitude conditions. Particular attention should be given to high-altitude, low-oxygen operating conditions, under which the simultaneous achievement of combustion efficiency, NO_x mitigation, and effective thermal management remains a major unresolved challenge.

Future emission-control technologies for aero heavy-fuel piston engines are expected to develop toward greater intelligence, higher efficiency, and deeper decarbonization. Intelligent combustion-control systems will likely become an important research focus, combining artificial intelligence (AI), big-data analysis, and real-time in-cylinder sensing to optimize fuel-injection parameters, EGR rates, and after-treatment settings. In addition, the combination of ultra-high-pressure injection with advanced control strategies may further improve combustion efficiency while reducing particulate and hydrocarbon emissions. Advanced combustion modes, such as low-temperature combustion (LTC) and ultra-lean combustion, also show promise for suppressing NO_x and improving fuel economy. For alternative fuels, future studies will continue to optimize the use of synthetic fuels and hydrogen-based fuels, including dual-fuel hydrogen–diesel combustion and hybrid propulsion concepts. Meanwhile, progress in catalytic materials, intelligent urea dosing, and low-temperature SCR systems is expected to further enhance after-treatment performance.

The present analysis indicates that future emission control of aero heavy-fuel piston engines will depend on the coordinated development of combustion optimization, intelligent control strategies, alternative-fuel utilization, and efficient after-treatment systems. Through multi-technology integration and systematic optimization, the emission performance of aero-diesel engines can be further improved to meet increasingly stringent aviation-emission standards and environmental requirements, thereby supporting the transition of aero-propulsion systems toward higher efficiency and cleaner operation.