Hydrogen Engine Conversion Aspects ^†

Abstract

The transition from traditional petrol-based combustion engines to hydrogen-powered systems represents a promising advancement in sustainable and clean energy solutions. This review paper explores the intricacies of converting a conventional internal combustion engine to operate on hydrogen gas. Key topics include the performance limitations of hydrogen engines, the role of water injection in combustion modulation, and the investigation of direct injection and port injection systems. This review also examines challenges associated with lean and rich mixtures, risks of backfire and pre-ignition, and the conversion’s overall impact on engine performance and longevity. Additionally, this paper discusses hydrogen lubrication to prevent mechanical wear and addresses emission-related considerations.

1. Introduction

The global automotive industry has reached a turning point where there is increasing pressure to transform traditional petrol-based internal combustion engines into more sustainable and environmentally friendly energy sources [1]. In this context, hydrogen-based systems offer a prominent alternative, capable of significantly reducing greenhouse gas emissions and mitigating the harmful effects of air pollution. This transition is critical not only for environmental sustainability but also holds economic importance by potentially leveraging existing manufacturing infrastructures for internal combustion engines, thereby preserving substantial industrial investments [2].

Hydrogen, as a fuel, possesses advantageous properties such as high energy density and clean combustion. However, it also introduces complexities. It is extremely high flammability poses risks of irregular combustion including backfire and pre-ignition, due to its wide flammability range and low ignition energy. Moreover, hydrogen lacks the lubricating properties found in petrol and other hydrocarbon fuels, which can increase mechanical wear on engine components. The higher combustion temperatures associated with hydrogen fuel also raise concerns about increased nitrogen oxide (NO_x) emissions, necessitating advanced combustion strategies and exhaust treatment systems [3].

2. Performance Limitations

Hydrogen combustion engines face several performance limitations despite their potential as a cleaner alternative to traditional fossil fuel engines. One significant challenge is the lower energy density of hydrogen compared to gasoline or diesel, causing reduced range and performance—as

Table 1. Hydrogen properties compared to gasoline and diesel [4].

Property	Hydrogen	Gasoline	Diesel
Density (at 1 bar and 273 K; kg/m3)	0.089	730–780	830
Volumetric energy content (at 1 bar and 273 K; kg/m3)	10.7	33 × 103	35 × 103
Stoichiometric air/fuel mass ratio	34.4	14.7	14.5
Octane number (R + M)/2	130+	86–94	-

. shows. This results in larger fuel tanks by volume and increased storage pressure requirements, which negatively influence vehicle design.

Because of the lower density, a higher amount of gas must be injected into the cylin-der to reach the same performance as a gasoline engine. The space taken up by the hydro-gen when using PFI reduces the amount of air available for cylinder filling, leading to performance loss. Therefore, direct injection systems are more favored for hydrogen ICE engines. Hydrogen has a much higher flammability than gasoline. At atmospheric pressure, in a stoichiometric mixture, around one-tenth of the energy is enough to ignite hydrogen compared to gasoline [5]. Additionally, the flame speed of hydrogen is higher, at 1.85 m/s (at 1 bar and 273 K), compared to 0.37–0.43 m/s for gasoline. This behavior leads to a high risk of backfire, knock, pre-ignition, and, eventually, undesired autoignition, which requires combustion modulations [6].

3. Injection Methods: Direct vs. Port Injection

When comparing hydrogen injection methods in engines, two primary approaches are often studied: hydrogen port injection and hydrogen direct injection (DI). Hydrogen port injection involves introducing hydrogen into the intake manifold, while hydrogen direct injection executes injection near the top dead center (TDC) to control combustion [7].

The current H2ICE prototypes mainly use port fuel injection (PFI), which allows easy conversion of existing engines. However, this has several drawbacks, such as pre-ignition and knocking. These problems can be reduced using direct injection (DI), as studies have shown that DI can increase efficiency, minimize knock, and reduce NOx emissions in hydrogen engines by enabling mixture-controlled combustion [4,7,8,9].

However, the results presented in

Table 2. Comparison of key features and parameters of PFI and DI fuel system [4].

Features and Parameters	Intake Manifold		Direct Injection (Low Pressure)	Direct Injection (High Pressure)
H2 injection	PFI single point	PFI open valve	Suction and beginning of compression stroke	Near TDC
Fuel injection equipment costs	Moderate		Best cost/benefit trade-off	Costly H2 injection system
Power density	Ca. −30% comp. to diesel		Comparable to diesel, resp., 0 to −20%
Efficiency	Slightly below diesel		Close to diesel
Further features	High risk of backfire	Risk of backfire	H2 LP—system as FCEV, allows high mileage	The H2 compression pump requires

. are contradicted by the following research, where dual-fuel diesel–hydrogen engines have shown promising results in terms of performance, with peak efficiencies reaching around 50% [10]. In Table 2, unfortunately, for petrol engines, there are insufficient data on power density and efficiency as a function of the injection system.

In addition, the researchers investigated the heat transfer characteristics and effects of hydrogen–oxygen mixtures in petrol internal combustion engines [11]. It is important to highlight that in both port and direct injection, injection timing is a key factor in avoiding complications such as inhomogeneous mixture formation, inefficient flame propagation, and increased NOx emissions [12]. The current studies mostly discuss the low engine speed range for the timing, and only a few research studies give some insight into the higher speed ranges [13].

4. Combustion Modulation: Stochiometric and Lean Mixtures

The choice of the equivalence ratio in hydrogen combustion engines is crucial for optimizing performance, emissions, and efficiency. Lean mixtures are favored for their environmental benefits and efficiency gains. However, mixtures targeting stoichiometric or near-stoichiometric ratios provide higher power, resulting in increased emissions and lower efficiency, and are primarily suitable for specific high-power applications with the added risk of abnormal combustion.

Most research on hydrogen engines concentrating on lean burning targets lambda values of 2 and higher [14]. The excess air helps lower combustion temperatures, thereby reducing the formation of nitrogen oxides (NO_x). This makes hydrogen a cleaner alternative to conventional combustion methods, aligning with environmental goals. To achieve higher performance with this combustion strategy, it is necessary to use turbocharging or super-charging; however, this approach also results in higher cylinder pressure and temperature. By using direct injection combined with a strict injection and valve timing strategy, it is possible to minimize backfire, knock, and pre-ignition behavior without the need for additional systems [15]. Considering the currently available H₂ direct injection systems, the EOI timing is limited by the counterpressure of the cylinder, not reaching the critical pressure ratio. Thus, SOI (start of Injection) is determined by the available H₂ pressure and required quantity. However, if the SOI occurs earlier than the IVC (intake valve closing), there is a risk of H₂ backflow into the intake system, which may result in inconsistent cylinder filling, leading to possible knock and backfire of residual H₂ in the intake manifold. For consistent and safe operation, the combination of these timings must be well-calibrated.

Moving towards the stoichiometric mixture is raising many issues in hydrogen combustion engines, including high temperature and high NO_x emission, but also increasing tendency of knock and pre-ignition. In these instances, while the previously mentioned strict injection and valve timing remain essential, they no longer address the previously highlighted issues. To maintain control over combustion, current research primarily focuses on two methods: using water injection for combustion modulation or employing an EGR system.

Water injection, also known as anti-detonation injection, is a method utilized to lower the temperature of the combustion chamber. The system typically mixes fuel with the air entering the cylinder, enabling the use of higher compression ratios and reducing the occurrence of knock, leading to an overall increase in engine performance [16,17]. EGR system recirculates a fraction of exhaust gases back into the combustion chamber, thereby reducing the amount of available oxygen for combustion and lowering the combustion chamber temperature. While a moderate amount of EGR can enhance indicated thermal efficiency, an increased level of EGR significantly reduces volumetric efficiency [4,6].

5. Hydrogen Lubrication and Mechanical Wear

In conventional 4-stroke internal combustion engines, there are several means of lubrication. There is no need to discuss the lubrication of oil; however, it is often not recognized how important the lubrication of the fuel used to run these engines. Fuel is responsible for cooling and lubrication of the fuel injectors in PFI systems, and the high-pressure fuel pump and the injectors in DI systems, but, besides this, it also has an important role in the lubrication of the valves and valve seats [18,19].

Research on hydrogen–diesel engine lubrication and mechanical wear reveals both benefits and challenges. Hydrogen as a fuel can improve thermal performance and reduce emissions of CO, HC, and smoke opacity [20]. Hydrogen further decreased the viscosity of the lubricating oil by 26%. Additionally, hydrogen gas produced increases of 17.7%, 29.27%, 21.95%, and 27.41% in metallic components, such as Fe, Cu, Al, and Cr, respectively [4,20,21]. However, hydrogen combustion can still produce particulate emissions containing metals from lubricating oil [20,21,22].

Lubricant contamination is a primary cause of engine wear, affecting fuel efficiency and component life [22]. Alternative lubricants, such as biodiesel and microalgae oil, show promise in reducing friction and wear [23,24]. Adding fatty acid methyl esters to bio-hydrogenated diesel can significantly improve its lubricating properties [25].

6. Emission Considerations

When considering the environmental impact of transport modes, the literature finds several measurement and calculation methods to be acceptable [26]. However, there is a consensus that achieving the most accurate results is worth taking a “well-to-wheel” approach, which effectively considers the period that integrates the entire life cycle of the fuels and products used. Essentially, this means that the calculations are based on how much fossil or electric “fuel” was burned or used and how much CO₂ emissions were required to produce the energy source [27,28], as

Table 3. Overview of the 2021 life cycle emissions of medium-sized electric, hybrid, and ICE vehicles in each stage of their life cycles, using tCO₂e. These numbers consider a use phase of 16 years and 240,000 km [30,31,32].

		Battery Electric Vehicle	Hybrid Electric Vehicle	Internal Combustion Engine Vehicle	Internal Combustion Engine Vehicle (H2)
Production emissions (tCO2e)	Battery manufacturing	5 [t]	1 [t]	0 [t]	0 [t]
	Vehicle manufacturing	9 [t]	9 [t]	10 [t]	10 [t]
Use phase emissions (tCO2e)	Fuel/electricity production	26 [t]	12 [t]	13 [t]	115/13 [t]
	Tailpipe emissions	0 [t]	24 [t]	32 [t]	0 [t]
	Maintenance	1 [t]	2 [t]	2 [t]	2 [t]
Post-consumer emissions (tCO2e)	End of life	−2 [t]	−1 [t]	−1 [t]	−1 [t]
	Total	39 [t] CO2	47 [t] CO2	55 [t] CO2	126/24 [t] CO2

shows as well. As mentioned above, when testing the emission factor of these new types of engines, particular attention should also be paid to the measurement of nitrogen oxides, as hydrogen gas has a higher combustion temperature than conventional fuels. It is also important to note that the results measured so far show that internal combustion engines using hydrogen can easily meet the EURO 6 standard [4,29].

The different types of vehicles (ICEV, BEV, FCBEV, HICEV) can have very different greenhouse gas emissions during their life cycle stages. By this, we mean, by way of example, that while an electric car emits less CO₂ during its operation, the production process requires more CO₂ emissions due to the production of the battery [33]. The final column of Table 3 contains calculated and imputed results and values, based partly on our measurements and partly on results from the international literature [34,35].

The data reveals that all-electric vehicles generate more CO₂ during production than conventional cars but offset this disadvantage over time. With a lifetime of 240.000 km and 16 years, electric vehicles hold about a 40% CO₂ reduction advantage at the end of their life cycle. While hydrogen internal combustion engines have similar CO₂ production emissions to conventional engines, the key difference lies in fuel production and exhaust gases. Moreover, the emissions from electric and hydrogen vehicles are heavily impacted by the method of hydrogen production [36]. The final column highlights future improvements if solar and hydrogen power plants replace the current global electricity grid, emphasizing the significance of ongoing studies by the European Environment Agency and the European Commission [37].

7. Conclusions

The conversion of traditional internal combustion engines to hydrogen-powered systems presents significant opportunities and challenges. A lean mixture is promising for heavy-duty applications, with stoichiometric mixtures showing the potential to match or surpass current engine technologies. While port fuel injection (PFI) is easier to implement, it faces issues like pre-ignition and knocking. DI offers improved efficiency, reduced knocking, and lower NOx emissions. Hydrogen as a fuel enhances thermal performance and reduces emissions but poses significant lubrication challenges. It reduces oil viscosity, decreasing friction but increasing wear on engine components. Exploring alternative lubricants can mitigate these issues.

In summary, hydrogen-powered combustion engines hold significant potential for reducing emissions and enhancing performance. However, several technical challenges need to be addressed to fully realize this potential. Future research should focus on refining injection methods, improving lubrication and mechanical wear resistance, and ensuring sustainable hydrogen production to make hydrogen-powered vehicles a viable and competitive alternative to traditional internal combustion engines.