Hydrogen Engines

Hydrogen engines represent a key development in combustion technologies, operating along the same principles as traditional internal combustion engines with key modifications enabling the utilisation of hydrogen as fuel. These key modifications include the adaptation of critical components such as fuel injectors, combustion chambers, and exhaust systems to handle the unique characteristics associated with hydrogen, such as low ignition energy and high diffusivity.

Hydrogen engines are divided into two categories, the Spark Ignition (SI) engine and the Compression Ignition (CI) engine. SI engines operate based on the principles of traditional gasoline engines, where hydrogen ignition is facilitated by a spark plug. CI engines operate on the principles of traditional diesel engines, but due to the high autoignition temperature of hydrogen, a pilot fuel is required to initiate combustion. The biggest advantage of hydrogen engines lies in the transferability of skills and infrastructure, allowing for an overall easier transition for the powertrain industry. Additionally, virtually zero carbon dioxide is emitted, replaced by water as a by-product. Having a high calorific value increases engine efficiency and provides versatile up- and down-scaling applications across a multitude of sectors.

On the back of these advantages, rapid technological advancements, and increased government incentives, the hydrogen engine market is poised for significant growth, valued at £14.22B in 2023 and increasing to £36.4B by 2033 (future market insights). For a realistic realisation and widespread adoption of hydrogen engines, several critical challenges need to be overcome. The foremost of these are NOx emissions, which are particularly high due to high flame temperatures arising from hydrogen combustion. While green hydrogen use makes hydrogen engine emissions carbon-neutral when considering well-to-wake emissions, local NOx emissions would significantly increase. Flashback and knocking are also significant issues that limit hydrogen engine efficacy. Other issues arise concerning material compatibility, including lubrication challenges, hydrogen embrittlement, and durability in high cycling conditions. Additionally, the limited development in the supply chain and the high costs of green hydrogen have also hindered the advancement of hydrogen engines. These issues need to be addressed to ensure a smooth transition away from fossil fuel engines.

Challenges Revealed Through Literature Review

Volumetric efficiency reduction
Excessive cyclic variation in power
High NOx emissions and exhaust temperature
High risk of knock and noise
Flashback
Hydrogen autoignition and pre-ignition
Hydrogen embrittlement
Durability and lubrication
Optimal compression ratio

Pure Hydrogen SI Engine

Hydrogen Storage and Supply: Hydrogen is a lightweight and volatile gas, necessitating solutions for its storage and supply, including high-pressure storage and safety measures.
Combustion Stability: Hydrogen combustion processes are more complex compared to traditional fuels, requiring special combustion chamber designs and combustion control systems to ensure stability.
Emission Control: While hydrogen combustion produces only water vapor as emissions, high temperatures can generate nitrogen oxides (NOx), requiring effective emission control systems

Hydrogen-Ammonia SI Engine

Fuel Supply and Mixture Control: Ammonia and hydrogen have different combustion characteristics and require different mixture ratios, necessitating adaptable fuel supply and mixture control systems.
Combustion Efficiency: Ammonia combustion produces complex gas compositions, potentially generating harmful substances such as nitrogen oxides and particulate matter, requiring optimization of the combustion process to improve efficiency and reduce emissions.
System Complexity: Dual-fuel systems are inherently more complex than single-fuel systems, requiring more sophisticated control systems and stricter operational requirements.

Hydrogen-Gasoline SI Engine

Fuel Supply and Mixture Control: Gasoline and hydrogen have different combustion characteristics and require different mixture ratios, requiring suitable fuel supply and mixture control systems to achieve stable combustion.
Ignition System: Gasoline and hydrogen have different ignition requirements, necessitating efficient ignition systems capable of accommodating dual-fuel ignition.
Emission Control: Combustion of mixed fuels can produce a more complex range of emissions, necessitating adaptive emission control systems to meet environmental requirements.

High risk of knock
High NOx emissions
Hydrogen autoignition and pre-ignition
Air-fuel mixing technologies
Optimal compression ratio
Durability and lubrication
Hydrogen embrittlement
Flashback

Pure Hydrogen CI Engine

Combustion Control: Hydrogen’s high reactivity and wide flammability limits pose challenges for controlling the combustion process to achieve optimal efficiency and emissions.
Ignition Timing: Achieving proper ignition timing in a compression ignition engine with hydrogen can be challenging due to its high autoignition temperature and fast combustion speed.
Emission Control: While hydrogen combustion generally produces low emissions, controlling nitrogen oxides (NOx) formation at high combustion temperatures remains a challenge.

Hydrogen-Ammonia CI Engine

Fuel Injection and Mixing: Achieving proper fuel injection and mixing of both ammonia and hydrogen in varying proportions to ensure homogeneous combustion and avoid combustion instability.
Combustion Efficiency: Optimizing combustion efficiency while minimizing emissions of nitrogen oxides (NOx) and unburned ammonia from the combustion of ammonia and hydrogen blends.
System Integration: Ensuring seamless integration of the dual-fuel injection system, combustion control, and emission treatment systems for efficient operation under varying engine loads and speeds.

Hydrogen-Diesel CI Engine

- Fuel Injection and Combustion Control: Achieving precise control of diesel and hydrogen injection timing, quantity, and mixing to optimize combustion efficiency and emissions performance.
- Ignition and Combustion Stability: Ensuring reliable ignition and stable combustion of diesel-hydrogen blends across a wide range of engine operating conditions to avoid misfires and combustion instability.
- Emission Reduction: Minimizing emissions of nitrogen oxides (NOx) and particulate matter (PM) from diesel-hydrogen combustion while maintaining high thermal efficiency and power output.

Hydrogen Compression Ignition (CI) Engine – Academic Capability Mapping

Word cloud

The word-cloud of the primary and secondary keywords is presented for the Hydrogen Compression Ignition (CI) Engine technology. These keywords were used as the input to Scopus for the purpose of the Academic Capability Mapping. The analysis underscores key research areas like combustion, dual-fuel capabilities, and nitrogen oxide emissions.

Documents by Country

The number of papers published worldwide pertaining to Hydrogen Compression Ignition (CI) Engine since the year 2000, divided into three decades. Only the top 10 countries are displayed. The UK is number 4 in Hydrogen CI Engine research globally. While the research into Hydrogen CI Engines is still new, the UK is cementing its place in global academic research in hydrogen combustion technologies.

Documents by Author (2000 – 2025)

Prominent UK academics and their affiliation is showcased. The y-axis represents the H-index of the authors, while the x-axis illustrates the number of papers published. It can be clearly seen that some of the UK’s top researchers are competitive against global researchers in the field of Hydrogen Compression Ignition (CI) Engine.

Documents by Affiliation

The number of papers published by affiliation in the UK since the year 2000 are showcased. University of Birmingham leads the way with the most publications, closely followed by Brunel University London. The figure specifically highlights the top 10 UK institutions in the field of Hydrogen Compression Ignition (CI) Engines, providing a definite ranking list of universities with excellent expertise in the Hydrogen CI Engine technology.

Hydrogen Spark Ignition (SI) Engine – Academic Capability Mapping

Word cloud

The word-cloud of the primary and secondary keywords is presented for the Hydrogen Spark Ignition (SI) Engine technology. These keywords were used as the input to Scopus for the purpose of the Academic Capability Mapping. The analysis underscores key research areas like combustion, injection methods, and nitrogen oxide emissions.

Documents by Country

The number of papers published worldwide pertaining to Hydrogen Spark Ignition (SI) Engine since the year 2000, divided into three decades. Only the top 10 countries are displayed. The UK is number 8 in Hydrogen SI Engine research globally. While the UK is top 5 in Hydrogen CI Engine research, research into Hydrogen SI Engine is lacking and requires further academic consideration.

Documents by Author (2000 – 2025)

Prominent UK academics and their affiliation is showcased. The y-axis represents the H-index of the authors, while the x-axis illustrates the number of papers published. It can be clearly seen that the UK’s top researchers are not competitive against global researchers in the field of Hydrogen Spark Ignition (SI) Engine.

Documents by Affiliation

The number of papers published by affiliation in the UK since the year 2000 are showcased. Brunel University London leads the way with the most publications, closely followed by the University of Birmingham. The figure specifically highlights the top 10 UK institutions in the field of Hydrogen Spark Ignition (SI) Engines, providing a definite ranking list of universities with excellent expertise in the Hydrogen SI Engine technology.

Hydrogen Compression Ignition (CI) Engine – Delphi Survey Analysis

Participant Identifiers

Participant Industry Collaboration

Participant Confidence Level

Participant Country Affiliation

Key Performance Indicators

Key technical target predictions were provided by the participants, expected to be achieved by 2030.

Challenges

The participants were provided with several options and were asked to rank these options from 0 (least critical) to 6 (most critical). They were also provided with a text option to suggest additional challenges.

Combustion Characteristics

Exhaust After Treatment

Further Research

Compression Ignition Engine System Integration

Marine applications
Railway applications
Distributed energy supply
Industrial processes and port infrastructure

Hydrogen storage
Green Hydrogen Production
Renewable energy sources
High-pressure hydrogen fuelling stations

Hydrogen specific nozzles
Exhaust after-treatment
Infrastructure

Hydrogen Spark Ignition (SI) Engine – Delphi Survey Analysis