Hydrogen combustion engines are real, but they are not automatically zero-emission. They avoid carbon dioxide from the hydrogen fuel itself at the engine, yet can produce nitrogen oxides (NOx), and their climate impact depends heavily on how the hydrogen is made. Development is most visible in heavy-duty and non-road equipment; evidence of engineering progress is not the same as broad availability.
What is a hydrogen combustion engine?
A hydrogen internal-combustion engine (H2ICE) burns hydrogen in a cylinder to produce mechanical power. A hydrogen fuel-cell vehicle is different: it uses an electrochemical process to make electricity, which then powers an electric motor. The two technologies use hydrogen in different ways and should not be treated as interchangeable.
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H2ICE development adapts combustion-engine technology to a fuel that contains no carbon. That removes the fuel-carbon source of engine-out CO2, but it does not settle the engine’s full emissions profile or the climate impact of its fuel supply.
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Are hydrogen combustion engines zero-emission?
No—not as a blanket claim. Burning hydrogen does not release CO2 from carbon in the fuel, because hydrogen contains no carbon. But that says nothing by itself about emissions from making, processing, compressing and delivering the hydrogen. Real engines may also emit some tailpipe CO2 from lubricating oil and other factors.
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A 2024 report on non-road mobile machinery reproduces JCB- and Cummins-attributed test figures for cited hydrogen engines. It lists the following CO2 values alongside an 800 g/kWh Stage V diesel reference:
| Reported case | CO2 figure shown | How to read it |
|---|---|---|
| Cited hydrogen-engine case | 2.567 g/kWh | Figure reproduced in the 2024 non-road mobile machinery report; the report attributes its data to JCB and Cummins. |
| Cited hydrogen-engine case | 0.43 g/kWh | Figure reproduced in the same report; it is a test result, not a universal H2ICE value. |
| Cited hydrogen-engine case | 0.2 g/kWh | Figure reproduced in the same report; it is a test result, not a universal H2ICE value. |
| Preliminary hydrogen-engine case | Less than 1 g/kWh | Marked preliminary in the report. |
| Stage V diesel reference | 800 g/kWh | The report’s cited reference value, not a comparison that should be applied to every engine or operating condition. |
The report also describes at least a 99.9% reduction against its Stage V diesel unit for the cited stakeholder engines. These are report-specific results, not an independent fleet-wide finding. The test conditions and comparison matter, and tailpipe CO2 figures do not measure lifecycle emissions.
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Do hydrogen engines produce NOx?
They can. A hydrogen flame reaches high temperatures, and nitrogen and oxygen in the combustion air can react to form NOx. The U.S. Department of Energy says, “Because hydrogen burns at higher temperatures than natural gas, combustion of pure hydrogen may result in comparably higher NOx emissions.”
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Is the hydrogen itself clean?
It depends on how it is produced. The International Energy Agency’s 2025 review says global hydrogen demand was almost 100 million tonnes in 2024, while low-emissions hydrogen accounted for less than 1% of production. A hydrogen engine’s climate claim therefore needs to identify the fuel pathway rather than assume that hydrogen is low-emissions simply because it contains no carbon.
The same IEA review identifies high costs, uncertain demand and regulation, and slow infrastructure development as constraints. It revised announced potential low-emissions production for 2030 down to 37 million tonnes per year from the 49 million tonnes per year estimated a year earlier. Projects already operational or at final investment decision could provide 4.2 million tonnes per year by 2030. These are global supply figures, not a calculation of emissions for any particular engine or fuel supplier.
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- During the experiment, please use 80℃ hot water for Combination reaction (if the water temperature is low, the amount of hydrogen and air pressure from the Combination reaction are insufficient, the fuel cell cannot be used for power generation), and then take off the plug of the vent pipe at the lower part of the fuel cell, release the gas in the rubber hose immediately, and then plug it back immediately, so that only pure hydrogen and air are in the fuel cell, so that the fuel cell can generate hydrogen air power.
Where are hydrogen combustion engines being used or developed?
JCB non-road machines
In a January 2025 announcement, JCB said 11 licensing authorities across Europe had approved its hydrogen engine for commercial use in machines, with the Netherlands’ RDW the first. The company also reported producing more than 130 evaluation engines for backhoe loaders, telescopic handlers and generator sets, and said customer-site real-world testing was at an advanced stage. These are company-reported milestones; approval does not establish that every model is widely stocked or available in every country.
U.S. heavy-duty development
In January 2024, the U.S. Department of Energy announced $10.5 million for three heavy-duty hydrogen-engine research, development and demonstration projects: PACCAR in Mount Vernon, Washington; Cummins in Columbus, Indiana; and Powertrain in Plymouth, Michigan. The funding shows active development work, not commercial deliveries.
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Together, these examples point to a development focus on heavy-duty and non-road applications. They do not establish a current global deployment total, broad passenger-car availability or fleet-wide efficiency. Historical DOE webinar material provides context on H2ICE designs and milestones, not proof of current orderability or an independent performance comparison.
Are hydrogen engines better than fuel cells or batteries?
There is no universal winner established by the available evidence. The practical choice depends on the vehicle or machine’s job and local conditions, not just on the fact that one option uses hydrogen or retains combustion-engine hardware.
- Duty cycle and power needs: Identify how much power is required and how often the equipment must respond to changing loads.
- Range and turnaround: Compare the required operating range and refuelling or charging time for the actual job.
- Fuel or electricity supply: Check local hydrogen price and production pathway, or the availability and source of electricity.
- Infrastructure: Confirm that refuelling or charging is practical at the equipment’s operating locations.
- Local emissions: Account for NOx and other pollutant controls where combustion equipment operates.
- Deployment and ownership: Verify equipment availability, certification, service support and lifecycle cost for the specific application.
The sources available do not provide an apples-to-apples cost or efficiency ranking across hydrogen ICEs, fuel cells, batteries and incumbent engines.
What about hydrogen safety?
Safety is a legitimate question, including storage, leak detection and the effects of a release. The U.S. Department of Energy’s stakeholder FAQ reflects those concerns, but the available evidence here does not establish comparative safety rates for hydrogen combustion engines versus fuel-cell vehicles, batteries or conventional vehicles, nor does it provide a full assessment of onboard hydrogen storage. It would be misleading to declare one option safer on that basis alone.
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