IDTechEx: Do Hydrogen Engines Truly Produce Zero Emissions?

The idea of hydrogen-powered internal combustion engines (ICE) eliminating harmful emissions while retaining familiar technology is appealing.

With electrification the leading path to decarbonising transportation, challenges like slow adoption rates are prevalent. In fact, according to Mika Takahashi, a technology analyst at IDTechEx, only 11% of all car sales in 2023 were fully electric.

"The IDTechEx report, Hydrogen Internal Combustion Engines 2025-2045, explores whether hydrogen could be a disruptive technology with zero-emission credentials," says Mika.

How hydrogen combustion works

Combustion in ICEs is a high-temperature chemical reaction between fuel and oxygen.

For petrol or diesel engines, it produces carbon dioxide (CO2) and nitrous oxides (NOx). With hydrogen, there's no carbon in the fuel, eliminating CO2 from the tailpipe.

Mika explains: "No CO2 is formed when using hydrogen, but there's a caveat to this zero-emission claim."

Despite hydrogen's clean combustion, small amounts of motor oil are burned during engine operation – essential for lubricating moving parts. Most motor oils are hydrocarbon-based, but a tiny amount of CO2 is still emitted.

However, Mika points out: "Compared to conventional ICEs, this amount is negligible." 

Hydrogen ICE (H2ICE) vehicles produce less than 1kg of CO2 per 1,000 miles, compared to 272kg of conventional petrol or diesel engines.

Nitrous oxides: The real challenge

While hydrogen combustion eliminates carbon emissions, it generates high temperatures in the combustion chamber, producing nitrous oxides (NOx).

NOx are recognised as greenhouse gases (GHGs) and harmful pollutants, subject to increasingly strict regulations worldwide.

"For H2ICE to achieve credible emissions reductions, it must substantially reduce NOx emissions," Mika highlights.

IDTechEx's report dives into how thermal NOx formation occurs due to high-temperature combustion, analysing factors like:

• Air-fuel ratios and the industry's trend toward lean burn spark ignition designs
• Engine speed's influence on NOx formation
• Existing exhaust gas treatments for hydrogen engines, including catalytic converters and exhaust gas recirculation (EGR).

The report presents real-world data on current H2ICE vehicles and compares NOx emissions with historical and current tailpipe limits in critical regions.

The importance of hydrogen's origin

Hydrogen's potential for zero emissions depends on how it's produced.

"Although hydrogen is carbon-free when burned, its lifecycle emissions vary based on its production method," Mika explains.

In the industry, hydrogen is categorised by colour, denoting its origin. Green hydrogen is produced using renewable energy, making it the only truly carbon-free source; yellow hydrogen, produced using solar energy, is another clean option, though both are expensive and difficult to acquire.

For other types of hydrogen, emissions during production can negate some of the environmental benefits. Producing hydrogen from fossil fuels (like grey hydrogen) leads to significant CO2 emissions.

Even blue hydrogen, made from natural gas with carbon capture, has higher emissions than green hydrogen. Also, hydrogen must be compressed, transported and stored, reducing energy efficiency.

Mika adds: "3kWh of renewable electricity makes roughly 1kWh of usable hydrogen. The result is energetic inefficiencies that amplify any emissions in production."

Comprehensive lifecycle emissions

IDTechEx's report evaluates the lifecycle emissions of H2ICE vehicles, considering the production of green, blue and grey hydrogen and comparing them to fuel cell electric vehicles (FCEVs) and traditional petrol or diesel engines.

It includes benchmark comparisons with battery-electric vehicles (BEVs) under different energy grid conditions.

While hydrogen engines promise to reduce CO2 emissions, challenges with NOx emissions and the environmental impact of hydrogen production must be addressed.

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