Are Hydrogen Engines Truly Zero Emissions?

Internal combustion engines (ICE) but without the emissions. The idea is certainly appealing. Keep the same internal combustion engines (ICE) that have powered cars, buses, and trucks for over a century, but eliminate the damaging and climate-affecting emissions that escape from the tailpipe.

Electrification is currently the primary route to decarbonizing transportation. Still, battery-electric solutions present many challenges, meaning adoption is likely to be slow. IDTechEx estimates that 11% of all car sales in 2023 were fully electric.

Could engines run on hydrogen offer the best of both worlds: familiar and mature ICE technology with zero emissions? IDTechEx’s report, “Hydrogen Internal Combustion Engines 2025-2045: Applications, Technologies, Market Status and Forecasts,” explores the emissions credentials of this potentially disruptive technology.

No Carbon in the Exhaust?

Combustion is a high-temperature chemical reaction between a fuel and an oxidant (in the case of engines, this is atmospheric oxygen). In a conventional ICE fuelled by petrol or diesel, the hydrocarbon reacts with oxygen and nitrogen in the air at high temperatures to produce heat, water vapour, carbon dioxide, and nitrous oxides.

No carbon is present in the chemical reaction when switching to a pure hydrogen fuel, so no CO₂ is formed and emitted into the atmosphere. However, there is a slight caveat to this zero CO2 level. IDTechEx research indicates that a small amount of motor oil will be burned in an ICE.

Motor oil is essential to lubricate the intricate moving parts and prevent damage from metal-metal contact. Most engines will burn a small amount of oil due to leakages or blowback in the system, and as all motor oils currently in use are hydrocarbon-based, this does lead to a small amount of CO₂ being produced.

However, the amount of CO2 emitted by a conventional ICE is negligible. IDTechEx estimates less than 1kg of CO₂ would be produced per 1,000 miles from burning motor oil in an H2ICE vehicle, compared with 272 kilograms in a conventional ICE. Thus, an H2ICE would have around 99.7% less CO₂ emissions than a traditional ICE.

Nitrous Oxides Pose the Greatest Challenge

While the fuel contains no carbon, hydrogen combustion causes high temperatures in the combustion chamber, forming nitrous oxide. Nitrous oxides (NOx) have been recognized as greenhouse gases (GHGs) and harmful to air quality for decades.

As the harms of NOx have become more apparent, increasingly strict regulations on the permissible tailpipe emissions of NOx have been implemented globally. For H2ICE to have credible emissions reductions, it must also show that it can reduce NOx emissions substantially. The formation of thermal NOx (NOx formed in high-temperature combustions) is a highly complex interplay of factors and engine parameters.

IDTechEx breaks down each of these factors in turn within the report, examining the latest academic research and industry trends such as:

• The influence of the air-fuel ratio on NOx formation and why the industry is tending towards lean burn spark ignition (SI) designs.
• Impact on engine speed on NOx formation due to pressure and reaction time.
• Applicability of existing compression ignition (CI) and SI exhaust gas after treatment to hydrogen engines.
• Performance and operational principles of 2 and 3-way catalytic converters, selective catalytic reduction, lean NOx traps, and exhaust gas recirculation (EGR).
• Real-world performance data for existing H2ICE vehicles.
• Contextualisation of H2ICE NOx emissions with historical and current tailpipe limits (g/km and g/kWh) in critical regions.
• Discussion of emissions for sectors beyond road transportation, including aviation.

Colour Is Key

Although the hydrogen fed into the engine may be carbon-free, its overall life-cycle emissions depend significantly on its origin. In the hydrogen industry, the ‘colour’ of hydrogen denotes the production method. Of the rainbow of colours available, green and yellow hydrogen is the only truly renewable source.

However, these are both hard to acquire and prohibitively expensive, partly due to the low energy efficiency conversions. 3kWh of renewable electricity makes roughly 1kWh in usable hydrogen. To be fed into an engine, it must then be compressed, transported, stored, dispensed, and finally burned in a heat engine with limited thermal efficiency.

The result is many energetic inefficiencies, which amplify any emissions involved in production. While the emissions produced for green hydrogen are minimal, this becomes a significant consideration for other colours of hydrogen.

In the report, “Hydrogen Internal Combustion Engines 2025-2045: Applications, Technologies, Market Status and Forecasts”, IDTechEx assesses the overall lifecycle emissions of an H2ICE vehicle, including:

• Green, blue, and grey hydrogen estimated CO₂ emissions (in g/km).
• Comparison with a fuel cell electric vehicle (FCEV, a hydrogen-powered electric vehicle) across the same colours of hydrogen.
• Benchmark comparisons of diesel/petrol and a battery-electric vehicle charged under various grid scenarios.