Are Hydrogen Engines Truly Zero Emissions?

­Internal combustion engines (ICE) but without the emissions. The idea is certainly appealing. Keeping the same internal combustion engines (ICE) that have powered cars, buses, and trucks for well over a century, but eliminating the damaging and climate-affecting emissions that escape from the tailpipe. Currently, electrification is the primary route to decarbonizing transportation, but battery electric solutions bring many challenges, meaning adoption is likely to be a slow process - IDTechEx estimates that 11% of all car sales in 2023 were fully electric.

Could engines running on hydrogen keep 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.

A carbon-free fuel, so 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 vapor, carbon dioxide, and nitrous oxides. No carbon is present in the chemical reaction when switching to a pure hydrogen fuel; therefore, no CO₂ is formed and emitted into the atmosphere. There is a slight caveat to this zero CO₂ however. 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, when compared to the amount of CO₂ emitted by a conventional ICE, the amount is negligible. IDTechEx estimates that less than 1kg of CO₂ would be produced per 1,000 miles from burning motor oil in an H2ICE vehicle, compared with 272kg in a conventional ICE. Thus, an H2ICE would have around 99.7% less CO₂ emissions than a conventional ICE.

Nitrous oxides pose the greatest challenge

While there is no carbon in the fuel, the combustion of hydrogen causes very high temperatures in the combustion chamber, and this does lead to nitrous oxide formation. Nitrous oxides (NOx) have been recognized as greenhouse gases (GHGs) and harmful to air quality for decades. Increasingly strict regulations on the permissible tailpipe emissions of NOx have been implemented globally as the harms of NOx have become more apparent. 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:

• Influence of 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 historic and current tailpipe limits (g/km and g/kWh) in key regions.
• Discussion of emissions for sectors beyond road transportation, including aviation.