Due to the complete separation of the scavenging processes from all other aspects of the engine, such as the lubrication system and moving parts, the stepped piston engine is very well suited to hydrogen fuelling. Many conventional engines have experienced corrosion challenges when operating on hydrogen. This is countered by the segregation offered by the stepped piston engine architecture.

UMA290 twin cylinder 290 cm³ Range-extender engine

Due to the inherently low combustion temperatures and pressures evident with operation on the two-stroke cycle emissions of oxides of nitrogen (NOx) are lower than emissions from a comparable four-stroke cycle engine. The model data below compares the NOx emissions for the UMA290 engine and a four-stroke twin cylinder engine of 374 cm³ (4S 374). Both engines are operating on indolene, a reference form of gasoline. The power output and operational speeds of both engines is similar.

Operation on hydrogen means that all emissions of unburnt hydrocarbons, CO and CO₂ are eliminated. NOx however can still form. The data in the lower graph (UMA290 H2) shows the modelled NOx at full load when operating on hydrogen. Further significant reductions over the four-stroke engine can be clearly seen.

Comparison of NOx emissions for UMA290 stepped piston engine and similar output four-stroke engine (4S 374) using indolene fuel, together with UMA290 operating on hydrogen

Hydrogen will ignite at significantly wider flammability limits than can be achieved with gasoline/indolene. The stepped piston engine has repeatedly shown an ability to operate at very lean Air:Fuel ratios on gasoline. If it is possible to operate on hydrogen at very lean conditions the emissions of NOx reduce even further as shown below.

NOx emissions for UMA290 stepped piston engine as a function of Air:Fuel equivalence ratio

As can be seen in the above data Air:Fuel equivalence ratios below 0.3 result in single digit emission levels. All tests were conducted at 4000 RPM.

More details of the research conducted so far can be seen in the following journal paper: -

• Hooper, P.R. (2024) - "Hydrogen fuelled stepped piston engine for ultra-low emissions hybrid and range extender electric vehicles" (Int. J. Powertrains, Vol. 13, No. 3, pp.313–335.) https://doi.org/10.1504/IJPT.2024.142182

Link to paper

A larger capacity twin cylinder 497 cm³ engine is also under consideration for hydrogen fuelling as shown below.

Twin cylinder SPX500 Engine

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