Thermal efficiency and hydrogen-assisted combustion: what the data shows

1. What thermal efficiency means in an internal combustion engine

Brake thermal efficiency is the fraction of the chemical energy in the fuel that the engine converts into useful mechanical work at the crankshaft. The rest is lost — predominantly as heat to the exhaust, heat to the cooling system, friction, pumping losses, and unburnt or partially-burnt fuel leaving the cylinder.

Spark-ignition gasoline engines under typical road load operate in roughly the 20–30% efficiency band. Compression-ignition diesels do better — 35–42% on a good day, lower under transient light load. Even modern engines waste the majority of the fuel they consume. Anything that lets the cylinder convert a larger share of that fuel into work, instead of into heat or unburnt hydrocarbons, raises efficiency directly.

2. Mechanism: what a small H₂/O₂ fraction does to the burn

Introducing a small flow of mixed hydrogen and oxygen into the intake air does not "add fuel" in any meaningful energy sense — the energy contribution is negligible at the flow rates these systems run at. The mechanism is different. Hydrogen has a laminar flame speed several times that of gasoline or diesel vapour, and a much wider flammability range. A small hydrogen fraction in the intake charge acts as a combustion accelerant: ignition propagates faster across the chamber, end-gas burns more completely before the exhaust valve opens, and the fraction of fuel that would otherwise leave the cylinder unburnt or partially oxidised is reduced.

The result the dynamometer literature consistently reports is higher brake thermal efficiency, lower unburnt hydrocarbons, and lower carbon monoxide at the same brake power. The free oxygen co-produced by the electrolyser slightly enriches the intake oxidiser, which matters more at higher load.

3. Why steady-load engines respond best

The strongest, most repeatable response in the published literature is on steady-load, partial-throttle test engines — exactly the duty cycle of a stationary generator set. El-Kassaby et al. (2016, Alexandria Engineering Journal 55(1) 243–251) is a clean example: a small spark-ignition engine on a dynamometer, swept across brake power with and without HHO assist, reporting a brake thermal efficiency lift of roughly two to three percentage points across the load range, with the largest absolute gain near peak efficiency.

That study used an alkaline electrolysis cell. The mechanism it characterises is the gas-phase combustion effect, which is independent of how the gas was produced. The distinction matters at the equipment level — modern PEM units avoid the caustic electrolyte, the carry-over risk, and the maintenance burden of alkaline cells — but for the combustion response itself, alkaline-cell dynamometer data is directly informative about what to expect from a PEM unit feeding the same volume of mixed H₂/O₂ into the same intake.

Generator sets sit on the load curve where this mechanism is most effective: continuous operation, predictable load, no transients, no cold-start cycling. That is why our genset trial protocol is so much shorter than a transport trial — the signal is clean.

4. The carbon-cleaning effect — why urban gains build over distance

On variable-duty road vehicles the picture is more complex, and one specific feature of the data deserves attention. In historical road logging on earlier-generation alkaline-cell equipment we no longer supply, any highway change appeared immediately, while the urban change was not immediate — it built progressively over roughly the first 1,000 km of operation. No percentage from that discontinued equipment is restated here as a figure for the PEM systems we sell.

That build-up profile is consistent with progressive in-situ carbon cleaning of the combustion chamber, valves and injectors. The faster, more complete burn reduces the rate at which new deposits form, and over distance the existing deposit layer thins. The engine returns toward the combustion geometry the manufacturer designed for. The effect is not magic; it is the absence of fouling.

Our evidence standard, the published dynamometer work and its stated limitations are set out on the field results page: /field-results.

5. Honest limits

Results vary materially. Engine condition, fuel quality, duty cycle, load profile, installation quality and ambient conditions all move the number. Reported outcomes in the published literature vary widely by test method, engine and load, with steady-load dynamometer work generally showing the clearest signal. None of that justifies an "expected savings" figure for any specific vehicle or genset.

The only valid commercial basis is a controlled field evaluation on the operator's own equipment, on the operator's own duty cycle, with baseline data captured before installation. That is what our Fleet Trial Programme is for, and that is the number an operator should plan from — not ours.

FAQ

Further reading

• Fuel Cost Scenario Calculator — model the economics for your fleet at your chosen assumption.
• Field Results & Trial Data — first-party road trial, manufacturer trials, redrawn El-Kassaby chart.
• Fleet Trial Programme — controlled evaluation protocol.
• Generator Sets — steady-load duty cycle, the strongest-response application.