Hydrogen is a very unusual motor fuel in the sense that it has high and low octane ratings at the same time. Or rather, it has a Research Octane Number, RON, of 120 but the Motor Octane Number, MON, of only 60. This means that in the conditions representative of RON testing, hydrogen can be considered a very high-octane fuel compared to regular gasoline (RON=95) and even the alcohols methanol/ethanol with 107/106. This should enable high efficiency Spark Ignition operation with a high compression ratio. However, the MON rating of hydrogen is only 60, far lower than that of regular gasoline (MON=85-90). This means that in this operating condition, hydrogen is much more prone to auto ignition and engine efficiency will be much lower.

There are three major differences between RON and MON test procedures;

1. engine speed is increased from 600 to 900 rpm,
    
2. inlet temperature is increased from 52 °C to 149 °C and now it is the charge temperature and not air temperature that is controlled, and finally
    
3. spark timing is now variable and not fixed.

Hydrogen’s Lower Heating Value (LHV) is approximately 120 MJ/kg, which is nearly 3 times that of gasoline (~44 MJ/kg) or diesel (~42.5 MJ/kg). However, under standard temperature and pressure (STP), hydrogen gas is extremely light (density of ~0.089 kg/m³), so the volume of hydrogen needed is high.

Port Displacement: When hydrogen is injected into the intake manifold (port fuel injection), gaseous hydrogen takes up about 30% of the combustion chamber volume, displacing air. This reduces the volumetric efficiency and lowers the maximum power output of the engine by roughly 15–20% compared to gasoline (unless direct cylinder injection or turbocharging/supercharging is used).

Hydrogen has an extremely broad equivalence ratio range for flammability in air, spanning from 4% to 75% by volume (air-fuel equivalence ratio lambda from 0.1 to 10). This allows for extremely lean engine operation, which can help improve overall efficiency and lower NOx emissions.