Effect of relative humidity on water injection technique in downsized spark ignition engines

Journal article, 2021

Combustion knock is a major barrier to achieving high thermal efficiency in spark ignition engines. Water injection was
recently identified as a potential way of overcoming this barrier. To evaluate its general applicability, experiments were
performed on a downsized three-cylinder spark ignition engine, varying the humidity of the intake air, the water injection
timing, and the engine speed. The minimum quantity of injected water required to maintain a given load (and thus level
of engine performance) was determined under each set of tested conditions. The knock-suppressing effects of water
injection were found to be related to changes in the fuel–air mixture’s specific heat ratio (kappa) rather than evaporative
cooling, and to therefore depend on the total quantity of water in the cylinder rather than the relative humidity per se.
The total quantity of water in the cylinder was also shown to be a key determinant of advancement in combustion phasing
and particulate emissions under various conditions.