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

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.

Automated summary

The study shows that water injection can effectively suppress combustion knock, with the effect being related to changes in the specific heat ratio of the fuel-air mixture rather than evaporative cooling, thus emphasizing the importance of the total quantity of injected water over relative humidity. The total quantity of water in the cylinder plays a crucial role in advancing combustion phasing and particulate emissions under different conditions.