A Study on Reduction of Cooling Loss by Water Addition in SI Engine by using Rapid Compression and Expansion Machine

抄録

Water injection has attracted attention as one of the techniques for improving thermal efficiency of SI engine. When liquid water is injected in cylinder or inlet manifold, water absorbs the heat of fuel/air mixture due to its evaporation and reduce temperature of mixture. Consequently, it is expected that water injection can reduce cooling loss and suppress knock.
The water injection in SI engine so far has been attempted mainly in inlet manifold. Port injection of water forms uniform water vapor distribution in cylinder. On the other hand, this research proposes in-cylinder direct water injection for reduction of cooling loss and suppression of knock effectively with small amount of water by forming low temperature water vapor layer on the wall.
In the present paper, in order to investigate the influence of in-cylinder direct water injection on combustion and heat transfer in SI engine, measurements of in-cylinder pressure and wall heat flux and combustion visualization are conducted simultaneously by using rapid compression and expansion machine(RCEM). In order to separate the thermodynamic and chemical effects of water addition, homogeneous mixture of fuel/air/water is introduced into RCEM, and the amount of water and in-cylinder gas temperature are changed independently. Firstly, in order to investigate the chemical effect of water, the amount of water is changed with a constant in-cylinder gas temperature. Secondly, in order to investigate the thermodynamic effect of water, the in-cylinder gas temperature is reduced for simulating the water absorbing heat due to evaporation in cylinder.
The obtained results show that addition of water vapor to in-cylinder gas uniformly leads to reduction of wall heat flux and suppression of knock, whereas flame propagation speed remarkably decreases.