Investigation of the effect of water addition on performance and emissions of an n-heptane fueled HCCI engine

The main purpose of current study is investigation on the effect of water addition on n-heptane homogenous charge compression ignition combustion. A multi zone model coupled to the semi-detailed chemical kinetics mechanism is used for simulation of n-heptane homogenous charge compression ignition combustion. First, the accuracy of the model was estimated for two different operating modes, and then seven different amounts of water were added to the fuel and its effects on n-heptane combustion were investigated. Thermal, chemical and dilution effects of water are studied using artificial inert species method. The results show that the start of combustion was retarded by water addition due to the thermal effect of water. Peak values of in-cylinder pressure and heat release rate decreases by water addition. Water addition has caused the maximum amount of radicals in the combustion chamber to be reduced and the time of their formation is delayed. Water addition increases the amount of unburned hydrocarbons at exhaust. Thermal effect of water on start of combustion and emissions formation is more significant than its dilution and chemical effects. Using small quantities of water will increase the thermal efficiency of the engine and reduce emissions from it.