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 homogeneous combustion. Reaching the goal a homogeneous charge compression ignition engine is simulated using a multi zone model. N-heptane is used as engine fuel and its combustion process is modeled utilizing a semi detailed chemical kinetics mechanism. At first the developed model is validated against the experimental data and its accuracy in prediction of engine performance and emissions is calculated. At the next step different values of water are added to the in-cylinder mixture and their effects are studies. Thermal, chemical and dilution effects of water are studied separately in current study. The results show that water addition decreases the in-cylinder peak pressure and temperature and retards the start of combustion. When exhaust gas is not recirculated to the combustion chamber, low contents of water increase the first law efficiency and decrease the exhaust carbon monoxide and unburned hydrocarbons. In the presence of recirculated exhaust gases, low contents of water has no significant effect on engine performance and emissions. Higher contents of water cause to misfire and increase the exhaust emissions and decrease the first law efficiency sharply. The main important effect of water on engine performance and emissions is its thermal effect. Chemical effect of water on heptane combustion is negligible.