Study on the effect of water addition on combustion characteristics of a HCCI engine fueled with natural gas

The coordinated control of combustion boundary conditions and fuel chemistry is the core academic idea of homogenous charge compression ignition (HCCI) combustion. Realizing controlled combustion is the current development goal of HCCI engine and adding additives to the cylinder cooperate with the change of boundary conditions is an effective way to achieve it. In the paper, using a mull-zone thermodynamic model coupled with a detailed chemical reaction mechanism (including 53 species and 325 reactions) to study. First, the rationality of the mull-zone model was verified for six different operating conditions. Then seven different mass fraction of water were added to the cylinder and examined its effect on natural gas (NG) HCCI combustion. The thermal, dilution and chemical effects of water on start of combustion (SOC) were carried out using artificial inert species method. It was determined that in-cylinder pressure and heat release rate (HRR) decreases, SOC retards with the increase of water addition at different loads and boundary conditions. Under high load which is knock and abnormal combustion, water addition controls SOC and reduces ringing intensity (RI) to normalize the combustion process. The results show that the sensitivity of combustion to water addition increases and the tolerance of engine to water decreases as the intake temperature (T-in) reduces and excess air ratio (lambda) rises. The thermal effect of water on SOC is greater than that its dilution and thermal effect. It is easier to achieve controlled combustion by changing T-in than lambda in NG HCCI engine with water to mitigate knock. In addition, the effect of water on important species in chamber was proposed to prove the above conclusion from internal mechanism.