An Experimental Investigation And Numerical Simulation About Effect Of Nitric Oxide On HCCI Combustion

A homogeneous charge compression ignition (HCCI) engine combines advantagesof a SI engine with those of a CI engine, but its ignition delay and combustion rate aredifficult to control, and working range is narrow and emissions of CO and HC are high,and so on. To overcome these difficulties and achieve effective control of HCCIcombustion process, researchers have conducted a lot of researches and searched someways, and the exhaust gas recirculation (EGR) is the most important among thesetechnologies. Apparently, the effect of EGR on HCCI combustion can be divided intofour parts: dilution effect, thermal effect, heat capacity effect and chemical effect. Thechemical effect is incorporated from the different components. In this paper, a veryimportant component-NO is chosen for research target, and effects of NO on HCCIcombustion of n-heptane, iso-octane, methanol, and ethanol are studied.Firstly, HCCI combustion is achieved on a single cylinder, four-stroke,water-cooling diesel engine. Effects of NO on performance and emissions of HCCIcombustion with n-heptane and iso-octane are researched by change concentrations ofNO in inlet airflow. The cylinder pressure, temperature, heat release rate, ignition delay,combustion duration and emissions of HC, CO, NOx are compared on differentconcentrations of NO.Due to HCCI combustion controlled by chemical kinetics of fuels, the core ofsimulation for HCCI combustion focuses on chemical reaction mechanism of fuels.Based on experiments and previous work, the co-oxidation chemical kinetic model ofNO-nC7H16is set (245reactions,27components).By using this mechanism andzero-dimension calculation program named SENKIN, effect of NO on HCCIcombustion with n-heptane is discussed and impact on main components from oxidationof n-heptane is analyzed. Effect of NO on main reaction paths of n-heptane oxidation isalso analyzed using reaction path analysis method.Because of alcohol alternative fuels research attract more and more attention, effectof NO on HCCI combustion with methanol and ethanol and constant-volumecombustion simulation are further carried out. The experimental results show that NOshorten ignition delay of methanol and ethanol, and the maximum value of heat releaserate of methanol and ethanol are improved by raising the NO concentration, and thecrank angle correspond to the maximum value of heat release rate is also advanced. By constant-volume combustion simulation analysis, main reaction paths about how NOand NO2participate in alcohol fuel oxidation process are exposed. Results show that theconcentration curves of NO and NO2can be divided into four regions and differentreactions work in each region.