Simulaiton Study For Cold Starting Operation Of Dieselengine With Exhaust Gas Recirculation

Improving cold starting of diesel engines is a primary task of diesel engine designers. There are many problems during diesel engine cold starting, such as long cranking periods, high emissions, because of low compression temperature and pressure. As emission regulations promoted, there is an urgent need to improve performance of diesel engine during starting process because this period is included in emission regulation test cycles.EGR (Exhaust Gas Recirculation) is widely used to reduce NOx emission of diesel engine because its inert components reduce the in-cylinder oxygen concentration and in-cylinder temperature. However, the composition of exhaust gas in starting process is greatly different from that of the normal operation conditions. There are lots of residual fuel and active intermediate in the exhaust gas. Introducing these products into cylinder will affect the combustion of next cycle. In this thesis, combustion under different EGR rates during cold start process was investigated by chemical kinetics calculation. The ratio of fuel reacted, CO2 concentration in the exhaust gas,the temperature and pressure of cylinder were analyzed. It has been found that the appropriate rates of EGR can obviously improve the combustion during the cold starting process, but heavy EGR may cause an opposite effect.The concentrations of all kinds of compositions along with crank angle change were calculated. With the theory of n-heptane oxidation,the mechanism of combustion improvement by EGR has been investigated.The key compositions that affect the reactions were confirmed. Meanwhile it has been found that reduction of compression terminal temperature caused by too much EGR is the reason for combustion deterioration. The ignition delays considering the effect of equivalence ratio, unburned EGR rate, temperature and pressure were calculated. It has been found that increasing equivalent ratio and EGR rate will reduce the ignition delay. And as the temperature increase, the ignition delay showed a negative temperature coefficient phenomenon. With the mathematical method of MATLAB, a new model of ignition delay calculation is obtained.Finally, the quasi-dimensional model——DIJET model is constructed by GT-Power software. With the user model of GT-power, the new ignition delay model was coupled with the DIJET model. The cold starting performances under different EGR rates were calculated. The model provides a effective tool for cold starting process optimization.