Investigation on the EGR effect to further improve fuel economy and emissions effect of Miller cycle turbocharged engine

摘要

Miller cycle and EGR have been proved to be the effective ways to improve the engine performance. In order to realize Miller cycle with high compression ratio, the piston and intake cam profile were redesigned for a turbocharged GDI engine. In-cylinder pressure was taken to clarify the difference of working cycles between Miller and Otto cycle. At full load, Miller engine with high compression ratio leads to the knock at a low speed. But at a high speed, engine can operate nearer stoichiometric conditions and get better economy. At partial load, engine can adopt higher compression ratio and intake pressure. The reduction of pumping losses and exhaust gas energy have become the key factors to improve fuel economy. On the basis, LP cooled EGR was introduced to study the further effect on Miller cycle. In-cylinder temperature is reduced due to the EGR dilution and heat capacity effect. Thus, engine can adopt larger ignition advance angle to obtain better combustion phase. For emission analysis, lower temperature can effectively reduce NO_x emissions. However, because of the extended combustion duration and complex piston shape, insufficient combustion will result in the increase of THC. The constant excess-air coefficient makes the CO emissions almost unchanged.