Study On The Effect Of Synergistic Effect Of EGR And Variable Compression Ratio On Gasoline Engine Performance And Emissions

Nowadays, petroleum oil is still the main fuel of engines, but the world have suffered from the shortage of oil resources and the greenhouse gas emission and atmospheric pollution result from oil burning. So, an increasing number of countries pay attention to the energy conservation and emission reduction of internal combustion engine. Engine Downsizing is the most promising way to solve the problem for SI engine. However, with the further increase of the thermal load, knock, NOx and the high temperature of emission gas become more and more outstanding. EGR (emission gas recirculation) can reduce both the tendency to knock and NOx emissions. This article is based on a variable compression ratio single-cylinder gasoline engine, tests and simulation works under different compression to explore the EGR influence of EGR on engine performance and emissions are carried out.Firstly, engine performance and emissions with EGR rate of0%-20%were compared under1200r/min, wide-open throttle and compression ratio of8:1-11:1. Results show that:with EGR rate increasing, maximum instant heat release rates and peak cylinder pressure both decrease, initial combustion duration and main combustion duration increase, thus lower combustion temperatures are generated. When higher compression ratios are used, combustion durations decrease and combustion temperatures rise. Based above researches, impacts of hot and cool EGR on engine performance and emissions were also compared. Engine tests indicate that: compared to cool EGR, hot EGR with higher temperature results in lower intake air density and volumetric efficiency, in consequence, engine output power decreases and higher NOx are also found with hot EGR. The combustion rates with hot EGR are higher than that of cool EGR, therefore engine coefficients of variation with hot EGR are relative lower.Three-dimensional numerical simulation was also used in engine combustion process to analysis the inner reason of EGR. The results show that, EGR reduces the laminar flame speed in cylinder and this make cycle-to-cycle variation increase and there little influence to turbulence intensity and mean flow velocity. Increasing the compression ratio can increase laminar flame velocity, turbulence intensity and mean flow velocity, thus reducing cycle-to-cycle variation in the engine. With the increase of EGR rate, the exhaust gases dilute the oxygen concentration and inhibit the combustion in cylinder, and also this make the temperature decrease. Thereby, the amount of NO decreased with increasing EGR rate in the combustion process.Based on the results of experiments and simulation, the following conclusions can be drawn: EGR effect the combustion process of the engine by reducing the speed of the mixture, combustion temperature and oxygen concentration, which result the reduction of the NOx emission and knock tendency. Contrast with hot EGR, the cold EGR has the better power and the ability of reducing knock tendency, though it will increase the cycle fluctuations. EGR Combine with variable compression ratio, the power and fuel economy can be compensated to some extent, and make NOx emissions at a low level. The optimization of matching will be the focus of future research.