LP EGR Influence on Performance of Turbocharged Direct Injection Gasoline Engine

摘要

To maximize the benefits of LP EGR on a turbocharged direct injection engine, it is important to calibrate the engine properly, which is a critical process. To achieve the best fuel consumption, it is necessary to find the optimal EGR rate, WT angle, injection phase angle, and fuel rail pressure. During our study, we observed that EGR influences engine combustion, pumping loss and exhaust temperature. Furthermore, the reduction of the combustion temperature and of the pumping loss led to a significant improvement of BSFC and emissions. However, engine power and torque declined. The main findings of this study are set out below: 1. The addition of cooled exhaust gas to filtered air lowers the oxygen concentration of air-fuel mixture in the combustion chamber and causes a slowing combustion, which leads to the reduction in combustion temperature and reduces NO_x formation. A high load also suppresses knocking tendency, which allows to advance ignition angle and to improve the fuel consumption benefit. 2. The pumping loss will decrease by adding the throttle opening at medium and low load. When the exhaust introduced into the combustion; at high load, the exhaust flow will limit the intake flow, and then the engine performance will be lower; 3. LP EGR also affects the intake/exhaust temperature: After the exhaust has been introduced into the intake system, the intake temperature increases a little. On the other hand, the introduction of exhaust gas to the intake system decreases the combustion temperature, which leads to a reduction of the exhaust temperature. The intake pressure will be higher, and as the duty cycle will increase when the EGR system work, it leads to the increasing of the intake and exhaust pressure; 4. Because of the optimization of the combustion parameters and pumping, the BSFC will be lower with the increasing of the EGR rate. The BSFC reduction at medium to high load is higher. We achieved 16.5% BSFC benefit at low speed and high load. Furthermore, some BSFC benefits at low load (3-4 bar) according to the dyno test results. 5. Our study demonstrates the best EGR rate for minimum BSFC at different load ranks between 14 and 20%. Because of the limit of the mixture gas dilution, the engine is going to misfire if the EGR rate is too high, which will adversely affect the BSFC benefit. 6. LP EGR technology has some effect on the emission, and the combustion temperature decreases significantly by adding LP EGR so that NO_x is reduced by 90%. On the downside, HC rises and CO shows just a little fluctuation but essentially remains constant. 7. Finally, the air flow is limited under high load conditions, if EGR used, which leads to a performance degradation of the engine.