Experimental Studyon A Turbocharged PFI Gasoline Engine Withlow Pressure EGR Technology

Gasoline engines have small vibration noise, high power density, low productioncost. Especially, gasoline engines have obvious advantagesin terms of exhaustemissions control. So they are used widely. However, due to their low compressionratios, high pumping losses at low and part loads, large coefficient of cycle-to-cyclevariation, and difficulty in burning lean mixture, the average fuel consumption ofgasoline engines is20%~30%higher than that of diesel engines. With the low pressureEGR（Exhaust Gas Recirculation）technology being applied to gasoline engines, thefuel consumption can be greatly reduced at low and part loads, primarily because ofthe reduction in pumping losses. The fuel economy benefit at high loads can resultfrom an increase in compression ratio, due to a decrease in knock tendency. And thethree-way catalytic converter can still be used to solve the problem of exhaustemissions.This paper mainly studied the effects of different EGR rates on combustion andemissions characteristics of a small-displacement PFI（Port Fuel Injection）gasolineengine, and found the largest fuel-saving effect with the addition of EGR. The engineused in this experiment was a4-cylinder,1.5L gasoline engine equipped with aturbocharger. The inlet and exhaust systems of this engine were modified to use watercooled EGR in low pressure loop configurations. The original engine operated at9.3compression ratio, which was increased for this experiment to11.5. In order to furtherimprove the engine’s fuel economy and meet the emissionregulation of Euro5at thesame time, an idea that we combined the three-way catalytic converter with EGRtechnology to burn lean mixture was proposed. And the feasibility was tested in thispaper. The main conclusions are as follows:1. Under the condition of fixed injection pulse width, tests of different EGR ratesand ignition timings were done. The research showsthat EGR leads to the combustionphase lag, makes the combustion duration longer and the coefficient of cyclic variation larger, and both of the two characteristics can be effectively controlled by optimizingthe ignition timing.2. In the equivalent mixture combustion mode, with the addition of EGR, the COemission decreases while the THC emissions increase,and both of them change in asmall range. However, the NOXemissions decrease significantly. The BSFC（BrakeSpecific Fuel Consumption）reduces by about4.5%at low speed and low load, andabout9.7%at medium speed and medium load.3. Through the study of EGR with different excess air coefficients, we find that atlow speed and low load, compared with burning equivalent mixture, burning leanmixture can make the BSFC reduce further by about3.8%under the condition ofcombining EGR with the three-way catalytic converter. However, at medium speedand medium load, burning equivalent mixture is much better.