Experimental investigations of gasoline partially premixed combustion with an exhaust rebreathing valve strategy at low loads

This paper investigates the combustion and emission characteristics using a commercial gasoline with Research Octane Number (RON) of 93 on a Partially Premixed Combustion (PPC) engine equipped with a Variable Valve Actuator (VVA) system. The effects of injection timing, internal exhaust gas recirculation (i-EGR) rate and intake pressure on combustion and emissions are studied. The i-EGR was achieved by an extra opening of the exhaust valve during the intake process. The study was conducted at an engine speed of 1500 r/min with cyclic fuel mass from 11.6 to 22.2 mg. The operating region for high-efficient, clean and stable gasoline PPC (i.e., COVimep < 5%, NOx < 0.4 g/kW h, Smoke < 0.1 FSN, CO and HC as low as possible) was explored and discussed. The results illustrate that proper i-EGR rates with a fixed injection timing of −24°CA ATDC could maintain optimal thermal efficiency while minimizing CO and HC emissions as the engine load decreased from 3.9 to 2.1 bar gross indicated mean effective pressure (IMEPg). However, high NOx at higher load was observed and the adoption of earlier injection timing with lower i-EGR rate has the potential to suppress the sharp Smoke increase at lower NOx levels. Improved combustion process can be obtained with later SOI timings as well as higher i-EGR rates as the intake pressure increased. It has been found that the low-load limit of stable gasoline PPC can be extended to 1.5 bar IMEPg through the optimization of VVA, injection strategy and intake boosting.