The Effects of Charge Motion and Laminar Flame Speed on Late Robust Combustion in a Spark-Ignition Engine

The effects of charge motion and laminar flame speeds on combustion and exhaust temperature have been studied by using an air jet in the intake flow to produce an adjustable swirl or tumble motion, and by replacing the nitrogen in the intake air by argon or CO2, thereby increasing or decreasing the laminar flame speed. The objective is to examine the "Late Robust Combustion" concept: whether there are opportunities for producing a high exhaust temperature using retarded combustion to facilitate catalyst warm-up, while at the same time, keeping an acceptable cycle-to-cycle torque variation as measured by the coefficient of variation (COV) of the net indicated mean effective pressure (NIMEP). The operating condition of interest is at the fast idle period of a cold start with engine speed at 1400 RPM and NIMEP at 2.6 bar. A fast burn could be produced by appropriate charge motion. The combustion phasing is primarily a function of the spark timing. At each spark timing, however, there is an optimal charge motion that could minimize the COV of NIMEP while not substantially changing the end of combustion (measured by the 90% burn location: CA90). The exhaust temperature is only a function of CA90, irrespective of whether the CA90 is changed by spark timing, charge motion, or laminar flame speed. By using an appropriate amount of charge motion, the exhaust temperature could be raised by 100 C while keeping the same COV of NIMEP. INTRODUCTION To meet the emissions regulatory requirements, it is essential to after-treat the spark-ignition engine exhaust with a catalyst. The treatment, however, is not effective before the catalyst reaches a light-off temperature (~300°C). Consequently, a major portion of the total emissions produced in the Federal Test Procedure (FTP75) occurs during the first tens of seconds of engine operation before the catalyst reaches light-off temperature [1]. The hydrocarbon (HC) emissions during this period are especially severe because the cold environment is not favorable for the mixture preparation process. Enabling the catalyst to light off quickly will substantially reduce overall HC emissions. One strategy to accomplish fast catalyst light-off is to increase the exhaust enthalpy flow by retarding the spark timing [2]. Then combustion occurs substantially late in the expansion stroke, and less work is extracted by the piston. Late combustion also results in a lower heat loss by the charge because the peak temperature is lower and the burned gas has less residence time in the cylinder. These factors contribute to a hotter exhaust. Furthermore, to provide the same torque output, a higher through put rate is required to compensate for the unfavorable combustion phasing, thereby increasing the exhaust sensible enthalpy flow. Retarding combustion