Research On Combustion And Emissions Characteristics During Engine Quasi-Constant Speed Start For HEV Application

Idle stopping is one of the important methods for hybrid electric vehicle(HEV) to realize energy saving and emissions reduction, therefore, the HEV will start and stop frequently during normal road condition. The engine of HEV is firstly cranked to the idle speed and then the fuel injection and spark ignition were triggered when it starts. During this process, as the engine speed and manifold pressure change dramatically in a short time, the quick start of HEV engine is described as highly transient, if not controlled properly, deteriorated combustion and emissions performance would happen. Moreover, due to the slow flame propagation speed of traditional gasoline, the spark-ignited engine would experience bad economic performance, increased cyclic variation and deteriorated emissions under low load and transient conditions. As an alternative fuel with better combustion and emission characteristics, methanol is believed to have the potential to improve the engine performance under the corresponding conditions. Focused on this problem, a quasi-constant speed start strategy for HEV engine was built in this paper and the effects of this strategy on the combustion and emission characteristics during engine start was experimentally investigated in a modified methanol-gasoline flex fuel spark-ignited engine.To determine the control parameters for quasi-constant speed start, as well as to investigate the effect of methanol on the combustion and emissions characteristics of engine under low load condition, the idle test of methanol-gasoline engine was firstly conducted. The test results show that, the indicated thermal efficiency of engine fueled with methanol was higher than that of gasoline and increased monotonically with the increase of excess air coefficient. Methanol availed to accelerate the in-cylinder combustion process, as well as shorten the flame development and propagation periods. Thanks to its faster combustion rate and wider flammability limits, the Co Vimep of the methanol test was lower and increased much slowly with excess air coefficient compared to the operation of gasoline. Methanol was propitious to decrease the HC, CO and NOx emissions under idle and lean conditions. The improved idle performance of engine fueled with methanol show that methanol has the potential to ameliorate the start characteristics of engine employing quasi-constant speed start strategy.Secondly, the effects of different boundary conditions on the first cycle combustion characteristic were evaluated. The test results show that, due to the lower intake pressure and better fuel evaporation performance, high starting speed avails to realize the successful firing of the first cycle with relatively less fuel injection mass and benefits to reducing the peak cylinder pressure and shortening the flame development and propagation periods. The molecule diffusion rate and flame propagation speed of methanol is faster than that of gasoline, therefore, under the condition that the first-cycle fuel injection mass is relatively less, methanol can improve work ability of fuel-air mixture and shorten the flame development and propagation periods. The main effects of the intake valve timing on the first cycle combustion characteristics lie on the fuel atomization ability and inducted air mass. When the intake valve was retarded opening, the piston was under downward movement and a certain degree of vacuum existed in cylinder, which was beneficial to the fuel atomization and diffusion; however, if the intake valve was advanced opened, more air mass would be entrained into the cylinder for the first cycle, thus resulting in improved combustion rate and increased peak cylinder pressure.Lastly, the start characteristics of engine employing quasi-constant speed start strategy were experimentally investigated. The test results show that, the quasi-constant speed start strategy can effectively reduce the engine speed overshoot and peak cylinder pressure as well as accelerate the variation of excess air coefficient to stoichiometric and decrease the HC, CO and NOx emissions during start process. Higher starting speed availed to reduce the engine speed overshoot and decrease the HC and NOx emissions; however, the CO emission displayed no qualitative difference between different starting speeds. Due to the better formation characteristic of air-fuel mixture and faster flame propagation speed, the test engine fueled with methanol showed better combustion stability, decreased cyclic energy consumption and reduced HC, CO and NOx emissions during the quasi-constant speed start process. The main effects of the first cycle fuel injection mass on the air-fuel mixture concentration of the subsequent cycles focused on the first five cycles. After adopting the cycle based lean fuel injection strategy, the speed overshoot and the HC emission of the test engine were both decreased during the start process.