Abstract
Abstract A single-cylinder test engine model was built by GT-Power software, and the effects of internal exhaust gas recirculation (i-EGR), external EGR (e-EGR), i-EGR/e-EGR coupling and the crank angle degree at which 50% of total heat loss has taken place (CA50) on combustion and emission characteristics of gasoline compression ignition at low-load condition were analysed. The results show that the ignition delay period with e-EGR was extended slightly with the increased EGR ratio, while that with i-EGR strategy first shortened and then extended, and that the optimised indicated thermal efficiency could be achieved using a small amount of i-EGR. With the same EGR ratio, nitrogen oxide (NO X ) emission is more likely to be suppressed by i-EGR, while soot emission was more deteriorated, and the superior trade-off relationship between carbon monoxide (CO)/hydrocarbon (HC) emissions and NO X emission was attained by the combination of lower i-EGR ratios and CA50 closed to top dead centre. When using i-EGR/e-EGR coupling with total EGR ratio being fixed, the indicated thermal efficiency was decreased by increasing i-EGR ratio, while the lower NO X , CO and HC emissions could be realised, but only with the consequence that soot emission increased.
Highlights
With great concern about the enormous demand for energy, several industry and policy initiatives aim at shifting the powertrain from internal combustion engine (ICE) to gasoline/plugin hybrid or fuel cell [1]
The results have shown that internal exhaust gas recirculation (i-EGR) realised by negative valve overlap (NVO) can effectively improve the combustion stability of gasoline compression ignition (GCI) combustion under low-load conditions, but the decreased gas-exchange efficiency leads to the deterioration of indicated thermal efficiency [25]
With injection timing fixed at −18◦CA after top dead centre (ATDC), Group 3 was set up to investigate the effects of i-EGR and external EGR (e-EGR) ratios on GCI combustion behaviour under the iEGR/e-EGR coupling strategy, while the total EGR ratio was kept at 50%, and i-EGR/e-EGR ratio of each case was set to 5%/35%, 10%/30%, 15%/25%, 20%/20%, 25%/15%, 30%/10% and 35%/5%
Summary
With great concern about the enormous demand for energy, several industry and policy initiatives (mainly in Europe and the USA) aim at shifting the powertrain from internal combustion engine (ICE) to gasoline/plugin hybrid or fuel cell [1]. Compared with diesel fuel, applying gasoline fuel with high octane number (long ignition delay period) and volatility to CI engines is more likely to obtain well-premixed or properly stratified mixture without very high injection pressure and EGR ratio [11,12,13], while the combustion is transited from fuel–air mixing control of the conventional diesel combustion to the cooperative control of fuel– air mixing and chemical kinetics. Considering the poor ignition quality of gasoline, it is difficult for the auto-ignition function to work, especially under low-load conditions, owing to the low in-cylinder temperature and pressure, which lead to the increase of cycle-to-cycle variation and even misfires. It is not suitable for the use of after-treatment technology because of the low exhaust temperature under such operation conditions [17]
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