Analysis Of A Deeply Downsized,Highly Boosted Spark-ignition Engine With Ultra-expansion Cycle

In order to improve the fuel economy and ensure adequate power output in automobiles simultaneously,downsized,highly boosted are regarded as one of the most promising technologies in gasoline engines.Due to increased throttle opening,downsized engine will lead to a shift in the typical engine operating range to higher loads,reducing the pumping loss and thus the fuel consumption.To meet vehicle requirements for the maximum power output,highly boosting is normally necessary for downsized engines,causing a more severe knock tendency compared with their naturally aspirated counterparts.In this study,a 1.5L four-cylinder,four-stroke engine is modified into two cylinders.And the thermal efficiency improvement mechanisms between five-stroke engine and Miller cycle engine with two-stage turbocharging and intercooler strategy are analyzed based on the first law of thermodynamics and one dimensional engine simulation.For five-stroke engine with the proper design,the fuel consumptions under the most-frequently operated conditions are improved by 9–26% and the low fuel consumption area on the operating map is drastically enlarged compared to the original engine.The energy balance analysis shows that the increased combustion efficiency,reduced exhaust energy and the extra expansion work in the auxiliary cylinder are the primary contributions to improving the fuel conversion efficiency at the middle and high loads.At the low loads,reductions in the pumping loss and exhaust energy are the main causes of the reduced fuel consumption,while the contribution of the extra expansion work in the auxiliary cylinder becomes small.Considering Miller Cycle can achieve better fuel economy than five-stroke engine in middle and low loads conditions owing to looser exhaust gas temperature and intake pressure limit.While in high and full loads conditions,Miller cycle performs worse because it can't achieve such a high intake pressure,and it also has to apply fuel enrichment strategy to lower the exhaust temperature to protect turbine blades,and obtains dissatisfied fuel consumption as a consequence,even worse than the original engine.