Feasibility Study of Operating 2-Stroke Miller Cycles on a 4-Stroke Platform through Variable Valve Train

摘要

A 2-stroke combustion cycle has higher power output densities compared to a 4-stroke cycle counterpart. The modern down-sized 4-stroke engine design can greatly benefit from this attribute of the 2-stroke cycle. By using appropriate variable valvetrain, boosting, and direct fuel injection systems, both cycles can be feasibly implemented on the same engine platform. In this research study, two valve strategies for achieving a two-stroke cycle in a four-stroke engine have been studied. The first strategy is based on balanced compression and expansion strokes, while the gas exchange is done through two different strokes. The second approach is a novel 2-stroke combustion strategy - here referred to as 2-stroke Miller - which maintains the expansion as achieved in a 4-stroke cycle but suppresses the gas exchange into the compression stroke. The first 2-stroke cycle generated a torque increment of 63% at 1000 rpm on a supercharged 4-stroke engine without increasing the maximum cylinder pressures. The second 2-stroke strategy - which uses the Miller cycle concept - generated a torque increase of 51.5% at 1000 rpm and higher thermal efficiencies than the first valve strategy. The 2-stroke Miller cycle has been proposed as a means of transitioning from 4 to 2-stroke with progressive torque delivery and without requiring a throttled intake. The 2-stroke cycle appears to be a potential solution to overcome some of the difficulties faced by downsized gasoline engines during high load operations. In particular this cycle will be beneficial in a condition where the engine is already operating at peak cylinder pressures and additional torque output is not easily achievable.