Measurement of short-circuiting and its effect on the controlled autoignition or homogeneous charge compression ignition combustion in a two-stroke poppet valve engine

摘要

Controlled autoignition or homogeneous charge compression ignition combustion in four-stroke spark ignition engines has been the subject of extensive research over the last decade. In order to extend the load range of controlled autoignition combustion for automotive applications, systematic research has been carried out on these combustion processes in a single-cylinder poppet valve engine that is capable of both four-stroke operation and two-stroke operation through flexible variable valve actuation. Since short-circuiting is a common issue for two-stroke engine operation, it is necessary to quantify the amount of short-circuiting. In this paper, a new method of measuring the short-circuiting rate during two-stroke engine operation is presented and applied to investigate the effect of short-circuiting on two-stroke controlled autoignition or homogeneous charge compression ignition operation. The cycle-resolved air short-circuiting rate is derived from instantaneous measurement of the carbon dioxide concentration near an exhaust valve using a Cambustion fast-response non-dispersive infrared analyser. It is found that the short-circuited air leads to fuel-rich combustion and hence high carbon monoxide and hydrocarbon emissions under closed-loop λ = 1 control. A method to calculate the in-cylinder air-to-fuel ratio based on the measured short-circuiting rate is presented and used to understand better the in-cylinder combustion and pollutant formation process during two-stroke operation of a direct-injection four-valve engine.