A Simple System for Sampling Cylinder Gases and Its Application to Measure Dilution by Residuals on an S.I. Engine with VVT

The level of charge dilution by residual (and recirculated) gases has a strong influence on many aspects of engine performance although it is generally difficult to measure. In common with previous investigations, the method used has been to sample the cylinder charge during the compression stroke and determine the level of dilution by gas analysis. Access to the cylinder has been made by modifying a spark plug to provide a sample path through a fine bore tube. The sample flow is controlled by a valve mounted close to the spark plug and passes through this to an on-line gas analysis set. The system makes use of a stock gasoline fuel injector operating in reverse flow as a high-speed sampling valve. This has been used without problem over a cumulative operating period of many hours. In the paper, the sampling and analysis system is described. The validation of accuracy based on test results for cases when intake CO₂ was varied is presented. The analysis of sample window, mixing and flow conditions is covered. The application of the system to measure the variation of residuals with valve timing changes is described. The engine has dual independent control of intake and exhaust valve timings. The influence of these is discussed.