Determination of Residual Gas Fraction in IC Engines

The objective of this study is to achieve a general understanding of gas exchange phenomena to develop a model for predicting the residual gas content. The knowledge of the cylinder-charge composition is important for the thermodynamic analysis of the combustion process of IC engines. Therefore, the amount of fresh air and fuel as well as the residual gas fraction has to be known. The residual gas mass strongly depends on valve train parameters and operating conditions.

In this study, the residual gas fraction has been determined by using in-cylinder gas sampling from the combustion chamber of a 4-stroke SI engine. The gas sampling valve was flush-mounted to the combustion chamber walls. The gas samples were taken after the gas exchange and analysed for its CO₂ concentration. In combination with the analysis of the exhaust gas composition, the calculation of the residual gas fraction is possible. The residual gas fraction of the tested engine was determined for a wide operating range including variable valve train conditions.

The prediction of the residual gas content using a thermodynamic gas exchange model correlates well with experimental results. The calculated mass flow is used for further investigations. Different phases during the gas exchange process could be identified, and additional information on the influence of different valve train parameters could be obtained.