Accuracy of Diesel Engine Combustion Metrics Over the Full Range of Engine Operating Conditions

Measuring and analyzing combustion is a critical part of the development of high efficiency and low emitting engines. Faced with changes in legislation such as real driving emissions (RDE) and the fundamental change in the role of the combustion engine with the introduction of hybrid-electric powertrains, it is essential that combustion analysis can be conducted accurately across the full range of operating conditions. In this work, the sensitivity of five key combustion metrics is investigated with respect to eight necessary assumptions used for single zone diesel combustion analysis. The sensitivity was evaluated over the complete operating range of the engine using a combination of experimental and modeling techniques. This provides a holistic understanding of combustion measurement accuracy. For several metrics, it was found that the sensitivity at the mid-speed/load condition was not representative of sensitivity across the full operating range, in particular at low speeds and loads. Peak heat release rate and indicated mean effective pressure (IMEP) were found to be most sensitive to the determination of top dead center (TDC) and the assumption of in-cylinder gas properties. An error of 0.5 deg in the location of TDC would cause on average a 4.2% error in peak heat release rate. The ratio of specific heats had a strong impact on peak heat release with an error of 8% for using the assumption of a constant value. A novel method for determining TDC was proposed which combined a filling and emptying simulation with measured data obtained experimentally from an advanced engine test rig with external boosting system. This approach improved the robustness of the prediction of TDC which will allow engineers to measure accurate combustion data in operating conditions representative of in-service applications.