PR-457-17201-R01 Residual Gas Fraction Estimation Based on Measured In-Cylinder Pressure

Developing a reliable means of predicting and controlling engine operation at various operating conditions may be a reasonable pathway for meeting increasingly stringent engine emission regulations. In order to do so, estimates of the mixture composition at the end of the gas exchange process (i.e. at exhaust port closure, or EPC) are helpful for having accurate models of engine combustion. A substantial fraction of the trapped charge mixture is composed of residual gases from previous cycles. This fraction, the trapped residual fraction, changes combustion characteristics of the trapped mixture. Residual gases dilute the fresh charge, thus decreasing the flame speed and temperature. Moreover, because of their spatial inhomogeneity, especially around the spark plug, these gases seem to have implications for engine stability. All these factors likely affect engine performance and emissions.

Unfortunately, it is not practical to directly measure the trapped residual fraction for engines operating in the field. To overcome this, computational models can be developed for control applications which estimate this fraction based on some measurable engine parameters e.g. manifold pressures, temperatures, etc. Currently, such models aren’t readily available for stationary two-stroke natural-gas engines. This report summarizes the results of phase I of a multi-phase project. In this phase a GT-Power based model was developed for an Ajax E-565 single-cylinder engine to study various factors that influence the removal, or lack thereof, of residual gases from the engine cylinder. The model was used to successfully simulate various engine operating conditions and study the scavenging characteristics of the engine. Various steps followed in the development of the model have been discussed in the report, and at the end some preliminary results and recommendations for the next phase of the project have been presented.