Torque modelling for optimising fuel economy in variable compression engines

Byline: Ylva Nilsson, Lars Eriksson, Martin Gunnarsson Fuel optimal control of a variable compression engine is studied and it is shown that a crucial component is the model for the engine torque. A model for the produced work that captures the important effects of ignition and compression ratio is proposed and investigated. The main task for the model is to be a mean for determining the fuel optimal control signals, for each requested engine torque and speed. The contribution is a model suitable for finding this optimal combination. This model consists of well-known components, and the novelty lies in the compilation and validation of the control-oriented efficiency model for a variable compression engine. The modelling and validation is performed on a multicylinder variable compression engine using two fuels with different octane rating. Despite the models simplicity, it describes the indicated work with good accuracy, and suits its purpose of finding optimal control signals. In the evaluation, it is shown that a fuel optimal controller based on the proposed model captures the optimal IMEP to within 1.2%. This corresponds to a loss in engine efficiency that is in the range of 0.5% units or less.