Wall Heat Transfer in a Multi-Link Extended Expansion SI-Engine 2017-24-0016
The real cycle simulation is an important tool to predict the engine efficiency. To evaluate Extended Expansion SI-engines with a multi-link cranktrain, the challenge is to consider all concept specific effects as best as possible by using appropriate submodels. Due to the multi-link cranktrain, the choice of a suitable heat transfer model is of great importance since the cranktrain kinematics is changed. Therefore, the usage of the mean piston speed to calculate a heat-transfer-related velocity for heat transfer equations is not sufficient. The heat transfer equation according to Bargende combines for its calculation the actual piston speed with a simplified k-ε model.
In this paper it is assessed, whether the Bargende model is valid for Extended Expansion engines. Therefore a single-cylinder engine is equipped with fast-response surface-thermocouples in the cylinder head. The surface heat flux is calculated by solving the unsteady heat conduction equation. By using a surface-ratio related weighting method, it is possible to determine a global wall heat loss from the local heat fluxes.
The natural-gas test engine has a multi-link cranktrain to achieve, based on a compression ratio of 12.2, an expansion ratio of 17.6. The cranktrain is later modified by a mechanical adjustment in order to set the strokes to equal lengths, establishing a “conventional” engine process. This enables the comparison of experimentally determined heat transfer characteristics of two different engine processes from the same test engine.
The comparison between the experimentally determined and the modeled heat flux at the conventional engine process shows a very good conformity. As well in the Extended Expansion mode, a good conformity of measured and modeled data is shown, so that the heat transfer model is also valid for this engine process.
Subsequently, the difference in wall heat losses by Extended Expansion is analyzed using engine process simulation. Compared to a base engine with an equal intake stroke, the Extended Expansion engine has a higher wall heat loss.