In this paper the heat transfer of the hot gases to the cylinder walls of DI Diesel engines is analyzed using Computational Fluid Dynamics (CFD) and compared to the predictions obtained with a zero-dimensional thermodynamic model based on a variant of the Woschni equation. The final objective is to improve the simple model by modifying the original equations to better take into account the influence of important parameters, such as swirl, on the heat transfer.
CFD calculations of the compression and expansion strokes have been made for two real DI Diesel engine geometries, a small one with a displacement of 0.4 l. and a heavy duty one of 2.0 l., and for different working points.
The total heat transfer rate to the cylinder surfaces is highly dependent on the mean flow behavior and turbulence levels. Hence, starting from initial conditions at Inlet Valve Closing (IVC), various parametric studies have been performed to take into account the influence on the heat transfer of the level of swirl on the one hand, of engine speed on another, and of modifying the clearance height on another.
Solutions obtained with both the CFD simulation and the diagnosis model are compared. They show that there are some important differences due to the assumptions made in the thermodynamic model concerning these parameters. Interesting conclusions are drawn from the more accurate CFD results and a simple modification of the terms included in the Woschni-like equation of the thermodynamic model is proposed to better take into account the significant influence of the swirl on the heat transfer.