Search Results

The significantly poorer efficiency of a spark-ignition engine in the part load range compared to its efficiency at full load is the result on the one hand of the restriction losses and on the other hand of the severe combustion lag. It would be a conceivable proposition to considerably improve part load efficiency (10 % at bmep = 3 bar) by increasing the compression ratio although this is impeded by the risk of knocking combustion at full load. In combination with variable valve timing (this requires variable compression ratio) even 19 % efficiency improvement are achievable (1). This has given rise to a joint development programme between Mahle GmbH and Daimler-Benz. Research aimed at producing a piston with a variable compression height which can thus offer optimum compression ratio at every point of the map.

The combustion process of a heavy-duty DI-Diesel truck engine has been investigated using numerical simulation. The numerical modeling was based on an improved version of the KIVA-2 engine simulation code, employing a modified characteristic time-scale combustion model and a modified Kelvin-Helmholtz spray atomization model. The NO-formation process was modeled using the extended thermal Zeldovich mechanism. The simulation efforts included the effects of different injection characteristics such as varying the injection rate profile or number of injection holes and sizes. The physical sub-models used to improve the simulation of the mixture-formation and the combustion process were validated through comparison with single-cylinder engine experiments. Special attention was given to accurately model the in-cylinder flame propagation of the individual sprays and their effect on thermal NO-formation. All simulations were based on full load cases at medium speed.