New Insights into the Mechanisms of In-Cylinder Heat Transfer in Diesel Engines

Measurements of instantaneous in-cylinder heat transfer have been performed at a selected location of the cylinder head in a super long stroke, low speed, two-stroke experimental diesel engine operating under various conditions. The temperatures of both the wall and the process have been varied either by locally using titanium instead of steel as wall material or correspondingly by operating the engine with an unusually high equivalence ratio. A standard case with a steel wall and a typical full load point has been used as reference. For all conditions, measurements of the wall temperature very close to the surface of the apparent radiation temperature of the combustion chamber contents and of the gas temperature across the boundary layer (the latter by means of a for this purpose specifically developed fiber-optical sensor) have been carried out. As a result, the total heat flux as well as its two components, convection and radiation, have been determined. Under normal operating conditions convection and radiation contribute with about equal magnitudes to the total heat flux at least for this location of the engine under investigation, but the former mechanism peaks earlier than the latter during combustion. The peak magnitude of the total heat flux is clearly lower than values reported for smaller four stroke diesel engines. Operating the engine on the unusually high equivalence ratio results in a significant increase of the local heat flux, while with the titanium wall surface the local heat flux decreases. The results for all cases other than the reference one are rather preliminary, therefore further measurements are necessary in order to clarify the influences of all parameters involved.