Effect of radiation on diesel engine combustion and heat transfer

Yoshikawa, T. Effect of Radiation on Diesel Engine Combustion and Heat Transfer. University of Wisconsin-Madison, 2008.

The main purpose of the present work is to improve the accuracy of the relevant submodels in the Computational Fluid Dynamics (CFD) code, which was coupled with a Heat Conduction in Components (HCC) code for the prediction of diesel engine performance. The Discrete Ordinates Method (DOM) radiation model was implemented into the CFD code, and was modified to exploit a computational sector mesh for computational time reduction. A grid-independent spray model and improved fuel combustion chemistry mechanism was also implemented and validated. An improved reduced NOx mechanism, that considers not only thermal NOx but also prompt NOx, was developed. Additionally, the soot formation parameters of the soot model were optimized using a Genetic Algorithm (GA) method, together with soot thermometry data.

The interaction between soot and NOx emissions was studied from the view point of a “NOx bump” characteristic behavior that has been observed by researchers on the Sandia optical engine. Although an obvious “NOx bump” was not observed in the model prediction, a reduction in the predicted NOx with retard of SOI was found. Additional analysis suggests that the reduction of NOx emissions due to radiation heat loss is not by suppressing NOx formation in the flame, but mainly by enhancing NOx consumption in regions of relatively high equivalence ratio.

The influence of spray angle and piston bowl geometry on soot radiation and wall temperature was examined. NOx emissions were reduced when the radiation model was applied, while soot emissions increased. The influence of the radiation model was larger on NOx emissions than on soot. The influence of wall temperatures, or heat loss from the walls, was significant on soot emissions.

The oil cooling gallery was found to significantly cool the piston crown in the computations. The variation due to the engine operating conditions (SO1 in this case), on the cylinder head temperatures was most sensitive and showed more than 100 K differences, while the temperature variations on the other surfaces, such as the piston surface and liner were only about 25 K.