Thermodynamic Analysis of the Effects of Fuel-Side and Air-Side Oxygen Addition on Diesel Engine Combustion Characteristics and Pollutant Formation

Abstract

A multi-zone combustion model is used in the present study to examine the effect of increased in-cylinder oxygen availability (either by using oxygenated fuels or by increasing the oxygen percentage of intake air) on direct injection (DI) diesel engine performance characteristics and pollutant emissions. Simulations are produced for a single-cylinder DI diesel engine (“Lister LV1”) by keeping constant the oxygen content of in-cylinder fuel/air mixture and the engine brake torque. The effects of the two oxygen-enhancement techniques on combustion characteristics, soot and NO concentrations inside the combustion chamber are examined using model predictions for a common diesel oil, a neat oxygenate and the case of increasing the oxygen fraction of intake air. The multi-zone model is also utilized to interpret the relative impact of fuel-side and air-side oxygen on soot formation mechanism by examining the temporal evolution of combustion characteristics and soot formation and oxidation rates inside the fuel jet zones. Evaluation of the theoretical results revealed that the increase of in-cylinder oxygen availability by both techniques resulted in earlier initiation of combustion, increase of peak cylinder pressure and increase of in-cylinder and exhaust NO concentrations. It resulted also in reduction of exhaust gas temperature and exhaust soot values. Fuel oxygen addition was proven to be more influential on combustion process and consequently, on soot and NO formation mechanism compared to oxygen-enhancement of intake air. This is attributed to the higher oxygen availability inside each fuel jet zone, which is observed in the case of oxygenated fuel combustion.