Effects of EGR on Heat Release in Diesel Combustion

The effects of Exhaust Gas Recirculation (EGR) on diesel engine exhaust emissions were isolated and studied in earlier investigations (1,2,3,4,5). This paper analyses the heat release patterns during the combustion process and co-relates the results with the exhaust emissions. The EGR effects considered include the dilution of the inlet charge with CO₂ or water vapour, the increase in the inlet charge temperature, and the thermal throttling arising from the use of hot EGR.

The use of diluents (CO₂ and H₂O), which are the principal constituents of EGR, caused an increase in ignition delay and a shift in the location of start of combustion. As a consequence of this shift, the whole combustion process was also shifted further towards the expansion stroke. This resulted in the products of combustion spending shorter periods at high temperatures which lowered the NO_x formation rate. In addition, the longer ignition delay period provided more time for the fuel top penetrate which could have led to larger amounts of gases in the flame envelop, thereby lowering the combustion temperature and resulting in lower NO_x formation.

Although the increased ignition delay period is expected to cause increasing amount of fuel being burned during the pre-mixed burning period, however, the reduction in oxygen availability associated with the application of EGR diluents reduced the rate at which the fuel burnt in the pre-mixed phase. Moreover, the shift of the combustion process towards the expansion stroke resulted in earlier quenching of the combustion process, that is, shorter combustion duration, which yielded higher levels of products of incomplete combustion in the exhaust.

The different effects of inlet charge thermal throttling (associated with the use of hot EGR) were simulated and the heat release patterns were analysed. The reduction in the inlet charge oxygen, resulting from throttling the inlet charge, caused an increase in the ignition delay; however, this increase was largely offset by reductions in ignition delay due to increased inlet charge temperature and decreased inlet charge mass.