The Impact of Intake Dilution and Combustion Phasing on the Combustion Stability of a Diesel Engine 2014-01-1294
Conventionally, the diesel fuel ignites spontaneously following the injection event. The combustion and injection often overlap with a very short ignition delay. Diesel engines therefore offer superior combustion stability characterized by the low cycle-to-cycle variations. However, the enforcement of the stringent emission regulations necessitates the implementation of innovative diesel combustion concepts such as the low temperature combustion (LTC) to achieve ultra-low engine-out pollutants. In stark contrast to the conventional diesel combustion, the enabling of LTC requires enhanced air fuel mixing and hence a longer ignition delay is desired. Such a decoupling of the combustion events from the fuel injection can potentially cause ignition discrepancy and ultimately lead to combustion cyclic variations.
This work investigates the impact of exhaust gas recirculation and combustion phasing on the combustion stability and cyclic variations of a diesel engine while entering into the LTC operation regime using a single-injection fueling strategy. Experiments are conducted on a laboratory instrumented high compression ratio (18.2:1) diesel engine. The combustion phasing control and the application of moderate-to-high EGR are used to achieve ultra-low NOx and smoke emissions. The combustion characteristics and emission performance are presented. Cycle-resolved cylinder pressure measurements acquired for 200 engine cycles are used to evaluate indicated engine performance and apparent heat release parameters including ignition delay, burn duration, combustion phasing, and the pressure rise rate. The results indicate that the enabling of single-injection diesel LTC demands precise control over operating conditions and it is sensitive to minor variations in the injection timing and the amount of EGR applied. However, when successfully controlled, this combustion mode exhibits cyclic variability comparable to conventional diesel combustion.