Effect of the application of an uncooled high-pressure EGR strategy in low-load diesel PCCI operation

The exploitation of new advanced combustion concepts referred to as low-temperature combustion (LTC) strategies gives the possibility to reduce the typical NOx and PM emissions from conventional diesel combustion. These strategies are implemented using high quantity of exhaust gas recirculated at the intake manifold, which allows to lower the peak combustion temperatures, thus reducing NOx, and advancing the fuel injection timing, thus keeping the PM emission under control. Cooling the EGR can be beneficial, as the increment in density of the burned gases downstream the EGR cooler reflects in a further decrement of NOx and PM emissions. On the other hand, the lower EGR temperatures may enhance higher in-cylinder formation of unburned hydrocarbon (HC) and carbon monoxide as well as negatively affect the exhaust gas temperature upstream the after-treatment line. This latter effect can be particularly detrimental especially at low engine speeds and loads and/or during engine warm-up. In these cases, in fact, the after-treatment system is not able to convert the incomplete combustion species because the light-off temperature of the catalyst is not reached. This work tries to present a comparison between cold and hot EGR strategies in the case of a short EGR route configuration. The results highlight a reduction in terms of tailpipe HC and CO emissions for low-torque engine working points, when using hot EGR strategies, especially due to the enhancement of the DOC working temperature. On the other hand, the area of the map where PCCI combustion is achievable with uncooled EGR shows to be reduced with respect to the regular cold EGR strategy.