Browse Publications Technical Papers 2019-01-0022
2019-01-15

Optical Investigation on the Combustion Process Differences between Double-Pilot and Closely-Coupled Triple-Pilot Injection Strategies in a LD Diesel Engine 2019-01-0022

The combustion processes of three injection strategies in a light-duty (LD) diesel engine at a medium load point are captured with a high speed video camera. A double-pilot/main/single-post injection strategy representative of a LD Euro 6 calibration is considered as the reference. There is a modest temporal spacing (dwell) after the first pilot (P1) and second pilot (P2). A second strategy, “A,” adds a third pilot (P3). The dwell after both P2 and P3 are several times shorter than in the reference strategy. A third strategy, “B,” further reduces all dwells. In all three cases, the fuel from P1 combusts via sequential autoignition. For the reference and A strategies, the ignition of P2 occurs at its interface with the existing combustion regions. This fuel burns as a propagating premixed flame. For B, the start of injection of P2 occurs before the autoignition of P1 becomes fully developed. This injection has a combination of ignition mechanisms; autoignition of the tip of the jet, and premixed combustion of the bulk of the jet. Extinguishing of the prevailing combustion by the fuel jets of later injections is noted in all strategies. This phenomena is confirmed by comparing the timing of each fuel injection with the behaviour of the heat release rate (HRR) and spatial luminescence over time. The dips in the HRR due to the extinguishing are larger than would be expected by the cooling effect of the injected fuel alone. Furthermore, not all dips in HRR are the result of this extinguishing, and it would not have been possible to determine if the dips are due to this extinguishing or a simple exhaustion of available fuel without this optical investigation. Even if the precise hydraulic injection timing can be known, the spatial relationship of the injected fuel and prevailing combustion is necessary.

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