Post-injection strategies for gasoline compression ignition combustion under high load conditions: Understanding the role of premixed, main, and post-injections in soot mitigation and load extension

Engine experiments and computational fluid dynamics modeling (CFD) were used to isolate and understand the role of premixed, main, and post-injections in soot mitigation and load extension for high-load GCI operation. Results showed that for all the three injection strategies soot emissions increased with increasing load. For the post-injection cases, soot emissions increased as the dwell time between the main and the post-injection increased. However, adding load through post-injections resulted in higher soot emissions compared to the baseline injection strategy irrespective of the SOI timing of the post-injection. Premixing a portion of the main injection fuel reduced the soot emissions for the post-injection cases, but they remained higher compared to the baseline injection strategy. The CFD modeling showed that the post-injection cases resulted in lower soot formation when compared to the baseline injection strategy. However, the increased injection durations at high-load conditions resulted in soot being formed late in the cycle from the post-injection where the temperatures dropped rapidly, slowing down the soot oxidation rates. This resulted in higher net soot production for the post-injection cases compared to the single long main injection cases. This temperature effect on soot emissions was enhanced, as the post-injection SOI timing was delayed, resulting in increased soot emissions with increasing dwell time. Premixing a portion of the main injection fuel reduced the soot emissions for the post-injection strategies, as the well-mixed premixed fuel combusts without forming any soot. When a similar study was repeated under low- and mid-load conditions using the validated CFD model, post-injections showed a benefit with a maximum reduction in soot of ∼62% compared to the baseline strategy. This was because, similar to the high-load conditions, the fuel from the post-injection was targeted at a different region in the combustion chamber relative to the main injection, which provided better access to the oxygen to both the main and the post-injections. However, compared to high-load conditions, since the duration of the main and the post-injection is shorter, it allowed the SOI timing of the post-injection to be advanced closer to TDC without overlapping with the main injection. The advanced post-injection timing, combined with the shorter duration of the post-injection, resulted in the fuel being delivered sufficiently early in the cycle. This provided enough residence time in the high-temperature regions to oxidize the soot formed from the post-injection completely, resulting in reduced soot emissions compared to the case without the post-injection.