Abstract:

Reactivity controlled compression ignition (RCCI) is a clean and efficient combustion mode of internal combustion engine. At present, the research of RCCI is mostly studied under conventional conditions, and cold start is one of the important problems faced by the application of RCCI in automotive engines. Based on a dual-fuel optical engine, combined with high-speed imaging technology, this paper carries out a visual study of the effect of in-cylinder injection strategies on hydrogenated catalytic biodiesel (HCB)/gasoline RCCI combustion under cold conditions. Firstly, for gasoline port injection and HCB single direct injection strategy, an RCCI cold combustion test with different HCB injection timings (-30--15°CA ATDC) is carried out. The results show that, with the delay of injection timing, the ignition timing is delayed, and the peak of cylinder pressure and heat release rate first increase and then decrease. -25°CA ATDC is the optimal injection timing under the HCB single injection strategy, but the overall flame area is small and the light intensity is high, so the ignition and combustion characteristics need to be optimized.

In view of the shortcomings of the HCB single injection strategy, an experimental study on the HCB double injection strategy is carried out. HCB main injection timing (-25--15°CA ATDC) and HCB pre-injection ratio (30%∶70%, 70%∶30%) are used as the control parameters. Compared with the optimal condition called S-25/100% under the HCB single injection strategy, the ignition timing is advanced and the peak points of cylinder pressure and heat release rate moveforward under the HCB double injection strategy. When the pre-injection ratio is 30% and the main injection timing is -25°CA ATDC, the combustion efficiency improves most significantly and the soot emission is low, which is the best condition under the HCB double injection strategy. Different HCB injection parameters have different effects on the process of combustion and the law of flame development. The pre-injection ratio has a significant impact on the combustion phase.The higher the main injection ratio is, the later the ignition timing is, the shorter the combustion duration is, and the more concentrated the heat release process is. The main injection timing has a significant impact on the flame development mechanism:the main injection timing is earlier, and the flame development is mainly due to spontaneous combustion from multiple points around the combustion chamber to the center; the main injection timing is delayed, and the flame development is mainly spread along the spray oil beam to the surrounding area.

The HCB double injection strategy can more flexibly adjust the overall reactivity and local reactivity of the combustion chamber by changing the injection timing and ratio, thereby optimizing the combustion performance.Compared with the HCB single injection strategy, the RCCI cold combustion performance can be improved. In this experiment, the combustion characteristics and the law of flame development under different HCB injection strategies are analyzed, and the appropriate HCB injection timing and proportion are determined. It is expected that the cold ignition and combustion characteristics of RCCI can be improved by regulating the high active fuel injection strategy, which provides experimental basis for the promotion and application of RCCI combustion mode in engines in the future.