Effects of diesel pilot-injection strategy on a methanol/diesel dual-direct injection engine

To verify the effectiveness of dual-direct injection technology in enhancing the performance of methanol/diesel dual-fuel engines and to explore the positive impacts of multiple-injection strategy on the in-cylinder mixture formation and combustion process, an experiment was conducted on a modified methanol/diesel dual-direct engine test platform in this paper. The research aimed to explore the effects of the diesel pilot-injection strategy, especially in terms of diesel pilot-injection timing and diesel pilot-injection duration, on the operating range, combustion characteristics, performance and emissions. Results indicate that the pilot-injection strategy reduces the maximum pressure and pressure rise rate of the engine, alleviating the rough combustion in the cylinder and broaden the operating range. As the diesel pilot-injection timing advances, the in-cylinder pressure curve during the combustion process shifts leftward as a whole, and the pressure rise rate curve gradually becomes an approximate single-peak shape. The ignition delay almost increases linearly and CA50 first decreases and then increases with the advance of pilot-injection timing. When the pilot-injection duration increases, the ignition delay shortens and CA50 advances. Additionally, the diesel pilot-injection strategy can optimize engine emissions. As the pilot-injection timing advances and the duration increases, the HC, CO and soot emissions decrease, while the NOx emission increases. Using the pilot-injection strategy, the highest indicated thermal efficiency of 42.94 % is obtained at the diesel ignition injection timing and duration of -28 degrees CA aTDC and 0.25 ms respectively, which is 7.54 % relatively higher than that under a single injection strategy. The research results show that the pilot-injection strategy better adjusts the distribution of high reactivity and low reactivity fuels in the cylinder, which is beneficial for improving the combustion and performance of the methanol/diesel dual-fuel engine.