Effects and mechanism of pilot diesel injection strategies on combustion and emissions of natural gas engine

The research objectives of pilot diesel injection (PDI) ignition natural gas technology include high efficiency, clean combustion, and low pilot diesel mass. This study is based on a single-cylinder thermodynamic engine, combined with the CONVERGE simulation model and CHEMKIN chemical reaction kinetics model. The effects and mechanisms of various PDI strategies on the mixture equivalent ratio, temperature, and characteristics of combustion and emissions were investigated. The experimental results showed that the best PDI mass was 8 mg/cycle. The thermal atmosphere and activity in the cylinder were improved with an increase in PDI mass from 2 to 8 mg/cycle, which stabilized the mixture combustion. Further, the effects of different pilot injection timing (PIT) on combustion and emissions were investigated via experiments and simulation by controlling the operating conditions and maintaining a constant PDI total mass. The results show that the diesel had a single low-temperature reaction path when the PIT was close to the top dead center, whereas the PIT at the early stage of the compression stroke (CS) changed the chemical reaction path and accelerated the transformation of CH3 to CH2O, accumulating numerous active groups and accelerating the combustion rate, which is difficult to control the ignition phase. The reaction path of the double PDI strategy was similar to that of the PIT at the early CS stage, and its combustion is closed to premixed combustion; however, the accumulation of active groups was relatively small, and the combustion rate was relatively slow because the ignition phase was controlled by the second PDI, making the combustion phase easy to control. Finally, with the double PDI strategy that had the advantages of efficient combustion and avoidance of knock, the gross indicated thermal efficiency reached 49.3% that involved a -60 degrees crank angle (CA) after top dead center (ATDC) first injection and -4 degrees CA ATDC second injection.