A Study of the Characteristics of Mixture Formation and Combustion in a PCCI Engine Using an Early Multiple Injection Strategy

As world attention has focused on global warming and air pollution, high-efficiency diesel engines with low CO 2 emissions have become more attractive. Premixed diesel engines in particular have the potential to achieve a more homogeneous mixture in the cylinder which results in lower NO x and soot emission. It is well-known that the injection strategies such as the injection timing and the spray included angle are important to create the optimal mixture formation for a PCCI (premixed charge compression ignition) engine. In this research, we investigated the effect of injection angle, injection timing, and frequency on the combustion and mixture formation in a direct injection type PCCI engine using an early multiple injection strategy. The experimental results showed that the mixture formation, IMEP, and emission characteristics in the PCCI engine were dominantly affected by the fuel injection timing and spray included angle. In particular, the injection timing of 65° BTDC with the spray included angle of 100° effectively reduced the smoke emission in the early single injection case. In other words, the smoke number was less than 1 FSN when the IMEP exceeded 3.5 bar. The multiple injection method also resulted in more homogeneous mixture formation due to decrease in spray penetration and increase in the total amount of fuel evaporation in the combustion bowl. In addition, a simulation was conducted in order to estimate the mixture distribution within the cylinder according to injection conditions such as spray included angle and injection timing. The simulation result is very effective to clarify the air fuel distribution of a PCCI combustion.