Spray impingement in the early direct injection premixed charge compression ignition regime

The main goal of this paper is to acquire more insight into the relationship between wall and piston impingement of liquid fuel and unburnt hydrocarbon emissions (UHC) emissions, under early direct injection (EDI) premixed charge compression ignition (PCCI) operating conditions. To this end, the vaporization process is modeled for various operating conditions using a commercial CFD code (StarCD). Predicted values for liquid core penetration, or liquid length LL, have been successfully checked against experimental data from literature over a wide range of operating conditions. Next, the correlation between the CFD results for wall and piston impingement and measured UHC emissions is studied. The diesel fuel used in the experiments is modeled as n-dodecane and n-heptadecane, representing the low and high end of the diesel boiling range, respectively. A distinction is made between liquid spray impingement on the piston surface and cylinder liner. For a conventional DI diesel nozzle, the high UHC emissions in the EDI PCCI regime correlate well with modeled cylinder wall impingement. Conversely, piston impingement is negligible in this regime. Accordingly, it may be assumed that the primary cause for high UHC emissions in the EDI PCCI regime, using conventional DI nozzles, is caused by liquid spray impingement against the cylinder liner. In this regime it was found that a higher intake and fuel temperature, as well as an elevated intake pressure have a positive effect on both UHC emissions and the spray impingement against the cylinder wall. This provides additional evidence that the two parameters (i.e., UHC and wall impingement) are linked. Lastly, the impact of nozzle cone angle is investigated. When adopting a narrow cone angle nozzle in the EDI PCCI regime, wall impingement is negligible and piston wetting becomes the dominant source of UHC emissions