Improvement of diesel combustion with multiple injections at cold condition in a constant volume combustion chamber

- Lower peak of flame luminosity was observed under cold start condition.
- Cold startability was improved by applying pilot injections before main injection.
- Increase in pilot injection quantity was advantageous for better main combustion.
- Fuel flow toward crevice volume was detected with first pilot injection.

A series of diesel spray combustion tests was carried out in a constant volume combustion chamber (CVCC) to investigate the effect of multiple injection strategies on cold startability. The experiments were performed under a simulated low temperature cold start condition. In-chamber pressure analysis and high speed flame imaging were conducted to compare the effectiveness of each injection strategy on cold startability. Spray targeting visualization was also performed to examine the wall impingement of injected fuel. The diesel fuel was injected into the CVCC with an injection pressure of 35 MPa. Multiple injection strategies with different amounts of pilot injection quantities were applied to improve the diesel combustion under simulated cold start ambient condition. The flame imaging and in-chamber pressure results showed that the multiple injection strategy provided better cold startability than the single injection condition. According to the pilot injection quantity, the peak of the flame luminosity and in-chamber pressure were gradually increased with larger pilot injection quantity cases. The peak of the variation rate of the in-chamber pressure with a multiple injection strategy was approximately 2 times higher than that of the single injection case. In terms of spray targeting imaging, the results indicated that the fuel impingement and flow to the piston crevice volume were increased with a larger amount of pilot2 injection. Therefore, the increment of the pilot1 injection quantity rather than the pilot2 injection quantity was suggested not only to improve cold startability, but also to reduce unburned hydrocarbon emissions under a cold start condition in a real engine.