Optical experiments on diesel knock for high altitude engines under spray impingement conditions

In high altitude regions (>4000 m), atmosphere pressure and thus in-cylinder ambient density is decreased significantly, leading to longer spray penetration. Affected by liquid spray impingement on cylinder wall, heavy-duty diesel engines operating at high loads may encounter abnormal combustion like diesel knock. However, with limited research data, the detailed combustion mechanism is not fully understood. To explore the origins of diesel knock and its correlations with spray impingement, optical experiments in a rapid compression machine were employed, and diesel spray impingement and subsequent combustion processes were investigated. High-speed direct photography and simultaneous pressure acquisition were synchronically performed to understand combustion evolutions and pressure characteristics. Meanwhile, the role of injection parameters, ambient conditions, and cylinder sizes were considered. The results show that early injection timing and spray impingement can lead to long ignition delay time. Depending on the premixed mixture formation within longer ignition delay, diesel knock manifesting supersonic detonation-like reaction front propagation can be observed, which results in strong pressure waves with an amplitude approaching hundreds of atmosphere. Further analysis on the critical conditions show that long ignition delay time does not necessarily result in diesel knock. It lies in whether there is sufficient premixed mixture formation before main combustion. The prevalence of diesel knock seems sensitive to large fuel injection pressure and small combustion chamber. Besides, depending on the ambient conditions, there are four kinds of combustion modes under spray impingement conditions.