Analysis of Diesel Knock for High-Altitude Heavy-Duty Engines Using Optical Rapid Compression Machines

Abstract

In high altitude regions, affected by the low-pressure and low-temperature atmosphere, diesel knock is likely to be encountered in heavy-duty engines operating at low-speed and high-load conditions. Pressure oscillations during diesel knock are commonly captured by pressure transducers, while there is a lack of direct evidence and visualization images, such that its fundamental formation mechanism is still unclear. In this study, optical experiments on diesel knock with destructive pressure oscillations were investigated in an optical rapid compression machine. High-speed direct photography and simultaneous pressure acquisition were synchronically performed, and different injection pressures and ambient pressures were considered. The results show that for the given ambient temperature and pressure, diesel knock becomes prevalent at higher injection pressures where fuel spray impingement becomes enhanced. Higher ambient pressure can reduce the tendency to diesel knock under critical conditions. For the given injection pressure satisfying knocking combustion, knock intensity is decreased as ambient pressure is increased. Further analysis of visualization images shows diesel knock is closely associated with the prolonged ignition delay time due to diesel spray impingement. High-frequency pressure oscillation is caused by the propagation of supersonic reaction-front originating from the second-stage autoignition of mixture. In addition, the oscillation frequencies are obtained through the fast Fourier transform (FFT) analysis.

Role of Altitude in Influencing the Spray Combustion Characteristics of a Heavy-Duty Diesel Engine in a Constant Volume Combustion Chamber. Part II: Impinging Diesel Jet

“…Wang et al 10 conducted optical experiments on diesel knock for high-altitude engines and confirmed that the origin of diesel knock correlated with diesel spray impingement, and they also found that the occurrence of diesel knock was sensitive to a high injection pressure and small combustion chamber, which could be attributed to the spray impingement and premixed mixture formation. In their companion paper, 11 they argued that the knock intensity decreased as the ambient pressure increased under lower altitude conditions for fixed injection pressures satisfying knocking combustion, which was attributed to the weakened liquid spray impingement. Moreover, Li et al 12 analyzed the effects of the altitude on the combustion characteristics with the large eddy simulation (LES) method and concluded that when the altitude was increased from 0 to 4500 m, the combustion changed from normal combustion to knocking combustion and the combustion efficiency decreased from 90% to 47% due to severe impingement.…”