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

Wall impingement, particularly liquid-wall impingement, has been demonstrated to be one of the critical causes of combustion deterioration in plateau diesel engines. Obviously, the complexity of wall impingement is exacerbated by the plateau scenario. However, fundamental studies specifically dedicated to this phenomenon are still inconclusive and insufficiently detailed, obviating the feasibility of the targeted design and optimization of diesel engines operating in regions with different altitudes. Consequently, the second part of this investigation, presented in this work, focused on the detailed physical and chemical processes of impinging spray combustion under different altitude conditions. A wall impingement system was designed to generate an impinging spray flame. The impingement distance was varied from 77 to 37 mm to cover different situations of wall impingement. The liquid spray, ignition, and combustion processes were visualized in detail by using different optical diagnostics. The results showed that the variation of the liquid length with the impingement distance was mainly dependent on the liquid impingement under the same altitude condition. The effect of the impingement distance on the ignition distance was more sensitive to the altitude. The quantitative analysis of the flame natural luminosity confirmed the decisive effect of the impinging flame morphology on the ambient entrainment and fuel-air mixing under different altitude conditions, and it also revealed that there was an optimal impingement distance under identical altitude conditions to achieve minimum soot emissions. And interestingly, the optimal impingement distance increased with altitude. Finally, the spray combustion processes of an impinging diesel jet were determined to occur in four typical regions, upon which a schematic diagram depicting the flame structure of an impinging diesel jet was proposed to phenomenologically describe the role of altitude in impinging spray combustion processes. Based on this, an attempt was made to explore some new perspectives beyond the popular solutions to recover and improve the performance of plateau diesel engines.