Experimental and numerical investigation on mixture formation, ignition, combustion and emission characteristics of aviation piston engines during cold-start under variable altitudes

Aircraft piston engines (APEs) serve as the core power source for general aviation (GA), plateau emergency rescue, remote area shipping and national defense equipment construction. However, performance degradation and emission deterioration during cold-start under variable altitude conditions (0–8000 m) have become critical bottlenecks, constraining the safety, environmental protection and environmental adaptability of high altitudes aviation operations. To uncover the evolutionary pattern and intrinsic mechanism of APEs' cold-start characteristics under variable altitude conditions, experimental platforms including a plateau low-temperature environment simulation chamber, an overall engine cold-start test bench and a constant volume combustion chamber (CVCC) system were established in this study. Combined with a three-dimensional CFD numerical model coupling fuel spray, combustion and emission, systematically investigated the changes in spray, mixture formation, ignition, combustion and emission pollutant generation during APEs' cold-start as altitude increased from 0 m to 8000 m. The experimental and numerical results demonstrate the following sequence of events. Firstly, as altitude increases, adverse effects such as decreasing ambient temperature, pressure, density and oxygen concentration cause the fuel spray penetration length to gradually lengthen and the initial Sauter mean diameter (SMD) to increase during cold-start. This leads to earlier spray-wall impingement and increased wall deposition mass. Subsequently, the deterioration of spray atomization quality and spray-wall impingement causes the rich mixture zone to shift from the tail of the spray core to the wall region near the bowl arc. The phenomenon of inhomogeneous mixture distribution becomes more pronounced. Then, as altitude increases, the ignition delay time (IDT) gradually prolongs, and the ignition process transitions from multi-stage ignition to single-stage ignition, and eventually to misfire. The in-cylinder combustion pressure gradually decreases with rising altitude. The peaks of pressure rise rate (PRR), heat release rate (HRR) and in-cylinder mean temperature show a non-monotonic trend of initial increase followed by decrease. Under conditions where fuel can auto-ignite, diffusion combustion intensifies and its duration prolongs with increasing altitude, while premixed and partially premixed combustion weaken and their durations shorten. Finally, as altitude rises, the peak and cumulative values of HC initially rise, then decline, and subsequently rise again. In contrast, both CO2 and NOx peak and cumulative values decrease progressively. The soot exhibits a non-monotonic trend, increasing initially before decreasing, reaching a peak at 1000 m. These findings provide crucial theoretical and engineering support for enhancing the cold-start reliability, safety and environmental adaptability of APEs under variable altitude conditions.