Investigation on the Effects of Low-Altitude Environmental Changes on Engine Spray and Combustion Performance

Liu Hui

Zibo Polytechnic University

Abstract：

The modern expansion of the low-altitude economy has driven an urgent requirement for reliable, high-density propulsion systems, particularly unmanned aerial vehicles and light aviation aircraft powered by internal combustion engines. However, operating within low-altitude envelopes from sea level up to 3000 meters exposes engines to significant thermodynamic shifts in ambient pressure, air density, and temperature. This study provides a comprehensive theoretical and numerical analysis of how these low-altitude environmental variations influence macroscopic spray morphology, microscopic droplet breakup, and thermodynamic combustion performance in aviation heavy-fuel engines. Based on the International Standard Atmosphere model and classical multi-phase aerodynamic theories, the interaction mechanisms between reduced ambient density and fuel injection dynamics are theoretically derived. The results demonstrate that ascending from 0 to 3000 meters causes a 25.8% drop in ambient gas density, which attenuates aerodynamic drag acting on the liquid jet. Consequently, spray tip penetration length at 1.5 milliseconds post-injection increases from 42.3 mm to 51.8 mm, while the spray cone angle contracts from 21.4 degrees to 16.8 degrees, significantly elevating wall-impingement risks. Concurrently, weakened aerodynamic shear elevates the average Sauter Mean Diameter from 18.5 micrometers to 22.7 micrometers, indicating severe atomization degradation. This deteriorated fuel evaporation, combined with lower in-cylinder compression temperatures, prolongs the ignition delay from 1.12 ms to 1.48 ms, causing a 15.7% drop in peak cylinder pressure and a 5.6% reduction in overall thermal combustion efficiency. These theoretical insights establish fundamental guidelines for optimizing injection timing and combustion chamber matching in low-altitude powerplants.

Key Words：

low-altitude economy; heavy-fuel engine; spray penetration; sauter mean diameter; ignition delay; combustion efficiency