Investigation of Combustion and Performance Limits of Liquid LPG Direct Injection in a Single-Cylinder Research Engine

This work investigates the direct injection of Liquefied Petroleum Gas (LPG) in its liquid phase in a single-cylinder research engine, focusing on the primary challenge of maintaining LPG in its liquid state throughout the fuel system to prevent vaporization and gas bubble formation. Due to propane’s low critical temperature (96.8°C), fuel-carrying components exposed to typical engine operating temperatures (80–110°C) are prone to outgassing, which creates pressure drops and disrupts continuous liquid injection. To overcome this, the 200-bar high-pressure fuel system was optimized by combining increased pressure, active cooling, gas separation, and recirculation strategies, ensuring stable liquid conditions upstream of the pump and injector. This robust system allowed for continued stable engine operation and enabled comprehensive comparative measurements—including in-cylinder pressure analysis and FTIR-based emission measurements—to systematically evaluate combustion behavior and environmental impacts. The results demonstrate that LPG’s superior knock resistance (RON 103–111) allowed for earlier ignition timing and higher engine loads compared to E10, with beneficial charge cooling effects further suppressing knock occurrence. In addition, LPG direct injection yielded significant reductions in particulate and CO₂ emissions relative to gasoline, confirming both the performance and environmental advantages of liquid LPG direct injection technology for future engine applications.