The Effects of Fuel Composition, System Design, and Operating Conditions on In-System Vaporization and Hot Start of a Liquid-Phase LPG Injection System

A liquid-phase port injection system for liquefied petroleum gas (LPG) generally consists of a fuel storage tank with extended capability of operating up to 600 psi, a fuel pump, and suitable fuel lines to and from the LPG fuel injectors mounted in the fuel rail manifold. Port injection of LPG in the liquid phase is attractive due to engine emissions and performance benefits. However, maintaining the LPG in the liquid phase at under-hood conditions and re-starting after hot soak can be difficult. Multiphase behavior within a liquid-phase LPG injection system was investigated computationally and experimentally. A commercial chemical equilibrium code (ASPEN PLUS™) was used to model various LPG compositions under operating conditions. Fuels with varying amounts of methane, ethane, ethylene (or ethene), propane, and butane were modeled, and the thermodynamic processes that the fuel experiences in the fuel pump, fuel delivery system, and fuel rail were simulated to determine how changes in fuel composition and conditions affect the performance of the system. Results show that as tank temperature and/or volatility decrease more heat input to the fuel rail is possible without producing vapor. The results of a simplified heat transfer model indicate that there is a critical time period during the engine warm-up process when vaporization is most likely to occur. This tendency is reduced with additional pump boost pressure but is also dependent upon other factors. In low-pressure regions generated by suction at the pump inlet, the sensitivity to pressure drop and heat input is reduced for more volatile LPG mixtures and high tank temperature. These effects are opposite of those observed at the fuel rail and are explained by the effects of composition and tank temperature on system pressure. As the tank liquid fuel level decreases, the space above it allows volatile components to boil off. This results in a more pure liquid in the tank, and problems with vaporization decrease. Start delays after a hot soak of the engine can be reduced by decreasing the thermal mass of the fuel rail, temporarily increasing the flow of liquid fuel to the injectors, and/or raising the vaporization temperature by increasing boost pressure during start-up. A simple model indicates that increased flowrate is more important than increased boost pressure for reducing hot start delay.