The injection spray characteristics and restart operation after hot soaking of a top-feed injector were investigated using liquid-phase injection of liquefied petroleum gas (LPG) in a spark-ignition engine. LPG is one of the well-known gaseous fuels with benefits of emission reduction. The most challenging aspect of the LPG injection system is the phase change of the fuel. LPG inside the pressurized fuel rail is liquid when the engine is in operation. However, it may become gaseous when the fuel pump is turned off. Vaporized LPG inside the fuel rail has to be removed using the fuel return line during the operation of fuel pump before cranking. The top-feed fuel injector does not have a fuel return line in the injector body. The vaporized LPG inside the fuel rail is not removed at the critical conditions and injected at the intake port instead of liquid LPG. However, the injection duration during the engine-start phase is limited, and the fuel amount is insufficient to start the engine. The resulting hot restart problem is very important and must be solved to adapt an injector for LPG liquid-phase injection systems. LPG engine test and flow visualization of the LPG injection system were carried out to investigate the feasibility of the top-feed injector. The LPG temperatures in top- and bottom-feed injection systems were measured and analyzed. Different operating pressures and temperatures of the fuel injection system were tested to identify the injection characteristics after hot soaking using an injection test rig and an engine test bench. The vaporized LPG was successfully removed with a bottom-feed injector. However, in the case of a top-feed injector, the vaporized LPG still remained inside the fuel rail, so that the engine may not be restarted. A modified design of a LPG delivery pipe with a larger volume at higher pressure was suggested to solve the vapor lock problem. A Mie-scattering technique was used to verify the successful liquid-phase injection after hot soaking. Fuel delivery pipe visualization was carried out to measure the position and size distribution of vaporized LPG inside the fuel rail. In the case of a bottom-feed injector, the injection was accomplished at every experimental condition. In the case of a top-feed injector, rail pressure over 1.2 MPa resulted in the need for an overly long injection duration, indicating that the injector solenoid was not powerful enough for such high pressure. The modified LPG delivery pipe was made with I x 10(-3) m(3) Of volume, and LPG injection pressure was 1.05 MPa. The engine with the modified top-feed fuel injection equipment was successfully restarted after hot soaking.
