Experimental Studies on the Effect of Vaporizer Heating and Transition Temperature in a Bi-Fuel LPG Vehicle 2011-26-0006

Liquefied Petroleum Gas (LPG)-powered vehicles use a pressure regulator/vaporizer to expand and modulate the gas pressure to meet the engine's operational demands. This expansion process is accompanied by a phase change wherein liquid LPG is converted to its gaseous form. This consequently reduces the temperature of the working fluid which may result in freezing (Joule-Thompson effect). In order to aid complete phase change and avoid any freezing, the vaporizer is heated either electrically or by the engine coolant circulation. Any inefficiency in the heating may lead to improper phase change and can result in a phenomenon known as "liquid carryover," wherein a liquid LPG gets entrained in the downstream gas circuit where the gaseous form is demanded.

The liquid carryover (if any) leads to the improper engine functioning leading to driveability and emission issues. In bi-fuel (two fuel options - LPG and gasoline) vehicles, operation in LPG is usually avoided until the coolant temperature reaches the optimum "transition temperature" wherein the engine shifts from gasoline mode to LPG operating mode.

This paper establishes an experimental technique to efficiently evaluate the liquid carryover phenomenon in LPG gas-fuelled vehicles using the conventional physical measurements of gas temperature and air-fuel ratio. The paper also presents the use of this experimental technique to effectively evaluate the optimum coolant temperature and its mass flow rate required for efficient phase change process.

The results presented in this paper are based on the experimental tests conducted on a passenger car powered by a 1.2 l MPFI Bi-fuel engine. The results indicate the presence of liquid LPG in the low pressure gas stream at transition temperatures below 40°C. The tests also establish the effect of transition temperature on catalyst light-off time and emissions. The results indicate that the catalyst light-off time increases as the transition temperature is lowered. The catalyst light-off time increases by 3.5% for a decrease in transition temperature of 15°C. Contrarily, the HC and CO₂ emission decreases by 18% and 2.6% respectively as the transition temperature is lowered by 15°C. CO and NOx emissions showed no perceivable change with changes in transition temperature.