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.