Design and development of an ECU anti its air-fuel ratio control scheme for an LPG engine with a bypass injector

Liquefied petroleum gas (LPG) is a mixture of propane and butane gas, and LPG has been used worldwide in commercial light-duty vehicle fuel. LPG is one of the most promising alternative fuels for light-duty vehicles because it has higher octane number, lower maximum combustion temperature and lower CO and HC emissions than those of gasoline. In this study, a prototype electronic control unit (ECU) for an LPG engine with a bypass injector is developed and evaluated. Hardware and software of the ECU are designed for air-fuel ratio control of an LPG engine. The ECU is built using a Motorola MCU MC68HC05. In order to handle appropriate air-fuel ratio control, a modified PI control scheme is designed and implemented in the ECU. At low engine speed operation, it is difficult to meet the stoichiometric air-fuel ratio due to poor fuel feeding which is the nature of LPG engines. Furthermore during the idle operation, it is more difficult to maintain the stoichiometric air-fuel ratio because of the increase of cycle time delay and poor combustion condition. However during high engine speed operation, the variation of air-fuel ratio from the stoichiometric is substantially decreased, thus the emissions of its engine are reduced. Experimental results show that the prototype LPG ECU and its control scheme provide superior engine performance in terms of idle stability and emission reduction.