Liquefied Petroleum Gas (LPG) is an alternative fuel in spark ignited premix combustion engine and emissions from LPG engines are lower than those in gasoline engines. This article presents a novel method of changing the ignition curve in an LPG/Gasoline bi-fuel engines which still use the converter and mixer models. The goal of this research was to get the best engine power in fuel operating mode both gasoline and LPG. It is known that the gasoline and LPG have different properties, especially burning speeds. In order to obtain optimum engine performance in both fuels, there should
be two ignition curves, one for gasoline and the other for LPG. A circuit Simple Electronic Spark Module (SESM) was applied to manipulate the feedback voltage from a Manifold Absolute Pressure Sensor (MAPS). In the gasoline mode when idle, feedback from the MAPS was 1.4 volts. In this study, the standard ignition curve was maintained for the gasoline operation mode, whereas, in the LPG operation mode, feedback from MAPS was varied at 1.4; 1.2; 1.0; 0.8; and 0.6 volts at idling respectively. The Toyota 5A-FE engine was tested on a chassis dynamo meter to confirm the performance of the circuit. Test results show that the feedback of 0.8 volts produced the best power when the engine running on LPG
