Development of a liquid-phase LPG MPI conversion system

For decades Liquefied Petroleum Gas (LPG) has been considered as one o f the most prominent alternative fuels to petrol. LPG typically consists of propane, butane and propylene, but also smaller quantities o f methane and ethane. LPG has a low price compared to petrol, the potential o f low emissions and has indigenous availability. It possesses approximately the same energy density, on a gravimetric basis, as petrol, vaporises easily, mixes readily with air, is resistant to auto-ignition, has wide flammability limits and a good laminar burning speed. Therefore it is also possible to achieve an acceptable efficiency and power from a spark ignition engine run with LPG. The state-of-the-art LPG systems used in spark-ignition engines are either mono-fuel systems, where the vehicle is solely operated using LPG, or bi-fuel vehicles, capable o f using either petrol or LPG. The objectives o f this work were to develop an aftermarket conversion bi-fuel LPG system, which would improve the efficiency o f the engine during LPG operation, with further improvements in the mixture preparation and control, the methods for LPG fuelling calibration and the methods to prevent premature vaporisation in the fuel rail. An additional objective o f this study was to investigate the performance and combustion o f LPG in a non-optimised spark-ignition engine. A prototype system was developed and demonstrated in a 4-cylinder research engine. This novel system uses a liquid LPG injection system, in contrast to the conventional vapour injection systems used in aftermarket LPG bi-fuel conversions. A significant improvement in engine power output was shown, as well as an improvement in mixture control. An optical diagnostics method was applied in order to study the mixture preparation using two alternative LPG fuel injection configurations. The results from both the mixture formation study and the engine experiments showed that the charge cooling effect can be used to improve the efficiency o f a non-optimised bifuel engine. It was also shown that from the mixture control point o f view, injecting the fuel directly to the manifold gives a significant advantage over systems where the fuel is injected to the manifold through coupling pipes. This novel LPG system also uses a fuel pressurising method that improves the fuel system performance in extreme conditions. In addition, a control method to prevent the premature vaporisation of the LPG fuel in the fuel supply line was developed. The method comprises an optical sensor which can detect migrating vapour bubbles in addition to complete phase changes in the fuel line. It was noticed during validation o f the sensor that vaporisation in the fuel rail starts in local hot spots, before the global saturation conditions in the fuel rail are met. This work has demonstrated the potential o f using non-optimised LPG systems in bifuel vehicles. However, the final validation o f the novel control system requires extensive testing on a fleet of test vehicles, and this was not possible within the scope of the work.