The pollution levels recorded in large urban areas are rising concerns for public health and substantial reductions in pollutant emissions have become an important issue.
In this context, engine manufacturers have developed a number of techniques to reduce emission vehicle levels and in particular are turning their attention to the use of clean fuels. One example is the Liquid Petroleum Gas, roughly a mixture of propane and butane, which gives a benefit in terms of toxic hydrocarbons emissions and ozone formation due to its composition (mainly alcanes and olefines from C2 to C5) and CO2 emission levels. The aim of this study was to provide a quick and low cost solution for downtown emissions of motor vehicle. In order to maintain a good autonomy, the development work was focused on a dual fuel vehicle using LPG as well as gasoline. For this purpose, a RENAULT CLIO vehicle powered by a 1.4 liter single-point gasoline engine (commercially available in 1996), was chosen and the technology used for the LPG conversion was based on conventional equipments such as a gas induction system. The use of pre-heated catalyst, secondary air injection and even air gap insulated exhaust pipes were banished to ensure a low cost and reliable solution, and no engine efficiency deterioration for increasing exhaust temperature was permitted.
To achieve the ULEV target, the whole LPG engine mapping was optimized on a test bed in terms of spark timing advance and equivalence ratio for the best catalytic conversion efficiency and control of exhaust temperature. On the vehicle, several catalytic layouts were tested to derive the best solution for fast light-off, high catalytic conversion and engine performances.
The results after optimisation on the test bed showed almost the same engine torque curve with a loss of less than 3 % between gasoline and LPG, and a good fuel distribution cylinder to cylinder with LPG. Finally, the vehicle passes the ULEV and the corresponding European emissions levels with a significant margin using a 5 000 km aged catalyst. The study also plans a 80 000 km endurance running test on the dynamometer and a cold start (-7 °C) characterization. A specific analysis of the N2O, hydrocarbons and aldehydes emissions confirm the very low toxicity of this solution combined with an ozone reactivity of about one half the gasoline one.