Performance and Emission Evaluation of CNG as an Alternative Fuel in a Bi-fuel SI Engine Powered Vehicle

Alternative fuels are of much importance because of strict emission regulations, increasing fuel cost and the dramatic increase in the rate of depletion of crude oil resources. Therefore, vehicle manufacturers are shifting their research to develop vehicle engines that use alternative fuels such as compressed natural gas (CNG). However, the purpose of the present study is to measure the performance and emission characteristics of a vehicle over a wide range of its operation conditions. All the tests have been done on a passenger vehicle chassis dynamometer under transient state conditions for both gasoline and CNG fuels, where a detailed comparison has been made between their results. A bi-fuel vehicle that has been retrofitted for both fuels together with a locally produced three-way catalytic converter (TWC) that was fitted on the exhaust manifold to reduce its emissions. A multi-point injection (MPI) system is used. The results indicate that the reason behind the increase of total hydrocarbon (THC) in CNG operated vehicle over that produced in gasoline operated vehicle is due to the difficulty in oxidizing the unburned hydrocarbons in the exhaust gases, where the oxidization of hydrocarbons is one of the functions of the TWC. The exhaust hydrocarbons of a gas operated vehicle have a significantly different composition to those of a gasoline operated vehicle. Furthermore, a small deviation in the air index (λ) can be observed in all the testing times. This indicates that the use of catalyser with gasoline or CNG produces nearly same λ.