Propane is considered to be a viable fuel alternative for low-emission heavy-duty vehicles in Finland. Natural gas and propane have roughly the same potential for reduced exhaust emissions. Since natural gas and propane are both imported fuels in Finland, there is no preference between these two fuels. Propane, however, is much more easy to distribute, refuel and store onboard the vehicle. This is why propane has received more attention than natural gas as an automotive fuel.
Work to develop a low-emission propane fueled truck started back in 1988 with engine tests. The first prototype, a 17-ton SISU truck was built in 1990, and was operated until September 1992. This truck was equipped with a 7.4-liter Valmet-engine, a closed-loop controlled IMPCO-fuel system and a three-way catalytic converter (TWC).
The experience with this propane fueled truck was good. The driveability was excellent, and both noise level and exhaust emissions were low. The following emission results were obtained in the European 13-mode (ECE R49) emission test with a three-way catalyst aged for 30.000 km: CO 1.8, THC 0.5 and NOx 0.5 g/kWh.
Encouraged by these good results the Finnish truck manufacturer Oy Sisu-Auto Ab decided to build a test fleet of seven propane fueled trucks. These trucks were built in late 1992/early 1993. One of these trucks is used for test and demonstration purposes, and six trucks have been delivered to five different operators for various applications like municipal waste collection, distribution work and street maintenance. A surveillance program concerning these trucks is currently under way.
In the years 1990-1993 a surveillance project on a propane fueled MAN-citybus was carried out concurrent with the truck operations. This vehicle also uses TWC technology. The surveillance work included both weekly checkups on the fuel system and catalysts. Furthermore, altogether six measurements were carried out on a chassis dynamometer according to a modified ECE R49 procedure.
In April 1993 the total accumulated driving distance was some 200.000 km and the bus has worked relatively well, although some minor technical problems have occurred. Judged by the emission test, the original catalyst had lost 50% of its efficiency at 80.000 km. This catalyst was then replaced with a converter from the Finnish company Kemira Oy. With this new catalyst, extremely low emission values were obtained.