Comparative exhaust emission tests were performed on Japanese light- and heavy-duty vehicles fuelled with a Fischer-Tropsch diesel derived from natural gas and two crude oil-derived reference diesels. Both vehicles were tested without and with an oxidation catalyst fitted to the engine.
In the case of the light-duty vehicle a current Japanese specification diesel and a future specification low sulphur diesel were used as reference while in the case of the heavy-duty vehicle only the low sulphur diesel was used as reference.
The emission tests were performed using the standard Japanese emission test cycle applicable to that vehicle type. In addition certain selected modes from a special test cycle representative of congested traffic patterns encountered in the Tokyo inner city environment were used for both the light- and heavy-duty vehicles.
In general, Fischer-Tropsch diesel reduced all the regulated emissions compared to both crude derived diesels. The exception was in the case of the light-duty vehicle where nitrogen oxide (NOx) emissions remained at roughly the same level with all the fuels.
Over certain modes of the Tokyo cycle which comprise a high proportion of idling and low speeds it appeared that the Fischer-Tropsch diesel improved the conversion efficiency of the oxidation catalysts.
Fischer-Tropsch diesel therefore offers environmental benefits compared to low sulphur refined diesel. Most importantly, in applications where vehicle densities are high, such as in inner city environments, and subsequently where low exhaust emissions are of paramount importance, Fischer-Tropsch diesel appears to provide further enhanced benefits.