EMISSIONS AND FUEL CONSUMPTION OF NATURAL GAS POWERED CITY BUSES VERSUS DIESEL BUSES IN REAL-CITY TRAFFIC

In the fkame of the IEA-AMF project "Real Impact of New Technologies for Heavy Duty Vehicles", three state-of-the-art city bus technologies were evaluated for fuel consumption and emissions in real city traffic and in a number of test cycles, both on engine and on vehicle level. The three buses were a diesel bus, a natural gas bus with stoichiometric fuel control and three-way catalyst, and a natural gas bus with lean bum fuel control. The paper will compare fuel consumption and emissions of the three buses measured in real-city traffic using Vito's VOEM measurement system. The measurements showed that the natural gas buses had clearly higher fuel consumption (in diesel equivalents) than the diesel bus, caused by the lower average engine efficiency. Concerning the emissions, the natural gas bus with stoichiometric fuel control totally fulfilled its expectations with about 10 times lower NO,, THC and CO emissions than the diesel bus. The natural gas bus with lean burn fuel control had low CO emissions, but rather high THC and NO, emissions. In order to lower NO, emissions, the lambda control system needed some adjustments. The buses were tested with three different loads. Vehicle weight had a clear impact on fuel consumption and CO2 emissions for the three buses. Considering the other emissions, only NO, emissions of the diesel bus clearly related to vehicle weight. The actual function of a bus is to transport passengers from one point to another, so it is justified to relate fuel consumption and emissions to the number of passengers. This allows a comparison with other vehicles like passenger cars, vans or minibuses.