Influencing Parameters of the Exhaust Gas Emissions of a Stoichiometric Natural Gas Bus in Real Use

In the frame of the IEA-AMF, Annex XVII project "Real Impact of New Technologies for Heavy Duty Vehicles,' three state-of-the-art city bus technologies were evaluated for fuel consumption and emission in real city traffic and in a number of test cycles, both on engine and on vehicle level. One of the three buses was a natural gas bus with multi-point fuel injection, stoichiometric fuel control and three-way catalyst. Compared to the other tested technologies, this engine reached very low exhaust gas emissions. The paper will discuss the results obtained with the stoichiometric natural gas engine and compare the emissions in real traffic versus various engine test cycles, based on a number of influencing parameters. Concerning cycle characteristics, it was based on a number of influencing parameters. Concerning cycle characteristics, it was the distribution of the engine-operating points which had most effect on the exhaust gas emissions. Also the follow-up of acceleration, cruising and deceleration phases had a direct effect on catalyst functioning and thus also on the emissions. Apart from the cycle characteristics, some other parameters like intake air temperature, coolant temperature, lambda value, ignition timing or engine and component condition may also influence the emissions. The effect of coolant temperature variation on exhaust gas temperature and emissions was researched on engine test bench at Istituto Motori. It appeared that coolant temperature had no significant effect for this technology; most variations were related to small shifts in lambda and the condition of the three-way catalyst. Other factors like intake air temperature were not researched within the IEA-AMF project, but from previous experiences their influence was clearly proven.