Analysis and empirical modelling to assess and predict the impact of catalyst light-off strategies on the exhaust gas temperatures of spark ignition engines

In order to conform to stringent emissions legislation, three-way catalysts have been routinely fitted to gasoline-powered vehicles in order to reduce the emissions of carbon monoxide, unburned hydrocarbons and nitrogen oxides. In Europe the majority of all personal car journeys are less than 4 km in length, with the emissions produced in the first kilometre accounting for 80% of the total emissions produced in that journey. This is due to the time taken for the catalyst to attain ‘light-off’ signified by the point at which a catalyst reaches a sufficiently high temperature to allow chemical reactions to occur. There are a number of methods used to reduce the light-off time such as retarded spark timing, higher idle speeds, artificial loading of the engine, secondary air injection and λ control. While numerous studies have investigated light-off strategies in isolation, or in limited combinations, this study is the first in the open literature to examine all the above factors simultaneously and to assess how their impacts on the exhaust gas temperatures varies with the distance away from the exhaust ports. Experimental work was undertaken in order to generate empirical models to predict the effects of the catalyst heating strategies of gasoline engines on the light-off times of the production three-way catalyst and the exhaust gas temperatures at various distances along the exhaust system up to, and including, the inlet to the catalyst. The empirical model demonstrated good accuracy with errors in the temperature predictions, when compared with experimental data, found to be, on average, less than −0.6% at the pre-catalyst location.