Flow characteristics of misfired gas in the exhaust manifold of a spark ignition engine

Misfiring in spark ignition engines should be avoided, otherwise unburned f uel and oxygen are brought into the catalyst, and subsequent combustion gre atly increases the temperature, possibly resulting in immediate damage to t he catalyst. As a new concept of misfire detection method, the signal fluct uation of a wide-range oxygen sensor has been introduced to monitor the flu ctuation of the oxygen concentration at the exhaust manifold confluence poi nt. The current research aims to develop a tool that is capable of predicti ng the variation in oxygen concentration at the exhaust manifold confluence point, and to investigate the how characteristics of the misfired gas in t he exhaust manifold under misfiring conditions in a cylinder. The oxygen co ncentration at the confluence point could be predicted by comparing the gas flowrate from the misfiring cylinder with the total exhaust gas flowrate. The gas flowrates from each of the cylinders were calculated using a one-di mensional engine cycle simulation including a gas dynamic model of the inta ke and exhaust systems. The variation in oxygen concentration was also dete rmined experimentally using a fast-response hydrocarbon analyser. The trend of the oxygen concentration fluctuation calculated by the analytical model was compared with the experimental results. The analytical model, could du plicate the measured trend of the fluctuation of oxygen concentration at th e confluence point, which was characterized by twin peaks for one mis firin g. The twin peaks are mainly caused by the mixing of the misfired gas with the burned gas from normally operating cylinders. The effects of engine loa d and speed on the characteristics of the variation in oxygen concentration were also investigated analytically and experimentally.