Exhaust gas of an SI engine flows through the exhaust system in a pulsating pattern due to the pressure difference between the cylinder and the exhaust system during the blow down process and piston displacement processes. However, when an abnormal combustion, such as misfiring, is encountered in a cylinder, the exhaust gas flow characteristic changes to a different pattern. Pulsation caused by the blow down process is omitted and a sudden change of exhaust gas concentrations is also accompanied.
This research aims to develop a tool that is capable of predicting the exhaust gas flow characteristics and the variation in oxygen and UHC concentrations in the exhaust manifold and at the exhaust manifold confluence point, under misfiring conditions. In this study, to apply a wide-range oxygen sensor for the detection of the sudden change of the gas concentrations caused by a misfiring, the response characteristics of the sensor to the flow of misfired gas were also investigated. And, to apply this sensor for the misfire detection requirement in the OBD-II regulation, real vehicle application studies were carried out.
Oxygen and UHC concentrations at the confluence point could be predicted by interpreting the gas flow from the misfiring cylinder along with the total exhaust gas flow. Gas flow rates from each cylinder were calculated using an one-dimensional engine cycle simulator including a gas dynamic model of the intake and exhaust systems. Variation in oxygen concentration was also determined experimentally using a fast-response hydrocarbon analyzer. The trend of the oxygen concentration fluctuation calculated by the analytical model was compared with the experimental results. The analytical model could duplicate the measured trend of the fluctuation of oxygen concentration at the confluence point, which was characterized with twin peaks for one misfiring. The pattern of twin peaks is mainly caused by the mixing of the misfired gas with the burned gas from normally operating cylinders. The effects of engine load and speed on the characteristics of the variation in oxygen concentration were also investigated analytically and experimentally.
To apply a wide-range oxygen sensor for the detection of sudden change of gas concentrations caused by a misfiring and to understand the pattern of the signal fluctuation, steady state and transient response characteristics of the sensor to the flow of misfired gas were investigated quantitatively, in this study. It was recognized that, when a sensor contacted the misfired gas, the steady state output voltage increased more than the voltage of normal combustion case. This was considered a result of the difference of exhaust gas concentrations between the misfired gas and the normal combustion gas. The transient response was compared at different engine speeds and the response speed was found to increase with the engine speed.
The signals of the wide-range oxygen sensor were characterized over the various engine-operating conditions in order to decide the monitoring parameters for the detection of the misfire and the corresponding faulty cylinder, which were requested by the misfire detection requirement in OBD-II regulation. Effects of the sensor position, transient response characteristics of the sensor and cyclic variation in the signal fluctuation on the performance of misfire detection were also investigated. Limited response time of a commercially available sensor barely allowed to observe misfire. It was found that a misfiring could be distinguished more clearly from normal combustion through the differentiation of the sensor response signal. The differentiated signal showed more distinct twin peaks for a single misfiring in a cylinder. Amplitude of the fluctuation of the differentiated signal was used as a monitoring parameter for misfire detection. Transient response was fast enough to detect the misfire even at a high engine speed of 5000rpm. The phase delay angle of the second peak of the differentiated signal from a reference signal was found appropriate for identification of the faulty cylinder. It was confirmed that the repeatability of phase delay was reliable enough to identify the misfiring cylinder since the deviation of the cyclic variation in the phase delay in 3040 misfiring cycles was small enough.