1995-02-01

Engine Misfire Detection by Ionization Current Monitoring 950003

Engine misfires cause a negative impact on exhaust emissions. Severe cases could damage the catalyst system permanently. These are the basic reasons why CARB (California Air Resources Board) mandated the detection of engine misfires in their OBD II (On-Board Diagnostics II) regulations.
For the last several years, automobile manufacturers and their suppliers have been working diligently on various solutions for the “Misfire Detection” challenge. Many have implemented a solution called “Crankshaft Velocity Fluctuation” (CVF), which utilizes the crank sensor input to calculate the variation of the crankshaft rotational speed. The theory is that any misfires will contribute to a deceleration of the crankshaft velocity due to the absence of pressure torque. This approach is marginal at best due to the fact that there could be many contributors to a crankshaft velocity deceleration under various operating conditions. To sort out which is a true misfire is a very difficult task. The fact that this is an “Inferred” method makes this approach much less than robust.
The two factors which make this approach a favorite for almost all the automakers are: First, this method does not require any additional hardware; Second, it works quite well under most normal operating conditions.
Chrysler has developed a Misfire Detection system by monitoring the Ionization Current at the spark plug gap. This is a direct in-cylinder measure of the combustion event. It is believed that the robustness of this system is much superior to the CVF system. The design and development as well as some key test results of the Chrysler Ionization Misfire Detection system will be presented in this paper.

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