Ignition Monitoring
Unburned hydrocarbons (HC) occurring during misfiring damage the environment and in large quantities may cause permanent thermal damage to a catalytic converter at operating temperature. Therefore, the control unit operates continuous ignition monitoring. This is achieved by analysing signals from a crankshaft sensor and a camshaft sensor. The camshaft sensor is required for cylinder recognition. The engine continues if one sensor fails because there are two rpm signals.
In order to determine an unsuccessful ignition (misfire), the rpm is measured constantly by the sensor on the crankshaft. There is a short drop in rpm until the next ignition in the event of a misfire. This drop in rpm is detected by the control unit and analysed statistically. With a correspondingly high occurrence (statistically relevant) of such misfires, the MIL is switched on.
If the occurrence of misfires is more than 5% but still below 9%, this causes the exhaust gas to deteriorate. Therefore, the MIL is switched on if this is established in the third successive driving cycle. In the case of misfires, from switching on the ignition to switching it off constitutes one driving cycle.
If the occurrence of misfires is more than 9%, the MIL is activated immediately and flashes. To prevent damage to the catalytic converter, the fuel supply is cut off at the cylinder where the misfires were determined. For safety reasons, the fuel supply is switched on again if the accelerator is depressed completely. This also occurs if the misfire rate drops below 9%.
Misfire detection is not active in the following cases:
- in the first 5 seconds after starting
- if the speed is more than 6000 rpm
- if the tank contains less than 20% of its total capacity
- when driving on very bad roads
- if the battery voltage drops below 9 volt.
Bad roads are detected by an analysis of the signals from the front right ABS sensor and the sensor on the crankshaft which detects any drop in rpm. In the event of severe signal fluctuations at the crankshaft sensor, it is determined, with the aid of the ABS signal, whether the signal fluctuations are caused by the effects of vibrations due to a bad road surface. Only when the effect of a bad road has been excluded, can a misfire be detected. The reference signal from an additional ABS wheel signal sensor is supplied if ABS is not available.
