Engine Speed Detection
Engine Speed Detection
Engine speed detection
Smooth idle speed can only be evaluated with the engine running at idle speed (cold or hot). An individual cylinder with poor combustion is detected exactly.
The poor-road-surface detection detects the poor-road-surface mode on a poor stretch of road.
Brief description of components
The following components are described for the misfire detection:
Crankshaft sensor
The crankshaft sensor records the position of the crankshaft with the aid of an increment gear screwed to the crankshaft. The DME uses this to calculate the engine speed. The crankshaft sensor is necessary for the fully sequential injection system (fuel injection takes place individually for each cylinder at the specific point of ignition).
The signal from the crankshaft sensor means that the DME also evaluates the crankshaft acceleration. The crankshaft acceleration provides an indication of the combustion quality of individual cylinders.
Power is supplied to the sensor by the DME with 5 Volts and earth. The sensor delivers a digital signal via the signal cable to the DME. As of approx. 20 rpm, a signal that can be evaluated is output. The tooth gap is detected by the control unit in that the measured distance between the tooth gaps is more than double the size of the previous and succeeding gaps. The tooth gap itself is assigned to a defined crankshaft position of cylinder 1. At this point in time, the DME synchronizes the crankshaft position. With each subsequent low signal, it counts the crankshaft position on by 6°.
System functions
The following system functions are described:
Misfiring detection
The smooth-running values of the individual cylinders are indicated for troubleshooting purposes. The engine must be operated at idle speed for at least 3 minutes to produce useful values. The smooth idle speed evaluation only works when the engine is running at idle (cold or warm). Individual cylinders with a bad combustion performance are pinpointed. Random fluctuations in the smooth-running value of an individual cylinder can only be detected by close observation of the value. In the case of an engine with theoretically even combustion, the smooth-running values are 0 (averaged over all cylinders). An increase in the smooth-running values may be caused by various factors (e.g. misfiring, excess air, mixture deviations, faults in fuel supply, low compression). For this reason, exact control limits cannot be specified. The crankshaft sensor is used to measure the engine speed at the increment gear. In addition to the engine speed detection, the smooth running of the engine (= misfire detection) is also monitored. For misfire detection, the increment gear in the DME control unit is divided into 2 segments according to the firing interval (between 2 ignition operations). Within the DME control unit, the periodic duration of the individual segments is measured and statistically evaluated. For each mapped value on the characteristic map, the maximum permissible values for irregular running are stored (as a function of engine speed, load and engine temperature). If these values are exceeded for a certain number of combustions, a fault code memory entry is stored for a cylinder regarded as faulty.
Poor-road-surface detection
The poor-road-surface detection detects the poor-road-surface mode on a poor stretch of road (driving over stones, gravel or potholes) on the basis of the transferred wheel acceleration. In the case of poor-road-surface detection, a fault is stored and the misfire detection is briefly disabled. The disabling is necessary, as vibrations in the drive train due to poor road surfaces can leads to erroneous misfire detection. Is it also possible that the poor-road-surface detection takes effect too late (only after a misfire has already been erroneously detected). In this case, the combustion misfires are recognized as an incorrect diagnosis by the poor-road-surface detection system.
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