Sensors desception-engine operation-6

Camshaft position (CMP) sensor

The intake and exhaust camshafts each have a sensor installed on them. The CMP sensors are used by the PCM for cylinder-1-identification and thus to determine the injection sequence. The signal is also needed to regulate the camshaft adjustment.
Based on the calculated camshaft position and information about the crankshaft position, the PCM is able to determine the correct ignition timing and the correct injection timing for each individual cylinder. Also it can accurately identify a cylinder that is showing a tendency to knocking combustion.
The CMP sensor is realized as a Hall effect sensor and is provided by the PCM with a 5 volt supply. The Hall effect sensor emits a signal when the pulse segments incorporated into the sensor wheel rotate past the tip of the sensor. Thanks to the composition of the pulse segments, and combined with the signals from the CKP sensor, the PCM is able to calculate the position of the individual camshafts at any time. If an increase occurs in the area of the sensor, the PCM receives a 'low' signal with a maximum voltage of 0.5V. If a gap occurs in the area of the sensor, a 'high' signal is sent to the PCM. In this case the voltage is approx. 4.5V.
If one or both CMP sensors fail, a fault is saved in the error memory of the PCM and the camshaft adjustment and knock control are deactivated.

Crankshaft position sensor

The CKP sensor is used by the PCM to detect the engine speed and for TDC (top dead center) recognition.
The periphery of the flywheel has 35 indentations on the engine side, one of which is twice as large as the others. This is used for TDC recognition. The CKP sensor works according to the induction principle and generates a sinusoidal signal voltage whose level and frequency are speed-dependent.
From the frequency of the signal the PCM calculates the engine speed. Due to the double gap in the flywheel, with every engine revolution an altered sinusoidal oscillation is generated, with the help of which the PCM recognizes the TDC position of the crankshaft.
The signal from the CKP sensor is used to determine:

• the crankshaft position
• the engine speed
• the ignition timing
• the fuel injection point
• the adjustment angle of the {Acronym.VVT} units

 

A---Higher engine speed

B---Lower engine speed

1---Zero transitions: Spacing small/amplitude large

2---Zero transitions: Spacing large/amplitude small

 

The acceleration of the flywheel at each power stroke results in a change in the CKP signal.
During the power stroke, the combustion pressure acting on the piston causes an acceleration of the crankshaft and thus also of the flywheel. This is apparent in the voltage curve from slightly higher frequencies and amplitudes of the CKP signal.
If the CKP signal fails, no substitute function is provided. The engine is switched off or the engine will not start and a fault is stored in the error memory of the PCM.

Knock Sensors (KS) 

 

1---Seismic mass

2---Piezoceramic

3---Housing

4---Piezoceramic contact

5---Electrical connection
The knock sensors convert mechanical vibrations of the cylinder block into electrical pulses which can then be processed by the PCM.
The KS consists of piezoceramic crystals which generate a voltage when a mechanical load is applied to them.
When fastening the KS, make sure the specified torque is adhered to. In this way a defined initial tension is applied to the crystals which exerts an influence on the operation of the KS.
When the engine is running, the pressure fluctuations arising due to the combustion process cause vibrations in the cylinder block. These act on the crystals in the KS, causing the sensors to produce an output signal. These signals are evaluated by the PCM and compared with stored data.
 

 

A---Normal combustion

B---Knocking combustion

1---Pressure characteristics in cylinder

2---Output signal from KS

The PCM is able to identify knocking combustion on each individual cylinder. If knocking occurs, the ignition point for the cylinder concerned is adjusted to late for a few crankshaft revolutions, until knocking combustion ceases. After that the ignition point is slowly returned to the calculated value.
If the signal from one or both KS is implausible or absent, knock control is deactivated. The PCM defaults to an ignition map which is further away from the knock limit, so as not to damage the engine. If a fault occurs, a fault code is stored in the error memory of the PCM.

Heated oxygen sensors (HO2S) and catalyst monitor sensors

 The HO2S are located upstream of the TWC. The catalyst monitor sensors are downstream of the TWC. The HO2S measures the residual amount of oxygen in the exhaust before the TWC.

The catalyst monitor sensor measures the amount of oxygen in the exhaust after the TWC.
Both the HO2S and the catalyst monitor sensors send this data to the PCM.

HO2S preheating

The heated oxygen sensor is only able to work at temperatures above 300°C. The normal working temperature in the vehicle is between 350°C and 850°C. If the temperature rises above 1000°C, the heated oxygen sensor will be irreparably damaged.
HO2S are installed so that the optimum operating temperature can be reached as quickly as possible. The heating also serves to maintain a suitable operating temperature while coasting, for example, when no hot gases are flowing past the sensor.
The heating element in the HO2S is a PTC (positive temperature coefficient) resistor. The heating element is supplied with battery voltage as soon as the Powertrain Control Module relay engages. The HO2S is earthed via the PCM. As the heating current is high when the element is cold, it is limited via PWM (pulse width modulation) in the PCM until a certain current value is reached. The PCM then permanently connects the heating element to earth.
The PCM is able to detect faults in the heating element and store these in the error memory.

Heated oxygen sensor

The two HO2S are installed before the TWC. The HO2S consists of a solid galvanic zirconium dioxide cell surrounded by a porous ceramic body. The output voltage of the HO2S depends on the amount of oxygen in the exhaust gas and at lambda = 1 lies between 300 and 500 mV. If the air/fuel mixture becomes richer, the voltage rises as high as 900mV. If it becomes leaner, the voltage falls as far as 0V.
The HO2S are used by the PCM to determine the amount of oxygen in the exhaust gas before it enters the TWC. This allows the PCM to regulate the injected volume of fuel so as to provide an optimum air/fuel mixture for combustion of lambda = 1 at all times.
The HO2S emit a linear voltage signal which corresponds to the ratio of oxygen in the exhaust gas to the oxygen in the ambient air. For this comparison, the HO2S has a small bore hole through which the ambient air reaches into the interior.

Catalyst monitor sensor

The two catalyst monitor sensors are arranged downstream of the TWC. They are used by the PCM to measure the amount of oxygen in the exhaust gas after it emerges from the TWC. If all the conditions for catalyst diagnostics are met, based on this information the PCM can check that the TWC is working satisfactorily. The information is also used to improve the air/fuel mixture adjustment.
The catalyst monitor sensors work in a similar way to the HO2S, except that they emit a binary rather than a linear signal to the PCM. This signal changes very markedly if the oxygen content of the exhaust gas changes. For this reason, catalyst monitor sensors are also called "jump lambda sensors".