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Author Topic: Knock Sensor  (Read 6940 times)

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Fuse 19

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Knock Sensor
« on: 06 September 2006, 17:41:44 »

All Omega Petrols engines utilise knock sensor(s) in the control of ignition timing.

Knock is very rarely heard on modern engines thanks mainly to the presence of knock sensors and knock control. Knock is more commonly known as 'pinking', a phenomenon many will know from the old distributor based ignition systems where the timing was to far advanced. Knock also occurs at different points depending on the quality of fuel used i.e. as a rule of thumb, the lower the Ron value of fuel the more retarded (closer to TDC) the ignition timing needs to be. Those who remember the introduction of unleaded fuel may remember how the timing had to be retarded by approx 5 deg to accommodate the lower RON value (if the valve seats could cope with it!).

The optimum point of combustion occurs just after the onset of 'knock' and it is important to ignite the fuel at this point to gain maximum power and maximum efficiency.

The knock sensor is little more than a microphone which is mounted onto the side of the engine block, the ECU advances the timing on every cycle until knock occurs (detected by the sensor) at which point it retards the timing (often by as little as 2 deg) before repeating the cycle. The detected timing values on some systems are used to update the timing Map and hence the ECU can adapt to engine wear etc. This method also allows the ECU to adapt to higher/lower octane fuels automatically.

Some later control systems with coil per plug set-ups also use the knock sensor to detect if the cylinder has fired which allows it record miss fires.

Points to note:

1) The sensor utilises a screened Coax cable to reduce the level of interference that can occur from the high voltage ignition systems and other high electrical noise engine systems. As such it is essential to try to maintain this screening if repairing damaged cables.

2) The knock sensor is a VERY reliable device and not one that I have ever known to truly fail. I have however seen them on V6 power plants where the cable has been trapped and damaged under the cam covers following gasket replacement (passenger side) or caught in the aux belt (drivers side).

3) On the V6 its worth noting that where there are more than one sensor (note also includes the lambda sensors) the numbering follows the cylinder numbering i.e. knock sensor number one is the drivers side on cylinder bank 1-3-5 etc.

4) Under failure conditions, the ECU enters a limp home mode where the timing is retarded to protect the cylinders. This results in the engine running and idling perfectly but, there is a BIG drop in output power.

5) A crude method for testing your DIS pack is to unplug the knock sensor and run the engine, the retarded timing will result in a higher voltage being required to ignite the fuel and hence a weak DIS pack will often miss fire. Note though, this can be terminal to a failing DIS pack!
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