Our X308’s are equipped with knock sensor. But does it actually control anything engine wise? (e.g. ignition timing). I know it can throw a code and also cause “restricted performance”, but I seem to recall it does not control or do anything beyond that. I have been looking in JTIS but nothing obvious springs out? Maybe I am looking in the wrong section.

Thanks

Jeroen

 

The knock sensors are quite important as they prevent engine detonation (self-ignition, knocking, pinging) through communication with the ECM. They also automatically adjust the engine ignition timing to whatever octane petrol you are using. Excessive detonation in the engine can cause serious damage like holes in piston crowns.

The workshop manual states:

"Knock Sensor

This sensor uses a piezo-electric sensing element to detect knock which may occur under acceleration at critical conditions.
Should detonation be present the ECM will retard ignition timing of individual cylinders."

 

Thanks for the quick response, I have just found it in the JTIS.
But how does it work, how can ECM on the basis of two knock censors (V8) know which ignition in which cylinder needs to be adjusted. Of course, the ECM knows the crankshaft position, so it knows where every cylinder is in its cycle I guess. Even so, to my understanding a knock sensor measures essential noise (i.e. pinging at certain frequencies. But they are mounted in the inlet manifold.

 

You are correct in all except the mounting location of the knock sensors - they are screwed to the engine block at each bank:

 

I think you're overthinking this. They're vital sensors, as are O2s, MAF etc.

You can either have working ones and be happy or non-working and be unhappy.

 

Sure, from a trouble shooting point of few, that is good enough. But I’m an engineer at heart. I just like to understand how stuff works in detail. Never gave much thought to these sensors yet, although many modern cars have them. But when you think of how they need to work, taking their cue from all cylinders indirectly and being able to correlate noise to a particular cylinder, that is actually a nifty piece of engineering. So interesting to understand how the ECM logic deals with it.

Jeroen

 

Every time I've wanted to know about sensor X or actuator Y I've found web searches provided multiple articles - not true of knock sensors?

 

Sure you can find lots of information. It is usually a general explanation on how it works, how you can test it. Which, to me earlier points tends to be fine for troubleshooting and so.

But I am a bit of nerd, when I say I would like to understand how things work, I mean right down to the last detail. I know how a piezo electric element works. I know how to test it. But how do you get it so it can detect knocking in individual cylinders? It must be designed in a very specific way in order to measure a phenomena like knocking indirectly. Knocking itself is a hugely complex and interesting topic alone. If you begin to think what knocking entails, at what stage does the knock sensor starts detecting it, and how would you retard the ignition, how much, how fast, what kind of control loop is used etc. Whereas the basic concept of how a knock sensor works can be explained in one sentence (as the JTIS does) it does not provide any details. They are not relevant for those who use the JTIS to trouble shoot and fix a Jaguar.

Rest assured, nothing that keeps me awake at night. When these things come up, sometimes my mind starts drifting and I (think) can see how complex the design of such an apparently simple and common piece of kit, actually is. (which is probably true of lots of stuff on cars)

Jeroen

 

As you also mentioned earlier, the ECU knows which cylinder the knock comes from by comparing the moment when the knock is registered by the position of the crankshaft and camshaft at the time. With regard to recognition of the genuine knock (detonation, self ignition) in the combustion chamber in the forest of many other knocks and vibrations present in the engine block, the ECU is programmed (tuned) to the specific "shape" and the complex frequency of the sound that is generated by the detonation (the knock sensor is basically a piezo-microphone). It will retard the ignition for the offending cylinder until the knock stops. This ECU circuit is active all the time, i.e. after the ignition retard, it will, at certain time intervals I guess, try to advance it somewhat and back off again on any repeat of the detonation.

These ECU trials with the ignition timing advance enable automatic correction of the timing advance to match the octane of the fuel you are using. If, after running on lower octane fuel (for which the ECU has set lower timing advance) you put in higher octane fuel, the ECU will determine new (higher) level of the timing advance as the fuel octane allows. Of course, the ECU will not go above the preset optimal timing advances across the rpm range.

 

Many thanks Mr. Stojanovic!

Regards, Jeroen