I have been looking at the knock module from various LT1 and V6 applications in the late '90s. The circuit is housed in a plastic carrier with eight contacts. Inside there is a hybrid circuit (I believe it is thin film). On the circuit are a number of capacitors (one looks to be a larger polarized tantalum cap), LASER trimmed resistors and two ICs. One of the integrated circuits is a soic 8 pin manufactured by philips (marked NE532D dual op-amp [edit]) the other has a higher pin count (perhaps 16 pin) and is not packaged. The second chip has no markings because it is not packaged. I believe the part is made with solder bumps and is mounted directly on the substrate.
I find it interesting that when manufactured, the hybrid is mounted in the top of the plastic housing with some very low density elastomer. This material is very soft and I believe is used as a sound deadener to prevent flexing of the substrate from becoming microphonic. That is to say that the module could under some circumstance pick up vibrations and turn them into unwanted signal.
I plan to look at this module in the future & try to find it's operation, but for now this is all I have found.
One question for the group... knock is when the compression cycle heats the mixture to the point where it ignites before the spark (pre-ignition). How does retarding the spark seek to cure this? I can see how making the mixture richer might cool things. Overall retarding the timing would cool things when knock isn't present. Perhaps someone could explain?
-Tom
Last edited by Tom H; 09-17-2020 at 02:44 PM.
Pre-ignition (ignition before spark) and "Knock" Detonation (after ignition) are two different events.Originally Posted by Tom H
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I think the trick is to prevent rather than recover. I did order the sensor, I just found it odd that the same style and engine displacement demanded a different part number.
Last edited by dfarr67; 09-17-2020 at 04:21 PM.
The sensor will be a different part number depending on whether it's OBDI or OBDII, because the OBDI version contains a resistor inline that the PCM uses to determine whether the sensor is physically present or not (but that can become damaged and fail without necessarily causing the sensor to fail, tripping a DTC when the system is in fact still functioning as designed). The OBDII version has no such resistor because the PCM determines whether the sensors are present based on their actual output signal.
The modules all have totally different part numbers because every single one is calibrated differently due to changes in packaging between various different engine configurations and vehicle layouts, which cause changes in the audio profile that need to be filtered in different ways. It's this part that is so tricky when you start making changes to the system, and why creating a replacement module would be the only "correct" way to fix the problem of not correcting detecting knock once you've created an engine that doesn't fit the stock GM profiles.
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Knock is the audible side effect of pressure increasing rapidly while the crank and piston are not in a position to be driven by the force created by combustion.Knock is a sudden explosion - detonation - rather than a smooth controlled burn. Instead of building, pressure spikes so fast it's like a hammer striking the piston or the cylinder or the valves.
Ultimately the game is to create pressure on the piston while the crank angle is optimum for creating motion from that pressure. It's a bit of a guessing game, but generally your fuel will burn at a rate that will allow you to time the beginning of combustion so peak combustion pressure occurs at or near optimum crank angle. If pressure is created at the wrong time, generally when the crank is not far enough ATDC, continued combustion will create sound and heat energy rather than motion. Pressure can be created at the wrong crank angle by beginning spark too early for the fuel's rate of combustion. It can also be created by altering the rate of combustion through temperature change, by using fuel more likely to self ignite, or by a point or points of high temperature in the chamber causing multiple flame fronts.
Retarding spark timing delays the beginning of combustion and will hopefully delay the onset of peak pressure. When spark is retarded as a result of knock it is hoped the crank angle is great enough ATDC to be drawing the piston away from compression, so combustion pressure is able to act on the piston to create movement. It is hoped during this time that the cooling system will draw heat from the combustion chamber which can help reduce the likelihood of continued combustion problems.
The ESC system is an active system which has the ability to delay onset of combustion in response to audible knock. The GM implementation will significantly delay combustion then begin to reduce the delay over time. If knock is detected the cycle of delay then reduce delay will repeat.
Water injection helps prevent knock by slowing combustion. Rather than delay the beginning of the combustion event, WI delays the onset of peak combustion pressure. WI can be very effective as liquid water draws heat energy from the chamber in order to convert to steam, and steam slows combustion.
Alcohol is sometimes added to water or injected by itself. Alcohol is more combustible than water but has a higher octane rating than most gasoline. In cases where combustion temperature is triggering fuel self-ignition, increasing the octane rating can help. Additionally liquid or vapor alcohol, like water, will draw heat from the chamber in order to change to gaseous form. This method also delays onset of peak pressure without delaying the start of combustion.
Fuel additives such as Methyl Ethyl Keytone, alcohol, and others attempt to increase the octane rating, the fuel's resistance to self-ignition. Additives generally work in the same way as alcohol above.
Ultimately you will lose more power when using ESC with very conservative settings. Upon detecting knock, combustion start is delayed, often far beyond what is needed to create peak pressure at optimum crank angle. As the system reduces the delay, it is possible for excess heat in the chamber to cause knock to occur again which triggers increased delay. Preventing knock in the first place generally means excess heat will not be created in the chamber so spark does not need to be delayed.
Excellent explanation Shannen :) One note discussed briefly earlier is detonation and pre-ignition are two different things. Pre-ignition can be fatal to an engine VERY rapidly.
-Carl
I understand a little more now... with knock the plug ignites the mixture and a flame front starts from it's tip. Normal ignition the front continues, pressure builds and all is good. With Knock what happens next. Could someone point to a good link that describes all this??
-Tom
Pre-ignition is when the electrical spark is not the start of the flame front. It often but not quite always leads to knock. Even if it somehow is not knock is still bad because pressure builds before the piston is ready to convert the pressure to useful work.
Knock is a sudden explosion - detonation - rather than a smooth controlled burn. Instead of building, pressure spikes so fast it's like a hammer striking the piston or the cylinder or the valves.
Depending on the amount of air & fuel involved in the knock event as well as when it occurs, has an effect on its acoustics, but it always sounds like some kind of metal on metal.
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Using this definition, knock isn't a result of the spark timing. How does retarding spark eliminate the knock?
I am interested in this because of some other info I came across. Some early aviation engines with turbos and so on had problems with knock. In some cases, this was cured by injecting a water/alcohol mix. In my PCM code I can see where spark retard is applied... wondering if one of the un-used outputs can be used to signal an external pump to inject water. I think this sort of setup would be quite easy in terms of setting up the pump and so on. Software would also be quite easy. Question comes as to how much to inject. This is where things get complicated.. Probably a constant pressure with some sort of jet to control the flow. Just thinking out loud.
-Tom
Using this definition, knock isn't NECESSARILY a result of spark timing.
Less-than-ideal timing - like, say, the factory spark timing maps we tend to improve on - does not cause knock in and of itself.
Some engines, regardless of timing, are very resistant to knock, whereas others are much less so.
Anyway, did you mean something like the BMW M4 GTS' water injection system?
https://www.bmw-m.com/en/topics/maga...00-horses.html
It's blatantly obvious how old this idea is.
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