Knock Detector

[dfabro]

Hello to all. (Sorry by my English)

This article is about a circuit capable of detecting detonation with excellent precision. It was tested in VW 1.8 Turbo Engine with the MS-Extra1 pcb v3.0.
This is an enhancement and much better of the existing knock detection circuit.

SPECS
RPM range: 2000-7200RPM tested.
Overall sensitivity adjustment according to the specific engine noise.
Knock Detection with variable sensitivity over RPM.
A 4.8kHz high-pass filter and a 7.4kHz low-pass filter of 40dB per decade.
Four LED's to indication of actual knock level.
A 0-5v analog output signal (for Datalog)

BASICS:
The knock detection as become increasingly important in the years. The principle of finding out if the engine is knocking or not is expected to be simple, but it can become much more complex when the subject is analysed with greater depth.
As all Engine Tuners know, the loss of many engines, particularly the high-
performance and boosted, can be attributed to knock. And as every Vehicle manufacturer knows, the control of knock is directly linked to longevity, economy and emissions of an engine.
Generally speaking, the engine power is limited to the amount of oxygen that it can accept, and the ability to burn the air/fuel mixture on the threshold of detonation. The advance spark control (time) must always be done within the detonation limits, that consumption and performance are optimized. If the curve is too late, the consumption greatly increases, and performance decreases. The occurrence of knock, even in sub-audible levels, can still be very engine harmful.

This circuit can detect the knock presence, long before a professional can hear. When you can hear the "knock" engine at a level higher than the engine noise, it’s already too late.
The benefits are the optimization of the combustion process, meaning excellent performance, maximum use of the power without compromising the longevity, with consumption and emissions always under control, thus, maximizing the potential of your vehicle and protecting your investment. Due to all these benefits, it becomes an essential tool, when the target is the maximum in performance, safety, economy and durability.

WHAT IS KNOCK?
The start of the fuel mixture burning in a gasoline engine, must be controlled by the spark plugs ignition. However, in many situations, the high temperatures and pressures that the mixture is subject during the end of the compression time, can lead to a self-ignition sudden and uncontrolled. For this spontaneous combustion and unwanted, there is the name of Knock,
that should be avoided at all costs, because if it occurs with high frequency can seriously damage the engine.
When something is not within the specifications - very high compression rate, ignition time well advanced, spark plug incorrect or poor quality fuel, for example. The flame spread so disorderly, which brings suddenly the pressure and temperature in the combustion chamber. There will be a spontaneous combustion, a second flame on another point of the chamber
This front of flames moves up to ten times faster than normal, causing huge pressures and creates a metallic noise, which is similar to that of balls of gude within a cup. It is the detonation or the common vocabulary "knock".
The detonation causes damage to produce a very rapid movement of the combustion gases, that beats against the extremes of the chamber and generate heat. If the detonation persists for some time, raising the temperature of components can lead to a head drop, a hole in the piston or the cylinder casting.


The effects caused by the detonation phenomenon depends directly of its duration and
intensity. While moderate, the knock does not produce significant changes in performance and engine durability, but when intense and/or for prolonged periods, can damage it.


AS THE CIRCUIT WORKS :
The noise generated by knock causes a vibration in the structure of the engine (block and Head). The Circuit receives electrical signals from sensors installed on the engine (knock sensor). The task of detecting the Detonation is to capture the sound of knock above the background noise of the engine, through a filter that allows the passage of a specific frequency range, corresponding to the frequency of the sound of Detonation. By filtering the noise of the engine, it allows detection of the knock as soon as possible. The level of background noise specific and the changes in RPM of each engine, make the process of filtering a difficult task.
Thus, for an efficient detection system, it’s important to vary the sensitivity throughout RPM range. If you have only one setting of sensitivity, when the detection of knock is sensitive to the noise of an engine at high RPM, it will not be able to detect knock at low RPM. The circuit conducts an increase of sensitivity at low RPM, noticing any appearance of noise across the engine RPM range.
The sensitivity will gradually decrease in accordance with the increasing speed of the engine (adjustable). If contrary, the circuit will detect false detonation, which is very harmful, so as not detect it. The dyno test showed that the circuit is able to detect the precise level of knock from 2000RPM until 7200RPM, in a 4 cylinder engine with 22 psi of boost, using ethyl alcohol (ethanol) with octane equivalent of 93oct. Alcohol from sugar cane (Brazil).
The circuit indicates four levels of knock, according to the intensity and duration. Small knock for long periods can be harmful.

The stage of filtering has a low pass filter and a high-pass filter in series, which will only allow the passage of the resonant frequency detonation.
The circuit reads the RPM of PCB3 output JS3(tach-out). According to the frequency of this signal, it generated a DC voltage (Vref) directly proportional to the RPM.
This voltage is used as a reference for determining the sensitivity of the detector.
You can connect two diodes with R29 and R26, and linking them in the knock circuit, I believe that this will not change the signal sent to igniter. the frequency of a voltage converter, must be calibrated so that the tension of reference is 1.20v at 1500RPM and 3.7v at 7000RPM. to facilitate, you must have a stimulator.
The diodes should be singled out for the knockcircuit, because it needs positive pulses.

KNOCK SENSOR:

The Knock Sensor is usually set in the engine block or in the head and has as sensor element, a piezo-electric crystal. This material when subjected to mechanical deformation, generates electrical voltages on its surface. Thus, the sensor is able to capture ("listening") the vibrations caused by the parties that move in the engine and also by the phenomenon of detonation, turning them into electrical signals.
The installation of the knock sensor must be made on a completely flat surface and free of impurities. The torque applied on the screw for fixing the Knock Sensor is between 2.0 to 2.5 Kgf.m.
The location and the tightness of the knock sensor should not be changed after the process of adjusting the sensitivity of the circuit
The knock sensor should be positioned in the block highest possible (the closest to the head).
Try not to assemble near the camshaft, valve train, rocker arms, rods, That is to reduce the background noise.
Keep the lead of the knock sensor away from the discharge, or use thermal insulating and not pass near the ignition system (spark plug, distrubutor and coils).
Use only knock sensor type non-resonant.


I am preparing a PCB
I have to do more tests, but until now, the unit was very efficient.
grateful for the opportunity to explain my work. I am available to answer any questions.

[Fred]

Very nice :-)

I have a simple comment :

Detonation is when the mixture burns very rapidly sometime during the propagation of the flame front. Usually this is after the spark event and due to pressure, heat etc. Pre-ignition is what you have illustrated in this diagram :



Where by even if you had not put a spark event in the chamber, you would have got ignition anyway. That diagram gives you exactly what you get in all twin spark engines.

Detonation can still occur when there is pre-ignition (and is likely to in fact because it is effectively a more advanced spark).

Preignition is best illustrated by running an old carboned up carburetored car hot and then removing ignition. The engine will run on unassisted, simply by the glowing carbon in the chamber.

I'm just being fussy with terminology :-) I realise you said "excuse your english", but maybe this will help? :-)

I look forward to seeing more results!

Admin.

[dfabro]

OK, but:
The illustration was copied from an article made by a Brazilian university professor.
Source Article NGK Spark Plug (2004).

ABSTRACT
Fernandez, B. O. (2006). Considerations on other jobs of the knock sensor for
the electronic control of Otto cycle engines using spectral analysis. Dissertation
(Master) – Escola de Engenharia de São Carlos, Universidade de São Paulo, São Carlos-
São Paulo, Brazil, 2006.
This dissertation analyses the suitability of using the knock sensor as a feedback element for the electronic control of internal combustion engines.
The proposed approach consists of using the knock sensor, originally installed by the engine maker in order to eliminate the spontaneous combustion effect, to sample the mechanical vibration produced by the engine. This vibration, which results from the moving parts and the natural oscillation of the system in combustion, produces an output in the range of audible sound. This research contemplates using the spectral variation of that sound to estimate information about an engine operating with different fuel blends.
http://op6wta.bay.livefilestore.com/y1p ... f?download
According to him: In a traditional Otto cycle engine, the mixture of air and fuel is carried by the combustion chamber and compressed by the piston movement. The burning of the mixture begins with the ignition spark plug at some moment near the TDC (when the volume in the combustion chamber is minimal).after that, the pressure wave spreads by the combustion chamber. The last gas to be consumed by a flame front receive the name: Final gases and are usually found away from the ignition point.The final gases are heated and compressed by the piston movement and the pressure waves, starting to react homogeneously.If reach a certain condition, the gas will come into spontaneous combustion. If reach a certain condition, the gas will come into spontaneous combustion. Under normal conditions, the flame front consumes all the mixture, rather than a spontaneous ignition can occur and thus observe a normal combustion.Otherwise, if the final gases start the combustion before the end of the flame front, the ignition explosive results in schock waves in the combustion chamber.



The Mystery of Detonation
By Robin Tuluie, Jun. 23, 2000
Detonation usually happens first at the pressure wave's points of amplification, such as at the edges of the piston crown where reflecting pressure waves from the piston or combustion chamber walls can constructively recombine - this is called constructive interference to yield a very high local pressure. If the speed at which this pressure build-up to detonation occurs is greater than the speed at which the mixture burns, the pressure waves from both the initial ignition at the plug and the pressure waves coming from the problem spots (e.g. the edges of the piston crown, etc.) will set off immediate explosions, rather than combustion, of the mixture across the combustion chamber, leading to further pressure waves and even more havoc. Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the engine.

In this case, the pressure waves generated by flame front, was that caused the second flame. Without the ignition of the spark plug, this would not happen.
The pre-ignition occurs when the air-fuel mixture in the cylinder (or even, to enter the cylinder), starts the flame, before the spark occurs. This pre-ignition is caused by another source of ignition, other than generated by the spark plug.

When you retard the ignition advance, this results in reducing the intensity of pressure waves, and that it does not coincide with the TDC, this hampers the occurrence of detonation.
The pre-ignition can occur regardless of the progress of ignition.

Thanks.

[ababkin]

thanks for the writeup dfabro

you say "... the power of an engine is limited to the amount of air that it can accept ..", but you're forgetting other alternative oxidants like the NAWSSS (just messin' with ya )


The problem with standard knock sensing is that, while you design and tune a knock-detecting system for a particular engine configuration, (one of the factors that can change the knock frequency is the bore size for instance) you'd have to blow up a few engines in order to collect the necessary data like knock signal patterns, etc. Hope you agree that while major car manufacturers can swallow afford this kind price for production cars, we enthusiasts cannot.
Thankfully there is a whole new direction for fighting knock while dynamically maximizing for best power/economy (not very well explored yet and generally not accessible to an enthusiasts due to lack of public domain about it), it's called ion sensing (IS) and pressure peak point (PPP) estimation.
The biggest difference between these two methods is that vanilla knock sensing only knows about it after the fact, whereas IS and PPP estimation can extrapolate on how far the ignition can be advanced to be right on the verge of knock but not quite there, as well as constantly optimizing advance for the ever-changing conditions like, for example, air humidity.

I made a local thread about it with various references: http://www.diyefi.org/forum/viewtopic.php?f=16&t=161
Maybe some of us once will make some advancements to make this possible and accessible. I know there is at least one person on this forum who has some good knowledge/experience with high-voltage stuff, which is needed for ion sensing.

[Fred]

Perhaps I need to do some more reading :-)

I see what you mean now. You are looking at it in more detail than I have bothered to in the past. I was assuming that overall pressure and heat was just too high and caused the explosion rather than burn. However, you are saying that it is the shock wave from initial combustion (regardless of how it occurs) that causes localised high pressure which causes the explosion. I buy those theories :-) At least for some cases.

Sorry for interfering with your thread, perhaps it will help some other uneducated soul like myself learn something too though.

An analogy is gunpowder. When sprinkled on the floor in a line, it simply burns. When compacted carefully in a cardboard tube, it destroys letterboxs and other things rather well. I bet in a heavy boosted situation (where damage is most likely to occur) it is possible that there is just so much heat generated from successive advanced burns that when the spark fires, it explodes on the spot (rather than after the pressure wave moves). But, I'm just guessing and speculating.

Thanks for tolerating my ignorance.

Admin.

[ababkin]

From what i've read while doing my knock research in the past, teh whole 'detonation' term is misnomer and was coined by auto engineers who just assumed it was detonation (just like your gunpowder analogy). Later, scientists found that it was not detonation but something else (i think they still don't know for sure what the hell it is) and called it "knock"

[dfabro]

You are absolutely correct, should say oxygen not air
It is very cool, talk about a subject so important, with people who can add knowledge to everyone

By WESTBROOK (1992): A detonation is a shock-induced combustion wave that internally consists of a discontinuous hydrodynamic shock wave followed by a smooth region of decaying combustion. The adiabatic compression due to the passage of the shock rises the temperature of the combustible mixture above the ignition limit. The reaction results in an energy release driving the shock wave forward.
PUZINAUSKAS (2004) found three theories as the most cited in literature. Although the theory of self-ignition, as the theory of detonation, has achieved general acceptance in the literature found, is the theory of self-igniting the most accepted as common cause of the detonation. But we will not enter much in theory.

Certainly the ion sensing method is more efficient, to achieve the knock detection prematurely. but has several problems in implementing the method in practice. But I believe worthwhile to be examined.

In an attempt to better the process of detection by piezo-electric knock sensor , I implement a band-pass filter. As the knock sensor non-resonant plays virtually all audible frequencies of engine noise, a filter is necessary to separate only the resonant detonation frequency.
The resonant frequencies excited by the presence of knock depend on the geometry of the combustion chamber and the speed of sound in the cylinder charge. These resonant frequencies are typically estimated by assuming an acoustic model for the combustion chamber. For a homogeneous gas filled, acoustically hard walled ideal cylinder, the resonance frequencies are given by Draper’s equation.
A document of SAE Technical Paper Series 900488, shows that the fundamental detonation frequency in a cylinder of 75mm is 7.6 kHz, one of 94mm is approximately 6.1 kHz, while a piston with 110mm is 5.2 kHz. The document shows a reasonable linear relationship , the frequency with a diameter of the piston. The following equation can be used to estimate the resonant frequency of detonation for a particular engine:



Where the resonant frequency is measured in Hertz and the cylinder radius in metres

According to this equation, the knockcircuit allows detection reasonably accurate, for engines with cylinder of 75mm to 120mm.

I am developing a project with HIP9011.
We have received six samples of Intersil. But first I want to extract the maximum this circuit

[whitenoise]

Haven't read through the entire thread. But here's some more useful data about knock sensors.

[dfabro]

What are the possible detonation prevent solutions?

After identifying the knock occurrence with Knockcircuit, you can make some steps to avoid them. Through MegaLogViewer, you can change the ignition time or Richening up the mixture (for example) only in the rpm/map range where the knock is occurring. Thus, the engine operates with the highest performance across of useful RPM range.

Below is some solutions in order of priority in knock prevent:

*Reduce the Spark Advance - As the spark advance increases, the knock tendency also increases.The engine rpm and manifold vacuum/pressure are the main factors that determine the volumetric efficiency, and this which sets the ideal spark advance.The spark point should occur in the exact moment, so that the maximum cylinder pressure happens between 15 and 20 degrees ATDC. We must reduce the advance just enough to stop knock, and only at the point where it is occurring (RPM, Map and Temp).

By Lars Eriksson.
Three different pressure traces resulting from three different spark advances. The different spark advances are:
SA1: spark advance 32.5 degrees (BTDC)
SA2: spark advance 22.5 degrees (BTDC)
SA3: spark advance 12.5 degrees (BTDC)
The optimal spark advance is close to SA2, With too early ignition timing the pressure rise starts too early and counteracts the piston movement. and this is a detonation provocative.

*Increasing fuel octane - The octane determines the fuel ability in resisting the self-ignition, that is, their ability to wait for sparks to then start the flame (combustion).

*Reducing the dynamic compression rate - There must be a balance between the cylinder VE and the compression rate. In an boosted engine, the pressure in the manifold should be consistent with the compression rate, avoid the combustion chamber overheating, it causing knock.

*The decrease in volume of the combustion chamber caused by charring, is a knock generator too.

*Richening up the mixture, AFR below 14.7:1 or lambda = 1. By Klaus Allmendinger - Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard. Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline

*Decrease the manifold air temperature

*Increase humidity admission air

*spark plug with adequate heat graduation

*Making more efficient the engine cooling system

*Combustion Chamber with High Turbulence

etc...

[Fred]

The obvious practical one you have missed is : Water Injection

We can't "just change the combustion chamber" but we can inject water, meth, toluene, etc.

Admin.