Or the compression ratio reduced or use fuel of a higher octane.
Knock sensors detect pinking. It's only another word for detonation.
Dave Baker - Puma Race Engines
No ... too late. I want to hear more about knock sensors. Years ago I tried to get comment on the consequences of using normal u/l rather than premium u/l in a 406 but got no meaningful information. The issue in this thread - pinging - and knock sensors - are obviously relevant. I, and most other owners of vehicles at the mercy of Bosch engine management systems, yearn for some insight into how these work - how they respond to stimulii such as knocks, wrong temps, etc. etc. This thread, addressing pre-ignition, offered hopes of shedding light. Which is why I stomped on people introducing irrelevant emotional issues. Particularly as the same people obviously had something to contribute to our enlightenment on engine management systems.
Actually Dave's last post was one of the most informative on the subject of vacuum advance that I've seen on UseNet.
I believe that a static ignition system will use a knock sensor to decide if the timing is correct - retarding it if knocking is occurring. Normal u/l has a lower octane rating and therefore ignites at a lower compression - this is of no consequence if your engine doesn't have a high compression ratio, but will probably mean that the ignition will be retarded if your engine does have a particularly high compression. This is all going from memory though, I'd like to hear other people's thoughts on the subject if I've got something wrong....
.... of course, then maybe we could turn into another emotional flame war ;)
Chris.
For about 50 years I have been aware of the pinking phenomenon. As early posts said, it is pre-ignition. The dwell angle used to define the point at which the spark appeared - somewhere around tdc - top dead centre. Pinging occurs when compression ignition occurs before the spark. As has been pointed out, the possibility existed that ignitiona took place before tdc so that the explosion tried to turn the engine backwards. Before we were offered a choice of fuels, the usual cause of pinking was build up of carbon - increasing compression and, maybe, actually maintaining hot spots in the cylinder head. Another possibiity was excessive spark advance. Another cause might have been when, after a head gasket failure, the head was shaved excessively. Like carbon build up, this increased the compression ratio. This thread has skated very lightly over knock sensors. My impression is that noone knows much about them. The problem is that computerised engine management systems degrade to a safe (read low)level of performance when they get an error message from a knock sensor or a temp sensor or, no doubt, various other sensors. With so many sensors talking to the ems one wonders if any cars on the road are actually operating in top performance mode. I may live to see a vehicle which will, on request, indicate precisly what the ems is doing and why.
No it isn't. That's a completely different phenomenon caused by hot spots in the combustion chamber such as glowing carbon deposits, sharp edges, exhaust valves or spark plug or by compression ignition if the temperature inside the chamber reaches the flash point of the fuel before the spark is initiated - in other words dieseling. Pre-ignition can cause an engine to continue running even after the ignition has been switched off.
The dwell angle used to define
Not necessarily or even usually. Abnormal combustion falls into two main categories. Pre-ignition abnormalities and post ignition abnormalities.
Pre-ignition is as described above. It splits into two further categories - surface ignition (hot spots in the chamber) and compression ignition as also described above. If pre-ignition is present then it can't be directly controlled by spark timing as it is essentially independent of the same. However, reducing the ignition advance can cause the temperatures to reduce also until the source of the pre-ignition dissipates. Pre-ignition abnormalities are very rare in modern engines and mainly tended to afflict older engines with cast iron cylinder heads and poor heat dissipation such as MGB and A Series Mini engines etc.
Post ignition abnormalities are caused after the spark has fired. In fact it's the increase in temperature and pressure caused by the burning gases ignited by the spark that then cause end gases to ignite before the flame front reaches them. In normal combustion the flame front advances through the combustion chamber in a steady fashion until it has reached all the fuel/air mixture, a process that takes a couple of milliseconds. In detonation some of the gases are ignited by the spark and then the rest auto ignites before the flame front reaches it. This leads to a far quicker burn than normal and much higher rates of pressure rise in the chamber. The resistance of a fuel to auto ignition is called its Octane value. Octane value has nothing to do with the energy stored in a fuel and higher octane fuel doesn't release more power. It simply allows higher compression ratios to be used before detonation takes place. The higher compression ratio is what creates more power.
The VAST majority of cases of detonation described as pinking or knock are post ignition abnormalities. They start when the throttle is opened largely or fully and subside immediately the throttle is backed off. If the cause was pre-ignition then throttle position would make no difference which is why running-on is a pre-ignition fault.
Sometimes detonation is audible. Generally that which occurs at low/medium engine speeds and high throttle openings. This audible detonation tends to be labelled pinking, pinging or knock. However high speed detonation is often inaudible and only discovered on strip down by the damage it does to piston crowns, gaskets and chamber material or by total engine failure during operation.
Maybe no one had time to post much about them :)
Detonation causes abnormally high pressures in the combustion chamber and pressure pulses which normally manifest at specific frequencies. Pressures can exceed 2500 psi (normal is under 1000 psi) and pressure pulses usually occur in the 5 to 10 kHz range. The high pressure pulses cause the block and cylinder head walls to vibrate. These vibrations can be detected by piezoelectronic accelerometers provided they aren't masked by other engine noise and vibration. Generally this masking noise makes knock sensing very difficult at above about
5000 rpm which is why high rpm knock can be so damaging.Dave Baker - Puma Race Engines
Hello Dave,
Pleased to read that ! If you read my post above, that was what I asked.
Mmmm, a good, piezoelectric resonator has a reduced bandwidth. And anyway they aren't placed at the opposite of the engine, I mean, not on the gearbox. The sensor's position (i.e, close to the cyl. block / engine head) and the fact it's close to the noise source, makes it work a more reliable way.
I suppose an engine which wasn't knocking at 3kRPM (at full load) won't knock, say, 10 secs later @5kRPM, as the ignition will be corrected at the same time, according to ECU setup &/or map. Of course, I may be wrong once again, here.
Regards, G.T snipped-for-privacy@worldonline.fr
205 Diesel & turbo-Diesel :
one thing nobody has mentioned is EGR valves, a major contributing factor of poor ignition and combustion. years ago these were unheard of, since nobody was concerned about our ozone layer exhaust gas was thrown out the back once it was burnt, nowadays with EGR valve engines a percentage of the gasses are introduced back into the chamber to be reburnt. BTW chris, some ppl are unable to put a brilliant point across without emmotional attachment. we don't all walk the same line and some are passionate about what they believe. some call it childish, others commitment. steve.
Hi Steve,
I merely said this could turn into 'another emotional flame war' because Ken had stated that people were bringing 'irrelevant emotional issues' into the thread. Yes, I was being rather irreverent/sarcastic but I was just trying to bring a little humour into this topic rather than stir things up again.
Chris.
much needed too, i don't take things too seriously even if my posts may show otherwise,
Dave Baker thus spake the following on 03/03/04 08:50:
I understood (and I'm a most certainly an uninformed amateur!) that the ECU (certainly the early ones) would retard the ignition until a "knock" was detected and then advance it slightly. In other words the ECU held the engine just at the point of "knocking".
Regards
Peter
So are pre and post-ignition equally damaging? Do they sound the same? How do EGR valves affect the matter?
This explanation would be an excellent addition to your website, as the terms are often abused unknowingly.
Thanks,
Thomas Hood
They can both produce similar damage and noise if they create detonation at normal engine speeds. Pre-ignition run on after the ignition has been switched off is usually nothing to worry about and doesn't affect FI engines as the fuel is cut off with the ignition switch. Carb engines often have an anti run-on solenoid to cut the fuel supply with the ignition switch.
EGR is primarily there to reduce emissions of oxides of nitrogen. It does this by lowering chamber temperatures by bleeding small amounts of exhaust gas back into the inlet manifold. Although hot these gases have a cooling effect in the chamber. Like any other contaminant this reduces flame speed which means a bit more ignition advance is required for best power. A side effect of the lower chamber temps is more resistance to detonation.
If an engine with EGR has a defective EGR valve then the chamber temps go up and the ignition advance which was required for EGR operation might now to be much for non EGR operation. Combine the two and detonation can result.
I'll have to keep something back for the book on race engine theory and practice or no one will need to buy the book :)
Dave Baker - Puma Race Engines
Engines are most prone to detonate at peak torque rpm where cylinder filling (volumetric efficiency) is at its highest. On a road car that will be normally be in the 3k to 4k rpm range and fairly easily detectable. Race engines have more of a problem with peak torque rpm coming much higher up and closer to peak power rpm. That's where knock is often inaudible and the first sign of trouble is when the engine blows.
Dave Baker - Puma Race Engines
Hello,
That seems evident, when reading your considerations about pinking (being @ max torque means being @ max cylinder filling I agree, I already knew that), this involving more of the air/fuel mix to burn.
I can understand it's more difficult to hear, or detect, but another point I'd like you to explain to me is, aren't race engines rather prone to pinking ? I mean, pinking could be a consequence of being close to engine's limits (due to setup / preparation). I know it may be due to fuel quality, but my question is (and I'll use an example which may not be perfect) "could a racing 106 Rallye @200HP pink easily compared to a standard 106 Rallye ?".
Regards, G.T snipped-for-privacy@worldonline.fr
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