and car pinks a tiny bit in idle
I'm trying to rescue my sanity as my granddaughter has the TV tuned to "simply K Pop" which is a Korean pop song channel. Music to slit your wrists to as Mrs J describes it.
Anyway I'm rereading some of the threads I've posted to recently and I suddenly realized this (the above) sounds not quite right? "Car pinks a tiny bit on idle"? This would be a very unusual situation in which to experience pinking.
Why? well, for a start, what is pinking? Pinking is a mild form of detonation. So what is detonation? Well, let's think about what happens to the air/fuel mixture when the piston is nearing the end of it's compression stroke. Very slightly before TDC the spark plug fires, a flame initiates around the plug electrodes and forms a flame front which advances very rapidly throughout the combustion chamber until the fuel/air mix is burnt. Although this process is very rapid indeed it is not instantaneous. The burn, and so the rise it causes in pressure, takes a finite time so the piston is starting to move back down the cylinder as the burn progresses which limits the peak cylinder pressure experienced by the mixture. A lot of careful calculation and design of combustion chamber shape goes into this to gain maximum work from the fuel.
Detonation comes about when the pressure in the combustion chamber rises to fast and to too great a pressure so that the rise in temperature caused by the compression of the gas - and often a wee hot spot such as a red hot carbon particle causes the unburnt fuel/air mix to explodes all in one go. ie, no flame front, no gradual burn, just a big, instantaneous, explosion of most of the mixture. As the piston will be virtually at TDC, or only very slightly after TDC, when this happens the forces on the piston, con rod etc are extreme. You won't mistake it once you've heard it, sounds rather like someone inside the engine going at it with small hammers! A bit like diesel knock, but different, and few engines will withstand actual detonation for very long before very serious damage occurs. Trouble is that if you are traveling at speed or, maybe have a noisy exhaust, you may not hear it before it's too late.
Pinking is a lesser version of this malady. Typically what happens is that most of the mixture will burn as intended. During the burn the pressure in the combustion chamber is rising of course as it is designed to do but something intervenes, late in the combustion process, which causes the last of the mixture to explode rather than the controlled burn completing. This causes the light tinkling ping which is so typical. Most commonly, if it's going to do it, you'll hear an engine pinking when pulling hard, with the engine fully up to temperature or maybe a little over heated - in heavy traffic perhaps, on a large throttle opening, at low engine revs. This is entirely understandable as this situation will promote very high cylinder pressures. What's causing it? Very commonly the engine will be coked up so combustion chamber volume has been reduced by carbon buildup which may also be glowing red hot and exacerbating the problem. but also poor quality, low octane fuel or ignition being too far advanced (not so common now with electronic control units monitoring everything and anti knock sensors on most engines which are there to specifically prevent pre-ignition (another word for pinking)
So now maybe you can see why I'm puzzled by your description of pinking at idle? If you can be bothered can you enlighten me any further?
While we are on this subject, many people think that diesel engines are noisy because they basically detonate their fuel. Not true. A diesel works by compressing it's air so tightly (compression ratios are maybe twice that of a petrol engine) that the air in the cylinder at TDC exceeds the temperature required to ignite the fuel. Then the fuel is injected (and that can be a quite complicated sequence) directly into this very hot air (until this point there is only air in the cylinder) People think that the fuel "explodes" as it hits the air, but in fact it's a controlled burn just like the petrol engine as the fuel takes a finite time to be injected (sometimes even injected in a delayed series of "spurts") so no instantaneous rise in pressure takes place. However it's all taking place under far higher pressures, hence the greater noise.
