riff_raff wrote:Edis wrote: ....F1 engines doesn't really have any problems with engine knock due to their very high speeds. The time available for the end gases to self ignite during combustion is so short that knock isn't a problem........
Edis- While appreciate that you are trying to provide a short answer to a complex question, I think you have oversimplified the situation. The mechanism of spontaneous detonation type combustion (ie. knock) in an SI engine is not really time dependent. Instead, it is more dependent on heat transfer and chemical kinetics. What was said about the thin layer of charge mixture trapped between the piston crown and cylinder deck in the quench area at TDC having a sufficiently high rate of heat transfer to those metal surfaces such that the mixture never becomes hot enough to spontaneously ignite is accurate.
Spontaneous detonation combustion in a homogeneous charge mixture can occur virtually instantaneously. This is demonstrated by HCCI combustion in recip engines.
Knock is very much time dependent!
I based my answer on Douaud and Eyzats induction correlation, a simple model that can be used to predict engine knock (it is often used by engine simulation software). The model is based on the principle that knock will occur when the induction time is less or equal to the flame front arrival time. The induction time is a calculated value that is dependent on fuel octane, cylinder pressure, cylinder temperature and a few multiplier values and the flame front arrival time is of course dependent on the speed of combustion.
The heating of the endgas during combustion is actually not that dependent on heat transfer - instead it is mostly a result of adiabatic compression. First in the form of compression during the compression stroke, then as a result of the rapid pressure rise during combustion. For an engine having a compression ratio of 13:1 the compression stroke will raise the pressure in the cylinder to about 40 bar, then the combustion will raise the pressure further, in the case of a F1 engine up to about 110 bar. This will cause the unburned endgas to exceed a temperature of 700 degC and since gasoline only need 300-400 degC to self ignite it's just a matter of time until it do just that.
As the temperature of the unburned air fuel mixture increase the fuel will start to break down, pre flame reactions will occur, radicals will be produced and given enough time self ignition with a rapid heat release will follow. As a side note - tetraethyl lead actually works by inhibiting these pre flame reactions in some way.
Air/fuel mixtures don't self ignite instantly, but depending on the conditions this ignition delay will vary. That's why HCCI engines are so difficult to control. With a HCCI engine you can't control ignition timing directly, instead the ignition timing is controlled by indirectly adjusting the ignition delay (usually HCCI engines operate with very lean homogeneous air/fuel mixtures). Diesels on the other hand are easier. Diesel fuel have a high cetane number - in other words, it self ignite quite rapidly if injected into a chamber containing hot air. So with a diesel you just control the start of fuel injection, often referred to as the alpha angle, then the fuel will self ignite after a few crankshaft degrees. Still, the ignition delay is much longer at low loads and low engine temperatures. That's why diesels tend to produce a "knocking" sound at low loads, but not at high loads when the engine is warm. This knocking sound is indirectly produced by long ignition delays - in a diesel a long ignition delay will lead to a more rapid pressure rise in the cylinders since more fuel has had time to premix with air before the combustion starts. That is also why diesels these days use pilot injection, a short fuel injection before the main fuel injection aimed to reduce the ignition delay.
