[2] Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion.
The combustion is started by the spark plug some 10 to 40 crankshaft degrees prior to top dead center (TDC), depending on many factors including engine speed and load.
This growth is due to the travel of the flame front through the combustible fuel–air mix itself, and due to Rayleigh–Taylor instability (resulting from the hot, low-density combustion gasses expanding into the relatively cold and dense unburnt fuel–air mix) which rapidly stretches the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have (this latter process is enhanced and accelerated by any pre-existing turbulence in the fuel–air mixture).
[3][4] When unburned fuel–air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur.
A local shockwave is created around each pocket, and the cylinder pressure will rise sharply – and possibly beyond its design limits – causing damage.
[citation needed] The addition of tetraethyl lead (TEL), a soluble organolead compound added to gasoline, was common until it was discontinued for reasons of toxic pollution.
As an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation.
Diesels actually do not suffer exactly the same "knock" as gasoline engines since the cause is known to be only the very fast rate of pressure rise, not unstable combustion.
[4] Due to the large variation in fuel quality, atmospheric pressure and ambient temperature as well as the possibility of a malfunction, every modern combustion engine contains mechanisms to detect and prevent knocking.
If the signal normalizes indicating a controlled combustion the ignition timing is advanced again in the same fashion keeping the engine at its best possible operating point - the so-called ″knock limit″.
This way performance is kept at its optimum while mostly eliminating the risk of engine damage caused by knock (e.g. when running on low octane fuel).
This then enables engineers to design ways to mitigate knocking combustion whilst maintaining a high thermal efficiency.
[citation needed] Since the onset of knock is sensitive to the in-cylinder pressure, temperature and autoignition chemistry associated with the local mixture compositions within the combustion chamber, simulations which account for all of these aspects[7] have thus proven most effective in determining knock operating limits and enabling engineers to determine the most appropriate operating strategy.
Therefore, the desired trade-off must be done in a stochastic framework which could provide a suitable environment for designing and evaluating different knock control strategies performances with rigorous statistical properties.