Editing Engine knocking (section)

==Abnormal combustion==
{{Main|Cool flame}}

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. Detonation is characterized by an almost instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket, and the cylinder pressure will rise sharply – and possibly beyond its design limits – causing damage.  (Detonation is actually more efficient than deflagration, but is usually avoided due to its damaging effects on engine components.)

If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effect is particle wear caused by moderate knocking, with the resulting particulate dispersing into the engine's oil system causing abrasive wear on other parts prior to being trapped by the oil filter. Such wear gives the appearance of erosion, abrasion, or a "sandblasted" look, similar to the damage caused by hydraulic [[cavitation]]. Severe knocking can lead to catastrophic failure in the form of physical holes melted and pushed through the [[piston]] or [[cylinder head]] (i.e. rupture of the [[combustion chamber]]), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system. [[Hypereutectic piston]]s are known to break easily from such shock waves.<ref name="b3" />

Detonation can be prevented by any or all of the following techniques:
* retarding ignition timing
* the use of a fuel with high [[octane rating]], which increases the combustion temperature of the fuel and reduces the proclivity to detonate
* enriching the [[air–fuel ratio]] which alters the chemical reactions during combustion, reduces the combustion temperature and increases the margin to detonation
* reducing peak cylinder pressure
* decreasing the [[manifold pressure]] by reducing the throttle opening or boost pressure
* reducing the load on the engine
* addition of a [[Antiknock agent|knock inhibitor]] to fuel, increasing the effective octane rating and resistance to detonation

Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures by [[compression ratio]] reduction, [[exhaust gas recirculation]], appropriate calibration of the engine's [[ignition timing]] schedule, and careful design of the engine's combustion chambers and cooling system as well as controlling the initial air intake temperature.{{Citation needed|date=December 2023}}

The addition of [[tetraethyl lead]] (TEL), a soluble organolead compound added to gasoline, was common until it was discontinued for reasons of toxic pollution. Lead dust added to the intake charge will also reduce knock with various hydrocarbon fuels. [[Manganese]] compounds are also used to reduce knock with petrol fuel.

Knock is less common in cold climates. As an aftermarket solution, a [[Water injection (engines)|water injection]] system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation. Steam (water vapor) will suppress knock even though no added cooling is supplied.

Turbulence, as stated, has a very important effect on knock. Engines with good turbulence tend to knock less than engines with poor turbulence. Turbulence occurs not only while the engine is inhaling but also when the mixture is compressed and burned. Many pistons are designed to use [[Squish (piston engine)|"squish" turbulence]] to violently mix the air and fuel together as they are ignited and burned, which reduces knock greatly by speeding up burning and cooling the unburnt mixture. One example of this is all modern side valve or [[flathead engine]]s. A considerable portion of the head space is made to come in close proximity to the piston crown, making for much turbulence near TDC. In the early days of side valve heads this was not done and a much lower compression ratio had to be used for any given fuel. Also such engines were sensitive to ignition advance and had less power.<ref name="b3" />

Knocking is more or less unavoidable in [[diesel engine]]s, where fuel is injected into highly compressed air towards the end of the compression stroke. There is a short lag between the fuel being injected and combustion starting.{{Citation needed|date=December 2023}} By this time there is already a quantity of fuel in the combustion chamber which will ignite first in areas of greater oxygen density prior to the combustion of the complete charge. This sudden increase in pressure and temperature causes the distinctive diesel 'knock' or 'clatter', some of which must be allowed for in the engine design.{{Citation needed|date=December 2023}}

Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce knocking greatly, and modern engines using electronic [[common rail]] injection have very low levels of knock. Engines using [[indirect injection]] generally have lower levels of knock than [[Fuel injection#Direct injection systems|direct injection]] engines, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air. 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. Diesel fuels are actually very prone to knock in gasoline engines but in the diesel engine there is no time for knock to occur because the fuel is only oxidized during the expansion cycle. In the gasoline engine the fuel is slowly oxidizing all the time while it is being compressed before the spark. This allows for changes to occur in the structure/makeup of the molecules before the very critical period of high temperature/pressure.<ref name="b3" />