"Knock" is an expresion you will often come across in the context of petrol (gasoline) engines, both on this website and elsewhere. There is a lot of confusion about exactly what it is, so I have provided some explanation on this page.

In normal engine operation, the fuel/aix mixture is ignited by the spark plug and the flame spreads through the combustion chamber as indicated in this diagram. Although the flame moves very quickly (passing through the cylinder in just a few miliseconds) it is nonetheless a relatively smooth process with a correspondingly smooth release of heat energy.

However, during this process, the "end gas" at the edges of the combustion chamber experiences a rise in pressure and temperature due to the spreading flame. In some cases this is enough to cause the remaining fuel/air mixture to self-ignite before the flame arrives, pretty much instantaneously. This causes a sudden very rapid release of heat energy causing an equally sudden rise in pressure and temperature - an "explosion" rather than a "smooth burn".

This explosive release of energy is what causes the characteristic "knocking", "pinging" sound from which the phenomenon gets its name. Unfortunately it is not just an unpleasant sound; the sudden rise in pressure and temperature can cause catastrophic damage to pistons and head gaskets, among other engine components, and so it is vital to avoid anything other than very light, occasional knock.

The main driver for knock is ignition timing. As the ignition is advanced - the spark is fired earlier in the engine operating cycle - the pressure and temperature during the combustion event become higher, thus making knock more likely. So sufficently retarding the ignition will always prevent knock; however this brings its own problems.

The torque output of an engine typically varies with ignition timing as shown. Due to the finite speed of the burn, the mixture must generally be ignited somewhat before the piston reaches top dead centre in order to achieve maximum output.

The point "M", where torque output reaches its maximum, is commonly known as MBT (Minimum ignition advance for Best Torque). However, in some cases knock begins to occur before the optimum point can be reached - for example at point "K". In this case the engine output is well below what is theoretically possible, but the ignition cannot be advanced further without risk of catastrophic damage. The engine is then said to be knock limited.

The gap between what is theoretically possible, and what is actually possible due to knock, depends very heavily on the details of the engine design. Some engines are not knock limited at all under normal conditions, that is to say that knock occurs beyond the point where maximum torque is reached. On a typical naturally aspirated engine the loss might be 5 - 10%; for highly turbocharged engines it can be 20% or more. Owing to the additional benefit of even just one or two degrees of extra ignition advance on a very knock limited engine, it is usual practice for modern engines to have an "active knock control system", that "listens" to the sound using accelerometers and precisely adjusts the timing of each individual cylinder to keep just away from danger.

Since it is pressure and temperature that drive knock, turbocharged engines inevitably suffer most - not only is the pressure in the cylinder higher, but the turbo also heats up the intake air (which is one reason why an intercooler is so important). To compensate, turbocharged engines almost always have a lower compression ratio than naturally aspirated ones.

Reducing air and coolant temperatures is always good for supressing knock (colder air is also more dense, allowing the engine to take in a greater mass of air.) This is one of the reasons why "induction kits" that take air from the hot engine bay rather than sucking it via a duct from outside are not generally to be advised.

The final strategy for reducing knock - and so allowing more ignition advance (and hence more torque) - is to use a fuel with a higher octane rating. Higher octane fuels contain more components such as benzene that are resistant to self-ignition, and less of components such as n-heptane that self-ignite relatively easily. In general, that's the main difference between "regular" and "premium" fuels - in particular, the rate of burn and the energy content are (contrary to popular opinion) broadly similar.

Knock should not be confused with pre-ignition, which is the tendency of the fuel/air mixture to start burning before the spark has been fired, typically due to a "hot spot" somewhere in the combustion chamber. However, since pre-ignition means effectively too much ignition advance, it frequently leads to knock.

A crucial point with respect to fuel consumption is that knock almost exclusively occurs at high engine load. Under normal cruising conditions (light load) knock is highly unlikely, and the engine can be operated at optimum ignition timing. So actions to reduce knock will generally improve engine power (assuming the ignition is then advanced to take advantage of this), but will generally not improve economy.

Diesel engines do not suffer from knock - or to be more precise, they always knock. In a diesel engine, air is compressed to a high pressure and temperature, and then the fuel spontaneously ignites as soon as it is injected without any actual ignition source. This is part of the reason for the characteristic "diesel clatter" - the very fast rate of burn causes an explosion-like pressure rise. Diesel engines are specifically designed to withstand this - their pistons are much stronger, for example - and so no damage results. The lack of any "knock problem" allows diesels to run very high boost levels, thus giving the high torque at moderate speeds that makes a modern diesel so "driveable".

Interestingly, diesel fuel is specificially optimised to have a high tendency to self-ignite. Rapid self-ignition at low temperatures improves noise and performance of a diesel, and so a high cetane number is desired. Roughly speaking, a high cetane number (high tendency to self-ignite) is exactly opposite to the high octane number (low tendency to self-ignite) required in a petrol engine. This is one reason why I find catalytic fuel "saving" devices, that claim to optimise both petrol and diesel, highly implausible.