What is Knock

Many of us have heard the term and also witnessed the final outcome of knock related engine failures. How then might we better understand this phenomenon? How might we then guard against it happening in our expensive road or race bred engines? These are good questions. A step to getting a hold of this concept will start with an understanding of normal combustion, where knock is not evident.

To begin, we shall concentrate on the ignition phase of the 4-stroke engine cycle. I will assume you are familiar with the Otto-cycle of intake, compression, combustion and exhaust strokes common to most engines we come into daily contact with, however the general principles apply to 2 stroke engines as well.

At the point of ignition, the mixture of fuel and air has been compressed by the compression stroke to a point where it can be efficiently ignited, usually by a single spark at just the right time. When the spark ignites the mixture, it normally causes the propagation of a flame front that moves like the ripple of a stone dropped in a pond, to envelope all of the fuel air mixture in the clearance volume at the top of the piston.

The key here is the orderly nature of flame propagation. It is not an explosion, which is an instant uncontrolled event. On the contrary, it is orderly and progressive, starting at one place (the spark gap) and proceeding until all the available mixture is burnt. This takes a finite amount of time. In order to extract the most work (power output) from each combustion event, the ignition is advanced to a number of degrees before top dead center to account to this delay. Exactly how many degrees of advance is optimum will depend on many factors, but the dominant one is engine speed. For a given (fixed) burn time, at high engine speeds the ignition will have to be advanced by more degrees than at low speeds, all other factors being equal. There is more to that story, as we shall find out, however the extent of this explanation shall suffice for now.

Remember now, how we talked about the orderliness of the combustion event? This is the crucial factor. The ready-to-burn mixture is ideally in a state of turbulence for good mixing and the when the spark sets it off; most of the combustion takes place away from the metal parts. In an optimum world, only a smallish portion of the heat of combustion finds its way to the cylinder walls and the other metal components such as the cylinder head chamber or the valves and it mostly does this where the burning gas touches the metal surfaces. (Remember this point).

So stable controlled combustion minimizes heat loss to the water jacket and allows the metal components to live without stress. This leaves a significant proportion of the heat left over to do what we want, and that is to expand the intake gas by making it hot and pressurized. Simple enough. The hot gas can then only become expanded when the piston moves down the bore, generates mechanical work turning the crankshaft and this wins you the race. The hot gas becomes cooler as it expands and the energy lost to the gas comes out at about 30% in exhaust heat, 30% in water jacket heat, and 30% (roughly) in energy to turn the crankshaft to push you forward and prepare any other cylinders on the crankshaft to fire.

So what of knock then? In simple terms knock occurs when the orderly combustion process breaks down. It turns out that petrol and most hydrocarbon fuels have distinct limits of pressure and temperature at which they will sustain orderly combustion. What happens when these limits are reached is that the fuel mixture will self ignite (a bit like a diesel) so that there are pockets of self-ignition combustion distinct from the spark-originated combustion.

This causes considerable disarray in the combustion event. The combustion is no longer ordered and stable. It often happens that a particular part of the combustion chamber will trigger a separate self-ignited flame origin. This causes a localized pressure event that pushes or distorts the remaining un-burnt mixture with such force it will wobble backwards and forwards in the combustion chamber and makes the combustion chamber contents ring like a bell for an instant. While that ringing or washing backwards and forwards is happening, of course the mixture is continuing to burn. This causes the burning mixture to contact much more of the surrounding metal parts than it would normally do (more surface area is exposed) and so it imparts far more heat by conduction to those metal surfaces. What results is a kind of runaway process. The super-hot component (valve edge, squish nose, poorly cooled patch of chamber floor/piston crown) will then likely cause another unstable combustion event, and that will add more heat to these susceptible areas and before long, pinging/knock/detonation become the rule and engine destruction becomes inevitable if left alone. Other knock causes are also related to temperature such as exhaust gas contamination. Here exhaust backpressure may cause hot exhaust gas to leak back into the combustion chamber during valve overlap and kick off premature ignition, causing knock. However poor cooling distribution around the combustion chambers or high inlet charge temperatures are often the most dominant causes of knock for most designs.