Knocking (also called knock, detonation or spark knock, pinking in UK English or pinging in US English) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases catastrophically. Spark knock can range from inconsequential to completely destructive.
Under ideal conditions the common piston internal combustion engine burns its fuel/air mix in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 5 to 40 crankshaft degrees prior to top dead center (TDC), depending on engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases.
The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size its heat output increases allowing it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning fuel air that have a much greater surface area than a simple spherical ball of flame would have. In normal combustion, this flame front moves throughout the fuel air mix at a rate characteristic for the fuel-air mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.
Detonation — abnormal combustion
When unburned fuel/air mixture beyond the boundary of the flame front is heated and pressurized by the advancing flame front for a certain length of time, detonation occurs. It is caused by an instantaneous, explosive ignition of pockets of fuel/air mixture. The cylinder pressure rises sharply beyond its design limits, and if it is allowed to persist, detonation will damage or destroy engine parts. The deleterious mechanisms range from particle wear caused by moderate knocking, to holes punched through the piston or head caused by serious knocking.
Detonation can be prevented by the use of a fuel with higher octane rating, enriching the fuel/air ratio, reducing peak cylinder pressure by increasing the engine revolutions (e.g., shifting to a lower gear), decreasing the manifold pressure by reducing the throttle opening, or reducing the load on the engine. Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures at the engineering level 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 an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation.
Knocking is unavoidable to a greater or lesser extent in diesel engines, 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. By this time there is already a quantity of fuel in the combustion chamber which will ignite first in areas of greater oxygen density, before the rest of the charge. This sudden increase in pressure and temperature causes the distinctive diesel 'knock' or 'clatter'. Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce knocking greatly- modern engines using electronic common rail injection have very low levels of knock. Engines using indirect injection generally have lower levels of knock than direct injection engine, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air.
An unconventional engine that makes use of detonation to improve efficiency and decrease pollutants is the Bourke engine.
Pre-ignition (or preignition) in a spark-ignition engine is often confused with engine knocking. In fact, it is a different phenomenon, when the air/fuel mixture in the cylinder ignites before the spark plug fires. It is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events. Dieseling or run-on is the same phenomenon, but refers to the engine continuing to run after the ignition is shut off with a hot spot as an ignition source for the fuel air mixture. Because both preignition and engine knock sharply increase combustion chamber temperatures, either effect can increase the likelihood of the other effect occurring. Given proper combustion chamber design, preignition can generally be eliminated by proper spark plug selection, proper fuel mixture adjustment, and periodic cleaning of the combustion chambers.
- Pre-ignition and Detonation by Bob Hewitt (Misterfixit) Accessed June 2007
- Engine Basics: Detonation and Pre-Ignition by Allen W. Cline Accessed June 2007
- Pre-Ignition at MetaGlossary. Accessed June 2007
- Charles Fayette Taylor, Internal Combustion Engine in Theory and Practice: Vol. 2, Revised Edition, MIT Press, 1985, Chapter 2 on "Detonation and Preignition", pp 34-85. ISBN 0-262-20052-X