Editing Engine knocking (section)

==Normal combustion==
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 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.<ref name="b2">{{cite book |url=https://books.google.com/books?id=U4TBoJB2zgsC&pg=PA630 |page=630 |title=Automotive technology: a systems approach |first=Jack |last=Erjavec |publisher=Cengage Learning |date=2005 |isbn=978-1-4018-4831-6 }}</ref>

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, which allows 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 [[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). In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the particular 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 force applied on the piston (from the increasing pressure applied to the top surface of the piston) can give its hardest push precisely when the piston's speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases, thus maximizing torque transferred to the crankshaft.<ref name="b2" /><ref name="b3">{{cite book|url=https://books.google.com/books?id=MFx4VRErHNoC&pg=RA1-PA171|pages=169–173|title=Fundamentals of Internal Combustion Engines|author=H.N. Gupta|publisher=PHI Learning |date=2006|isbn=978-81-203-2854-9}}</ref>