<P> The addition of certain materials such as lead and thallium will suppress detonation extremely well when certain fuels are used . The addition of tetraethyllead (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 . </P> <P> Knock is less common in cold climates . As an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation . Steam (water vapour) will suppress knock even though no added cooling is supplied . </P> <P> Certain chemical changes must first occur for knock to happen, hence fuels with certain structures tend to knock easier than others . Branched chain paraffins tend to resist knock while straight chain paraffins knock easily . It has been theorized that lead, steam, and the like interfere with some of the various oxidative changes that occur during combustion and hence the reduction in knock . </P> <P> Turbulence, as stated, has 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" 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 engines . A considerable portion of the head space is made to come in close proximity of 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 . </P>

Difference between normal and abnormal combustion in si engine