Burn-rate and knock reduction in the spark ignition engine

The dependence of knock on combustion chamber shape is investigated. Knock originating from the spark plug's ignition of the air-fuel charge ('spark knock') is assumed. Four general parameters that influence knock are isolated. Examination of the parameters reveal that manipulation of the burn history is favoured in curbing knock. By utilizing an existing model of knock, it was shown that slightly larger pressures and temperatures can be applied to an air-fuel charge for shorter times without knock. This explains the well-known observation that reducing the burn time in a spark ignition (SI) engine decreases the end-gas's propensity to knock. It was also shown that the increase in end-gas pressure and temperature associated with a higher burn rate is insufficient to re-introduce knock. Results for the engine data examined indicate that for a pressure trace at the onset of knock (reference trace), a twofold reduction in burn time can allow the combustion pressure to be scared up by about 23% without knock occurring.