An audible shuddering sound produced by an engine, because of the uneven burning of its air/fuel mixture. A high-compression ratio spark ignition engine gives good fuel consumption for a given power output. However, a high-compression ratio also interferes with the fuel's combustion process, leading to uneven ignition, unsteady combustion, and eventually damage. Diesel engines tend to have knocking characteristics opposite to those of a spark ignition engine. To overcome these problems, antiknocking fuels are used.