Engine Roughness - the Key to Lower Octane Requirement

Improvement of engine octane requirement by combustion chamber design has been blocked by engine roughness noise. This noise is caused by high rates of pressure rise in the cylinder due to fast-burn combustion. This produces transient noise bursts with an uneven amplitude pattern. The noise amplitude may be reduced by lowering the burn rate or by structure modifications; however, the low rates cause higher octane requirement. With a stiffer engine structure, higher burn rates can be used without roughness noise, but appreciable gains are not practical because of the weight and cost penalties. A new way to eliminate this noise is proposed. If the amplitudes of the noise bursts from each cylinder are made the same, the roughness noise changes to an acceptable sound. This means that the rate of pressure rise in each cylinder must be the same.