Numerical Investigation of Increasing Turbulence through Piston Geometries on Knock Reduction in Heavy Duty Spark Ignition Engines

Knock in heavy duty (HD) spark ignition (SI) engines is exacerbated by a large bore diameter and a higher flame travel distance. An increase in turbulence close to TDC can improve combustion speed and reduce knock through low residence time for end gas auto-ignition. Since HD SI engines are usually derived from diesel engines, it is common to have a swirl motion that does not dissipate into turbulence. To increase flame speed and limit knock, squish can be used to produce turbulence close to TDC. In this study, two different piston bowl geometries are examined: the re-entrant and quartette. The influence of squish area on turbulence production by these piston geometries were investigated using motored simulations in AVL FIRE. The effect of increased turbulence on knock reduction was analyzed using a calibrated 1D GT-Power model of a HD SI engine and the performance improvement was estimated. The effect of clearance height and input swirl level on turbulence was studied for both piston geometries to determine their sensitivity. A lower squish area quartette piston provided the same knock advantage corresponding to a higher squish area re-entrant piston due to additional turbulence production by swirl breakdown. With zero swirl, there was no difference in the turbulence produced by re-entrant and quartette pistons, however, a considerable increase in TKE was observed compared to the baseline swirl level re-entrant case as piston driven flow imparted more turbulence early in the compression stroke.