The injector in a DI diesel engine has been modified to allow rotation. The injector speed was varied within ± 4,000 rpm in the current study over 13 testpoints. Air swirl levels tested are 1.5 and 2.8.Rotating the injector adds a free parameter to the combustion system and enables lowest possible smoke emission from each given loadpoint.Smoke reduction up to 74% has been encountered. The mean reduction over all 10 testpoints with a swirl ratio of 1.5 is 55%. Increasing the air-swirl level decreases the smoke level with static injector. The further smoke reduction with counter-swirl rotation is significant albeit not as large as for the lower swirl case.A relationship between the injector speed and effective swirl ratio at TDC is strongly supported by the results and maximum spray stagnation onto the piston bowl wall explains the maximum smoke level when rotating co-swirl.Both counter-swirl rotation and increased air-swirl decreases the ignition delay. Increased swirl leads to lower premixed peak whereas counter-swirl rotation increases the premixed peak. This shows that the rotating injector enhances the initial air/fuel mixing more than air-swirl do.The sensitivity of smoke to changes in injector speed increases with: aLower air swirl level. bIncreasing engine load and fuel/air-equivalence ratio. cDecreasing aromatic content of the fuel. dDecreasing engine speed.