4 Versus 8 Counterweights for an I4 Gasoline Engine Crankshaft - Analytical Comparison

This paper presents results of an analytical comparison between two alternative versions of a crankshaft for a 2.2L gasoline engine. The first version had 8 counterweights and a bay balance factor of 80.3%. The second had 4 (larger) counterweights giving a bay balance factor of 56.6% and a crankshaft mass reduction of 1.42 kg.

The results presented in this paper relate to the main bearings in terms of specific loads, oil film thickness and shaft tilt angle under full load and no load conditions across the speed range. Torsional vibration analysis and crankshaft stress analysis were also performed but the results are not presented here.

The differences in bearing force and oil film thickness were very small and the only major difference in terms of shaft tilt angle occurred at Mains 2 and 4 (increase of ∼ 20% compared with 8 counterweight version). This increase could result in extra edge wear of the bearing shells but the crankshaft mass reduction achieved by using the 4 counterweight version was significant. It is not currently possible to determine with certainty the acceptability of the bearing durability at Mains 2 and 4 by analysis. Testing of both versions is planned and may be the subject of a further SAE paper.

The highest value of shaft tilt angle was predicted to occur at Main 5 at high speed and this was caused by whirling of the flywheel to be used for testing (this is heavier and has larger overhang than flywheel intended for production).