Internal Combustion Engine Cylinder Volume Trace Deviation

Heat release analysis is a widely used cylinder pressure-based method for evaluating combustion in engine development, and it is also being investigated as a means to control engine combustion. Heat release analysis has been shown to be sensitive to errors in the calculated cylinder volume, but despite this one of the most common assumptions is that the cylinder volume is nominal and can be calculated solely by the geometrical relations among the measures of the engine components. During engine operation, the components surrounding the combustion chamber are exposed to thermal forces, pressure forces, and mass forces from the reciprocating components. Due to these forces, the components will deform and the volume of the combustion chamber will deviate from its ideal volume. The volume will also be affected by the production tolerances of the engine. This article investigates the validity of the assumption of nominal cylinder volume in a heavy-duty engine context and considers the effect of production variations, static distortion of the cylinder liner, and dynamic deformation of the cylinder and crank mechanism. A detailed plant model was used to show that there was a significant deviation from nominal cylinder volume. In the crank angle domain around top dead center, the cylinder volume could deviate as much as 6% due to static distortion and dynamic deformation. Production tolerances were found to give an additional +/- 2% deviation. Based on the plant model study, a virtual sensor (VS) was developed to enable better cylinder volume estimation. The complexity of the VS was limited to enable the VS to be used in an embedded system. The VS decreased the cylinder volume error from static distortion and dynamic deformation to below 0.4%. Because the VS does not account for the effect of production variations, these variations will define the accuracy of the VS.