THE VARIABLE STROKE ENGINE--PROBLEMS AND PROMISES

The concept of the variable stroke engine (VSE) is explained and problems related to its use are discussed. Single-cylinder combustion data are combined with published multi-cylinder VSE friction data to formulate an engine model. This engine model is coupled with a vehicle model to project 55/45 fuel economy (based on Environmental Protection Agency urban/highway schedule calculations) and NOx (nitrogen oxides) gm/mi which are compared with similar projections for a throttled engine. At an NOx level of 0.93 gm/km (1.5 gm/mi), VSE fuel economy improvements ranged from 2% to 20%, depending upon the frictional losses and the vehicle power-to-weight ratios used in the models (the higher the ratio, the greater the improvement). The calculations suggest that with the VSE, the lower the NOx level, the less the fuel economy improvement. Variable displacement engines using cylinder deactivation have been projected to offer fuel economy improvement competitive with that of the VSE.