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In the twenty-first century, exhaust emission control will remain a major technical challenge especially as additional pressures for fuel and energy conservation mount. To address these needs, a wide variety of engine and powertrain options must be considered. For many reasons, the piston engine will remain the predominant engine choice in the twenty-first century, especially for conventional and/or parallel hybrid drive trains. Emissions constraints favor the conventional port fuel-injected gasoline engine with 3-way exhaust catalyst, while energy conservation favors direct-injection gasoline and diesel engines. As a result of recent technological progress from a competitive European market, diesels, and most recently, direct-injection (DI) diesels now offer driveability and performance characteristics competitive with those of gasoline engines. In addition, DI diesels offer the highest fuel efficiency.

A staged combustion engine has been evaluated in which pairs of cylinders are coupled in series. The first cylinder of the pair inducts and burns a homogeneous, fuel-rich mixture which produces exhaust products containing substantial amounts of combustibles (CO, H2, and HC) and only small quantities of NOX. These products are then cooled, mixed with additional air, and inducted into another cylinder for a second stage of combustion. Additional work is extracted in this second stage, where substantial cleanup of CO and HC occurs while maintaining a low level of NOX. Experiments with a two-cylinder research engine showed that low NOX emission could be obtained without sacrificing engine efficiency. However, approximately 40 percent more displacement is required to produce the same power as conventional SI engines. The sources of HC, CO, and NOX emissions were investigated, as were the effects of major engine variables on these exhaust emissions and fuel consumption.

The concept of the variable stroke engine (VSE) is explained and the problems related to its use are discussed. Single cylinder combustion data are combined with published multi-cylinder VSE friction data to formulate engine model. This engine model is coupled with a vehicle model to make projections of 55/45 fuel economy and NOx g/mile which are compared with similar projections for a throttled engine. At an NOx level of 0.93 g/km (1.5 g/mile), VSE fuel economy improvements ranged from 2 to 20%, depending upon the frictional losses and vehicle power-to-weight ratio used in the models.