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The paper explores an alternative for hybrid powertrains: the use of two independently operating two-stroke engine modules in a common housing as the primary and secondary energy converters. A low-energy-capacity regenerative braking system rounds out the total hybrid powertrain. The reasons to favor two-stroke engines in the modular engine hybrid approach are that the one-per-revolution firing frequency allows smooth engine operation when operating with low numbers of cylinders; and there is a cost and packaging advantage with piston ported engines when compared with the added valve gear complications associated with multiple four-stroke engines.

Real-world tailpipe emissions from properly-functioning, model year 1991-94 conventional gasoline-fueled cars associated with vehicle operations not emphasized in the FTP are analyzed. Tailpipe emissions are expressed as the product of three factors: fuel rate, engine-out emissions index, and catalyst pass fraction, which are modeled using empirical data from the FTP-Revision Project and applied to in-use driving survey data to estimate real-world emissions. Average tailpipe emissions due to fuel enrichment in warmed-up vehicles are estimated to be 8 g/mile for CO, and 0.3 g/mile for HC. For NOx, the contribution due to incremental loads on the engine (i.e. air conditioner, grade, high acceleration, and high speed) that are not accounted for in the FTP but are encountered in real-world driving are estimated to be roughly 0.3 g/mile.