Half the engine displacement of popular cars and light trucks would be adequate for most driving. The split engine (SE) is introduced here as a concept to improve the fuel economy of light-duty vehicles with large spark-ignition internal combustion engines. It operates with a small-displacement portion of the engine for typical driving and activates the secondary portion of the engine to assist with high-power driving. SE is different from cylinder deactivation; the two portions of the engine have independent crankshafts which connect through a one-way clutch, a mechanical diode with indexing features to achieve the correct relative phase of the engine sections. For illustration, 6- and 8-cylinder SE are proposed and simple versions are modeled analytically. The 6-cylinder SE consists of two inline 3-cylinder engines of equal or near-equal displacement. The 8-cylinder SE consists of two opposed horizontal 4-cylinder engines of the same displacement. SE and cylinder deactivation are also compared. Moments of inertia and the time to connect both engine sections smoothly are estimated. Fuel economy improvements with SE are estimated for the EPA urban and highway cycles.

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