HEHC Cycle
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As the name implies, the High Efficiency Hybrid Cycle' (HEHC) attempts to combine the best features of numerous thermodynamic cycles including Otto, Diesel, Rankine, and Atkinson to create a highly efficient engine.
Referring to the figure, HEHC can be described as follows:
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High Compression Ratio: Air (with no fuel) is compressed to a high ratio (> 18). This is similar to the compression process in a Diesel engine.
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Constant Volume Combustion: Fuel is injected into the combustion chamber and auto-ignites, also similar to the Diesel engine. However, instead of immediately expanding, the burning fuel and gas mixture is constrained so that combustion occurs under constant volume. The combustion is held under isochoric conditions, and the mixture burns for an extended duration of time before the expansion stroke begins. This ensures that fuel and air has a lot of time to mix and burn. The Otto cycle, standard in a gasoline engine, is also modeled as constant-volume.
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Over-expansion: Combustion products are expanded to near-atmospheric pressure, which requires a larger expansion volume than compression volume, as in the Atkinson Cycle.
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Internal cooling via water injection: A small amount of water may (optionally) be injected during combustion or expansion. Water may facilitate the cooling, lubricating, and sealing of combustion chamber and pistons. Water used for cooling from within the engine turns to steam, which in turn increases the cylinder pressure, allowing for some of the cooling losses to be recuperated.
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The combination of high compression ratio, true constant volume combustion, expansion into a larger volume than intake, and (optionally) water turning to high pressure steam cumulatively add to the efficiency of the engine. An air-standard analysis predicts thermodynamic efficiency of 75%.
