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A rich mixture is used to maximize power output and cool the intake charge in order to minimize the risk of detonation, not cool the catalytic converter, which has typical operating temperatures around 1500-1600F, right around the maximum EGTs SI most engines will make. Having the larger 1.8L engine and taller gearing could very well help out with SFC, although the four-stroke Atkinson cycle is exactly like the SI Otto cycle minus the delayed closing of the intake valve after the piston has started to travel upwards again. The four-stroke versions allowed Toyota to cheaply and effectively de-stroke their 1.5L engines for use in the Prius, unlike the original Atkinson cycle that supposedly used a modified crank and had the intake, compression, power, and exhaust strokes in a single turn of the crankshaft.
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