Meanwhile, auto makers are proving there's still plenty of potential in the gas-powered engine to meet customer's fuel-efficiency demands.

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It is a cleaner, more fuel-efficient engine than the gas-guzzling original, which the carmaker stopped producing in 1971.

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It is powered by a hydrogen fuel-cell engine, which converts hydrogen gas into energy.

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When the battery is depleted (or when driven at very high speeds), the premium-gas internal combustion engine powers the car, but its fuel mileage suffers from having to shove around 400lbs of batteries.

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