During periods of high demand, the heat pump runs in reverse as a heat engine, expanding and cooling the argon and generating electricity.

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The system is like a clever version of a car radiator, but in reverse, keeping the air rushing into the engine hot on the inside, and cool on the outside.

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Ford's first attempt at a genuine four-wheel vehicle, which he called the Quadricycle, used valves adapted from a steam engine and didn't have a reverse gear.

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What rattles the markets is how slumping exports might combine with weaknesses elsewhere in the economy to throw the lone engine of domestic growth, business investment, into reverse.

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