Porsche may not be the first name that springs to mind when you think of hybrids, but the company has a growing portfolio of eco-friendly Earth savers including, as of this week, the 918 RSR concept.

The 918 RSR combines elements from two of Porsche's previous hybrid supercars. It uses the same gorgeous body shape as the epic 918 Spyder hybrid (albeit this time with a roof) and is powered by the ludicrously potent engine and motors found in the 911 GT3 R Hybrid race car.

Most of the 918 RSR's grunt comes from a 563bhp mid-mounted V8 petrol engine that drives the rear wheels. A further 204hp is produced by a pair of electric motors connected to the front wheels, giving the 918 RSR a total power output of 767bhp.

Unlike the Porsche 918 Spyder, the 918 RSR's motors aren't powered by lithium-ion batteries. Instead, power generated during braking is stored in a 150kW flywheel accumulator -- a spinning wheel that briefly stores kinetic energy before unleashing that power, on demand, to the electric motors.

The power stored in the flywheel can be used to produce a sudden surge in acceleration or to reduce fuel consumption without significantly reducing the level of performance compared to using the petrol motor alone.

When fully charged, the flywheel can power the electric motors for 8 seconds at a time -- ideal for one long straight per lap, or for accelerating briefly out of multiple slow corners.

The use of the flywheel has a couple of advantages over battery hybrid systems. Flywheels can be lighter than equivalent batteries, are not affected by changes in temperature and don't suffer from memory effect -- the phenomenon that causes batteries to lose their maximum energy capacity after repeated recharging when only partially discharged.

There are drawbacks, too. The 918 RSR's flywheel is positioned where the passenger seat would have been, so this car is strictly a single-seat affair. Also, the flywheel rotates at up to 36,000rpm and spinning it any faster could cause it to explode and shatter into a million tiny pieces, with each shard flying faster than a speeding bullet -- not ideal when the thing's sat next to you.

The Porsche RSR 918 is still in the concept stage and so isn't available to buy. If you want a slice of this high-performance, low-emissions action, you should keep an eye on the 918 Spyder on which it's based. The car doesn't have an official release date as yet, but it is green-lit for production.

Peep the Porsche RSR 918 pictures in our gallery above.

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