Vauxhall-Opel is unveiling a new diesel hybrid concept version of its Corsa supermini at September's Frankfurt motor show.The concept car, which is based on the already frugal 74bhp 1.3-litre CDTI, has a newly developed stop-start system that cuts the engine when the car is stationary. The Corsa Hybrid uses a belt-alternator/starter motor, a powerful lithium-ion battery, regenerative braking and an automatic gearbox.As soon as the car comes to a complete halt, the engine is cut, but it is restarted as soon as the driver lifts their foot from the brake pedal. GM says the belt-alternator also provides a torque boost when extra power is needed, such as when accelerating hard, though no figures are currently available. When braking, generators transform the car's kinetic energy into electrical energy.In 74bhp form, the Corsa has already been tweaked to produce only 119g/km of Co2 and manage 62.8mpg combined. The Hybrid cuts that to 99g/km and 75mpg.