Researchers have published plans to develop a thermoelectric generator which converts heat from car exhaust fumes into electricity.

Boffins at the Fraunhofer Institute for Physical Measurement Techniques said that two-thirds of the fuel's energy is emitted in the form of heat.

About 30 per cent is lost through the engine block, and a further 30 to 35 per cent as exhaust fumes.

To use this wasted energy the scientists are developing thermoelectric generators that convert heat into electrical energy by making use of a temperature gradient.

"The temperatures in the exhaust pipe can reach 700 degrees Celsius or more, " said Dr Harald Böttner, head of the Thermoelectric Systems department at Fraunhofer.

"The temperature difference between the exhaust pipe and a pipe carrying engine cooling fluid can thus be several hundred degrees Celsius."

The thermoelectric converter makes use of this huge differential. Driven by the flow of heat between the hot exhaust fumes and the cold side of a coolant pipe, the charge carriers pass through special semiconductors, thus producing an electric current similar to a battery.

Dr Böttner predicts that the device could cut petrol consumption by between five and seven per cent.