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The new system is eight tonnes lighter than the diesel engines and hydraulic motor it has replaced.

As he drove the car around the company's Midlothian factory and demonstrated how it worked, Caldwell told Reuters: "Conventional electric hybrid mixes the power of the engine with that of an electric motor and an electric battery. In contrast our hydraulic hybrid mixes the power of the engine with a hydraulic motor  and a hydraulic battery, which is otherwise known as an accumulator, which stores the energy of braking the vehicle in the form of compressed gas."

This is a car that has a small, ordinary petrol engine sitting alongside a hydraulic motor that runs on compressed nitrogen.

It is a box of steel, 31,000 pounds of it, powered by a hydraulic motor and three big pistons known as hydraulic actuators.

Peugeot's Hybrid Air system, developed with German supplier Robert Bosch, will use a separate hydraulic motor driven by nitrogen compressed by energy recovered from braking.