You may have an air compressor pump on your car – perhaps a portable unit to inflate your tyres when you’re carrying a heavy load, or to pump up a flat in an emergency.

But the Hybrid Air system from PSA Peugeot Citroen puts an air compressor pump at the very heart of the powertrain, creating a hybrid engine that, thanks to its primarily mechanical design, the manufacturers say is highly reliable.

In principle, it works similarly to kinetic energy retrieval systems; when the vehicle brakes, energy is recovered from the braking process, and used to compress air into a cylinder, rather than to recharge a battery.

When the vehicle is running in zero-emissions mode, it is this compressed air which is released in order to accelerate, rather than burning any petrol.

According to PSA Peugeot Citroen, the powertrain can run in zero-emissions mode for up to 80% of the time in city environments, with urban fuel savings of up to 45%.

Even under gasoline-powered operation, sound levels are reduced, 5% less fuel is consumed, and the compressed-air cylinder is still replenished under braking.