Paris, France - the Citroën C4 Cactus Airflow two-litre concept, scheduled to premiere at the Paris motor show in a couple of weeks, does not, as you might expect from its name, have a two-litre engine.
It's a lightweight hybrid, using petrol and compressed air as power sources, and it's a runner; it gets its name from the maker's claim that it will complete the EU revised cycle on less than two litres of petrol per 100km.
To do that it is 100kg lighter than the production Citroën C4 Cactus, itself 200kg lighter than the C4 hatchback on which it's based, thanks to an aluminium upper cowl panel, inner side members and rear floor pan, high-strength steel front side rails and heel board, and composite floor-panels.
Carbon-fibre is used for the suspension springs, tailgate, rear bench, side panels, roof, roof cross-members, wings and doors, as well as the lower side sill, wheel arches and the lower part of the front bumper, its characteristic woven texture providing an attractive contrast with the pearlescent finish of the painted sections.
The bonnet and engine cradle are aluminium, the panoramic sunroof is made of translucent polycarbonate and even the Cactus' signature Airbump panels are made of carbon fibre.
The whole car weighs just 865kg ready for the road
GOING WITH THE FLOW
The front bumper has three air intakes that continuously adjust themselves according to the engine temperature and the car's speed through the air, while the wheels have self-adjusting shutters controlled by centrifugal force.
Small aerodynamic slats on either end of the front bumper channel the air along the wheels to create an 'air curtain', smaller, slimmer rear-view cameras in place of conventional door mirrors, a longer spoiler and an air extractor on the rear bumper to reduce turbulence.
The whole underbody has been streamlined so the air flows smoothly under the car and LED light modules at the front and rear replace the existing lights. Consuming little power, these new modules save energy and benefit fuel economy.
Alongside a three-cylinder PureTech petrol engine (tweaked for this application with diamond-like carbon coatings, lightened reciprocating components, needle roller bearing in place of plain bushes and very low-viscosity oil), there's a hydraulic pump/motor unit and automatic transmission with an epicyclic gear train; in the rear there are two compressed-air tanks made of composite materials.
An electronic management manages driver input to deliver the best use of combustion energy (which is, in effect, the car's only source of power input), alternating as it sees fit between petrol power, using the only the combustion power, air power, using only compressed air stored in the tanks, or a combination of the two.
Of course it takes energy to compress the air in the first place - but a lot of that energy comes from gravity, when running downhill, and from braking - energy that would normally be wasted as heat - which is how the concept achieves its impressive fuel consumption.
What's even more impressive, however, is that Citroën says it should be possible, in the medium term, for a production car to achieve the same fuel-consumption.