Citroen has released details of its fuel-efficient C4 Cactus AIRFLOW 2L concept and suggests many of its features could make it onto regular C4 models in the medium term.

LESS THAN A DAY after Renault revealed details of its EOLAB concept, spruiking 1.0L/100km fuel efficiency, Citroen has released details of its own fuel-efficient concpet, the C4 Cactus AIRFLOW 2L concept to be revealed in full at the 2014 Paris Motor Show.

Like the EOLAB concept, the C4 Cactus AIRFLOW 2L concept is full of clever innovations by Citroen’s boffins to try and make the cars we drive, well, now, more fuel efficient. Similar to the EOLAB but perhaps not quite as radical looking, or should we rather say, less like a concept car, the C4 Cactus AIRFLOW 2L concept returns 2.0L/100km using:

• tweaked design with a 20% improvement in aerodynamics;
• low rolling resistance “tall and narrow” tyres;
• lighter parts to reduce overall vehicle weight by 100kg;
• Hybrid Air technology, which cuts fuel consumption by 30% when compared to the garden-variety C4 Cactus.

Like the EOLAB concept, the C4 Cactus AIRFLOW 2L concept was born out of the, and take a deep breath, “2L/100 km vehicle programme set up by the Plateforme de la Filière Automobile industry group in France”. This group set car makers the task of “reducing the impact of vehicle running costs on household expenditure and to reduce the eco-footprint of car travel”.

Citroen says it took up the challenge with its newest model, the C4 Cactus because it best lent itself to what the brand wants to achieve: “more design, more comfort and more useful technology, combined with an affordable cost of ownership”.

Exactly like the EOLAB, Citroen concentrated on three areas of development: aerodynamics; lightweight materials; and hybrid technology.

Aerodynamics explained

On the C4 Cactus AIRFLOW 2L, Citroen’s boffins modified some of the existing design elements and built new ones to optimise vehicle aerodynamics. These include:

• Variable-geometry components:
• The new front bumper features three air intakes that continuously adjust in accordance with vehicle use, both for engine cooling and to regulate air flow over the vehicle;
• Mobile side deflectors have been added to effectively guide the air flow around the vehicle (see pictures); and
• The wheels feature mobile shutters activated and controlled by centrifugal force.

Fixed-geometry components

• New generation 19-inch low rolling resistance tyres that are tall and narrow. Their design improves both energy efficiency and aerodynamics;
• The wheel arches feature an “Air Curtain”. This is achieved with small aerodynamic slats on either end of the front bumper to channel the air along the wheels;
• The spoiler has been lengthened and an air extractor has been added on the rear bumper in order to reduce the turbulence that can increase drag;
• The conventional door mirrors have been replaced by smaller, slimmer rearview cameras to reduce the impact on air flow;
• The vehicle sub-structure has been entirely streamlined so the air flows smoothly underneath the car; and
• Energy-efficient LED light modules at the front and rear replace the existing lights.

Citroen says these changes result in an overall 20% improvement in aerodynamic performance compared with the production C4 Cactus.

Lightweight materials

• The production C4 Cactus is 200kg lighter than the C4 hatchback, yet the C4 Cactus AIRFLOW 2L concept is a further 100kg lighter (kerb weight:865kg). That means its 11% lighter than the production model.
• To achieve this, the boffins reduced the weight of many structural components, with the body sub-structure featuring new materials:
• Aluminium, for the upper cowl panel, inner side members and rear floor pan;
• High-yield steels, for the front side rails and heel board; and
• Composite materials for the front of the vehicle floor.

Citroen says composite structural parts are a promising development area and will be essential to make cars lighter in the future, but that large-scale production is one of the key challenges to flowing them into car production.

• Carbon-based composite materials have been used for the suspension springs, tailgate, rear bench, side panels, roof, roof cross-members, wings and doors. On the lower side sill, wheel arches and the lower part of the front bumper, the “textured” look of the carbon brings out the matt appearance of these parts, providing an attractive contrast with the pearlescent appearance of the surrounding features;Aluminium is used for the engine cradle. The bonnet specification is the same as for the production C4 Cactus, which already uses aluminium;
• Owing to their significantly lower bulk and density (around 2700kg/m3 for aluminium and around 1200 kg/m3 for carbon, compared with 7800kg/m3 for steel), these materials contribute significantly to reducing overall vehicle weight;New processes to reduce the thickness of the tubes and cups in the exhaust system; and
• Translucent polycarbonate, for the panoramic sunroof. This material is even lighter than multi-layer glass, but has the same properties in terms of thermal and acoustic insulation and ultra-violet filtering capability.

Hybrid Air technology

The C4 Cactus AIRFLOW 2L concept uses the same Hybrid Air engine from the Citroen C3 shown at the 2013 Geneva Motor Show. This is a 3-cylinder PureTech 82 petrol engine (this appears on the production C4 Cactus), “a compressed air energy storage unit, a hydraulic pump/motor unit and an automatic transmission with an epicyclic gear train”.

Three operating modes are available:

• Air power (zero emissions) where the compressed air motor takes over from the petrol engine (the two compressed air tanks are mounted at the rear of the vehicle;
• Petrol power, using only the internal combustion engine; and
• Combined power, drawing upon both the combustion engine and the compressed air.

According to Citroen, the PureTech 82 engine has been optimised for this new compressed air hybrid drivetrain. Friction losses, which account for 20% of the power consumed by the engine, have been reduced by using a Diamond-like carbon coating; making moving parts lighter; and using bearings to guide rotating parts. Further improvements were made by adopting new polymer pads and using very low viscosity oil. Overall engine efficiency has been improved by 5%.

While Renault’s EOLAB concept is a future glimpse of what a fuel-efficient car might look like, Citroen says the C4 Cactus AIRFLOW 2L concept offers enough real-world technology that “It could therefore be possible, in the medium term, for a production vehicle such as C4 Cactus to reach this target [2.0L/100km]”.