Background. EOLAB forms part of the “fuel consumption of 2 litres/100 km for all” plan introduced within the framework of France’s New Industrial Plan. However, EOLAB goes further than the fuel consumption target set by the French government since it sets its sights on the much longer term. In the course of the prototype’s design, Renault developed the technologies necessary for the introduction of a car with fuel consumption of just 2 liters/100 km by 2020.

To achieve this, Renault collaborated with French automotive industry partners such as car glass manufacturer Saint-Gobain, seat supplier Faurecia, tire company Michelin and Continental (brake system). Renault also worked closely with major partners such as the Korean company Posco (magnesium components).

The Z.E. Hybrid system. The Renault gasoline-electric plug-in hybrid system supports 60 km of electric range plus extended range due to the gasoline-fueled internal combustion engine. Renault said that the system, which can be carried over to entry-level vehicles, will be incorporated in the range by 2020.

For the EOLAB prototype, the Z.E. Hybrid powertrain combines a small three-cylinder 999cc gasoline engine with a power output of 57kW (75hp) and peak torque of 95 N·m (70 lb-ft), with a novel clutch-less transmission based on a compact and economical three-speed gearbox sufficient to cover the vehicle’s speed range.

This is a notable advantage compared with the majority of the market’s hybrid technologies which make use of CVT- or DCT-type transmissions (Continuous Variable Transmission, or Dual Clutch Transmission), said Renault. These tend to be bulky, heavy and costly, and consequently ill-suited to small cars.

The chief feature of the concept lies in the clutch casing which houses a permanent magnet electric motor (axial flux discoid motor in the case of the prototype). This motor is compact and economical and covers the need for the availability of extra torque since it is capable of instantly delivering peak power of 50 kW and 200 N·m (148 lb-ft) of torque.

Z.E. Hybrid system. Click to enlarge.

Fed by a 6.7 kWh battery, the motor is sufficient to power the light weight car under electrical power. As a lighter car needs less energy to be moved forward, the battery can be both smaller and less expensive.

The first two ratios of the three-speed gearbox are mated to the electric motor, while the third ratio is linked to the ICE. These three ratios permit nine different combinations for the electric and hybrid modes combined.

One of the concept’s major innovations is the fact that gearshifts do not necessitate the use of a clutch thanks to a specific control unit designed by Renault’s engineers.

The lithium-ion battery differs from the batteries that equip Renault’s Z.E. range of electric vehicles. While electric vehicles are designed to store a high amount of energy because of the fundamental need to maximize the vehicle’s range, hybrid vehicles like EOLAB need to cover the same power requirement with a limited quantity of electrical energy. This entails using a different type of battery cell with a higher power/energy ratio.

The resulting battery pack is the fruit of active cooperation between the teams at Renault and the CEA who rose to the challenge of finding smart solutions to come up with a battery pack that is capable of storing a high amount of energy in a smaller volume, while at the same time minimizing weight.

Aerodynamics combined with active systems. OLAB’s CdA is 0.470 m² (A = 2.00m²/ Cd = 0.235) which represents an overall reduction of 0.200 m² (around 30%). (tThe measure of a vehicle’s overall aerodynamic efficiency is its CdA—the vehicle’s frontal section (A) expressed in square meters multiplied by its drag coefficient (Cd).)

This lower drag coefficient results in a significant fuel consumption reduction at higher speeds. At a stable speed of 130 km/h (81 mph), for example, it accounts for a fuel consumption saving of 1.2 liters/100 km in comparison with the benchmark vehicle.

Renault achieved the aerodynamic efficiency by combining several factors:

• An innovative vehicle architecture which includes a narrower rear track and a lower roofline without detracting from cabin space.

• Carefully designed rear body panels.

• Incorporation of active systems for even greater aerodynamic efficiency.

EOLAB Aerodynamic elements. Click to enlarge.

EOLAB’s front bumper is equipped with an active spoiler that lowers by 10 cm at speeds in excess of 70 km/h (43 mph) in order to restrict airflow beneath the car. Even when a vehicle is designed with a flat underbody, there are still a number of asperities that can detract from aerodynamic efficiency.

Another mobile feature of EOLAB’s aerodynamic package is the 40cm x 10cm vertically-positioned flaps that are visible on the rear bumper, rearward of the rear wheels.

At speeds in excess of 70 km/h, these flaps open by 6cm in order to ensure that as much of the airflow as possible stays attached to the vehicle as it moves forward. Without this solution, passing air has a tendency to become detached from the vehicle’s sides too early after passing the rear wheels and this has a negative effect on drag.

In the open position, airflow remains attached to the car as far rearward as possible, right to the trailing edge of the bumper.

These flaps tauten the airflow and prevent turbulence which otherwise acts as a sort of aerodynamic brake.

—William Becamel, the aerodynamics expert who worked on the project

The EOLAB took a close look at the wheels, too. In a perfect aerodynamic world, the rims would be covered and smooth; this is rarely the case, however, essentially for design- and brake cooling-related reasons. The team designed a system whereby the rims are covered whenever the brakes do not need to be cooled, thereby reconciling design and aerodynamic efficiency considerations. The system is controlled by a temperature sensor built into the rims.

To further perfect EOLAB’s aerodynamic performance, it is fitted with particularly narrow, 145mm-wide tires—40mm narrower than the smallest tires available for the Clio IV. Michelin and Renault’s designers worked on the tread pattern to give a visual impression of width, while the sidewalls were designed to exude an impression of light weight.

At the same time, tire supplier Michelin optimized the casing and tread to minimize rolling resistance while maintaining excellent levels of safety and performance. The tires’ rolling resistance is 15% lower than those of the Clio IV which itself is quite strong in this area. These tires are also lighter, and their profile has been honed to minimize drag.

Meanwhile, Renault has changed the technology used for the wheel bearings in favor of a more efficient solution that has achieved a CO₂ emissions saving of 1g/km.

Body shell. EOLAB weighs 955kg (2,105 lbs). That’s more than 20% lighter than the benchmark vehicle. Total weight savings amounted to 400 kg (882 lbs) due to the teams’ holistic approach.

While delivering the same level of performance, lighter vehicles require less energy to move forward. One of the chief focuses in the case of the EOLAB project consequently consisted in reducing the vehicle’s overall weight. This in turn meant that the vehicle’s principle assemblies (powertrain, brakes, running gear, cooling system, fuel tank, etc.) could also be lighter, and these additional savings compounded the initial groundwork.

The savings achieved by taking a fresh look at the size of the different mechanical assemblies enabled us to cover the cost of using more expensive materials and technologies elsewhere without losing sight of the aim not to add to the overall cost.

—Laurent Taupin, EOLAB Project Leader

Because it is lighter and more streamlined, EOLAB needs less energy to be moved forward. It was therefore possible to revise the amount of power necessary to drive it in order to arrive at a result that ensures running costs comparable with those of a current vehicle, without scrimping on acceleration performance. Equipped with a battery of just 6.7kWh (total energy), EOLAB supports an all-electric range of 60km at speeds of up to 120 km/h.

To reduce a car’s weight, a well-trod path is to replace steel with lighter materials. All-aluminium and aluminium/carbon solutions already exist but, in addition to the high cost associated with such materials, they often necessitate an in-depth review of assembly and production processes, and there is a price to pay for that, Renault noted.

You can always save weight if you’re prepared to pay the price, but that would be contrary to Renault’s philosophy. Our strategy is to reduce weight in a way that benefits everybody. That means finding economically viable solutions that our customers can afford. Our approach can be summed up by the phrase: “the right material for a given job”.

—Laurent Taupin

The Renault team designed an innovative body shell that combined different materials selected as a function of their weight, cost and necessary production processes. EOLAB’s body shell consequently combines steel, aluminium, magnesium and plastic composites.

Materials in the body. Click to enlarge.

Renault turned to Very Very High Elastic Limit (VVHEL) steels which have a yield strength of between 1,200 MPa and 1,500 MPa, an improvement of between 200 MPa and 500 MPa over the VHEL steels used for current Renault models. With a tensile strength of up to 150kg/mm², these grades were employed wherever their use served a real purpose, notably for the front part of the cabin. Their production requires a hot stamping process which Renault already uses for its production cars.

EOLAB’s shell also features a significant proportion of aluminium in different, readily available forms such as sheeting, castings and profiles. Like steel, upgrading the mechanical properties enables thinner grades to be used with no detriment to a part’s function but with an additional weight saving over conventional aluminium. Here again, these grades were selected in cases where it was possible to optimise the trade-off between function, mass, cost and the required manufacturing processes. Some of them necessitated a new warm stamping process (approximately 250/300°C, compared with hot stamping which is closer to 900°C) but this was compatible with existing stamp shop facilities.

A combination of castings, sheet metal and profiles was chosen for EOLAB’s all-aluminium rear circumferential chassis member. Their use represented a good compromise between mass and body rigidity. Moreover, using the same material for this part of the shell was an attractive proposition in terms of thermal expansion. Indeed, body shells are exposed to temperatures of up to 180°C in the paint shop curing ovens and certain multi-material assemblies can have a negative impact on body shell’s geometry.

Advantage was also taken of the use of cast parts of varying strength to cover multiple functions—i.e., the use of large, one-piece components to replace a number of smaller parts. This made it possible to compensate for the technology’s additional cost compared with conventional stamping.

While using steel and aluminium for a car body is itself not usual, Renault went further down this path in the case of EOLAB by making use of magnesium, too. This light weight metal (density: 1.7) is prone to corrosion which is why it has only been used for interior components until now. Some cars already feature magnesium but only for moulded parts made from magnesium powder (e.g. the majority of the market’s steering wheel armatures).

Renault said that EOLAB breaks new ground in the automobile industry by using magnesium sheeting. Developed thanks to the expertise of the specialist supplier POSCO, this innovation permitted the production of bulky items such as the roof, cowl panel and parts of the seat frames. As a result, EOLAB’s roof tips the scales at just 4.5kg, compared with 10kg for an equivalent steel roof.

Magnesium sheeting cannot be used for as wide a range of parts as steel, or even aluminium. However, where the compromise between function, mass, cost and manufacturing processes allows, there are spectacular savings to be achieved, especially as the manufacturing process is the same as it is for certain new grades of aluminium, i.e. warm stamping.

—Vincent Desmalades, Deputy Project Leader in charge of Processes

In order to cover as many realistic solutions as possible to minimize EOLAB’s weight, the project team also took a look at polymers. Assisted by university laboratories and specialist suppliers, the team behind EOLAB worked on a new family of thermoplastic resins which are easier than thermoset resins to recover at the end of the vehicle’s life cycle and consequently recycle.

The demonstration car’s front, rear and central floor pans, B pillars and lower cross member are all made from hot-stamped composite thermoplastic, while the skins of the one-piece wings/hood assembly and doors are made from injection moulded thermoplastic. The type and proportion of the glass fibers mixed with these polymers vary as a function of the properties required for each end-use (e.g. longer fibers for structural components).

Ultra-light chassis. EOLAB’s designers also focused on the car’s main assemblies with a view to saving weight. The brakes, steering, suspension, wheel and tires were all looked at carefully to shave off kilograms without penalizing performance.

A vehicle’s running gear alone accounts for almost 20% of its weight. The most efficient solution involved replacing steel with aluminium for a significant number of heavy parts, such as the subframe, which has shed 5.3 kg (11.7 lbs) compared to that of the Clio IV, suspension arms (1.8 kg, 4 lbs saved), hub carriers (5 kg, 11 lbs saved) and rear arms (9 kg, 20 lbs saved).

EOLAB’s designers took a different approach to the architecture’s resistance to head-on impacts while maintaining the same overall level of safety performance. Normally, programmed chassis deformations affects three areas in an impact: the upper part of the chassis, the side members and the lower part of the chassis.

In the case of EOLAB, the latter’s add-ons were eliminated but this was compensated for by relocating and strengthening the side members. The resulting saving was between seven and eight kilos.

We also looked at making the springs from a glass fibre/thermoset plastic composite, which would have produced a saving of three kilograms. Using a different composite for the rear beam would have saved another three kilos or so, too.

—Laurent Taupin

For the brake system, Renault and Continental developed three advanced innovations. The most visible contribution was to suppress disc rub, whereby the brake pads remain in contact with the discs even when there is no pressure on the pedal. This constant rubbing adds slightly to fuel consumption. In the case of EOLAB, the pads do not touch the discs whenever the driver’s foot is off the pedal. In addition to being beneficial to fuel consumption, this also extends pad life.

Ultra-light chassis. Click to enlarge.

To maintain the same speed of response, and even improve it, EOLAB’s system eliminates slack by moving the pads closer to the discs the instant the system detects that the driver intends to apply the brakes.

Furthermore, the system’s new brake control unit has replaced the control units of several previously distinct functions (ABS, ESP, emergency brake assist) while allowing decoupled braking which is indispensable when it comes to recovering braking energy to reduce energy consumption.

Meanwhile, the front brake discs have been lightened by combining steel and aluminium. The part of the discs in contact with the pads is made of steel, while the central part is made of aluminium. Also, since the body shell is lighter, it was possible to reduce the diameter of the discs for a total saving of 4.7 kg (10.4 lbs).

A similar solution was employed for the rear drum brakes. The friction zone is still made of cast steel but the remainder of the drum is made of aluminium. The lighter rear brakes are more compact and equipped with more efficient bearings for a saving of 8.5 kg (18.7 lbs).

Also in partnership with Continental, Renault took advantage of this arrangement to make the automatic parking brake act on the rear drums. This is more economical than the market’s existing system. Suppressing the manual mechanism (lever, cables, etc.) adds another 1.3 kg (2.9 lbs) to the total weight saving.

Optimized trim and equipment. The EOLAB team also secured appreciable savings by reviewing the car’s glazing. Work to reduce the glass’s weight led to collaboration with Saint Gobain Sekurit with the aim of saving between 30 and 50% compared with present-day standards.

The thickness of EOLAB’s windows was reduced to 3mm (1.5mm less than the current norm), equivalent to the thickness of a pencil line. The thin windscreen, the form of which is particularly aerodynamic, is an automobile industry first. The side windows use laminated glass (instead of tempered glass), while certain non-moving windows make use of polymers which have become a widespread material for optical glass, but are still rarely seen in the car industry.

Lastly, the rear screen uses polymères vernis (instead of tempered glass). This technology made it possible to produce a one-piece polymère verni screen that also incorporates the rear lights. This solution serves as an example of how different functions can be grouped together to not only save weight but also enhance aerodynamic performance.

Combined, these solutions brought the total weight of EOLAB’s windows down to 21 kg, a 25% saving over a Clio.

EOLAB’s seats also came under the scrutiny of the car’s designers who not only endeavored to make these generally heavy items lighter but also sought to make them slimmer to free up extra room for rear passengers. To achieve these two objectives, Renault called on the expertise of the

Working with seat specialist Faurecia, Renault was able to reduce the thickness and weight of the front seats by using different materials for the frames (steel, non-ferrous alloys such as aluminium, carbon fiber composite and magnesium). The result was a 35% saving over the seat frame of a conventional B-segment vehicle.

The cushion structure was optimized thanks to the use of a semi-rigid shaped trim and a compliant seat back. The resulting seat is 30% more compact, and this has freed up extra leg and knee room for rear passengers with no detriment to the comfort enjoyed by the front occupants who can still adjust the position of their seats.

The seat shells were lightened using Faurecia’s Cover Carving technology which consists in embedding a rigid 3D pattern onto a textile support to enable the cover to match the seat back’s forms as closely as possible. The use of this technology shaved 40% off the weight of the seat’s shell and provided rear passengers with three centimeters of extra room for their knees, legs and feet.

Renault also paid close attention was paid to EOLAB’s smaller plastic fittings, such as the B-pillar trim. Although these part may only weigh a few hundred grams each, they add up to about 10 kg in the case of a production car if you include the trunk lining, said Taupin.

With input from suppliers, Renault’s specialists looked at alternative ways to make these plastic parts. One idea was to replace solid parts less than 2.5mm thick with parts comprising an ultra-thin skin (1.8mm) and injected with foam which is lighter because it contains air bubbles. Thanks to the incorporation of ribbing, they boast the same structural strength. Until now, this technique has been restricted to large visible parts or parts that customers cannot see. In the case of EOLAB, the challenge was to find a process that would be appropriate for smaller visible parts.

Meanwhile, for EOLAB’s trunk trim, Renault used another experimental technique which again uses a sandwich structure comprising a foam insert between the two outer surfaces. This sandwich is obtained by slightly opening the mould during production to allow the foam to spread.

Use of thinner fittings achieved a weight saving of between 20% and 30%.

Air intake ducts are generally made of compact polypropylene and weigh about 3 kg (6.6 lbs). The idea was therefore to replace this material with expanded polypropylene which is much lighter since its density is 0.06 compared with 0.96 in the case of compact polypropylene. However, if it had simply been a case of swapping one for the other, the switch would have been made a long time ago. In reality, it calls for duct walls that are five times thicker and that in turn means taking a fresh look at the car’s architecture. This is effectively what happened in the case of EOLAB and its ultra-light air ducts have been patented. They weigh 700 grams (1.5 lbs)—a savings of 2.3kg (5 lbs).