Aero - 1 / Narrow car

Being on the road and traveling slowly in traffic some time ago, it became clear that most drivers travel alone in large cars. This is where the idea was born for a vehicle project which would allow to reduce congestion and increase vehicle efficiency.

The idea

We can think of road traffic as volume flow rate in a pipe which is defined as the cross section multiplied by the flow speed. The cross section here would be the number of cars passing at a defined point and the speed would simply be the traffic speed.

If we consider that the road width and car speed are limited, the only element we can change to increase the flow rate is the number of cars on the road.

If most passengers are alone in their vehicle, it means that most of the car width is unused on the road. What if we would reduce the width of cars to half, we could have the driver cabin and a passenger seating in tandem.

We would then increase the number of cars on the same roads, increase the number of vehicles per parking space and increase the aerodynamic efficiency of the vehicle with a low drag coefficient and low frontal area.  Of course, motorcycles exist but they do not offer the same safety and comfort as a car.

This idea brought the following requirements

-        Design a narrow car about 900mm wide.

-        2 passengers should be able to sit comfortably in tandem configuration.

-        Use similar dimensions to a “standard” car to conserve the crash safety for the occupants.

-        Use the current regulation for light position, passenger location and safety equipment as for current cars.

-        Minimize the drag force.

-        Design a stable vehicle for 0 yaw condition and cross wind condition.

-        Reduce the impact of drag increase with yaw condition.

This project can start a virtuous cycle in car design, as low energy consumption means a small battery size which in turn allows a low weight and lower cost. The low weight and thinner tires would reduce tire resistance which then increases range further.

Note: Aerodynamics is our speciality, while we have broad knowledge of automotive engineering, electric vehicle development is not our expertise and assumption are made in some areas.

Initial exploration from aerodynamics

Using CFD calculation, our initial design exploration brought us to a 3-wheel vehicle design with 2 in wheel motors in the front.

The drag coefficient is low and frontal area is reduced to only 1m2 (compared to 2,2 – 2,6m2 for modern vehicles).

In comparison to for example the current lowest drag production car, the Mercedes EQS with a Cd.A of 0,502m2, this concept would have about 70% less drag force.

The low drag allows for a small amount of batteries (around 15kWh) that can be placed under the driver's feet.

Results of the design exploration

Dimensions

The internal dimensions (length / height), front / rear seat position and crash structure dimensions are similar to a Volvo C30.

The cabin is lowered to reduce frontal area without compromising passenger comfort.

Battery

The battery module is based on the one from the Peugeot e208.

Peugeot uses 18 6s2p modules for a total of 50 kWh (45kWh usable) and a weight of 350 kg.

Each module has a dimension of 390 x 150 x 115 mm.

We are using 5 of these modules for a total of 13.9 kWh and 97 kg. The 5 modules can fit under the drivers’ feet.

Motor

For this project we do not need high power thanks to the low drag and limited top speed from the tires.

Mass

The weight is kept low with an estimate at around 800 kg. With the in-wheel motors in the front wheels and the batteries under the driver seat, the weight distribution is estimated at 62% front similar to a front wheel drive hatchback car.

Tires

Readily available low resistance tires are selected for the front and rear axles.

The front wheels have a large diameter rim for the in-wheel motors while the rear wheel has a smaller diameter to increase cargo space.

The wheel width is selected to match the weight distribution which gives us a larger rear tire.

Design

The rear features a vertical fin for lateral stability under cross wind condition and a horizontal plane to decrease lift on the rear axle. The horizontal plane allows for the integration of indicators and brake lights.

The headlights and taillights positions are according to the European regulation.

The front licence plate is a standard European licence plate while the rear uses a motorcycle size plate.

The car features a gullwing door to reduce the footprint necessary to open the door in tight parking.

In green is the battery under the driver seat, in orange the front crash structure, in pink the seat position.

The blue outline represents a generic SUV shape.

The Aero - 1 is 900 mm wide which allows more vehicles on the road and increases the amount of parking spots available by fitting two cars in one spot.

When placed next to a standard size vehicle, the Aero - 1 is particularly small. At 1330 mm high, it is only 100mm taller than a Mazda MX5

CFD results at 0-degree yaw

The Mercedes EQS is currently one of the lowest drag cars available for purchase with a Cd.A of 0,502 m2. The Aero 1 has 70% less drag force than the Mercedes.

The drag coefficient is comparable to the Volkswagen L1 concept car with a Cd.A of 0,159.

Surface static pressure image of the car front shows good suction areas at the front and windshield.

The rear of the car shows a pressure increase which helps to reduce drag.

Slice colorised with velocity magnitude shows little disturbance around the front wheels and tail.

Cross wind conditions

We rarely have a pure 0-degree yaw wind on the roads; it is therefor important to look into cross wind conditions.

Front / rear lift is important for driving dynamics as are front / rear side forces for a neutral behaviour in cross wind. Ideally the distribution centre of both vertical and side forces should be applied slightly behind the centre of gravity to limit any unwanted behaviour.

The simulations showed an interesting result. Usually when a car is facing cross wind, the drag coefficient increases due to less efficient shapes such as a larger impact from the wake at the rear of the car.

In the case of Aero - 1, the drag coefficient is reduced with cross wind due to the shape of the windshield and roof. At large angles, the drag coefficient becomes negative (creating thrust). 

A trade-off of the Aero – 1 shape is the lateral stability in cross wind. The car would tend to steer into the wind requiring compensating input from the driver. To compensate for this effect, a vertical fin is fitted on the back.

A consequence of the vertical fin is the increase in side force. As seen by the side force coefficient, the fin increases side force but is still equivalent to the SUV design.

Still the side force is much smaller than the tire tangential friction force and there should be no risk of slipping sideways in cross wind.

The vertical fin increases drag at 0-degree yaw while it decreases drag with yaw.

A second impact of the vertical fin is the increase in rolling moment. 

This moment is important for rollover calculation.

The horizontal plane on the rear deck is creating downforce on the rear axle even in crosswind due to the low-pressure underneath and the high pressure above it. This allows for a lift distribution around the centre of gravity.

Similarly, the vertical fin moves the centre of pressure rearward and improves cross wind stability.

The results show that some more work could be done on the vertical and horizontal fins to further improve stability.

What do these results mean for the energy consumption?

We can estimate the yaw angle on the road based on wind speed, wind angle and vehicle speed.

-        Vehicle speed increases -> yaw angle decreases

-        Wind angle or wind speed increases -> yaw angle increases.

We take 2 scenarios to compare Aero – 1 to a generic SUV:

-        Car = 120km/h / moderate wind at 20km/h (max yaw = 9.6deg)

-        Car = 100km/h / fresh-strong wind at 40km/h (max yaw = 23.6deg)

The graphs of drag coefficient vs wind angle show a decrease in the drag coefficient of Aero – 1 for any wind angle while the SUV increases.

In the 2nd scenario, Aero – 1 drag becomes negligeable at 110deg wind angle.

By limiting the impact from wind on Aero – 1 design, the range is not as affected by wind conditions increasing its efficiency.

Range estimate

We can estimate and compare the range of Aero – 1 to the generic SUV at highway speed of 120km/h. For this we need the following data:  

The power consumption and range estimate does not take into consideration the accessories consumption as they are unknown.

Aero - 1 has 82% lower consumption than the generic SUV and allows to reach an equivalent distance with an 82% smaller battery.

Range is highly dependent on the travelled speed (not taking into consideration energy recuperation)

Reducing the speed to 90km/h, the range increases to 476 km or an increase of 149 km.

Performances

The top speed is limited by the motor speed + gear ratio at 161 km/h and the front tires rated to 160 km/h.

In theory, due to the low drag, the car could reach 284 km/h if we would use a different gear ratio and more appropriate tires.

Acceleration and braking are dependent on the friction coefficient, a value of 1 for dry asphalt conditions is used for the calculation.

The high motor torque and gearbox ratio coupled with low mass and low drag allows very quick acceleration.

The top speed is limited by the motor.

Charging

The Peugeot battery modules are 6s2p. With the selected 5 modules, the maximum charging power is 27,8 kW.

On a 22 kW home charger, the battery can be recharged in 36 min, which equals 9,1 km/min if traveling at 120 km/h. 

Using the Peugeot module specifications, the charging limit is a maximum of 11kW on a home charger which would increase the charge time to 1h12.

At full charging power, the charge time lowers to 28 min which equals a speed of 11,5 km/min.

What about solar?

Using the bonnet and roof we can add about 2,4 m2 of solar panels.

Using a solar radiance in the Netherlands with a yearly average of 1000 kWh and a solar panel efficiency of 20% we can get a 10% range increase, which equals 33 km at 120 km/h.

In the summer time with a radiance of 1700 kWh and reducing the speed to 90 km/h, the added range is 82 km per day.

Running cost

If we take a European average of 0,3€ / kWh, charging the Aero - 1 cost 4,17€.

Travelling at 120 km/h, the cost comes to 1,28€ / 100km.

Conclusion

Aero – 1 might be an odd car in the sea of SUVs currently flooding the market but it brings efficiency together with a similar level of safety for the occupants.

This car was primary designed for people going to work by car alone in their car everyday. The narrow size allows more vehicles on the roads therefore reducing the risk of traffic congestion while maintaining the driver and passenger comfort.

The dimensions together with the gullwing door allows to park 2 cars on a regular parking space, doubling the parking capacity.

On top of Aero – 1 high energy efficiency, the small battery means less resources are needed reducing environmental impact even further. A 77-kWh battery used in the generic SUV would have the possibility to power 5 to 6 Aero – 1 vehicles.

In the early 2000's, several manufacturers showed similar concepts but these disappeared quickly. Introducing an electric powertrain with the ease of home charging / solar and low maintenance might be the future solution that would bring high efficiency back.

Volkswagen presented the 1-litre concept in 2002 (1,25m wide) which ended up as the costly production version XL1 in 2011 (1,67m wide).

Honda had the 1st generation Insight (ZE1) from 1999 to 2006 (1,7m wide) which was never replaced.

What we have shown here would need further refinement and optimization but it shows a good basis for a different concept.

The small battery alone reduces the mass and the cost; the system turns into a virtuous cycle with smaller and lighter components necessary for the car.

Aero – 1 in a few numbers