The impact of frontal area on vehicle propulsion energy consumption

Michal Sura [email protected]

The EU aims to be a climate-neutral economy by 2050, with net-zero greenhouse gas emissions. Carbon neutrality means achieving a balance between emitting greenhouse gas emissions and the emissions that are able to remove the planet's natural absorption systems. Transport represents almost a quarter of the EU's greenhouse gas emissions and therefore these emissions need to be reduced. We must replace petrol and diesel vehicles with zero-emissions vehicles to achieve our goal. Although zero-emission vehicles do not produce any direct tailpipe emissions, there are greenhouse gas emissions associated with their production, and the propulsion energy of these vehicles also still has its own carbon footprint. Ways to reduce the carbon footprint associated with vehicle production as well as propulsion energy consumption must be sought. Reduction in a vehicle’s weight can reduce the carbon footprint associated with vehicle production and reduction in aerodynamic drag of a vehicle can result in substantial propulsion energy savings. In this study, we look at how a vehicle's frontal area affects its aerodynamic drag force and energy consumption.

Aerodynamics is one of the most beautiful and complex aspects of vehicle design. It includes incredibly complex physics and mathematical models. It is a colossal puzzle where it is necessary to reconcile so many things. Designers would like to achieve a good coefficient of air resistance, practicality of design for users, car manufacturability, safety, etc. Because it is the result of many compromises, the final design of the car may differ from the original design. The aerodynamic design of the vehicle is accompanied by many hours spent in the wind tunnel and computer calculations (Figure 1). Minimizing aerodynamic drag can save huge amounts of propulsion energy; therefore, great attention is already being paid to this.

Figure 1.

Aerodynamic drag of the vehicle Fair depends on the air density ??air, the aerodynamic drag coefficient of vehicle cd, the projected frontal area of vehicle Af , and square of vehicle’s velocity v.

Fair = 1/2 ??air cd Af v2

Drag coefficient? - cd - is a dimensionless parameter that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. The real drag coefficient can be obtained experimentally using a wind tunnel.

Frontal area - Af - is the largest cross-sectional area of the vehicle; it is the projected frontal area of the vehicle (Figure 2.)

Figure 2.

In recent years there has been a remarkable growth in the market for Sport Utility Vehicles (SUVs). SUV is a term used to describe vehicles that have, or better say “pretend” to have off-road driving capabilities. Many manufacturers state that the air resistance coefficient cd of their SUV models is 0.30 - 0.35;? even some of them claim to have reached values below 0.30. These values are already comparable to those of typical classic cars. A customer considering purchasing such an SUV may believe that it is a very economical vehicle, because the coefficient of aerodynamic drag is very low. Global SUV sales have increased from around 10 million in 2010 to more than 35 million in 2021, according to data from the International Energy Agency (IEA) (1). Almost all carmakers now sell electric versions of SUVs. It looks like this trend in SUV sales will continue with their electric versions as well.?

Electric vehicles are known for their low aerodynamic drag coefficient, so let's compare the values of aerodynamic resistance of a classic electric mid-size car and an electric SUV. We chose the Tesla 3 and the Audi e-tron for our comparison. The Audi e-tron is highly praised for its low drag coefficient.

The drag coefficient of the Tesla 3 (Figure 3) cd = 0.23 and the frontal area Af = 2.22 m2 (2).

Figure 3.

The drag coefficient of the electric SUV Audi e-tron (Figure 4)? cd = 0.28 and the frontal area Af = 2.65 m2 (3).

Figure 4.

The results show that the Audi e-tron has a significantly higher aerodynamic drag force due to its approximately 20% larger frontal area and 22% higher drag coefficient? (Figure 5).

Figure 5.

At 50 km/h, the aerodynamic drag force of the Tesla 3 is 63 N and the Audi e-tron has 91 N. At 120 km/h, the aerodynamic drag force of the Tesla 3 is 362 N, while the Audi e-tron has the aerodynamic drag force of 526 N.?

The aerodynamic drag force of Tesla 3 increased from 63 N at speed 50 km/h to 362 N at speed 120 km/h. This also explains why the consumption of propulsion energy notably increases when cruising speed is increased.

We found out Audi e-tron has a 45% higher aerodynamic drag force than the Tesla 3 at that speed of 120 km/h. This is due to a combination of a higher drag coefficient and a larger frontal area of Audi e-tron.kWh

When the vehicle is driving at speeds of 120 km/h, the air resistance is responsible for about 75% of its total energy consumption (4) (Figure 6).

Figure 6.

This means that the Audi e-tron must consume significantly more energy (~34%) than the Tesla 3 at 120 km/h due to its size. As can be seen, the weight of the vehicle does not play a very significant role at this speed (Figure 6). Despite the fact that the Audi e-tron has a very good drag coefficient cd = 0.28, its energy consumption at higher speeds increases a factor of its frontal size - frontal area Af.?

Car manufacturers typically publish only the drag coefficient value and almost no information about the frontal area. However, as seen in the case of SUVs, it significantly contributes to higher energy consumption, particularly at higher speeds.

The real life measured energy consumption of the Tesla 3 is 19 kWh and the Audi e-tron is 26.1 kWh (5). As can be seen, the consumption of the Audi e-tron, based on measured values, is approximately 37% higher than that of the Tesla 3.

When we compare the curb weights of the Tesla 3 (~1700 kg) and the Audi e-tron (~2500 kg), the Audi e-tron has a curb weight nearly ~50% higher than the Tesla 3. It is very likely that its production is associated with ~50% more CO2 emissions than the production of Tesla 3.

As we mentioned, the general trend of almost all car manufacturers is to produce electric SUVs. The question is whether this trend is sustainable given the higher emissions linked with their production and the higher energy consumption associated with their operation.

References:

1,https://www.iea.org/commentaries/global-suv-sales-set-another-record-in-2021-setting-back-efforts-to-reduce-emissions

2, https://arxiv.org/pdf/1908.08920.pdf%5D

3, https://avtotachki.com/en/unikal-nyy-cw-vsego-0-28-dlya-audi-e-tron/

4, https://www.sciencedirect.com/book/9780750650540/the-automotive-chassis

5, https://www.spritmonitor.de/en/evaluation/most_economic_electric_vehicles.html (FEB 2022)