Many Roads Lead to Driving Range – Energy Management in EVs

 

A HOLISTIC EXAMINATION OF THE OVERALL VEHICLE

 

Even if the same type of battery and power train are used, there is a significant difference in energy consumption and driving range depending on the vehicle’s basic concept and design. For example, SUVs are designed for greater spatial proportions, better aptitude for rough terrain, and powerful tractive output for pulling trailers. Of course, these use cases do not allow much leeway for energy efficiency. Hence, an SUV will never range the same efficiency levels as an aerodynamically advantageous, minimal weight vehicle optimized for high driving range.

Nonetheless, even the energy management of a vehicle with high consumption can be improved by optimizing elements of all divisions. One such approach to improve driving range is the efficient construction of the engine. In the next step, frictional resistance is minimized in the remaining power train. Then, tractive resistance (which consists of aerodynamics, rolling resistance, and overall vehicle weight) is reduced as much as possible. Thermal management also plays an important role in increasing the efficiency and therefore, the driving range of a battery electric vehicle (BEV) because unlike with ICE vehicles, energy for heating must also be borrowed from the battery.

 

A DRIVE FORMULA, TAILORED TO THE DESTINATION – ALL-WHEEL DRIVE CAN BE EFFICIENT

 

A power train’s basic concept must be suited to the needs of the intended target customer group. Hence, it should be designed in a way that solves target conflicts regarding the different customer’s expectations of the vehicle. One customer group might prefer good driving performance while another group is focused more on energy efficiency. A modular power train building block system offers a great solution for this problem. With this modular system, it is easily possible to build a version of a vehicle with one powered axle and another version of the same vehicle with two powered axles. While the vehicle with one powered axle can be offered as an entry level model build for driving range, the all-wheel drive vehicle acts as an alternative with higher overall performance. Moreover, the all-wheel drive feature provides an attractive additional selling point for any battery electric vehicle. The active torque distribution between the two drive axles presents another possibility to positively influence driving dynamics.

Optimizing power train design

On the surface, an electrified all-wheel drive has certain disadvantages conditioned by principle, like how two powered axles can increase overall weight and decrease available space within the vehicle’s design. However, higher performance potentials also enable better output levels in BEVs, since they perform better in partial load scenarios than internal combustion engine vehicles. This is already in use today with serialized vehicles. Also, even vehicles with one powered axle are usually outfitted with an overdesigned engine since the real use of BEVs incurs fewer high loads. This is especially important for constant driving as this use case is the most common type of normal driving operation.

There are more advantages to harness from all-wheel drives by decoupling one axle, for example the secondary axle, during phases of constant driving. This proves especially effective in combination with complementary rating of the primary and secondary drives.

Here, the rating of the drives occurs in a way that the efficiency sweet spots lie in different velocity ranges, which makes them complementary to each other. This way, two drives can create a far broader efficiency sweet spot through the complementary usage than only one drive ever could, resulting in a simultaneous increase of performance potential.

These advantages are of particular use to vehicle segments that need high performance anyway due to high driving range and consequently, heavier batteries. Usually, this also coincidently overlaps with vehicle use cases which are defined by higher customer expectations of traction and drive dynamics. Hence, it depends on the individual use case which solution makes more sense. However, contrary to ICE vehicles, an electric all-wheel drive can actually prove more economical.

 

CONVERGING NOMINAL RANGE AND REAL DRIVING RANGE

 

How much the WLTP nominal range and the real driving range of a BEV deviate from one another heavily depends upon usage profile: How similar is the expected vehicle usage to the standardized measurement methods according to the WLTP driven run? Generally, nominal range can be useful information for the end customer to objectively compare different vehicles. However, standardized measurements can never accurately reproduce the real vehicle usage of a specific end customer since nominal range is determined by a fixed run simulating urban traffic, rural traffic and highway traffic. That is why there can inevitably be higher values if the real usage varies from this particular route type allocation. For example, driving primarily on the highway will increase energy consumption and shorten the driving range. Thus, the aim of holistic energy management is to converge these two values in the development phase already.

The way this is achieved varies greatly depending on the vehicle’s concept and must be specified in the cooperation between OEM and development partner. On one hand, aerodynamics and the tires’ rolling resistance are much more important for vehicles meant for frequent long-distance travelling at higher velocity. On the other hand, a battery electric vehicle meant primarily for urban short-distance traffic with frequent deceleration, stagnancy, and subsequent run-ups must be optimized in terms of minimizing overall weight, maximizing the recuperation of braking energy, and optimizing energy demand for the vehicle’s A/C.

ARE TIRES PART OF ENERGY MANAGEMENT?

Minimizing tractive resistance as a central element of energy management determines the choice and design of individual vehicle components. Tires play a key role in that matter. On one hand, wheels and tires are an important design element and must match the vehicle’s overall style. On the other hand, they also heavily influence tractive resistance. This not only applies to rolling resistance itself, but also to air resistance and weight. A tire that bears on a small surface area and barely churns minimizes not only rolling resistance, but also decreases air resistance due to its low width. However, a tire with relatively hard rollover reduces driving comfort and negatively influences driving dynamics. Therefore, a higher effort must be made while developing the chassis and modulating control systems for driving dynamics to ensure an acceptable comfort experience and safe driving behavior of the vehicle. Depending on the OEM’s requirements, tradeoffs in one area are necessary in order to optimize another area.

That is why OEMs can profit greatly from a development partner like MAGNA since they not only provide decades-long experience in all areas of vehicle development and adjusting components, but can also validate the systems’ performance and robustness in on-location test sites and test rigs.