ELECTRIC MOTORS: DRIVE TRAINS OF THE FUTURE
Generally, the first difference you will notice between EVs and ICE vehicles, is that there are no gears to operate as such and are in effect automatics, with the vast majority of EVs designed as ‘direct drive’ rather than powered through manually geared transmissions.
Rapidly evolving EV motor technology is redefining and setting the electric vehicle landscape. EV motors account for less than 10% of the total car cost. There are many new and established companies entering the EV propulsion system market. One such new EV entrant is the Dyson corporation, for years dominant in the vacuum cleaner, hairdryer, fan and hand dryer business, centred primarily around its expertise in its motors. As part of a 2.5bn global investment, Dyson has now set up its first electric car manufacturing plant in Singapore [21].
Aside from Tesla, one of the main reasons that no other company has scaled up its EV manufacturing output, is that there is an incapacity to match the powertrain efficiency on a small scale. Tesla’s key to its success in part has been its development and use of a modified version of the switched reluctance motor. This new breed of motor delivers very high power density at low cost, making them very appealing for EV propulsion and transmission systems.
Though, there are drawbacks too, including high torque ripple: in one revolution, the difference between maximum and minimum torque often creates difficulties in precision low speed control and may cause unwarranted noise, especially when operated at low speed.
There are some commentators who forecast that precision EV component costs will decrease fast over the next decade to the point where commoditisation allows them to be obtained from low skill bases such as China. But most industry experts believe that this will not happen, just as jet engines remain in high skill manufacturing bases after almost 100 years. This may point to batteries becoming a commodity as production techniques and costs fall, but motor development and R&D costs may well level out as more efficient power trains become the norm.
There is an emerging trend of propulsion systems available in the EV market now. For example, some new four wheel drive EVs use an electronic differential and place motors with integrated hubs on all four wheels, lowering efficiency losses and simplifying the power train design. There is also a growing trend of integration in the form of ‘motor in axle’ solutions. Both of these solutions reduce overall cost, increase efficiency and lower complexity of the current generation of EVs.
Currently there appears to be two distinct choices in motor technology: the switched reluctance motor option, and advanced permanent magnet motors, now specified in many e-axles solutions. The more immediate breakthroughs can be seen in the rising use of Permanent Magnet motors, presently specified for more EVs than reluctance motors [22].
There is a growing trend to move away from one to four traction motors in all electric vehicle types. Certainly, growth in motor types, efficiency and integration is powering a myriad of options for the consumer, seemingly making your choice of EV an impossible task. Nevertheless, you can be assured that with few exceptions, car manufacturers and their designers will provide you with the best possible integrated system for your new vehicle.
Regenerative Braking
Most new BEVs use a clever technology known as regenerative braking. This technology uses the momentum and mass of the EV to charge the battery pack when you brake. Most regenerative braking also enables the driver to literally drive with a one pedal operation. When you take your foot off the accelerator, the regenerative braking initiates, slowing the car down as the vehicle reverts to power generation, charging the battery and creating electrical drag on the drive system, providing you with zero cost power and physical disc free braking whenever you release pressure of the accelerator.
Almost all new generation BEVs integrate a recuperation function that enables you to adjust the driving profile of your car. This facility enables you to regulate the intensity of your braking and its harnessing of power through regeneration. In essence, when set correctly, it is feasible to drive with just the accelerator pedal, known as ‘one pedal driving’.
Drag Coefficient and Aerodynamics – does it really matter?
In reality, only cars designed primarily for road use really need to reduce drag force, particularly in this era of low carbon output and overall fuel efficiency. The main factor that determines whether a car is aerodynamically efficient is known as the drag coefficient, or CD. This calculated number simply provides a value to how effectively a vehicle can force its way through air. Currently, the best performing cars are averaging 0.26 CD, while most generally sit between 0.30 and 0.35.
Aerodynamic drag increases with speed in any object; it therefore becomes key at higher speeds. The higher the speed, the more aerodynamic drag. By decreasing the drag coefficient in a car, the designer improves the performance of the vehicle as it relates directly to speed and fuel efficiency. There are numerous methods to reduce the drag of a vehicle within the design phase of development.
So, should we be heading towards the magic, if not impossible 0.1 CD number? Well not exactly. A degree of drag force is very much needed to maintain grip on all four wheels in a controllable manner. Additionally, we need to ensure that some airflow is forced on to the heat exchangers that your car may have to maximise cooling of the battery packs, for example. It appears that the conventional ICE motor industry has taken the pragmatic view, producing cars between 0.30 and 0.35, from a design and practical viewpoint. But electric vehicles need to extract every last Volt of electricity to increase its range. Thus, any CD factor below 0.30 will reward the owner by significantly reducing your monthly charging costs, and as a by-product of better CD design, will allow increases in potential top speed.
Body design and construction
Car manufacturers are masters of the phrase ‘if it isn’t broke, don’t fix it’. Most simply enhance their car frame designs over years of research and development to prevent a complete redesign for a new EV and this throws up all sorts of compromises. Conventional design has been optimised for Combustion engines, exhaust systems and geared transmission systems.
Though, EVs require a completely new way of design approach to the benefit of end-users. The by-product of custom-made designs for EVs is that you will get more interior room and expect to see EV costs continue to fall over the coming years as OEMs develop better, more efficient means of producing them, with lower battery prices starting to make an impact on forecourt prices.