In this on demand webinar, our Chief Technology Officer Chris Hilton, will explain the benefits of using in-wheel motors to achieve modular all wheel drive vehicle platforms, covering how to:
– Create superior all-wheel drive on a 2WD Platform with in-wheel motors
– Win the advantages of designing a vehicle platform for 2WD only, while offering an AWD variant
– Achieve space and range optimisation with a pure in-wheel motor platform.
Webinar: Benefits Of Using In-Wheel Motors To Achieve Modular All-Wheel Drive Vehicle Platforms
I’m Chris Hilton, the chief technology officer at Protean Electric. this presentation is about the benefits of using in-wheel motors to achieve modular all wheel drive vehicle platforms.
I will describe an evolutionary approach to adopting in-wheel motors, to create all wheel drive vehicle platform without compromising space or range. Firstly, I’ll talk about creating all-wheel drive on an existing two-wheel drive platform. Secondly, considering a new vehicle platform, how to win the advantages of designing for two-wheel drive only, while still being able to offer all-wheel drive as an option. And finally, we’ll look at the opportunities that you get if you design a platform from the ground up based on a pure in-wheel motor powertrain.
The way in which all-wheel drive is achieved has changed fundamentally in the transition from combustion engines to electric drive. In electric vehicles (EVs), all-wheel drive is created with a second drive unit in the form of an e-axle. It’s a simpler system than the mechanical solutions that were required in the ICE vehicles. Some platforms today are designed to accommodate four driven wheels, examples being the Tesla Models, VW id4, Audi E-Tron, and some of these offer two-wheel drive options. On the other hand, some are designed specifically and only for two-wheel drive, for instance the VW id3, BMW i3, Mini Electric, Nissan Leaf. Where the vehicle is designed for two-wheel drive option only, the space between the wheels of the undriven axle is used to house other components of the vehicle, or luggage, or passenger space, or even battery. It’s likely to be more cost and space optimized than a two-wheel drive variant of an all-wheel drive vehicle. The penalty for this design advantage is the inability to offer an all-wheel drive variant, which can serve as a halo vehicle, and it can command a premium price. But in-wheel motors can solve that problem for you
A pair of in-wheel motors is an alternative drive unit to a conventional e-axle. So here we can see the anatomy of Protean’s pd-18 4000 series motor which comprises a direct drive motor, an inverter and a complementary friction brake. It all fits entirely within the envelope of a standard 18-inch wheel rim. So looking from the outside in we have the rotor, this is an outrunner machine, and we have the standard wheel bearing. A stator incorporating the stator laminations and windings on a heatsink, behind that the inverter with its power modules, control and capacitor electronics cover which protects the electronics from the outside world, and a conventional but inside out friction break with standard hydraulic actuation. Each of these motors delivers up to about 1400 newton meters and 90 kilowatts output, so a pair of them is roughly equivalent to a 180-kilowatt e-axle. But in-wheel motors eliminate the need to mount the motor the inverter the gear and diff and the drive shafts on the vehicle. So this is one particular product, but this is a technology that’s scalable to other sizes of wheel rims and other performances.
So, in-wheel motors can be integrated into an existing vehicle with minimum disruption where an e-axle can’t be accommodated. Here we can see on the left the main in interfaces to the in-wheel motors. The HV cables are shown connected to the battery, HV goes directly into the package since the inverter is integrated. The powertrain control unit communicates with the motors typically via CAN or CAN FD to issue torque demands and receive status information back from the motors. A cooling circuit with a pump and a radiator provides the motors with water glycol cooling, and the original brake controller with hydraulic lines to the brake calipers is retained. For many suspension types, integration of in wheel motors can be achieved with little or no change to suspension members, giving identical kinematics and track width. On the right you can see an example of converting a front-wheel drive vehicle to all-wheel drive by adding in wheel motors to the rear. But equally, in wheel motors can be installed on the front wheels to convert a rear-wheel drive vehicle to all-wheel drive. We’re finding that customers see this as a low risk route to introducing in-wheel motors, which are not yet established in the mainstream.
Creating all-wheel drive is mostly about performance. We looked at the performance of a c-segment, two-wheel drive passenger car that’s on the market. In its standard configuration it has a 0 to 100 km/h time of 7.3 seconds. We simulated adding two of the pd-18 in wheel motors that I showed you earlier to that vehicle to create all-wheel drive. And that reduces the 0 to 100 km/h time to 4.6 seconds. In some circumstances increased traction is also important, particularly where winter conditions are prevalent and generally for the OEM, these can be halo products that can command a premium price, typically 10 to 30% more than a two-wheel drive version of the same car. In any case, all-wheel drive is clearly a popular choice with the public. Using in-wheel motors as a solution to create all-wheel drive, offers some other advantages over an e-axle. Firstly, an in-wheel motor solution is typically lighter than an equivalent e-axle. We’ve benchmarked two of our PD-18s against an e-axle in a production battery electric vehicle with very similar torque and power output at the wheels, and the in-wheel motor solution reduced the powertrain mass by about 30%. It’s not that the motors in themselves are lighter, in fact you’ve got two of them instead of one now, but the mass saving comes from the removal of the gear and the differential and the shafts and the CV joints. As a second additional advantage, you’ve got torque vectoring, which comes for free, it’s a software function and even applied to just two wheels only, it can significantly modify the over and understeer behaviour. That means that the handling characteristics can be tuned according to the OEM’s wishes in software. Add to that obstacle avoidance is improved even turning circle can be reduced. Thirdly, these motors have very high bandwidth control. They have a response time of the order of a millisecond and that high bandwidth control individually in each wheel improves the traction for launch on gradients for emergency stops and for operation on low traction surfaces.
So, I’ve described how in-wheel motors can allow you to create an all-wheel drive car on a platform that was designed for two-wheel drive only. The question is, if I’m designing a new vehicle platform, why would I design it for two-wheel drive only, since all-wheel drive is clearly a popular option in the market. And the answer is, if you design to support all-wheel drive, a two-wheel drive version of that vehicle is going to be compromised. It won’t be optimal in terms of cost or usable space. So the proposition here is, design your vehicle platform to support two-wheel drive only, and use in-wheel motors to create all-wheel drive. And here’s what that offers.
So if we look at the diagrams on the left here, considering the advantages of designing a vehicle with a rear e-axle only. As opposed to on the top, designing a vehicle to accommodate both front and rear axles to allow four-wheel drive. This is a simplified diagram obviously, but if you look on the top, the e-axle itself is an incompressible volume, so it needs to sit behind and out of the crash zone. The drive shaft then constrain the wheels to sit substantially either side of that drive unit, and in turn the position of the wheels and the drive unit constrain how far forward the cabin and the battery can reasonably extend. Clearly, removing the need to protect for the front drive axle, offers design opportunities that vehicle designers can take advantage of. For instance more cabin or luggage space, more space for battery packaging, or space for packaging of other components. Additionally, suspension design constraints associated with accommodating drive shafts are removed. As are structural requirements for supporting the drive unit on the body of the vehicle. So this space opportunity that you get from designing to accommodate only one e-axle, in this example an e-axle on the rear is apparent on some vehicles such as the BMW i3 and the VW id3, which really have impressive interior space compared to their overall footprint.
So we’ve covered converting an existing two-wheel drive platform to all-wheel drive with in-wheel motors. And then we looked at designing a new platform and doing so to accommodate only one e-axle. Taking advantage of the packaging opportunities that creates, and then allowing in-wheel motors to give you the all-wheel drive option to offer to customers. But the ultimate destination to gain maximum advantage, is to design the vehicle platform for in-wheel motors only and that’s phase three here. The modular all wheel drive vehicle solution easily supports front-wheel drive, rear-wheel drive or all-wheel drive without compromises to the packaging of the vehicle is obvious. But some other opportunities are less obvious.
The primary advantage of in-wheel motors is the additional space that is created on the given footprint. So here’s a picture that looks very like the one I showed you two slides back, but now I’ve removed both e-axles in the bottom scenario so this vehicle is to be driven by in-wheel motors only. Without the e-axles, the wheels can be pushed to the corners of the vehicles, with the space between the crash zones being fully available for non-powertrain components, such as battery, passengers and luggage space.
So at Protean we’re experts in electric drive but not in vehicle design and engineering, so we’ve engaged some experts in those areas to understand the opportunities here. So they’ve been able to achieve a 3400 millimeter wheelbase in a vehicle with four thousand eight hundred millimeter overall length, maintaining front and rear crash zones of a benchmark five-star NCAP vehicle so that’s the wheelbase of a Maybach approximately on the footprint of a Tesla Model Three.
Now there are many design opportunities created here by creating all that extra space but i’m going to focus on two here. Firstly aerodynamics and secondly battery packaging. It’s a very simple thought experiment, what’s the opportunity if we simply replace an e-axle with in-wheel motors and we use the space between the wheels to package battery. On the left here you see a 150 kilowatt e-axle, which is replaced on the right by a pd-18 motor in each wheel. Where the drive unit gear and differential sat we’ve packaged 39 kilowatt hours of battery modules, mounted on a double isolated subframe. The crash structure is maintained, performance is equivalent. These are senate sv59 modules for those that know about such things, so nothing exotic, in fact we were able to package 64 kilowatt hours in a different arrangement but that didn’t seem like an attractive option overall. So here we see in a simple form, an opportunity for what we can do with the space freed up by removing 150 kilowatt e-axle and replacing with in-wheel motors offering the same performance. In reality, if you were designing a vehicle from the ground up with in wheel motors this is not likely to be the best way of taking advantage of not having an e-axle.
A more likely arrangement is shown here. It’s to use the extended wheelbase concept to create more space to package the battery under the floor, which is a more common arrangement. So here we’ve created designs for a vehicle at the overall length 4400 millimeters that’s approximately the length of an id3, in which we’ve been able to create a wheel base of 3 300 millimeters. The numbers here are based on packaging a Nissan Leaf 1.67 kilowatt hour battery modules, the crash zones at the front and rear are generous, 730 millimeters and 580 millimeters respectively, and there’s a side impact deformation zone of 225 millimeters. So on the left is a straightforward layout of a 138 kilowatt hour battery, so clearly there’s an opportunity here to create a very long range vehicle on a small footprint with all-wheel drive as an option.
Perhaps a more cost effective solution is shown on the right. Again, using the extra length made available by using in-wheel motors, but this time to create footwells for the occupants of the cabin which allows the roof height to be lowered by about 140 millimeters. In this case, while retaining the same interior headroom, we’re still packaging 82 kilowatt hours of battery here. Worth noting really, we only care about range of electric vehicles on long journeys which are typically done cruising on highways rather than doing anything like a WLTP cycle. Reducing the vehicle height as shown like in this example reduces the aerodrag by about eight percent and so the cruising range will be increased by a similar amount
So, in summary, in-wheel motors enable the conversion of a two-wheel drive vehicle to all-wheel drive without disrupting the platform.
Designing a new vehicle to accommodate only one e-axle and using in-wheel motors to create all-wheel drive variants creates a more optimal platform.
An all-in-wheel motor platform is a modular solution with significant potential advantages of usable space and vehicle efficiency and range.
Thank you for watching.