IN-WHEEL ELECTRIC MOTORS The Packaging and Integration Challenges

One of the fundamental advantages of in-wheel motors is that they free up packaging space on the vehicle platform. From retrofits to new vehicle designs, the freedom to add batteries, range extenders, or simply preserve cargo and passenger space is a major competitive advantage for all styles of in-wheel motor over more conventional electrical machines. This advantage needs to be preserved by not having to re-locate the friction brake to a position on the sprung mass of the vehicle, so both motors and brakes must be accommodated inside the wheels of a vehicle. To preserve as much of the retrofit market as possible, it must be ensured that the packaging solution inside the wheel does not require significant platform tear-up. In addition, the whole system must be safe, reliable, and provide credible vehicle performance. This has led to some radically different in-wheel motor concepts being developed by various organisations around the world. Protean's unique solution to this packaging problem was, on the face of it, a bold one for a start-up company; displace the brake to provide space for a motor, and develop a new packaging philosophy for the unsprung assembly incorporating both brake and motor. By working with an expert partner on the braking side, this, we believe, results in a much more harmonious use of the valuable space inside the wheel than other in-wheel solutions, and meets or exceeds all the requirements outlined above. This presentation will provide details of the process that resulted in the Protean design, as well as highlight the engineering of the solution fitted to the front axle of the high performance Protean/Brabus E-Class vehicle. Figure 1: Unsprung assembly, Protean/Brabus E-class front-left corner. TARGET MARKETS AND REQUIREMENTS OVERVIEW Protean Electric is targeting both passenger vehicles and light commercials with a blank sheet electric motor design wholly dedicated to in-wheel applications. Principally, the motor must be capable of being the sole source of tractive power for an electric vehicle or series hybrid, but must also work alongside conventional internal combustion powertrains to enable parallel hybrid architectures. To keep costs down, 2WD is an absolute must on smaller vehicles and hybrids, and 4WD should lend credible performance to larger, heavier executive saloons and SUVs. The performance of these vehicles should be able to squarely address the market segment the vehicle is in with the performance, range and interior space that a demanding customer requires. A key requirement that drives the Protean motor design is retro-fit ability. Clearly an in-wheel motor equipped powertrain opens up so many possibilities for sprung mass packaging and design changes that any vehicles that are designed from the ground-up around in-wheel motors will have their own protracted development cycles measured on the scale of several years. Balanced against this, OEMs need to remove significant amounts of carbon emissions from their entire fleet right now, and a handful of compromised niche vehicles will not achieve this. The ability to engineer a hybrid or full EV on an existing platform, with minimal tear-up of existing hardware or intrusion into passenger, storage or accident-vulnerable space cannot be underestimated and is one of the main advantages of choosing an in-wheel motor to propel a vehicle. It is therefore highly desirable for an in-wheel motor to be able to be fitted to a car without requiring any special wheel design or requiring risky suspension modifications, and this played a large part in the development of the requirements for the Protean motor. Regardless of the general architecture and form of an in-wheel motor, the unsprung environment is a harsh one compared with the on-body requirements for a motor. The wading requirements of the vehicle mean that the motor needs to operate whilst essentially submerged, and the wheelhouse is subject to great extremes of temperature thanks to the friction brake and rejected heat from any on-body ICE powertrain. For a road vehicle the motor must survive repeated shock loads of up to 50g with continuous vibration spectra of up to 20g RMS for severe rough roads. Stones are expected to impact any exposed face of the device and the cabling to the motor must be able to survive repeated bending and articulation with suspension and steering movement at temperatures well below zero. [1] The main challenge however will always be package space, especially with a retro-fittable design. In appreciating this, it is clear that Nm/litre and kW/litre have to be maximised. It is already well known that the best motor technology for volume specific performance is liquid-cooled brushless permanent magnet, and so early-on in the programme the decision was made to focus on this technology for the first high volume production motor that Protean is developing. TORQUE AND POWER REQUIREMENTS The principal purpose of the drivetrain is to deliver tractive motoring effort to the tyre-road interface. As a free bonus, an electric drivetrain has the potential to apply similar tractive effort in the braking domain and hand a sizeable chunk of the vehicle’s kinetic energy back into the battery. Safety and/or regulatory concerns aside, on first consideration it is tempting to believe that friction brakes can be replaced entirely with the electric drivetrain but a few simple calculations show the cost of such a move. Take for example a relatively light 2WD passenger vehicle at 1500kg. Assuming a certain few key parameters it is quickly shown that the torque and power requirements are much greater in the braking domain than they are in motoring for a given wheel:  Torque o Motoring Pull-away torque for 30% grade = 650Nm (per motor) o Motoring Maintaining 100mph at 6% grade =300Nm (per motor) o Braking 1g braking = 1800Nm (per front wheel)  Power o Motoring Maintaining 100mph at 6% grade = 40kW (per motor) o Braking Deceleration at a rate of 1g at 100mph = 250kW (per front wheel) Fundamentally this is because the vehicle is always required to be able to perform an emergency stop from all speeds during normal operation, but forward tractive forces do not need to be anywhere near as large as braking forces in order to provide credible or even sporty forward vehicle performance. A universal truth of passenger vehicles is that the most powerful actuator on the vehicle is not the power-train, it is the brake system. It is important to note that the majority of this braking effort is carried out by the front wheels. In targeting 4WD applications, and not discerning (at the requirements level) between RWD or FWD in the case of a 2WD drivetrain, Protean clearly has to cater for, or not prevent, the provision of these levels of braking power and torque in its motor design If the motor were designed to be able to create the levels of retarding torque required in braking, it is shown by the example above that the motor would need to produce around three to four times more wheel torque and over five times more wheel power than if it were sized for even sporty forward performance. By inspecting Figure 2 below it can also be seen that, further to the above design difficulties, an emergency stop is very rarely conducted and so motors which are sized for full braking performance are very rarely of any use, and reclaim practically no extra regenerative energy than a drivetrain sized to regenerate up to 0.3g. The information displayed in Figure 2 was collated during initial developments on braking systems in conjunction with Alcon Components Ltd. Clearly, sizing the motor to meet emergency braking requirements will result in large implications for the cost, size and mass of the motor that are of little use. Figure 2: Brake utilisation curve for 2 million brake applications There are also some other show-stopping constraints – you need somewhere to put this energy you are generating with your motors. This will require either a much higher performance battery or other energy storage/dissipation devices which will add to the cost of the vehicle. Managing the failure modes of such a high performance motor will require some very restrictive safety requirements to be applied at both motor and vehicle levels, further increasing cost. The conclusion to this section is that Protean believes in-wheel motor driven cars still need friction brakes and one of the major requirements on our motor is that it is sized for forward performance only, and a friction brake is used to “top up” the braking torque when under rare demanding braking circumstances, or when the battery has restricted charge current. By making the vehicle perform adequately in the motoring domain, almost all of the available regenerative energy is captured over the majority of real-world drive cycles anyway, especially on front-drive applications where higher regenerative brake torque can be applied without stability concerns. There is optimism within Protean Electric that the present motor design will enable the safe removal of friction brakes from the rear axle in future applications, but this is not currently permissible under present road vehicle legislation. So this leaves us to set torque and power requirements based on forward performance. When deriving torque requirements it is tempting to base the motor requirements from existing ICE-driven vehicles. This would go along the lines of matching the torque at-the-wheels of the target vehicles when in a low gear. However these low gear ratios (especially 1) are sized more around the limitations of the powertrain and clutch life – for example allowing low-speed crawling in traffic or around cark parks without having to “ride the clutch” constantly. Second or third gear would be a more realistic target, but then an electric motor torque curve is substantially different to an ICE, and the torque of an in-wheel motor can be changed on the scale of single milliseconds, as opposed to hun