Fuel economy regulations of 35.5 mpg by 2015 and 54.5 mpg by 2025.
Increasingly stringent global emissions regulations.
A limited choice of costly, complex hybrid systems.
Scarce engineering resources and even scarcer engineering budgets.
Whether you’re in engineering, advanced technology, commercial fleets or any related field, if you impact vehicle fuel efficiency at all, you’ve no doubt heard something about in-wheel or wheel hub motors.
However, much of what’s considered common knowledge about in-wheel motors doesn’t accurately describe the Protean Drive™ system. That’s because wheel motors demonstrated to date haven’t been ready for today’s demands of fuel efficiency, vehicle handling, performance and durability. That’s led many engineers and technologists to dismiss the technology out of hand as not ready for real-road prime time.
But we’d like you to Reconsider the Wheel.
A quick history
The concept of having an individual in-wheel motor driving each wheel of a vehicle is not a new idea. In 1900 at the World’s Fair in Paris, the ‘System Lohner-Porsche’ was debuted. This vehicle was designed by Ferdinand Porsche; the man who gives his name to the Porsche car company, at his first job in the automotive field working with Jacob Lohner. The ‘System Lohner-Porsche’ was an electric vehicle driven by two in-wheel motors; this vehicle was capable of over 35mph and set several Austrian speed records.
Following on the success of this vehicle, Ferdinand Porsche then utilized Daimler’s and Panhard’s internal combustion engines connected to generators to provide power for the in-wheel motors; thus creating the world’s first series hybrid vehicle (SHEV), the “System Mixt.”
Porsche’s in-wheel motor–driven vehicles in both electric and series hybrid configurations continued to claim more speed records in both two- and four-wheel drive configurations, eventually resulting in Ferdinand Porsche winning the 1905 Poetting Prize as Austria’s outstanding automotive designer. The image at right shows one of Porsche’s vehicles from this period – the in-wheel motors can clearly be seen mounted in the front wheels.
Eventually Porsche ended up with a fleet of over 200 delivery vehicles that utilized in-wheel motors. Porsche’s recognition (from the speed records, design prizes and commercial vehicle fleet) led to Porsche’s work with Mercedes, Volkswagen and eventually the formation of Porsche Engineering (and ultimately today’s Porsche car company).
Over the last 110 years, in-wheel motors have been used in a wide variety of different industries including mining equipment, earthmoving equipment, rail vehicles and military applications. In the first decade of the new millennium, when battery electric and fuel cell vehicles were coming back into the limelight, the wheel hub motor popped up again. This very well could be the decade where in-wheel motors return to their automotive origins with the Protean Drive™ design – an elegant and comparatively simple concept that achieves fuel economy and emissions improvements while maintaining (or improving) vehicle performance and reliability.
Protean Drive™ is a unique, patented system that addresses the needs of this demanding application in a variety of ways:
Protean’s in-wheel motors were designed from the beginning to be in-wheel motors. Earlier iterations by automakers were not optimized since they simply took a typical industrial motor and modified it to fit into a wheel.
Typically the engine of a vehicle is designed to deliver high speed and low torque (a traditional car engine would operate around 3,000 to 6,000rpm). A gearbox is used to lower this speed (and increase the torque) to that required at the wheels, for example, an 18-inch road wheel on a vehicle doing 75 miles per hour will rotate at approximately 1,000rpm. In conventional electric/ hybrid vehicles the same principles are applied: A high speed, low torque in-board electric motor driving through a gearbox to deliver the lower speed and higher torque required at the wheels.
Protean Electric™ took a completely different approach and designed its motors specifically for in-wheel applications. The Protean Drive™ motors are a low speed, high torque, direct drive design (where no gearbox is required). Torque is simply a force acting at a radius, in order to maximize the torque, the parts of the motor that generate the torque (the coils and magnets) are mounted on as large a diameter as possible. The Protean Drive™ motors are an ‘inside-out’ design with the magnets mounted to the rotor which sits on the outside of the stator (instead of having a rotating central shaft in a conventional electric motor).
A conventional electric motor usually has an external inverter/ power electronics box – often this inverter is a similar size and weight to the electric motor itself, resulting in the difficulty of packaging two ‘boxes’ (the electric motor and inverter) in the limited space inside a vehicle. One of the major advantages offered by in-wheel motors is the amount of space freed up inside the vehicle, resulting in more cargo and luggage capacity and/ or more space for batteries. This advantage is diminished if the in-wheel motors need separate inverters mounted inside the vehicle.
Protean’s external rotor-internal stator design leaves a toroidal (doughnut) shaped space inside the motor which Protean has utilized to house the power electronics/ inverters. A conventional inverter needs to switch high currents (hundreds of amps) at high voltage (several hundred volts), which requires large, specialist, expensive, heavy components that generate a lot of heat and usually require a separate cooling system. Protean’s approach was to use several smaller micro-inverters, each switching several tens of amps at several hundred volts. This design means that the switching components can be smaller, lighter, cheaper, nearer to an ‘off-the-shelf’ design and take up less space.
The flexibility offered by the multiple micro-inverter concept not only means that the micro-inverters can be packaged inside the spare ‘toroidal’ space inside the Protean Drive™ motors, but also gives multiple levels of redundancy. If one sub-motor fails (for example in a Protean Drive™ motor with 8 sub-motors) then the motors continue to operate at 7/8 of their maximum performance (unlike a conventional electric motor design with a separate inverter, where any inverter failure causes the entire motor to be inoperable).
Another benefit of Protean’s integrated micro-inverter concept is the fact that both the coils and the micro-inverters can share the same liquid cooling system and heatsink (also integrated into the motor). The low-speed, high-torque, direct-drive design of the Protean Drive™ in-wheel motor concept (with integrated micro-inverters) both reduces the size/weight of the electric drivetrain and simplifies vehicle integration; the Protean Drive™ motors can be mounted to virtually any conventional vehicle, without the need to modify the drivetrain.
A common challenge to the packaging advantages of in-wheel motors is the perceived trade-off required by moving the drivetrain mass from the sprung to the unsprung mass. This increased unsprung mass is often challenged with the belief that there is an unsprung mass/sprung mass ratio threshold, beyond which the vehicle will become difficult to control and uncomfortable riding.
Lotus Engineering, one of the most well respected authorities on vehicle suspension and handling, performed a study to test this notion, using the specifications of several typical in-wheel motors. These papers used objective, subjective and numerical analysis to investigate the effects of increased unsprung mass on a vehicle. The outcome of their investigations was that unsprung mass is far less of an issue than some of the prevailing opinions in the automotive community had believed. The studies also concluded that top-of-the-segment performance can be achieved with already defined development techniques. For more detailed information, please click here to see the full white papers.
The Lotus study concluded that while perceptible differences emerge with increased unsprung mass, on the whole they are small and unlikely to be apparent to an average driver. The nature and magnitude of the changes can be easily overcome by applying normal engineering processes within a product development cycle. The study also indicated that control of individual wheel motors shows good potential for substantial improvements in vehicle dynamics, safety, performance and handling.