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MTSU group seeking to commercialize new wheel motor plug-in hybrid retrofit kit

A prototype plug-in hybrid retrofit kit is applied to a research vehicle’s rear wheels. After switching on the traction motors, the vehicle fuel economy increased from 50–100%. Click to enlarge.

A group at Middle Tennessee State University (MTSU), led by Dr. Charles Perry, who holds the Russell Chair of Manufacturing Excellence, has developed and demonstrated a proof-of-concept for a wheel motor plug-in hybrid retrofit kit for almost any car.

The core of the system is a new wheel motor invented by Perry and Paul Martin III; four patents are pending. They took the space that exists around the rear-wheel brake structure and packaged in a 3-phase DC brushless traction motor on each rear wheel. The stator magnets are packaged all the way around the backside; these are electromagnets switched on and off at the proper intervals as controlled by three Hall effect sensors. An array of 40 permanent magnets are on the backside of the rotor.

Design of the new wheel motor. Click to enlarge.

When the system is actuated, says Perry, each motor delivers 200 lb-ft (271 N·m) of torque, without modification to the wheel mounts. For the research vehicle, the system uses an 80V, 100A Li-ion phosphate battery pack with dual DC brushless motor controllers (one for each wheel motor). The production version pack will be smaller, Perry suggests.

The system is targeted at lower speed and city cycle driving; at high speeds the system deactivates and is transparent to the vehicle, Perry says. Perry and his current five-member team saw gas mileage increase anywhere from 50 to 100% on a 1994 Honda station wagon retrofitted with laboratory prototype plug-in hybrid capability.

Perry is now talking with several potential investors—companies with vehicle fleets—to solicit funds to build and demonstrate a manufacturing version of the plug-in hybrid technology.

The whole point was to demonstrate the feasibility of adding the electrical motor to the rear wheel of the car without changing the brakes, bearings, suspension—anything mechanical.

We have gained proof of concept in terms of feasibility. We need quite a bit of money to have proof of product. What we’ve achieved is a demonstrated technology, not a proven technology. Investors want to see proven field-tested performance and reliability. We have to pass through this transition, from feasibility to true, viable product.

—Charles Perry

Perry, who had 40 patents in a 28-year career with IBM before coming to MTSU, said a manufacturing partner has stepped forward “and is totally committed to us” and will accompany him to anticipated upcoming presentations.

Perry said Lou Svendsen, university counsel with the Tennessee Board of Regents, will join him in approaching companies that have both US and worldwide fleets of vehicles, especially those “interested in green technology, reducing carbon footprint and savings in fuel costs.



Lemme see, 200 Nm torque. What can you expect from that?

Assuming the radius of a car wheel is 0.25 m, that means a force of 800 N per wheel, 1600 in total. That could accelerate a 1600 kg car at a speed of 1 m/s^2. Or from 0-28 m/s (=100 km/h) in 28 s. That's not accounting for rolling resistance and aerodynamic drag. Enough to make a noticeable difference, given the limitation that it will only help the engine and never run electric only.

That's is probably the only thing you can do with an existing car that was not designed for start/stop operation. The engine must be running to provide air conditioning/power steering/power brakes.


Using 2x of these direct drive wheel motors will give approx 540 Nm (peak torque presumably) at the rear axle. That is approx 25% the torque available on the front axle after ICE flywheel torque is multiplied by the gearbox and final drive ratio to something around 2,000 Nm (depending on gear and rpm)

For direct drive wheel motors to be effective in EV mode they need closer to 700 - 1000 Nm peak torque and ideally on all four wheels to maximum energy efficiency from brake regeneration.

william g irwin

OK! A good idea. I wonder about dynamic braking, and heat tolerance, and cost. Retrofit is good if the installation is in a simple kit form, but I suspect the geometry of the brake varies from car model to car model, making kit parts car specific. I bet at this development level - proof of concept - the controller only handles the basic thrust issues. I would want dynamic braking too. I would like to see more as this progresses down the road (pun intended!).


Rear brakes don't do much braking, but a dump-load resistor integrated with the engine radiator could allow some amazing fade-free braking performance.  Not that this mod would be retrofitted to vehicles which do lots of mountain descents....

1 m/s² of accel is enough for operation in slow traffic.  Further mods to the vehicle, such as an A/C evaporator core with integrated ice storage, electric power steering and brake boost and a DC-DC converter to run 12 V loads from the HV battery, could allow long periods of engine-off operation.  Eventually you'd have to re-start the engine to circulate fluid through the transmission and lube all the parts rotating with the output shaft.


It could be more effective to install new standardized (lower cost) in-wheels e-motors on all four wheels. The extra in-wheel weight could be reduced with lighter materials and lighter re-enforced rubber tires.


50,000 miles of tire pouncing
wander what the affect on the motors

also you would want to make sure you dont hit any potholes, replacing motors and tires


bouncing i mean

Roger Pham

A 271-lb-ft motor if used to replace the torque converter, can generate much more torque than the engine can at the wheel, due to the gear reduction effect of the drivetrain. This is also retrofittable to existing vehicles, as I've outlined many times here in GCC.


Anne, the traction motor in the current Prius (Gen III, 2010-2013) puts out a maximum of 207 Nm, and it's the only motor-generator that propels the vehicle. You can so a lot with 200 Nm.


Why would you do this with a old, clapped-out Honda? If you are serious, try it with a relatively new vehicle. Oh well, it was probably a good learning experience for a few undergrad students but not much else.

Trevor Carlson

1994 Honda Accord Wagon
Tire Dia.= 614.93 mm
Bolt Circle Diameter= 100 mm

274 Nm @ 50 mm = 5480 Newtons
/.30734 meters= 891.52 Newtons at each wheel
891.52*2 wheels= 1783.04 Newtons of force

vehicle weight= 2,855 lbs = 1,295 kg

acceleration = 1783.04/1295 = 1.38 m/s^2 (.14 g)

at 15 seconds velocity from zero equals 20.7 meters/sec
convert -> 46.3 MPH

Velocity from 5 mph after 15 seconds of acc. is 51.3 MPH. (street start)

*perfect for engine-off city driving. Power steering is usually not needed unless maneuvering in a parking lot or other twisting roads. Power brakes would be essential in high traffic densities and A/C would be essential in hot climates.

Personally the vast majority of my driving day to day would fall within the engine-off parameters listed above. For $4,000 this would be a much better investment than a turbo upgrade.

Roger Pham

Why not just replace the torque converter with a single motor of 200 ft-lb? You can double the acceleration that way while using only one motor and one motor controller. No potential issues with shock damage and excess inertia of in-wheel motor.

Roger Pham

Correction: You can triple to quadrupple the acceleration using just a single motor when replacing the torque converter. You are getting 6-8 folds mechanichal advantage from the drive train.


There are much better ways of transmitting energy from the vehicles energy storage to the wheels. A transmission need not be more than a simple conductor; perhaps made of copper or aluminum. The rotor/stators don't have to be in the wheels with all of their disadvantages. A short shaft from each motor mounted nearer the center of gravity outboard to the wheels makes much more sense. A controller between the accelerator petal and the drive motors allots the appropriate energy distribution.

Unless and until we find a way of generating or storing significantly more energy than we can today, we are just spinning our wheels to think we will change the American mind set to adopt EV's to the degree needed.

If you doubt this, just look through any of the various motor magazines and stop and reflect on what you just read.

Just don't get caught up in their propaganda.


They need to get endorsements from moonshine runners


This is where a 2-speed gearset would be perfect.  Imagine multiplying that 200 ft-lb by 2.5x for takeoff, and the greater efficiency during low-speed operation due to higher back EMF and reduced coil currents.

If you could get 2.5 m/s² from zero to 15 MPH, you could do it in under 3 seconds with electric power alone.  That would do a lot of traffic-jamming right there.


The fact is that currently no major car maker plans to release a car with in-wheel motor. Some went with in-wheel concept (VW) or racing car (Toyota) and stopped there. They probably learned something, and are no longer mentioning it. At the same time they experiment with fuel cells and hydrogen. It implies that major car makers are less optimistic about in-wheel motors than even about fuel cells.
In-wheel motors must be high-torque motors (they spin at low speed, below 2,000 rpm). The fact is that for the same power output, high rpm e-motors have significantly lower mass than low-speed (high-torque) ones.
No wonder that Tesla roadster, Toyota Prius (post 2010 model), Nissan Leaf, all use e-motors with max rpm over 10,000 rpm.


Just FYI, MG, increased pole count substitutes for rotational speed; the power of a motor is proportional to the drive frequency, and torque is proportional to pole count.


Let's assume your statement (at 04:06 AM) is true.
But is it possible to increase the pole count (say to double or triple them) without increasing the motor diameter (assuming the same physical size of new and old poles to produce the same pulling force)?
The visually simplest case would probably be the switched reluctance motor.

But is it possible to increase the pole count (say to double or triple them) without increasing the motor diameter
Just look at the picture of the motor above.  Those poles are quite small.
(assuming the same physical size of new and old poles to produce the same pulling force)?
You have a misunderstanding, and I'll try to explain.

The magnetic force is conservative, like a potential field; moving from A to B yields the opposite in energy change from the move from B to A, and is independent of the path.  For instance, if you have four linear foot-long magnets in opposite-pole pairs aligned N-N and S-S at a certain spacing, then allow them to move so that they are aligned N-S and S-N, you'll release X amount of energy.  If you make each magnet into a semicircle so this shift is achieved by rotation, you'll get the energy released over 1/2 revolution.

Now consider breaking up each of these magnets into 10 pieces, and aligning them N-S-N-S along their respective sides.  Now it doesn't take 1/2 revolution to go from N-N and S-S alignment to all N-S, it takes 1/20 revolution.  Roughly the same amount of energy is released (magnetic leakage aside), so the torque is about 10 times as much (energy = torque * rotation).

For a motor, the torque is far higher and the power at any given rotational speed is greater because the same amount of energy can be applied more times per revolution.  The downside is that poles are smaller, magnetic leakage is higher, higher drive frequency requires more costly electronics (or did, I haven't checked lately), and eddy-current losses in core materials goes up.


Michelin's in-wheel uses 2 small higher speed e-motors to reduce weight, increase acceleration and increase braking energy recovery. Secondly, it becomes a fail soft design because both e-motors would not/very rarely fail at the same time. Standardized mass produced in-wheel would most probably be cheaper, at least when made in you know where!


I tried to follow what you are saying. I assumed you talked about a radial machine, not axial one like the one used in this text. Apparently you assumed that there were enough room to insert more magnets among existing ones, along circumference. I understood properly, you made new magnets by cutting the original cylindrical magnet so that circular surface remained the same, only you reduced the height, so new magnets are shorter in height.
If it is the case (ie there was empty space for new magnets around), then original design was inefficient. On the other hand if you reduced the surface (and not height) along which two magnets face one another (rotor and stator), then you also reduced the pulling/repealing force, and it is not a right way to prove something here.

I still believe that you cannot add new poles, that generate the same magnetic flux as old one (with the same current), without increasing the diameter of the motor.

Put it the other way around:
Say you removed 1/2 of poles from a pancake motor (say Honda IMA), so holes appear between poles. You can then simply reduce the radius, pack remaining poles tightly, and make the motor smaller/lighter. Drive it at the same rpm, but now with half torque, so it would produce only 1/2 the original power at given rpm?


No, MG, it's got nothing to do with axial vs. radial.  It's just physics.

How do you think AC Propulsion gets 200 horsepower out of an induction motor weighing just 70 pounds?  It's a 4-pole motor (not your typical 2-pole) driven at up to 400 Hz, for a max shaft speed of 12,000 RPM.  If you drove it at 60 Hz you'd never get such power out of it, and if it had only 2 poles you wouldn't get the torque.

I still believe that you cannot add new poles, that generate the same magnetic flux as old one (with the same current), without increasing the diameter of the motor.
A foot-length of rectangular permanent magnets is going to generate the same total flux if it's arranged as all N or S poles or N-S alternating every inch; it's just the arrangement of that flux that's changed.

I can't explain it any better than that.  The facts speak for themselves, go consult some texts on motor design if you have further questions.


How do you think AC Propulsion gets 200 horsepower out of an induction motor weighing just 70 pounds?
I know about that motor, used in Tesla roadster, first generation. Later they increased mass to 110 lbs. Max rpm is over 13,000 rpm meaning they run it up to about 450 Hz.
Isn't it a prime example of an induction motor having such power density thanks to high max rpm. Two pole, or 4-pole, but not 16 poles.

Quote from the Austin Hughes book on e-motors, 3rd edition, page 41:
Output power per unit volume is directly proportional to speed.
Low speed motors are unattractive because they are large, and therefore expensive. It is usually much better to use a high-speed motor with a mechanical speed reduction.

I admit, I'm not a motor expert, but know something. The guy 'T2' was probably the most knowledgeable to comment on motors, especially induction ones.

I mentioned axial just to avoid confusion.
Did you think of that Honda IMA pancake motor example I mentioned, would it work that way?


Could one of the highest power density (5hp/lb), high efficiency (95%) @ 8400 rpm e-motor, with Halbach arrays be adapted as a lighter in-wheel unit?

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