# Displacement on Demand (DOD) for electric motors?

1879 Views 3 Replies 3 Participants Last post by  DavidDymaxion
I used to have a Pontiac G8 GT which used GM's displacement-on-demand technology to give you 28MPG highway out of a 361 horsepower ICE. I sold the car when I got married, but the concept made me wonder if it could be applied to electric motors ("somehow") to get lots of power and lots of range too.

So the way DOD works (also called Active Fuel Management, or AFM) is that when you're accelerating, all 8 cylinders are a-bangin'. But when you're just cruising without changing speeds much, you don't need all that power that's available. So it shuts off 4 of the cylinders and the gas isn't injected to them. When you stomp on the pedal, a computer activates the other 4 cylinders again, and within 250ms all 8 are active to give you torque. It works amazingly well.

So is there any way to apply the principle to electric motors? I'm an electrical engineer by profession, but unfortunately I'm not very familiar with motor theory. I know it can be modeled by a resistor and inductor. Anyway, what I was thinking was, if I calculate, say, that my EV CRX needs 160 ft-lbs of torque to burn rubber and make my friends jealous, then maybe I can buy two DC motors with peak torque of 80 ft-lbs and connect their driveshafts together. When you stomp the pedal and the computer senses a massive delta in what you want it to do (that's the part I know how to do ), the controller feeds both motors thus giving peak torque and most battery drain. But if you are just cruising along and don't need much power to maintain speed, the computer tells the controller to direct that current to just one of the motors instead, thus halving the current requirements... right?

I'm not sure how to make the idea work in my head. From reading the parallel/series motor threads, it seems like a controller is modeled by a constant current source at variable voltages (through PWM). Is that a good summation? So I'm not sure if the above idea would actually work or if, being a constant current source, it will just divert all of the current through one motor instead. So instead of saving power like I want it to, it would just overload motor 1.

Similar concept to above is picture that inside your DC series-wound motor, you have access to the very middle of the coil... can you do anything with that? It seems like if you run current through half the coil, you should get half the magnetic torque, right, because if the controller is a current source then you get half the voltage drop... I'm not sure if you'd get half torque or half RPM.

It's too late, my eyes and mind is tired. Please post any thoughts you have on the idea, I think it can work.
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Further, how would scenarios A, B, and C differ in performance, where:

A) Two motors connected at the shaft. Controller can either run motor 1 alone, or can run motors 1 AND 2, in series connection.

B) Two motors connected at the shaft. Controller can either run motor 1 alone, or can run motors 1 AND 2, in parallel connection.

C) Two motors connected at the shaft. Controller can either run them in parallel or in series. But either way, BOTH motors are on. i.e. White Zombie.
I think DC series motors kind of do what you're suggesting automatically. ICE's have pumping losses and friction associated with each cylinder which is why deactivating cylinders allows them to work more efficiently. DC series motors have very low rotating friction and losses are proportional to I^2. It takes a certain amount of force (i.e. torque i.e. amps) to propel a vehicle down the road at a certain speed and I don't think it matters if that torque is created by one or two motors. Hmmm, maybe two motors are more efficient than one in this case? Not sure...

I do know that max output with DC series motors depends totally on batteries and controller. Think of the motor(s) as acting as a mechanical transformer: it/they will try to convert everything the controller/batteries can throw at them into mechanical motion.

I hope I understood your questions correctly!
That's a beauty of electric -- you give up almost nothing to have a fast one, unlike a gas car. You'll do maybe 100 or 200 lbs more motor, and 15 pounds more controller (~7% more weight), but the bigger motor is a bit more efficient so you get a little bit of that back.

An electric that has a high top speed will also tend to have good range.
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