Electric cars -- Does periodic coasting help efficiency?

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1. Aug 2, 2017

Domenico94

Hi. I'm a student of electrical engineering, and I've always been interested in the idea of electrical cars, so I came up with a question: let's suppose we want to have a normal trip with car, and then we're not interested in going so fast with it. When we are driving in this condition, it isn't always necessary to be always powered by the engine, at a given point, the car will keep moving because of inertia,and after that while, it will start decelerating, and losing speed. A similar thing happens in a bicycle, we don't need to always pedal, especially in a street with no climbs, because the bike as well would move for inertia for a while, and then it will start losing speed.
So my point is: why can't we implement a similar system in cars, so that they will not have to generate current continously to produce energy, but they could rely on inertia as well? This I think could be achieved with a circuit that gives a pulse at given time interval, or example one every 3 seconds. When there's no pulse, the circuit will rest, and will save energy, thus providing a longer autonomy of cars.
If we do need full power from car, we can make a circuit in which the timer is disabled, and the car engine will receive full power.

Last edited by a moderator: Aug 3, 2017
2. Aug 2, 2017

Staff: Mentor

You don't save energy with a pulsed system. To a good approximation, it does not matter if you supply 20 kW for half of the time or 10 kW the whole time. You have to counter the losses, and the losses won't depend on how exactly you power the car (assuming you still want to have the same average speed). A car will decelerate immediately if you stop powering it.

A side effect: A continuous lower current is better for the batteries, and probably better for the motor as well.

3. Aug 3, 2017

Domenico94

That's perfectly true, but my point was, that in 2 seconds, maybe the car won't lose so much speed ( sorry if I m too approximate, but I never had experience with design), and car would start to gain velocity very fast.
But from a merely electrical point of view, if you supply current, for, say, 8 seconds, and leave engine without current for 2 seconds, we save.current for 2 sec in a 10s time interval (20% of current saved).
That was my point.
We would have losses in speed, but wouldn't them be negligible compared to gains in saved current( about 20%?). Thanks.

4. Aug 3, 2017

Prophet

If power from the engine ceases then the car will not "keep moving because of inertia" and then start losing speed. It will start decelerating immediately. I seriously doubt that providing power in a series of pulses would be beneficial. I think it more likely that such a system would be less efficient than simply reducing the voltage.

5. Aug 3, 2017

Staff: Mentor

Well, sorry, but "too approximate" is exactly the problem here. Yes, it "won't lose much speed", but the speed you lose and therefore have to gain back represents a specific amount of energy that doesn't change when gaining or losing it. If "won't lose much speed" is 10 joules due to friction losses, then you have to apply 20 joules in the same amount of time to get back to the speed you started at (10 joules for the friction you have now, 10 for the energy you lost due to friction when you weren't applying power before).

6. Aug 3, 2017

Staff: Mentor

You would go slower. On average you would go as much slower as with a car running at 80% power continuously.

7. Aug 3, 2017

Domenico94

I understand now :) thanks :)

8. Aug 3, 2017

sophiecentaur

It's the best way for speed control. Pulse Width Modulation (PWM) is much less wasteful than the old method of putting huge (physically) resistors in series with electric locomotives. The resistors used to get hot and waste a lot of power. Varying the Duty Cycle of the pulses can provide any average input power you want from off-most-of-the-time to on-all-the-time and all the Energy goes into driving the vehicle. Good value and the speed reached at any setting will be where the losses are equal to the Power supplied (equilibrium speed).

9. Aug 3, 2017

Domenico94

Exactly my reasoning :)

10. Aug 3, 2017

2 additional points - Drag is proportional to the square of the speed, so what you feel on a bike at 10 mph or the "rate of slowing" is quite different then a car at 45 of 50 mph.

An interesting point regarding RMS current and losses (and the drag for that point). 50% duty at 2x current has higher losses than 100% duty at 1x current.

While the mean speed and current are the same - the RMS current and drag are higher. Very pronounced on the current becasue we are looking at 200% and Zero, vs 100% Continuous.

Example - using I2 * R losses, lets assume 5% losses (in the battery, cabling, inverter and motor windings) at 100A, 100V ( 10KW) - not a bad round number for a car steady state at ~40 mph

5% of 10KW = 500W.

but if we reduce to 50% duty, and have 2 x the current, we would have 2K Watts loss for 50% .... 1000W loss average.

11. Aug 3, 2017

anorlunda

Years ago, Saab used to offer "free wheeling" on cars. When they coasted downhill, it was like automatically putting the transmission in neutral. They said it saved fuel.

Years later BMW offered the same thing, but they ruined their market in the USA with an advertising mistake. The ad showed a BMW going downhill and in big block letters it said, BMW 0 MPG. It was (mis)translated from 0 liters per kilometer.

For most people, the savings were minor. I don't think anyone offers that option any more. But if the only driving you do is from the top of a mountain to the base of the mountain and back, your savings might be more.

12. Aug 3, 2017

sophiecentaur

My Dad had a Riley 1.5, in the 50's. That had a freewheel, I believe (and a Wilson Pre-selector gearbox - epicyclic gears, even!) Silver Grey and a lovely smell of leather seats. I sat on the handbrake lever once (alone in the car) and, when I got out, the car set off on its own, downhill. It demolished a wall at the bottom of the hill. Just bent the bumper a bit, I seem to remember. For some reason I never got a rollicking for that.

13. Aug 3, 2017

Staff: Mentor

It is completely unrelated to your reasoning, because it has nothing to do with the speed of the car. It is only about the electronics: how to transfer power to the motor without losses in the electronics.

14. Aug 3, 2017

sophiecentaur

Thee must be some misunderstanding here. Altering the proportion of Power On : Power Off times will directly affect the speed of the vehicle. The switching frequency is not particularly relevant as long it is 'smoothed' by the mass of the car and the torque of the wheels.

15. Aug 3, 2017

Domenico94

I didn't understand what you mean sorry..could you please explain it better?

16. Aug 3, 2017

sophiecentaur

PWM (i.e. switching the supply on and off periodically) is a way of speed control. That was what my post with which you 'Agreed' was about. The actual rate at which the switching is done is not particularly relevant. What counts is the proportion of On and Off times.
"Periodic Coasting" can be for very short periods.

17. Aug 3, 2017

CWatters

I believe this is key point. Consider the two ways of making the same trip in that same time..

1) At a constant speed.
2) At a varying speed about same average.

In the varying case you burn more energy when going faster than average and save energy when going slower than average. The question is does this cancel out? The answer is no because drag is the square of speed. So overall you burn more energy than you would going at a constant speed.

18. Aug 4, 2017

sophiecentaur

It is only relevant when the speed of the car varies 'significantly' over the cycle. If you flick the switch every 1/10 second - or even 1s, then the speed changes so little that the non linear losses can be neglected. Also, those losses will be less than losses introduced by other speed control methods (e.g a Fat Resistor)

19. Aug 4, 2017

Domenico94

Now I understand :)