What acceleration curve is most efficient?

[fatallightning]

In an ICE car, using fairly big throttle openings to accelerate to your desired cruising speed, while short shifting, is usually the most efficient way to accelerate. In this way you minimize the pumping losses of a typical motor. What about in an FFE? Is it more efficient to say, open up the throttle to 75-80%, getting to a cruise speed quicker, or to use a more conservative, say 40% throttle, taking longer to get up to speed?

 

[dmen]

It is more efficient but much less fun to accelerate as gradually as traffic allows. Similarly more efficient to start slowing earlier than you might in an ice car and decelerate as gradually as possible.

 

[WattsUp]

I don't know... the sensibility from the ICE world that "heavy acceleration burns up fuel inefficiently" may not carry over to the EV world.

If you Google for images of "electric motor efficiency curve", you'll see one general characteristic of most of the graphs is that electric motors are less efficient when not fully loaded.

Here's an example:
http://www.isa.org/Images/InTech/2007/April/20070432-2.gif

Perhaps that characteristic translates into the notion that heavy acceleration in an EV (which presumably more fully loads the motor) is not as "inefficient" as you might think. In fact, very slow acceleration (up to a set speed) might be actually be quite a bit less efficient versus than an equivalent, but quicker burst of acceleration (up to the same speed). The main point here being that slow acceleration seemingly would not place as high a load on the motor as heavy acceleration, which for an electric motor (unlike an internal combustion engine) would lead to it operating less efficiently.

Anecdotally, I haven't noticed my FFE range estimates being very much affected by light acceleration versus heavy acceleration (assuming the same speed is eventually reached and then held) as long as the rest of my driving style is conservative (slow efficient braking, etc.).

Of course, if you accelerate so heavily as to spin the wheels, that will not be very efficient. But, assuming the wheels never lose traction, I don't think extremely slow EV acceleration is any better than moderate, or even heavy, acceleration.

 

[dmen]

Wattsup-

That graph you linked is nice and simple. It shows hyperbolic curves for different powered motors, all rising sharply at low loads, then leveling out at their respective max capacity over the rest of the load range. The curve for 100 hp approaches max efficiency at like 30% load. FFE's 143hp is close so this curve might be a good fit.

I would think it doesn't take very hard acceleration to require 30% of the motor's power. It would seem to me then for the motor's efficiency, accelerating moderately would be as efficient as accelerating hard. But there's more to energy efficiency than the motor. If I accelerate hard and get to 60mph in 10 seconds then drive at steady speed for 2 miles, I'm pushing into air at 60mph for all but the first 10 seconds. If on the other hand I accelerate gradually and take 20 seconds to get to 60mph, then I have to push air at 60mph over a shorter distance and that takes less energy. Since the motor efficiency is the same in both scenarios, and everything else should be the same, energy efficiency is better in the gradual acceleration scenario.

And I can't claim it's scientific, but in my experience with the FFE, on a set route (evening commute) with attempts at controlling other variables, more aggressive acceleration results in higher Wh/mi compared to more gradual acceleration.

Granted, the effect of acceleration style on efficiency in an EV is much less than with an ICEV, which is probably your point. But if the original poster is trying to really maximize efficiency, sober acceleration will help.

 

[jmueller065]

I agree: Those curves are pretty sharp and I would suspect that any speed and/or acceleration over 10-15 mph would put us past the knee of the graph which would cancel out the "efficiency due to acceleration rate" on the entire vehicles efficiency cost.

 

[paw160]

Faster acceleration means higher motor current. Electrical power increases with the square of the current P=I^2 * R. So you will have more power turning into heat with higher acceleration. Motor resistances are generally small but it's not negligible.