Most efficient speed

Mitsubishi i-MiEV Forum

Help Support Mitsubishi i-MiEV Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

HParkEV

Well-known member
Joined
Jun 13, 2013
Messages
85
Location
Toronto, Ontario
Just wondering if anyone with some time to kill and maybe the Canion app would be willing to determine the most efficient speed* for the US/Canadian version of the i-MiEV?
With ice cars its quite easy - for most its the speed at which you get into the top gear that is most efficient, usually around 45mph (70km/h). What would it be for the i?
I know electric motors are generally most efficient at relatively high loads, around 75% or more, but at that load the i is going way too fast and too much energy is wasted on aerodynamic drag.
At very slow speeds on the other end, we run into having the base electrical load (that is not used to move the car) becoming a large portion of the total energy consumption, so I'm pretty sure the most efficient speed is not going to be just crawling speed.
It would be interesting to determine the most efficient speed with all accessories off, versus having the heater and lights on. How much difference would that make in the optimal speed? Because the heater load is so significant, I think it could make a significant difference in our most efficient speed.

* 'most efficient speed' - I mean the speed at which you can get the most distance traveled on a given amount of energy from the battery.
 
HParkEV, good topic, and perhaps can pick up where this one left off: Speed v. Power

When I tried to quantify this (primarily with the help of WeeJohn), we found it virtually impossible to maintain a constant speed on a perfectly level (not undulating) road surface with zero unobstructed wind (tremendous effects from other vehicles in front). Just note how much your red needle varies while attempting to keep your speed constant.

As you pointed out, we have fixed power consumption consisting of the vehicle's electrics and electronics, with rolling resistance usually considered a fairly constant load on an unchanging road surface. Motor/drivetrain efficiency I *think* is a fairly constant percentage of applied power. The big variable is heating and aircon, whereas the gorilla in the room is air resistance.

The most-efficient speed curve can be plotted a number of ways. One example is Range (ordinate) vs. Speed (abscissa), where the curve initially climbs steeply to a peak and then tapers down as speed increases.

There were a number of discussions on the Tesla site about this. Their curve shows a peak of around 18mph for the Roadster and 22mph for the Model S

http://www.teslamotors.com/blog/model-s-efficiency-and-range

As constant loads (such as heating) are applied, the most efficient speed will increase somewhat before the air resistance takes over. Before we go there, perhaps we can try to establish a baseline.

Your suggestion that we can use Canion to measure the power consumption at various speeds is a good one. The trick is to try to hold all the other variables constant - perhaps start off with everything else turned off.

We could then plot this information as Power (ordinate) vs. Speed (abscissa), and see what that gives us.

I sort of burned out trying to do this the first time around, and am happy you can now pick up the ball and run with it. Trying to wrench the iMiEV from my wife in order to perform some more experiments will have to wait until she leaves town... :geek:
 
Thank you Joe for the link to the very useful curves for the Teslas. I suspect our curves have the lowest point around the same speed, but probably rise a little steeper as speed increases since the i's aerodynamics are so poor.

I didn't even think of how hard it would be to maintain constant conditions at different speeds in order to make accurate measurements. Maybe a better solution would be to measure just the two end-points on the curves and plot the rest from theoretical calculations, the effect of aerodynamic drag being well known.

I've already seen here the estimates for resting power consumption, although the range of estimates was quite wide with the lowest being around 120W and the highest in the 400W range.
If someone who has a Canion (I don't) could find a stretch of flat road and measure the current draw at 81mph, that would give us an accurate point on the other extreme end of the curve. At least at 81mph you don't have to worry about maintaining a steady speed, the speed limiter will do that nicely for you. Just have to find and empty stretch of flat highway on a windless day, and average a few instantaneous readings taken over a stretch of a few minutes.
I could do that using just the big power gauge on our cluster, but that has no markings on it so it would be hard to translate the position of the gauge into a reasonably accurate Watts figure.
 
HParkEV said:
I've already seen here the estimates for resting power consumption, although the range of estimates was quite wide with the lowest being around 120W and the highest in the 400W range.
I ran a test for 14 hours using Canion and got 126W resting power consumption.
http://myimiev.com/forum/viewtopic.php?f=23&t=1789&p=13009#p13009
HParkEV said:
If someone who has a Canion (I don't) could find a stretch of flat road and measure the current draw at 81mph, that would give us an accurate point on the other extreme end of the curve.
But officer, I was conducting a scientific experiment.
Seriously though, I've found the wind to be a big factor. You use much less energy at higher speeds with a tail wind, and much more energy at higher speeds with a head wind. I guess it would be best to try it on a relatively calm day.
 
This was the 'awakening moment' of Joe's testing . . . . at least for me

-----------------------------------------------------------------------------------------------------------------------
Mitsi22mph1dot7A.jpg

Interpreting what you're looking at: at 22mph the iMiEV is drawing 2.0Amps out of its (nominal) 330V pack, thus consuming 0.66kW at 22mph.

What this means, if we get really greedy and say that we have a fully-usable 16kWhr pack, then we could go (16/0.66)*22 = 533 miles at 22mph on one charge. Let me repeat that: a range of 533 miles at 22 mph on a single charge. :!: :!:

Update - note the fallacy here: the current being measured at the controller relates to only motive power and does not take into account all the other vehicle energy consumers. See BlueLightning's post below: http://myimiev.com/forum/viewtopic.php?p=7080#p7080
-----------------------------------------------------------------------------------------------------------------------
We have Zero public charging stations where I live, so ALL charging is done at home. After reading and analyzing Joe's results, I realized way back then that speed is the BIG enemy of EV's, and that we really could go pretty much anywhere we liked with Zero 'range anxiety' *if* we just kept the speed down. Over the last year and a half, we've demonstrated that to ourselves dozens of times

Yesterday we made a run to Sam's Club, which is only 17 miles from the house. We needed to make a stop at Harbor Freight (to get one of their little 12 volt heater/defroster units to use defogging the windshield this winter) so the trip to Sam's was actually 25 miles . . . . but it was all city streets and 35 mph or so, much of it less. We did the 25 mile outbound leg using only 12 miles off our Range Remaining meter, but when we left Sam's, we made the 17 mile trip home on the freeway at 70 MPH and that used 30 miles off the Range Remaining meter

To the point of this discussion - I'm betting our 'optimal speed' for maximum range will be some number less than the 22 MPH Joe found in his tests. While his 500+miles of range may not be possible in actuality, if you keep the speed down and avoid jack-rabbit starts, you really can make your 'effective range' any number you need it to be . . . . within reason of course - You're never going to go 250 miles on a charge in real world driving conditions, *but* you CAN go farther if you take 35 MPH roads to your destination and back (even if you must drive some extra miles) as opposed to taking the faster, but shorter roads. If you're 'anxious' at all about any trip, STAY OFF THE FREEWAY!

Don
 
RobertC said:
HParkEV said:
If someone who has a Canion (I don't) could find a stretch of flat road and measure the current draw at 81mph, that would give us an accurate point on the other extreme end of the curve.
But officer, I was conducting a scientific experiment.

In Euro zone, 130 km/h (81mph) is the authorized speed on the highway in many countries. Many days I reach the limiter. But Euro i-MiEV is narrower, and perhaps slighty more aerodynamic than the American.

RobertC said:
Seriously though, I've found the wind to be a big factor. You use much less energy at higher speeds with a tail wind, and much more energy at higher speeds with a head wind. I guess it would be best to try it on a relatively calm day.

There is an old method: go both ways and get the average.
 
I tested on my commute yesterday. Going to work, I can do 48-50 mph with the needle on the E(co). On the way back in the same stretch of road with the needle on the c in ECO, I went down to 40 mph before traffic caught up to me (don't dare do less then the speed limit around here). This was accounted for in headwind and a very slight downgrade towards work (usually hover on the c for the home trip and do 47).

How does 106 Wh/km (Canion) at 48-50 mph compare?
 
Barbagris said:
RobertC said:
......

RobertC said:
Seriously though, I've found the wind to be a big factor. You use much less energy at higher speeds with a tail wind, and much more energy at higher speeds with a head wind. I guess it would be best to try it on a relatively calm day.

There is an old method: go both ways and get the average.

Don't think that average two directions trick works.
The wind resistance is the square of the speed ... Didn't do the calculations but offhand looks like you'll lose more mileage against he wind than you'll gain with tailwind.

I fly a light aircraft and know very well that when I fly against a headwind I do not make us the lost time (or recoup the excess fuel burned) on the return trip with the tailwind.

Think of it this way for quick and dirty:
If you tried to drive, say, 20 mile out and back test route against a 80mph headwind you'd use up the full charge essentially crawling almost motionless and get close to zero miles per kilowatt hour on the outward leg ... And probably never get the 20 miles.
 
acensor said:
Barbagris said:
RobertC said:
......

Seriously though, I've found the wind to be a big factor. You use much less energy at higher speeds with a tail wind, and much more energy at higher speeds with a head wind. I guess it would be best to try it on a relatively calm day.

There is an old method: go both ways and get the average.

Don't think that average two directions trick works.
...

I was suggesting doing the testing on a windless day. I know, hard to come by during the northern hemisphere fall, but we often have them here in Ontario during the summer and winter.
The averaging of the two runs is just to eliminate any effect of elevation change on the test runs.
 
"...The averaging of the two runs is just to eliminate any effect of elevation change on the test runs...."

Oh. Fair enough. :)
Still need to be avoiding a route with too much elevation changes as while those offset each other significantly better than tail and head-winds do they still don't quite balance/average out.
 
Even with head/tail winds, averaging a run in both directions will give you a more accurate measure of energy consumed than just using one or the other

If you were driving north/south with a 10 mph north wind, averaging the two runs should give you something pretty close to the energy you would use on a calm day I would think

Don
 
Thought I'd link this thread to the iMiEV Range Chart from jumpjack which shows the (theoretical) decrease in range at very low speeds due to all the fixed loads powering the system. Without climate controls, the speed is about 12.5mph (20km/h). Has anyone taken any CaniOn data which might corroborate this?
 
"… There is an old method: go both ways and get the average.…"
Has it been mentioned by others that doesn’t work out you never gain as much by the tailwinds as you lose from the headwind. For various reasons including exponential increase.

By the way, that fallacy that you can compensate for a nasty headwind with a good tail wind on the return trip is particularly lethal in aircraft trip planning. I fly a very small aircraft.
Short version: you never make up in hours from the good tailwind what’s your lost From the headwind in the other direction. And you can get in serious danger of running out of fuel in route by assuming you can.
 
I love the detailed analysis and nuances.
But in all the words I seem to have somewhere nice seeing a real world number.
I just semi arbitrarily figured in my MiEV at someway between 20 and 25 mph.
I don’t own it anymore, and figure about the same in my Chevy Bolt.

BTW For what it’s worth: on the Chevy Bolt forum The report of documented maximum range real world what is someone who got something just over 400 miles. If I remember right he had to drive somewhere around 35 mph. The Bolt has a 65 kwhr pack And a nominal advertised range of about 260 miles.

Alex
 
Back
Top