Revised gearing for highway commuters?

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nader

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I haven't been able to find the spec on final drive ratio, but i was thinking it might be interesting to have a new ring and pinion cut to lower RPMs at highway speeds. I know wind resitance becomes a bigger issue but I cant help but think that lowering RPMs at highway speeds by 15-20% will increase range. Of course acceleration will suffer but i think its got enough torque that it won't make that much of a diffefence.

Thoughts?
 
Unlike on an ICE vehicle, it wouldn't make much difference.

nader said:
I haven't been able to find the spec on final drive ratio, but i was thinking it might be interesting to have a new ring and pinion cut to lower RPMs at highway speeds. I know wind resitance becomes a bigger issue but I cant help but think that lowering RPMs at highway speeds by 15-20% will increase range. Of course acceleration will suffer but i think its got enough torque that it won't make that much of a diffefence.

Thoughts?
 
About the only significant difference, I expect, would be a higher top speed-on a downgrade.

I was going to suggest, that a far cheaper and easily reversible way to test this, would be to get higher-profile tires, to lower freeway RPM.

But that got me wondering about aerodynamic drag. Normally a higher profile tire would, by raising a car, very significantly increase drag. But with the LEAF's smooth underbelly, it might not increase drag a great deal (though increased lift might significantly affect handling if you changed the final drive ratio enough, AND found a grade steep enough to produce very high speeds...).

I owned a Citroen DS21 with an (almost) smooth belly plate, and a hydro pneumatic adjustable suspension, giving it several inches of ride-height adjustment. I don't recall ever driving long distance on the high setting, to check for how much fuel economy or top speed was reduced, though...
 
mogur said:
Unlike on an ICE vehicle, it wouldn't make much difference.

+1 it might actually be the opposite. Many times there is a high rpm point where the motor will drop off rapidly in its abitlity to put out power, and the efficiency at that point seems to go up just a little, but for the most part it makes little difference what speed your are running. You'd have to get the motor curves to know.
 
it would not make a difference, the Leaf's motor has top efficiency over a wide range of RPM. Going to a narrower 185 size tire may reduce your tire drag, also lowering the car should reduced air drag. You should also inflate the tires to the max sidewall ratings.
 
palmermd said:
+1 it might actually be the opposite. Many times there is a high rpm point where the motor will drop off rapidly in its abitlity to put out power, and the efficiency at that point seems to go up just a little, but for the most part it makes little difference what speed your are running. You'd have to get the motor curves to know.
The motor curves are published somewhere by an engineering publication - can't find the link right now, but there's a thread somewhere.

Efficiency is at it's peak of appx 95% at load (a bit more than very light throttle) and moderate speed (between 55 mph or so). Efficiency drops to 85% outside of that sweet spot. Note that this doesn't take into account other losses - such as the motor controller or battery losses at higher currents.

Edit: Here's the link to the thread: Nissan LEAF featured in Feb '11 SAE Newsletter and a quick screengrab of the chart.

screenshotnissanleafsae.png
 
Doesn't look to me like you'll gain much at (legal) freeway speeds by changing the final drive ratio, and would also lose efficiency in low-speed use. But, unless I'm missing something, looking at that efficiency curve, I cant help thinking that The LEAF's overall efficiency could be improved-a few percent overall?-by a variable final drive ratio, even as simple as two "speeds".

Obviously, Nissan decided cost and complexity outweigh benefits on the LEAF. I wonder if Nissan is considering it, though, for future higher cost and performance cars, such as the ESFLOW based on the LEAF drive-train?
 
edatoakrun said:
But, unless I'm missing something, looking at that efficiency curve, I cant help thinking that The LEAF's overall efficiency could be improved-a few percent overall?-by a variable final drive ratio, even as simple as two "speeds".

Are we looking at the same graph?.. there is a possibility of improving eff below 40mph, but little power is consumed at those slow speeds anyways, it may gain you about 1/10 mile range if you drove at 30mph (that was a WAG).. it is already optimized for speeds from 40-80mph with its single speed transmission.
 
derkraut said:
I would imagine that Nissan tried all the stuff you guys are talking about, when they developed/tested the Leaf. :?:
x2,

with all the resources of Nissan I am sure a few different motors and ratios were tried in various conditions.
The last thing I want is a two speed transmission. The 90mph top speed is way beyond what I need.
 
Herm said:
edatoakrun said:
But, unless I'm missing something, looking at that efficiency curve, I cant help thinking that The LEAF's overall efficiency could be improved-a few percent overall?-by a variable final drive ratio, even as simple as two "speeds".

Are we looking at the same graph?.. there is a possibility of improving eff below 40mph, but little power is consumed at those slow speeds anyways, it may gain you about 1/10 mile range if you drove at 30mph (that was a WAG).. it is already optimized for speeds from 40-80mph with its single speed transmission.

Yes, and the rest of my post you quoted reads:

"Obviously, Nissan decided cost and complexity outweigh benefits on the LEAF. I wonder if Nissan is considering it, though, for future higher cost and performance cars, such as the ESFLOW based on the LEAF drive-train?"

Clearly the LEAF motor and the rest of the drive train are designed to maximize performance in an economy car. I doubt Nissan would re-engineer the drive train for a performance car produced in far lower numbers. I think that, in a vehicle class where 0-60 performance, and high speed capability (how many buyers of $50 k sports cars likely to be happy with a two digit top speed?) in addition to vehicle range, are all "selling points", a variable final drive ratio, might be an effective modification.

Just an (off-topic) thought.
 
Granted I am new to EV's and still have a lot to learn but doesn't a motor turning at lower RPM consume less electricity? Hence all of these newer cars having a tall top gear to improve mileage at highway speeds. I imagine Nissan chose a gear ratio that gave a decent top speed and acceleration while staying within the motors efficiency range. My question is if I was willing to give up some acceleration on the bottom end could I make enough of a difference in power consumption on the top end by having the motor turn at a lower RPM.

As far as tire size changes that isn;t going to make much difference. You can't go with a much larger overal diameter than the stock 25" diameter before you'll run into clearance issues with struts, fenders etc.
 
So aside from aero mods and weight reduction what could be done to the motor/gearing to increase range at highway speeds? I'm not saying it has to be without compromise either. I just want to know what will make a difference.

-nader
 
nader said:
So aside from aero mods and weight reduction what could be done to the motor/gearing to increase range at highway speeds? I'm not saying it has to be without compromise either. I just want to know what will make a difference.

-nader

There's just about nothing you can do to the gearing or motor to get a meaningful range increase. What can easily increase or decrease your range is your driving style, use of AC/heater etc.

IOW; mod the driver, not the car ;)
 
A mod that would work would be an extra aggressive ECO mode, the faster you speed over 55mph the harder the gas pedal pushes against your foot.. after a while (or if you get a cramp) you get the hint and slow down :)
 
ICEV cars run at very low levels of energy efficiency at ALL speeds, so the same range/efficiency penalty produced by air resistance at higher speeds is often less apparent to the driver, than with a BEV. Operating in the most efficient narrow RPM range for an ICEV engine will give much greater efficiency, so, if properly geared, an ICEV vehicle may lose little range/mpg by increasing speed up to (legal) freeway speeds, and final drive ratios are often set to realize this benefit.

Unlike an ICEV, the LEAF drive train is within a few percent of maximum efficiency in a very broad range, from about 4,000 to 9,000 RPM, which if i correctly recall the calculations done by others previously, is about 35 to 85 mph. You could shift the LEAF efficiency curve to the right (higher speed) by altering the final drive ratio, but the tiny gain in drive train efficiency, would only be at very high speeds, and be nearly insignificant.

If you take a look at the drive train efficiency curve drees posted yesterday he took from P 17/18 here:

http://www.electricauto.org/resource/resmgr/media/nissan_leaf_sae_2_11.pdf

Look at the vertical scale on the same chart. It seems the LEAF is only significantly lower than optimum efficiency in high torque demand/low speed operation. Shifting the curve to the right by altering the final drive ratio would decrease the efficiency in these conditions, probably more than offsetting high-speed gains.

So at low speed, a light foot is your best "range extender", and the best "range extender" at high speeds, is avoiding high speeds, and the large aerodynamic drag penalty incurred.

As I said in your "drive home" thread, for trips exceeding single charge length (until we get DC charging) the slower you drive, the faster you will get to your destination.


nader said:
Granted I am new to EV's and still have a lot to learn but doesn't a motor turning at lower RPM consume less electricity? Hence all of these newer cars having a tall top gear to improve mileage at highway speeds. I imagine Nissan chose a gear ratio that gave a decent top speed and acceleration while staying within the motors efficiency range. My question is if I was willing to give up some acceleration on the bottom end could I make enough of a difference in power consumption on the top end by having the motor turn at a lower RPM.

As far as tire size changes that isn;t going to make much difference. You can't go with a much larger overal diameter than the stock 25" diameter before you'll run into clearance issues with struts, fenders etc.
 
Non-diesel ICE cars usually are most efficient at higher throttle openings due to reduced vacuum pumping losses. (Some engines, such as BMW's Valvetronic, no longer uses conventional throttle butterflies and this does not apply to them to the same degree.) Thus, that is why they try to gear as high (numerically low) as possible at high way speeds. By the way, the relative useable efficiency of a ICE and BEV are not that much different if you look at the numbers carefully. It is just that the ICE has a higher level of residual and overall energy loss. Speed and load affects both of their mileage similarly.


edatoakrun said:
ICEV cars run at very low levels of energy efficiency at ALL speeds, so the same range/efficiency penalty produced by air resistance at higher speeds is often less apparent to the driver, than with a BEV. Operating in the most efficient narrow RPM range for an ICEV engine will give much greater efficiency, so, if properly geared, an ICEV vehicle may lose little range/mpg by increasing speed up to (legal) freeway speeds, and final drive ratios are often set to realize this benefit.
 
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