dm33
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Idea for better highway efficiency

Tue Jul 16, 2013 9:11 am

The Leaf seems to get particularly poor mileage at high speeds. Although much of this can be attributed to normal air resistance, the power drain seems to be more than that alone would explain. The Chevy Volt doesn't seem to have this severe a range drop at highway speeds. On the Tesla website, they have a calculator which estimates range for city and highway where you can set the speed. They estimate a higher range for highway at 65mph than in the city. The Leaf is no where near the same. I find that going slower gets much higher range.

My assumption is that the reason for the inefficiency is that the motor is over-revving. Reaching RPMs that are close to its limit and becoming less efficient.

The Leaf has plenty of torque from a stop. On the 2013 model Nissan apparently thought it had too much torque. They actually programmatically reduce the torque from a start. I think they did this to try to eek out better range. Similar to ECO, it just alters habits to use less power from a start.

However, theres another alternative that could help both problems. A reduction gear is used to reduce the RPMs from the motor to the wheels. The current end to end ratio is 7.9377.

If the reduction gear ratio was lowered it would adjust both items above. Most importantly it would allow the motor to run at lower RPM for improved efficiency at high speeds. It would also lower the effective torque from a start.

The worst effect I can think of is that it would lower the effective torque throughout the powerband making the car feel somewhat less powerful. Maybe trying to do a higher torque motor like the Chevy Spark would help.

Thoughts? Any engineer types out there with design knowledge of the Leaf's powertrain?

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TickTock
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 11:27 am

Take a look at the graph Nekota provided below. Efficiency stays pretty high throughout the range.

http://www.mynissanleaf.com/viewtopic.p ... h&start=32

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UkrainianKozak
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 1:12 pm

Reducing gear ratio will have another side effect - Leaf might not be able to effectively climb/accelerate on steep hills under full load...
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LeftieBiker
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 4:37 pm

As I understand it, Nissan didn't just lower the torque through the whole speed range on the 2013. They reduced a couple of peaks that weren't needed for good acceleration, while keeping torque at other speeds the same. Changing the gear ration would lower torque across the board, as it were, hurting performance under all conditions except maybe "slippery road acceleration."

What's really needed is some kind of overdrive unit or second gear. A programmable CVT would really solve all the performance and economy problems, but that would mean redesigning the whole drivetrain.
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dm33
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 7:23 pm

LeftieBiker wrote:As I understand it, Nissan didn't just lower the torque through the whole speed range on the 2013. They reduced a couple of peaks that weren't needed for good acceleration, while keeping torque at other speeds the same. Changing the gear ration would lower torque across the board, as it were, hurting performance under all conditions except maybe "slippery road acceleration."

What's really needed is some kind of overdrive unit or second gear. A programmable CVT would really solve all the performance and economy problems, but that would mean redesigning the whole drivetrain.
Tesla uses a single reduction gear, yet they seem to have good efficiency throughout. How did they do it?
Or is it really the inverse; they have poor efficiency through the hole range and its masked by a large battery and large heavy vehicle.

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ggulik
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 8:31 pm

I wonder how much the Tesla's much better drag coefficient (.24 vs .28) affects efficiency at higher speed?

I see a huge drop in efficiency at 60mph+ in my LEAF but when I watch the efficiency gauge in my Model S I don't see much efficiency drop off until about 70mph and even then it's pretty slight.
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 10:16 pm

dm33 wrote:
LeftieBiker wrote:As I understand it, Nissan didn't just lower the torque through the whole speed range on the 2013. They reduced a couple of peaks that weren't needed for good acceleration, while keeping torque at other speeds the same. Changing the gear ration would lower torque across the board, as it were, hurting performance under all conditions except maybe "slippery road acceleration."

What's really needed is some kind of overdrive unit or second gear. A programmable CVT would really solve all the performance and economy problems, but that would mean redesigning the whole drivetrain.
Tesla uses a single reduction gear, yet they seem to have good efficiency throughout. How did they do it?
Or is it really the inverse; they have poor efficiency through the hole range and its masked by a large battery and large heavy vehicle.
Electric motors own much of their individual performance characteristics to the controller module. It's the "fuel system" that feeds the "engine," in ICE parlance. Controller programming is just as important as the physical characteristics of the electric motor, and Nissan has always tried for a compromise in "chip tuning" that everyone can live with but almost no one loves. I find the NISMO controller upgrade to be interesting: more power in Normal mode, and more regen (hopefully with less accelerator pedal resistance) in Eco Mode. At $1500, if it does what it claims it's a bargain, at least to Leaf *owners* as opposed to lease holders...
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DaveEV
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Re: Idea for better highway efficiency

Tue Jul 16, 2013 11:42 pm

dm33 wrote:My assumption is that the reason for the inefficiency is that the motor is over-revving.
The Volt is significantly more aerodynamic than the LEAF (less frontal area, lower coefficent of drag), so it can drive faster with lower penalty.

Unless the hatch is tapered down like the Volt or Prius, you're going to suffer in the Cd department. The funny shaped humps/arches over the rear wheels don't help, either. Compare the rear end of the LEAF to a Volt, Prius, or even GT-R (Nissan's car with the lowest drag). You'll see a significantly tapered rear end with sharp corners - all necessary traits to reduce drag.

The LEAFs wheels are not at all aerodynamic, either. Look at the wheels on most hybrids where the wheel is much flatter. The LEAF also has huge gaps around the tires in the fenders - all other efficient vehicles have much tighter gaps.

It's pretty clear that major compromises with the LEAFs design were made to favor utility over efficiency at least with aerodynamics.

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Re: Idea for better highway efficiency

Wed Jul 17, 2013 12:04 am

LeftieBiker wrote:...

What's really needed is some kind of overdrive unit or second gear. A programmable CVT would really solve all the performance and economy problems, but that would mean redesigning the whole drivetrain.
This is the approach the Volt uses. It uses two motors connected to a planetary gear set and will bring the RPMs of the main motor down at cruising speeds. (Since the motors are not the same size, maximum power is produced with just the main motor, with a loss of efficiency of course.)

One trick that drag racers have used for years is a dual motor setup, but with both motors tied together on the same shaft. Same simple gear reduction like a Leaf, no extra parts.

[The Leaf motor is not really over-revving, it's entirely safe at the speeds that car will drive at. What's happening is that electric motors become less efficient at higher RPM's do to the back-EMF they generate. They actually turn into generators, and that generated voltage is "fighting back" against going any faster. So efficiency drops as RPM increases.]

The trick with two Siamese motors is that you can run them in series at low speeds and switch to parallel for higher speeds. It's a little like having a two-speed trans without any extra moving parts. This really works great on a drag strip, I think the same idea could be used to lower the high speed back-EMF, and thus improve efficiency.
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Re: Idea for better highway efficiency

Thu Sep 12, 2013 9:38 am

Agreed on the range drop at speeds over mid 50's. Moving from 55 to 65 has a large drop. Also at freeway speeds if back windows are open as wierd and painful wind echoing noise happens.
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