Idea for better highway efficiency

My Nissan Leaf Forum

Help Support My Nissan Leaf Forum:

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

dm33

Well-known member
Joined
May 18, 2013
Messages
690
Location
Raleigh, NC
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?
 
Reducing gear ratio will have another side effect - Leaf might not be able to effectively climb/accelerate on steep hills under full load...
 
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.
 
LeftieBiker said:
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.
 
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.
 
dm33 said:
LeftieBiker said:
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...
 
dm33 said:
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.
 
LeftieBiker said:
...

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.
 
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.
 
dm33 said:
... Thoughts? Any engineer types out there with design knowledge of the Leaf's powertrain?
As others have said, it is mostly due to air drag. https://en.wikipedia.org/wiki/Drag_(physics" onclick="window.open(this.href);return false;)#Drag_at_high_velocity . As your speed increases the power needed increases at a squared rate, rather than linearly.
Some solutions:
1. Have a prius or tesla model s look and make the car very aerodynamic
2. Electrify the freeways so vehicle is pushed along by magnets and can recharge the batteries. Similar to how maglev works: https://en.wikipedia.org/wiki/Maglev" onclick="window.open(this.href);return false;
3. Larger battery pack.
 
I think part of the effect is the battery is too small.
The Peukert effect is hurting the range as LEAF draws 10+ kW continuous.
Probably hurting longevity also.
JMHO.
 
smkettner said:
I think part of the effect is the battery is too small.
The Peukert effect is hurting the range as LEAF draws 10+ kW continuous.
Probably hurting longevity also.
JMHO.
In the case of the LEAF vs. Volt (in CD mode), it's got to be aerodynamics plus gearing. The Voltec powertrain really is kind of trick. IIRR, at around 70 mph clutch 2 is engaged, which brings MG-A into the circuit (MG-B is the main traction motor. MG-A is the motor/generator) and unlocks the ring gear, halving the speeds of both MG-A and B. It's supposed to be good for about a 15% efficiency improvement.
 
Is the rate of discharge part of the issue at high speeds?

does draining at 20KW for 30 minutes drain the same amount of energy from the batter as 5KW for 2 hours?

Faster charge (and discharge?) creates heat, which comes from somewhere.
 
No, due to internal battery resistance and other factors, higher drain for a shorter period depletes the battery faster.

Yogi62 said:
does draining at 20KW for 30 minutes drain the same amount of energy from the batter as 5KW for 2 hours?
 
smkettner said:
The Peukert effect is hurting the range as LEAF draws 10+ kW continuous.
JMHO.
Actual testing data says that round trip efficiency goes up at higher speeds:

http://avt.inel.gov/pdf/fsev/fact2011nissanleaf.pdf" onclick="window.open(this.href);return false;

Peukert only applies to lead-acid. At moderate discharge rates lithium does not suffer from similar effects. The LEAF's battery can discharge at over 200 kW until it's nearly empty and it's internal resistance is under 0.15 Ohms.

At 10 kW and 360V, the pack is pushing about 30A. At 0.15 Ohms, you're only losing about 135W (P=I^2 * R) to internal resistance.

Even at 30 kW and 360V, the pack is pushing under 100A. Say it's 100A, that's still only 1500W. Pretty reasonable.
 
I still think field-weakening has something to do with this, as I noted in;

http://www.mynissanleaf.com/viewtopic.php?f=31&t=13767" onclick="window.open(this.href);return false;

Tesla has an induction motor. Renault (Fluence/Zoe) use a coil rotor. Volt has a gear set system that could, in theory, shift the motor's performance so it keeps its perm mag motor to its design for V/Hz.

Leaf flies out of its V/Hz range from some >35 mph, so I am lead to believe from its torque figures.

Tesla and Fluence have a Cd of ~0.25. Volt and Leaf are ~0.29. Real aerodynamics is not as linear as a bald Cd figure might lead you to believe (SCx for all are around the 0.66 m^2 figure), but the lower Cd should mean relatively lower impact from high-speed effects. Leaf might seem to suffer from relatively high aerodynamics, combined with the adverse effects of field weakening above its constant V/Hz range.
 
drees said:
Actual testing data says that round trip efficiency goes up at higher speeds:

http://avt.inel.gov/pdf/fsev/fact2011nissanleaf.pdf" onclick="window.open(this.href);return false;

hmm... interesting.

I can't say that is quite my experience with the Renault battery. Keeping to/below ~15kW seems to be fairly optimal.

I would say you should look suspiciously at anyone saying drawing/charging above 1C is no less efficient than below. I guess if you aim for 1C at 22kW, then inevitably in practice you will be occasionally drawing 10kW and sometimes 30kW. Over that 1C hurts the range/capacity. I guess when I aim for 15kW I still get good mileage because my occasional 'power excursions' stay generally below the 25kW mark. These quoted tests probably benefit from being able to pull a steady 22kW, on some flat test track, without excursions above 1C
 
donald said:
drees said:
Actual testing data says that round trip efficiency goes up at higher speeds:

http://avt.inel.gov/pdf/fsev/fact2011nissanleaf.pdf" onclick="window.open(this.href);return false;

hmm... interesting...

See the footnote:

9. The Battery Roundtrip Efficiency is calculated by dividing the DC energy out of the battery (A) by the DC energy from the on-board charger into the battery (D).

If I understand correctly, much or all of the variation reported as "Battery Roundtrip Efficiency" could also be explained by variations in the "100%" charge allowed by the LEAF's BMS/LBC in the post test "100%" recharge session.

This is also, IMO, one of the most likely explanations for the lower recharge capacity shown by the LEAF following the higher-speed tests.

CONSTANT-SPEED RANGE AND CHARGE TESTING

http://avt.inel.gov/pdf/fsev/fact2011nissanleaf.pdf" onclick="window.open(this.href);return false;
 
Back
Top