Add 4 speed transmission for 15% gain in range on highway?

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jsongster

Well-known member
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Sep 3, 2013
Messages
197
Location
MTZ CA
Just saw this... looks promising

http://www.greencarcongress.com/2013/07/4sed-20130725.html

I'm guessing the geniuses at Nissan have been thinking this over already... hopefully it is viable as a way to get more range on highways. Seems that an improved battery formulation giving 10 to 20% combined with a 15% improvement from a transmission or at least an overdrive highway ratio mode of some sort... would be awesome. Get real range into the 120 to 150 mile zone. :geek:
 
I don't know how beneficial this would really be. Transmissions in an ICE car only work because the engine has a very limited power band. Electric motors are near 100% from 0 to redline, and thus don't benefit from gearing.

The only significant advantage this could have is lowering the RPM, which reduces heat output. Considering everything is liquid cooled, I don't think it would have much effect.

Taken at an extreme, reducing the gearing could actually have a negative effect. This is because the motor must work harder, meaning more amps, and creates more heat and line vdrop losses.

In all scenarios (ICE and electric), wind resistance is the number 1 enemy for range and economy. Gearing just helps ICE motors stay in their peak efficiency range, which electrics do not have as much. That said, horsepower is a function of torque and RPM. Given a flat torque in most electric motors, you could manipulate the RPM (gearing) for a more preferable horsepower rating.
 
But wouldn't something like an overdrive help with highway range? That is the argument these inventors are claiming.
 
jsongster said:
But wouldn't something like an overdrive help with highway range? That is the argument these inventors are claiming.

Overdrive is only a gear in which your practical top speed is lower than the previous gear. As I said before, this is usually done to keep ICE's in their peak efficiency range. Electric motors become slightly more inefficient at high power/RPM, but not nearly as much as their ICE counterparts.

These numbers are made up, but are otherwise reasonably accurate:

ICE @ 0RPM: 0% efficiency, 0% power (this is why you need a starter, and it idles ~1000RPM)
EV @ 0RPM: 100% efficiency, 100% power (no power, no loss)

ICE @ 1000RPM: 80% efficiency, 10% power
EV @ 1000RPM: 99% efficiency, 99% power

ICE @ 2500RPM: 100% efficiency, 75% power
EV @ 2500ROM: 98% efficiency, 99% power

ICE @ 5500RPM: 85% efficiency, 100% power
EV @ 5500RPM: 95% efficiency, 98% power

ICE @ 6000RPM (redline) 70% efficiency, 95% power
EV @ 11k RPM (redline) 90% efficiency, 95% power


With the high 90's efficiency and power, the EV doesn't benefit much from gearing, as it's always at an efficient output, and high torque (power) rating. This is a stark contrast to ICE at the lower and upper bands.
 
Overdrive is only a gear in which your practical top speed is lower than the previous gear.

That's usually a side effect, but not the definition. OD is a gear in which the output shaft spins faster than the input shaft. There are add-on OD units that can lower the input speed for more than one gear, but they usually only work with the highest gear. I have yet to see an explanation for *why* they seem to always have a lower top speed...
 
This has been discussed in great detail here before but the bottom line is that there is little drive-train efficient left to harvest in the Leaf... Certainly not enough to justify the cost or complexity...

jsongster said:
Just saw this... looks promising
 
Because, in an ICE, the engine rpm falls far enough in OD to reduce the power output of the engine (torque times rpm) and thus the car goes slower with less actual power reaching the wheels...

LeftieBiker said:
I have yet to see an explanation for *why* they seem to always have a lower top speed...
 
TomT said:
Because, in an ICE, the engine rpm falls far enough in OD to reduce the power output of the engine (torque times rpm) and thus the car goes slower with less actual power reaching the wheels...

LeftieBiker said:
I have yet to see an explanation for *why* they seem to always have a lower top speed...



Given the wide variety of vehicles I'd driven/ridden with OD, it seems unlikely that this happens with them all. There must be some OD-equipped vehicles with powerbands that go "low" enough to accelerate at the top end of OD. My Suzuki Madura 1200 came very close...
 
kieranmullen said:
Good enough! Looks like we're done here.

TomT said:
Tesla considered a two speed gearbox for the Roadster and gave it up...

kieranmullen said:
WWTD (What would Tesla Do?)
They did more than consider it, they installed and sold the early ones that way. The problem was that the transmission couldn't handle the torque and kept self-destructing, so they switched to a single speed transmission and tried to accomplish the same thing through electronics.

From the Roadster wiki:

"Subsequent to completion of production car number one at Hethel, the company announced problems with transmission reliability. The development transmission, with first gear enabled to accelerate 0 to 60 mph (0 to 97 km/h) in 4 seconds, was reported to have a life expectancy of as low as only a few thousand miles. Tesla Motors' first two transmission suppliers were unable to produce transmissions, in quantity, that could withstand the gear-shift requirements of the high torque, high rpm electric motor. In December 2007, Tesla Motors announced plans to ship the initial Roadsters with the transmissions locked into second gear to provide 0 to 60 mph (0 to 97 km/h) acceleration in 5.7 seconds. The first production car was not delivered with this interim solution; P1 has both transmission gears enabled. According to the plan, the initial transmissions were to be swapped out under warranty when the finalized transmission, power electronics module (PEM), and cooling system became available."
 
Here's a good way to see how little friction the leaf's drive train has. Go 60 then shift into neutral. See how far you coast. Now go to a manual transmission ice car and push the clutch in at 60 leave it in gear.. See the difference? You can't possibly gain anything by adding a transmission. The absence of a need for one is a factor in what makes these cars so efficient! If they could only figure out two low speed high torque motors at each front wheel to get rid of the gear box.
 
The friction in the above scenario comes from the primitive dry clutch, which is not meant for coasting while disengaged. There are better ways of coupling to a transmission, some of them manual (including a modified dry clutch) and some automatic.
 
TomT said:
Tesla considered a two speed gearbox for the Roadster and gave it up...

kieranmullen said:
WWTD (What would Tesla Do?)

Not only did they consider it, it was the original production design. The incredible off-the-line torque chewed up 1st gear and, iirc, they tried with 2 separate companies to get a redesigned transmission that would work. They finally found it easier to modify the motor to get the desired performance with a single-speed. The initial batch of production roadsters shipped with 2-speed boxes. Tesla replaced the transmissions when the new design was available.
 
Performance and range/efficiency are two very different things... It is going to buy you next to nothing in efficiency... Later on, the engineers admitted that with the redesigned single speed system, that was no reason to have considered a two speed transmission in the first place...

Nubo said:
Not only did they consider it, it was the original production design. The incredible off-the-line torque chewed up 1st gear and, iirc, they tried with 2 separate companies to get a redesigned transmission that would work. They finally found it easier to modify the motor to get the desired performance with a single-speed. The initial batch of production roadsters shipped with 2-speed boxes. Tesla replaced the transmissions when the new design was available.
 
A better test would be to simply shift the manual transmission in to neutral. In modern manual gear boxes all the gears (with the exception of usually reverse) are always in mesh anyway and simply selected by the appropriate gear cone.

LeftieBiker said:
The friction in the above scenario comes from the primitive dry clutch, which is not meant for coasting while disengaged.
 
TomT said:
A better test would be to simply shift the manual transmission in to neutral. In modern manual gear boxes all the gears (with the exception of usually reverse) are always in mesh anyway and simply selected by the appropriate gear cone.

This actually makes going into reverse a bigger pita than it needs to be. Sometimes from a stop, or especially after moving forward, my Accord does not like going into reverse. I let out the clutch a little, and it jumps in. I understand why it's like that, doesn't mean it's not annoying.
 
mctom987 said:
TomT said:
A better test would be to simply shift the manual transmission in to neutral. In modern manual gear boxes all the gears (with the exception of usually reverse) are always in mesh anyway and simply selected by the appropriate gear cone.

This actually makes going into reverse a bigger pita than it needs to be. Sometimes from a stop, or especially after moving forward, my Accord does not like going into reverse. I let out the clutch a little, and it jumps in. I understand why it's like that, doesn't mean it's not annoying.

Annoying was the first time I drove a vw bug and no one told me you had to push the knob down to select reverse.
 
Some real world info, via ABG:

"Schaeffler Step2 asks: To gear or not to gear an electric vehicle"


http://green.autoblog.com/2014/07/21/schaeffler-step2-asks-to-gear-or-not-to-gear-electric-vehicle/" onclick="window.open(this.href);return false;
 
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