Transmission, 7", 42 lbs, Gear Ratio=7.065

From: http://www.greencarcongress.com/2011/10/imiev-20111010.html

http://bioage.typepad.com/.a/6a00d8341c4fbe53ef0153923408eb970b-popup


"The fixed gear ratio transmission of the 2012 North American-spec Mitsubishi i is engineered to make the most of the electric motor’s high torque output and a wide power band. The 7.065 gear ratio has been selected for use in this transmission as Mitsubishi research and testing determined this to be the best ratio for optimum dynamic performance from the electric motor. This fixed gear ratio, along with a simplified two-stage parallel shaft reduction and a conventional differential unit, distribute motor torque to the vehicle’s rear wheels."

"Other technical highlights include a single open-type ball bearing that helps to reduce drag and an output shaft layout position that helps to reduce friction of the transmission. The transmission measures less than 7 inches in length and weighs less than 42 lbs. When mated to the electric motor, the combined transmission/motor measures less than 19 inches long and weighs 143 lbs."

FiddlerJohn, thank you for posting this, as this article answered a lot of questions I had about our car. I had previously read in some review that reverse was physically a reverse gear, whereas this article debunks that and shows that we indeed have a single gear with the motor electrically driven in reverse. Must say, the more I drive my (wife's) iMiEV, the more I'm appreciating it.

JoeS said:

FiddlerJohn, thank you for posting.

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JosS, Thank you. Your postings have been an inspiration. I enjoy the detailed, technical side.

I am trying to understand the iMiEV transmission mode of "Neutral."

Is there an electric clutch on the big pink gear of the cutaway drawing? That is what it looks like, and there seems to be an electrical connector on the transmission, but that would waste power. That housing attached to the big pink gear must be the differential. Then why is there an electrical connector on the transmission?

It is too easy to simply not drive the motor when in neutral, but that means the motor is spinning 7 times faster than the rear axle. The stress on the gears is different. The stress is on the opposite side of the gear teeth. If towing at high speed with rear wheels down the transmission may not have good cooling.

ANSWER: The motor is simply electrically disconnected when the transmission is in neutral and the electrical connector on the transmission is for something else (ABS or Traction Control or temperature).


Study in GEARS: http://www.epi-eng.com/propeller_reduction_technology/gear_design.htm

"If the same gear in the idler mechanism contacts both driving and driven gears, its teeth are subjected to the most severe kind of fatigue loading: fully-reversing loads. The idler teeth are loaded first in one direction by the driving gear, then in the opposite direction (with the same force) by the driven gear. "

In the example "it takes only 36 hours for the driving gear to reach 10 million fatigue cycles."

Manual of Gear Design By Earle Buckingham
http://books.google.com/books?id=Qa...A#v=onepage&q="Manual of Gear Design"&f=false

Is there an electric clutch on the big pink gear of the cutaway drawing?

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No, that is the differential housing. The electric motor is ALWAYS connected to the rear wheels.

It is too easy to simply not drive the motor when in neutral, but that means the motor is spinning 7 times faster than the rear axle

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.

True, just as it does any time you are driving.

The stress on the gears is different. The stress is on the opposite side of the gear teeth.

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Yes, but in neutral the drag of the motor is small so the stress on the gear teeth will be small. Compared to the stresses during regenerative braking (which are also in the "opposite" direction), the stresses when in neutral will be so small as to be insignificant to the gear teeth.

If towing at high speed with rear wheels down the transmission may not have good cooling.

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Well, the owners manual dose say never to tow with the rear wheels on the ground, but I don't actually see a problem with it if you have the transmission in neutral and the key in the ACC position. The heat generated by a gear set is roughly proportional to the power being transmitted through it - NOT to the rpm at which it is spinning. Since in neutral the power being transmitted is very small, the transmission will produce very little heat. Further,I think the transmission is just passively air cooled. So, the cooling capacity will be the same if the car is on or off.

"If the same gear in the idler mechanism contacts both driving and driven gears, its teeth are subjected to the most severe kind of fatigue loading: fully-reversing loads. The idler teeth are loaded first in one direction by the driving gear, then in the opposite direction (with the same force) by the driven gear."

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True. But I think gear tooth failure is almost unheard of in today's cars. As you sited, the cycles go up very quickly on the gear teeth. Thus the gears are basically designed for an infinite number of bending cycles. Fatigue failure of the gears (in bending) would be VERY low on my list of things to worry about. At high mileages, pitting of the gear tooth face is more likely, but this doesn't suffer from the reverse-load problem as the face of the tooth taking the load (and thus subject to the pitting damage) changes when the loading reverses.

Why is there an electrical connector on the transmission?
It must be ABS or Traction Control. Probably Hall effect sensors for axle rotation.


The darker grey (almost brown) electrical connector is on the top, back left portion of the above drawing.

Isn't that connector for the gear lever linkage, it looks like the linkage goes right through the center of the electrical connector.

Wee John said:

Isn't that connector for the gear lever linkage, it looks like the linkage goes right through the center of the electrical connector.

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There isn't a gear lever linkage, in the traditional sense, since there are no gears to shift. I think the most likely thing is that it is an electrical connector for the solenoid or motor for the parking pall. I think speed detection is unlikely as this is already checked at each wheel. And since it is all fixed gearing, knowing the speed of each wheel lets you know the speed of every other part of the drivetrain. Temperature is a possibility, but I doubt it as not a lot of heat that can be generated by those gears when the maximum power going through them is only 66 hp. Note that manual transaxles don't often have temperature sensors even on cars with much higher horsepower.

How does the iMiEV transmission Gear Selector mode of "Park" function?

Does Park simply short out the motor? That would be the simplest solution, but the car might slowly creep down hill if the parking brake wasn't set.

Is there be a park mode in the Transmission that the Gear Selector enables? Maybe Wee John is correct?

Quixotix is saying that the electrical connection to the Transmission is probably the parking pall. I like that.

Just for fun, I calculated approximately what the RPM of the motor would be at 81 mph.

I calculate a rear tire circumference at a hair over 60 inches. I didn't actually measure the tires, so lets just go with an even 5 feet. Now 81 mph is 7128 feet per minute. Dividing gives us 1425.6 RPM at the wheel. Times 7.065 gives approximately 10,070 RPM for the electric motor.

Or 7460 RPM at 60 mph. That's buzzing right along for a motor of that size.

FiddlerJohn said:

How does the iMiEV transmission Gear Selector mode of "Park" function?

Does Park simply short out the motor?

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No. An electric motor/generator can't generate any power at zero rpm. Thus, the force to START the car moving from a stop will be the same whether the windings are shorted or not. It's not until the car is moving that that shorting the windings could produce any drag. Further, motors/generators turning at very low RPM's produce very little power, and thus could produce very little drag. I'd bet that on even a modest hill the motor could not keep the car's speed below 5 mph if the motor was being used strictly as a generator (i.e., not using power from the batteries to actively fight the car from moving).

Now I must confess to having a bit of unintentional experimental data here. My 15 year old daughter was driving (with me in the car). As she parked the car, she shifted into park just before the car was completely stopped. The car lurched to a stop, and she set the parking brake. When we went to leave, she started the car and shifted into reverse. The car made a little jerk (that it doesn't normally do) as the transmission "shifted" out of park. Even though the car had sat for 5 minutes, the transmission still had a load on it from the way she put it in park and set the parking brake. There is no way the motor (acting as a shorted generator) could have held this load for five minutes as just a few degrees of rotation would have relieved all of the stress.

There is definitely a parking pall or mechanical brake of some sort somewhere in the drivetrain. The transmission is the most likely place for it.

Quixotix said:

...rear tire circumference at a hair over 60 inches...

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You mean a hair over 6 feet.

I get 8019 rpm for a 24" OD rear tire, but tirerack says 869 revs per mile which is 8288 rpm. I guess the tire compresses 3% in radius under load.

For the front tires 145/65-15 with 22.4" OD, from:
http://www.tirerack.com/tires/tires...465TR5EC&vehicleSearch=false&fromCompare1=yes
Wrongly says: Revs per mile= 781 but it should say Revs per mile= 901 (minus 3% under load?)
WRONG: (81 miles/h x (781 r/mile) x (h/60minutes) x 7.065 = 7449 rpm)

Pi x D = 3.14 x 22.4" = 70.34" = 5.86'= 0.001110101 miles is 901 r/mile

81 miles/h x (901 r/mile) x (h/60minutes) x 7.065 = 8586 rpm

for the REAR tires 175/65R15 with 24" OD, Revs per mile= 869 from:
http://www.tirerack.com/tires/tires...HR5EP422&vehicleSearch=false&fromCompare1=yes

81 miles/h x (869 r/mile) x (h/60minutes) x 7.065 = 8288 rpm for the REAR tires

Pi x D = 3.14 x 24" = 75.36" = 6.28'= 0.001189393 miles is 840 r/mile

81 miles/h x (840 r/mile) x (h/60minutes) x 7.065 = 8019 rpm

Quixotix said:

FiddlerJohn said:

How does the iMiEV transmission Gear Selector mode of "Park" function?

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The car made a little jerk (that it doesn't normally do) as the transmission "shifted" out of park. There is definitely a parking pall or mechanical brake of some sort somewhere in the drivetrain. The transmission is the most likely place for it.

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Yes. Thank you. Great experiment.

Correction!

FiddlerJohn said:

for the REAR tires 175/65R15 with 24" OD, Revs per mile= 869

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REAR tires are 175/55R15
http://www.tirerack.com/tires/tires...755TR5EC&vehicleSearch=false&fromCompare1=yes Overall Diam. 22.6" Revs Per Mile 919
81 miles/h x (919 r/mile) x (h/60minutes) x 7.065 = 8765 rpm

The electrical connector on the transaxle is a Shift Position Switch. While no actual gear change occurs when the shift position is changed (park, neutral, drive, etc), the shift position detected by the Shift Position Switch is transmitted to the EV-ECU.

Thank you, archie_b

FiddlerJohn said:

Correction!

FiddlerJohn said:

for the REAR tires 175/65R15 with 24" OD, Revs per mile= 869

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REAR tires are 175/55R15
http://www.tirerack.com/tires/tires...755TR5EC&vehicleSearch=false&fromCompare1=yes Overall Diam. 22.6" Revs Per Mile 919
81 miles/h x (919 r/mile) x (h/60minutes) x 7.065 = 8765 rpm

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Sorry, I calculated the tire diameter based on the 175 width and the 60% height ratio.
That is 175(mm) / 25.4(mm/in) * .6 (60 series tire) + 15 (wheel dia.) = 19.1 in. I knew this was only an estimate, but I thought it would be closer than it really is. Your numbers from Tire Rack are obviously much better.

PS - I just double checked and my rear tires they show the size as 175/60R15 on the sidewall.

archie_b said:

The electrical connector on the transaxle is a Shift Position Switch. While no actual gear change occurs when the shift position is changed (park, neutral, drive, etc), the shift position detected by the Shift Position Switch is transmitted to the EV-ECU.

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Can you give a few more details?

1) What moves in the transmission that has it's position detected?

2) How is it moved? Is it moved by a mechanical linkage or by an electrical device?

3) Does anything change position in the transmission when you shift between N, D, Eco, and B, or does it only move when you shift out of P?

Thanks

Quixotix said:

archie_b said:

The electrical connector on the transaxle is a Shift Position Switch. While no actual gear change occurs when the shift position is changed (park, neutral, drive, etc), the shift position detected by the Shift Position Switch is transmitted to the EV-ECU.

Click to expand...

Can you give a few more details?

1) What moves in the transmission that has it's position detected?

2) How is it moved? Is it moved by a mechanical linkage or by an electrical device?

3) Does anything change position in the transmission when you shift between N, D, Eco, and B, or does it only move when you shift out of P?

Thanks

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OK I checked under my car, and can answer two of my own questions, and guess at the other one:

A-2) It is moved mechanically. It has a rigid cable connected to a lever on side of the transmission where the connector is. I think the other end of the cable is connected directly to the shift lever in the car, but I can't actually see it thanks to the battery.

A-3) The cable and lever move with every move of the shift lever (not just when you move out of park).

A-1) I suspect that the sensor is just sensing the position of the lever as it goes into the transmission (rather than sensing something internal to the transmission). But I can't really tell.

I can't think of anything in the transmission that needs to be moved by the lever other than the parking pall. I made the mistake of assuming the parking pall would be electrically operated, but it now looks like it is probably done mechanically. I'll bet the ICE car already had the mechanical cable running from the shifter to the transmission and it was easier to just use it rather than going all electric.

PS - I took a picture, but It looks like I have to upload it somewhere else on the web first and then reference it here. I'll try to do that later.

Quixotix:

It is difficult to fully understand how the transaxle operates as nothing I have found explains exactly how some functions are performed. Here is what I have been able to figure out:

1. On the input shaft of the transaxle is a parking gear. This gear has numerous detents in it. A park roller is used to engage the parking gear, thus locking the transaxle like a standard automatic transmission. The engagement or disengagement of this gear is a function of the transaxle control cable.

2. A transaxle control cable is used to mechanically transmit the location of the selector lever (P, R, N, D, etc.) to the transaxle control lever. Attached to the control lever is the Shift Position Switch which provides input to the EV-ECU electronically. As far as I can tell the mechanical linkage only engages or disengages the parking gear inside the transaxle. All other movements of this cable translate into a electronic signal to the EV-ECU. At the selector level end are two other mechanical cables for the key switch and brake pedal. Both of these cables must interact with the selector lever before the lever can be moved out of park.

3. As stated above, the only change within the transaxle occurs when going in or out of P on the selector lever. Everyhing else is done electronically by the Shift Position Switch. Reverse is accomplished by the motor turning in the opposite direction, not by a reverse gear.

Hope this answers your questions.

Thanks for the details archie_b. That makes sense.

Here is the picture under my car (if I did this right). It is looking forward from the drivers-side rear corner. At the center of the picture, you can see the shift position switch on the left side of the transmission. The rubber bellows covers the cable that goes forward to the shift lever in the cabin.