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Discussion Starter #1
Found this on Youtube. Tried to see if it has been posted already - sorry if this is a repeat. This video shows the two motors and the ICE as the owner floors it and hits almost 100mph. Great explanation of the operational modes of the motors and engine.

http://www.youtube.com/watch?v=M6ssU278Uk0
 

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In the first video and referring to the left gauge only, is motor A (yellow indicator) the larger, main traction motor and motor B (blue) the smaller motor/generator?
 

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My video - I used the standard GM naming scheme: MGB, main motor. MGA is the generator. Opposite of what I think most people would have called them. Besides helping out at steady highway speeds, you really only see it in action when the gas engine is employed.
 

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Dash Video

I am a new owner and would like to learn a lot more about the inner workings of the Voltec powertrain. What past issues of what magazines have the best diagrams, write-up, etc. Thanks.
Tim Shevlin Anaheim, CA '13/3,500 mi. Wundercar
 

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Discussion Starter #10
My video - I used the standard GM naming scheme: MGB, main motor. MGA is the generator. Opposite of what I think most people would have called them. Besides helping out at steady highway speeds, you really only see it in action when the gas engine is employed.
So, at 1:30 in the video, the MGA has spun up because you enter mountain mode, and MGB has gone down - and speed didn't change. This tells me that neither motor is locked to the wheel. Rather, somehow both motors are able to cause the wheel to spin. Is that through the "planetary gears"? Are they somehow connected to both motors? Sorry, I am not a mechanical engineer but this is fascinating. Thanks again for the video.
 

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So, at 1:30 in the video, the MGA has spun up because you enter mountain mode, and MGB has gone down - and speed didn't change. This tells me that neither motor is locked to the wheel. Rather, somehow both motors are able to cause the wheel to spin. Is that through the "planetary gears"? Are they somehow connected to both motors? Sorry, I am not a mechanical engineer but this is fascinating. Thanks again for the video.
Have a look here:

http://gm-volt.com/2010/10/11/motor-trend-explains-the-volts-powertrain/

At low speeds and high loads, the main motor (MG B) is "locked to the wheels" as you put it - it turns 7 times for every time the wheels spin. However, at higher speeds the other motor (MG A) and engine can be connected to part of the gearing that produces the 7:1 ratio - and them driving the ring gear forward reduces the speed of MG B.
 

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Discussion Starter #12
Thanks, that motortrend article helped, but mostly studying the gears in the picture cleared it up. What was puzzling me was how the wheels spin (what drives them) if the traction motor is almost stopped, which it is at 1:30 in the video. The answer is the generator is spinning very fast and driving the ring gear to drive the wheels, in mountain mode. The traction motor does very little in this mode. Which makes sense - you want to both drive the wheels but generate power to put back in the battery. I'd guess your gas milage in mountain mode - if you stayed in it for 5 or 6 hours, would be very low - maybe 20mpg?
 

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Thanks, that motortrend article helped, but mostly studying the gears in the picture cleared it up. What was puzzling me was how the wheels spin (what drives them) if the traction motor is almost stopped, which it is at 1:30 in the video. The answer is the generator is spinning very fast and driving the ring gear to drive the wheels, in mountain mode. The traction motor does very little in this mode. Which makes sense - you want to both drive the wheels but generate power to put back in the battery. I'd guess your gas milage in mountain mode - if you stayed in it for 5 or 6 hours, would be very low - maybe 20mpg?
It's not specific to mountain mode - you can be in the exact same drive situation in normal mode (with less charge in the battery, of course) - and no, if you stayed in mountain mode for several hours you wouldn't get horrible mileage. The only time mountain mode yields poorer mileage is when you engage it with little or no battery remaining - in that case, it runs the engine harder to rebuild the battery charge, and the mileage looks bad because you're only covering half of the miles that you're paying for (once you got back to the same state of charge you started at, the overall mileage would look pretty good, very close to what you get in other modes.)

Once the car moves into power split mode (with the engine driving the wheels,) the car uses a variety of strategies to optimize economy.

At higher speeds (over about 65,) the car runs the engine continuously at fairly low RPMs, attempting to minimize/balance the battery power flow (higher rpms climbing hills, lower rpms descending them.)

At lower speeds, though, the engine output at the lowest running rpm (~1400) is higher than the average road load. So instead, the car opts for driving entirely with engine power while slowly charging the battery, and then cycling the engine off when it's built a little charge.

This leads to what we have taken to calling "virtual 6th gear." The car more or less stops MG B (still using it to provide reaction torque, but using less than 200 watt to do so,) and uses the engine driving the ring gear to provide all the power to move the car. The gear ratio from the ring to the wheels with the sun held fixed is 3.1:1, close to 6th gear in a lot of cars (the more Eco minded 6 speed in the Cruze is 2.9:1.) The engine sizing is such that the engine produces somewhat more power in this situation than the car needs on a flat road throughout the (35-65 mph) range it uses this mode, and so the excess is pulled into the battery (with the actual amount going into the battery varying moment to moment depending on the accelerator position or the hill if on cruise.)

It's still an eCVT, able to set any engine RPM the car chooses - this is just what GM believes is best (most efficient, presumably,) which happens to mimic a fixed gear for part of the operating range.
 

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Discussion Starter #14 (Edited)
At higher speeds (over about 65,) the car runs the engine continuously at fairly low RPMs, attempting to minimize/balance the battery power flow (higher rpms climbing hills, lower rpms descending them.)
Thanks very much for that explanation. I'm still struggling to understand. One question - why don't I see this behaviour you mention in this part of your quote - running the engine continuously at fairly low RPMs at higher speeds? If my battery is charged the engine does not run.

I drive to work every day about 30 miles, most of it highway. I get to work with 10 to 15 miles left on the battery - except in the winter when I will drain the battery if I run the heater and keep my toes warm, which I like to do. In these cases, the engine runs only the last mile or two of the trip. I don't usually drive over 65 mph, but I have done it enough times to know it does not cause the engine to run. I tested it because I have read this before - the engine supposedly running when driving at higher speeds.

Is there any way the engine could run but the animation in the center stack screen not show the little engine icon? I admit I could not hear the engine at highway speeds, but surely that little icon would show it, wouldn't it?

I also checked for the engine running by resetting the trip meter to see if the gallons used changed from zero, but it didn't. But maybe my 30 mile trip just didn't use enough gas to register. I've noticed several times if the engine only runs a short period it doesn't even register a tenth of a gallon. Seems it has to stay on for several minutes to use a tenth and show on the display.

Thanks again for your explanation. It helps alot.
 

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Thanks very much for that explanation. I'm still struggling to understand. One question - why don't I see this behaviour you mention in this part of your quote - running the engine continuously at fairly low RPMs at higher speeds? If my battery is charged the engine does not run.
You missed the initial caveat - maybe I didn't explain clearly enough. The different strategies I was discussing are all for powering the car with the engine - in Hold mode, or after you drain the battery to the threshold for mountain or normal mode. There is no speed or power level at which the engine comes on if you have battery above the threahold and aren't in Hold mode. (though there is ERDTT if it gets cold enough outside.)

Once you're using gas to power the car, and after the car has decided to link the engine to the ring gear (GM calls it power split mode, which it generally engages immediately at speeds over forty or so,) the strategies I discussed above apply.
 

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Discussion Starter #16
Oh, now I see what you were saying very clearly - "Once the car moves into power split mode".

I hadn't read anything about a "virtual 6th gear" before either. Thanks.

The more I learn, the more I love my Volt!
 
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