Hi guys, serious question I have been mulling around for a few months, but have been a little reluctant to ask in case I come off a little naive. I am trying to better understand the efficiency of the Volt when the ICE is in use at high speeds (60-70 mph) versus low speeds (30-40 mph)

As I understand it, a typical transmission allows a vehicle to become more efficient in a higher gear relative to a lower gear in terms of miles driven per unit of energy used, because the higher gears move the car a greater distance than the lower gears for essentially the same RPMs.

The Volt does not have a typical transmission, it essentially has one "gear". So the electric motor can only make the car move faster by operating at higher and higher RPMs. This one "gear" means the electric motor is less efficient the faster the car is driven because of wind resistance, and it is most efficient in terms of miles that can be driven at speeds somewhere around 30 mph.

The Volt ICE, as I understand it, is usually uncoupled to the drive train. So most of the time, it is simply generating a specific quantity of electrons per unit time at set RPMs. Lets say the Volt's computer decides it needs the ICE to run for 10 minutes at a continuous RPM. If the electric motor is propelling the Volt down the road at 65 MPH, it would use up that specific quantity of generated electricity much earlier than it would if the Volt were diven at 30 MPH. So the way I am picturing it in my head, I would use up more gas driving at higher speeds than I would at lower speeds, which seems to contradict what I have read on other posts.

If the ICE is generating MORE electrons than the electric motor is demanding, are the surplus electrons captured and saved by the battery, or are they just lost? IF the battery saves the surplus, it would seem to me that lower speeds are more efficient regardless of the mode in which the vehicle is operating. If the ICE creates surplus which is lost, then I can see why higher freeway speeds would be more efficient by reducing the difference between the electron demand and surplus production.

Please help me understand what I am missing. Thanks guys!

Good questions.

Check out these things for starters.

There for 4 modes:
- Electric Low-Speed [in Charge-Depleting (CD) mode]
- Electric High-Speed [in Charge-Depleting (CD) mode]
- Extended Range Low-Speed [in Charge-Sustaining (CS) mode]
- Extended Range High-Speed [in Charge-Sustaining (CS) mode]

http://gm-volt.com/forum/showthread.php?6611-Voltec-drive-unit-has-four-basic-modes-of-operation


http://www.youtube.com/watch?v=IWSK8BR6LT8

Hi guys, serious question I have been mulling around for a few months, but have been a little reluctant to ask in case I come off a little naive. I am trying to better understand the efficiency of the Volt when the ICE is in use at high speeds (60-70 mph) versus low speeds (30-40 mph)

As I understand it, a typical transmission allows a vehicle to become more efficient in a higher gear relative to a lower gear in terms of miles driven per unit of energy used, because the higher gears move the car a greater distance than the lower gears for essentially the same RPMs.

The Volt does not have a typical transmission, it essentially has one "gear". So the electric motor can only make the car move faster by operating at higher and higher RPMs. This one "gear" means the electric motor is less efficient the faster the car is driven because of wind resistance, and it is most efficient in terms of miles that can be driven at speeds somewhere around 30 mph.

The Volt ICE, as I understand it, is usually uncoupled to the drive train. So most of the time, it is simply generating a specific quantity of electrons per unit time at set RPMs. Lets say the Volt's computer decides it needs the ICE to run for 10 minutes at a continuous RPM. If the electric motor is propelling the Volt down the road at 65 MPH, it would use up that specific quantity of generated electricity much earlier than it would if the Volt were diven at 30 MPH. So the way I am picturing it in my head, I would use up more gas driving at higher speeds than I would at lower speeds, which seems to contradict what I have read on other posts.

If the ICE is generating MORE electrons than the electric motor is demanding, are the surplus electrons captured and saved by the battery, or are they just lost? IF the battery saves the surplus, it would seem to me that lower speeds are more efficient regardless of the mode in which the vehicle is operating. If the ICE creates surplus which is lost, then I can see why higher freeway speeds would be more efficient by reducing the difference between the electron demand and surplus production.

Please help me understand what I am missing. Thanks guys!

It's complicated - and the Volt is certainly an exception to the usual. I actually got into the discussion of the Volt vs a typical car about a week ago here:

http://gm-volt.com/forum/showthread.php?10210-Holiday-100-Mile-Drive-Mountain-Mode..../page2

With a typical gas car, it's fairly easy to guess the most efficient speed to drive it - the lowest speed it'll hold reliably in the highest gear ratio available.

For the Volt, it's less clear. There's a hump in the efficiency curve in the 45-50 mph range, because the engine and/or second electric motor connect to the wheels, lowering the main motor speed, and in CS mode allowing some power to go directly to the wheels instead of through the generator-inverter-motor path.

I'm not sure it's a large enough hump to overcome the increased losses from air drag - the Volt's most efficient speed may be lower, like the Tesla and the Leaf (the Tesla's best speed is supposed to be 17mph, the Leaf's in the 20s.)

It's complicated -

With a typical gas car, it's fairly easy to guess the most efficient speed to drive it - the lowest speed it'll hold reliably in the highest gear ratio available.

For the Volt, it's less clear. There's a hump in the efficiency curve in the 45-50 mph range, because the engine and/or second electric motor connect to the wheels, lowering the main motor speed, and in CS mode allowing some power to go directly to the wheels instead of through the generator-inverter-motor path.



Aero drag is the square of the velocity times the Cd.
So as you go twice as fast it takes 4 times as much energy to overcome the drag force.
The drag force at speeds less then 45 - 50 mph is practically negligible. At speeds greater than this the drag force becomes a factor quickly.

Typically for a gas powered vehicle it's most efficient state it will be the lowest speed in the highest gear at speeds less then 50mph. For example a Mercedes Benz with a 7 speed transmission when driving at it's lowest speed at it's highest gear is nowhere near in an efficient state.

The Volt's electric motor will also have an efficiency RPM curve. I suspect the Volts most efficient speed will be when the electric motor is at it's most efficient RPM at speeds less then 50 mph (which I don't know. Perhaps someone else does?). The second electric motor typically does not kick in unless speeds are near 70mph (or under heavy load).

The Volt's electric motor will also have an efficiency RPM curve. I suspect the Volts most efficient speed will be when the electric motor is at it's most efficient RPM at speeds less then 50 mph (which I don't know. Perhaps someone else does?). The second electric motor typically does not kick in unless speeds are near 70mph (or under heavy load).


You are mis-understanding the second motor. It is for improved efficiency not for heavy loads. Its is effectively the high-gear. It can engage at much lower speed (30mph I believe). If you engage in a heavy load, e.g. to pass, it will disengage the second motor and switch to just traction motor.

Thanks for the responses, I think I see some light. From what I have read on this and recent similar threads, tell me if you all would agree to the following in regards to a standard load under 70mph (when only the main electric motor is needed):

1. The Volt is more efficient at slower speeds regardless of the mode of operation.
2. While the ICE is converting fuel to electrons, none of the generated electricity is lost, any surplus is stored by the battery. Therefor the lower demand of electrons from the electric motor at lower speeds allows the Volt to drive farther than than the higher demand from the motor at higher speeds.
3. Although the ICE converts fuel to electrons more efficiently at higher RPMs than at lower RPMs, travelling at 60-70 mph is less fuel efficient than travelling at 45-60 mph due to loss of efficiency from wind resistance.
4. When Mountain Mode is engaged while the battery is near depletion (less than 14 miles range left), the ICE will start up at its loudest and most efficient RPM. When the ICE drops down into a quieter and less efficient RPM, disengage the MMode and continue for 10-14 miles on the partially recharged battery until needing the ICE again. Cycling MMode in this fashion while driving at or under the freeway speed limit will optimize the fuel efficiency of the ICE.

Thanks again for the comments guys, your post help me be a more fuel efficient driver. :)

Thanks for the responses, I think I see some light. From what I have read on this and recent similar threads, tell me if you all would agree to the following in regards to a standard load under 70mph (when only the main electric motor is needed):

1. The Volt is more efficient at slower speeds regardless of the mode of operation.


These two are the only part I have a problem with. The Volt never "needs" the second motor due to power or speed - in fact, if you keep it floored right up to 100 mph, it will never engage the second motor. Instead, at speeds over ~45 mph, it engages the second motor when there is less power demand, for efficiency reasons (two motors turning at low RPMs require less power than one at high RPMs for the same road power.) The second motor can't generate ad much torque, so it limits the system output while engaged.

The other item is a matter of principle - there is some speed where all the power you are using for the heater and your wonderful tunes exceeds the power needed for moving the car down the road, and below that speed it costs more to go slower. In decent weather, that is probably in the low 20s, but in bad weather (either hot or cold) it likely moves into the 30s. When you're talking about driving down the highway, your statement is basically correct.

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The attached graphs were published on this site some time ago. They give a good view of where the best efficiency is for what mode is being used.

Notice in extended range mode that the car can shift to dual electric motor mode under light load. I have experienced this when decending long grades whree the regen puts lots of juice in the battery.

Pat

-Inflate tires to max (if you don't mind the firmer ride).
-Keep speed down (it's hard I know). On backroads doing constant 50mph I can land some numbers but then on the highway 75+ is a different story.
-Keep the Climate off or down as low as you can stand. When my g/f is with me my efficiency is off the chart (in a bad way).
-Drive along with a 40mph tailwind! lol!

I personally don't do all the mode and range changes when driving, I want to enjoy my drive not worry about what I have to do next to get optimal range. I use normal econo L for city driving and D for highway, SM is nice to play with and never had it in MM yet, maybe on a trip that requires a big upcoming grade/climb.