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Discussion Starter · #1 ·
At least once a month, I am disappointed to get into my Volt and find that I forgot to plug in the charging cable the day before. This morning I find an estimated 4 miles of electric range left. My roundtrip commute to work is 30miles but yesterday I had to run a few errands(typically EV range is 50miles). I decided to immediately switched to mountain mode to try to recharge the battery for the return trip. I arrive at work and switch to normal mode with 17miles of range showing. I don't quite remember the exact fuel used, but I think it was at least 0.5 gallons. I used the remaining range to get back home. My question is whether or not this actually saved any gas compared to driving the whole way in normal ranging extending EV mode. I assume more gas is used in mountain mode to charge up the battery.
 

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I haven't experimented with this, but my understanding was that the advantage of Mountain Mode was that keeping the battery charged allowed you to utilize the full torque/power when in hilly areas as opposed to running straight off ICE-generated electricity which doesn't provide as much immediate horse power. I'm not sure if it was an efficiency deal.
For what it's worth, this video shows how much gas is used to charge the battery if you put it in mountain mode and let it sit and charge itself.
 

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Let's see.

Mountain Mode runs the ICE to fill the battery to power the electric motors.

In contrast, when the battery is empty the Volt runs the ICE to fill the battery to power the electric motors.

MM is simply "banking" the juice. In charge Sustaining mode the ICE does the same thing, but it seems to be in smaller chunks. There may be some efficiency difference in running the ICE harder in MM, but if so I would think it would be small.

Maybe someone with a dash daq can share some data on this?
 

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My understanding is that each time you convert energy from one form to another you "lose" some of it (heat, friction, what have you).

By using the engine to blast the battery with charge at a very high rate (40kW?) I presume it's wasteful. Then of course you're just using the stored energy later. The alternate would have been to have the engine running to send the bulk of its generated electricity straight to the wheels with the big battery pack used as a buffer for regen and excess generation and so forth. Less conversion.

I would bet a dollar that just using the car in regular mode would be more efficient given the described scenario.

-Drew
 

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I believe that you are wasting gas. In mountain mode, the engine runs at high RPMs to get the battery back asap; because you need the juice for mountain climbing. If you are doing city driving, when you go from EV to ICE, the motor doesn't run at high. RPMs are low and the engine will even shut down occasionally.

The new question is when you have no EV miles left and put it in mountain mode running at a sustained 75+ mph. The engine is already running at high rpm.
 

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MTNdrew1 is correct, using MM to increase the charge in your battery is less efficient than using the electricity to directly power the wheels. This is also why the Volt can mechanically connect the wheels to the engine. No point in converting mechanical work to electricity and then back to mechanical work to power the wheels. When you use MM to increase the charge in your battery, you are converting mechanical work from the engine, to electricity in the generator , to chemical energy in the battery, to electricity in the motor, and finaly back to mechanical work to power the wheels
 

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Discussion Starter · #7 ·
thanks for clearing that up, guys. So I also should not use MM for driving for highway driving when I know I can't get home on electric only?
 

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you can engage MM before your battery gage reaches about 10mi remaining. This will hold the charge not add to the battery charge. I do this when i know i will completely deplete the battery before i can recharge, and i want to use the engine heat for the cabin. Engine heat is free. using your battery to heat the cabin uses a lot of energy. I also us MM when i am on the Hyw and want to preserve the charge for city driving.
 

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If you put it in MM on the flats and downhills I think it helps, take it out going up long hills and use the battery.... I do that for long highway runs and notice a difference versus just leaving it in MM the entire highway trip..

MrEnergyCzar
 

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Discussion Starter · #10 ·
wow, I was doing just the opposite. Using the MM for going up hills and acclerating. I guess it's probably better to drain the battery a bit than to use the gas engine for the least fuel efficient parts of the drive
 

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So long as you are not using MM to increase the charge level in your battery. Doing so will always consume more gas. The Volt requires 12.8 Kwh for a full charge, you only get 10.4 kwh of usable energy out of a full charge. The energy is lost in the form of heat when converting electrical energy ,to chemical energy, back to electrical energy.
 

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When "gaming" MM in my 2011 Volt, I am less concerned with fuel efficiency and more focused on driving pleasure. (The efficiency differences are marginal at best, unless you arrive with extra SOC, which then is a waste.)

So, for me, if I know that my route exceeds the available range, I run EV almost exclusively in stop-and-go conditions. At highway speeds, I always switch to MM (regardless of SOC) unless or until I have enough EV range to reach my destination.

I just love the silent smoothness of EV driving, and I'm not bothered in the least by the ICE running on the highway.
 

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slapshot28, I'm with you on EV mode for city driving. I would also sacrifice a few ounces of fuel for running EV mode in stop and go conditions. However, the OP question was in regard to efficiency. I think owners using MM in an attempt to increase efficiency, without a complete understanding of the losses, often end up operating less efficient.
 

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As others have said, the differences are small, but GM added power split mode because it was more efficient. That is when the engine clutches to the wheels through the generator. In MM, I don't think this would ever happen until the threshold SOC has been met, because the engine is always running at top speed to charge the battery, so it seems to me it is impossible for MM to be more effecient than normal mode under any circumstances. It might not be any worse if high power demand requires the engine to run at top speed anyway, but then what's the point? As has also been said, MM converts more electrical energy to chemical energy, introducing more loss.

The differences are 5-10% at the most during parts of the trip, so it might not be noticable in fuel consumption for one trip, since the granularity of the gauge in the center display is about 1/30 gallon, and the difference in fuel consumption would likely be less than that. Variations in driving conditions (temperature, traffic, etc.) may make a greater difference.
 

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[MM] might not be any worse if high power demand requires the engine to run at top speed anyway, but then what's the point?
I'm am a bit amused that a few folks (not you, Steve-o) scream bloody murder that their Volt does not have Hold Mode, when there is no doubt that prudent use of Mountain Mode can be an effective substitute for Hold Mode.
 

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wow, I was doing just the opposite. Using the MM for going up hills and acclerating. I guess it's probably better to drain the battery a bit than to use the gas engine for the least fuel efficient parts of the drive
Sorry, I'm not sure if energyczar is correct, for most cases there should be no difference if not a slight penalty in the described scenario (running mountain mode on the flats/downhills).

1) Mountain Mode will do nothing different if your battery is above the mountain mode threshold (10% in reserve if i recall correctly)
2) If your battery is <10% it will immediately go into higher RPMs than it would have in a "normal" scenario to generate [more] power to put into your batery, regardless of incline, decline, or flats.
2b) in all those situations, this is less efficient than using that energy from the gas to directly power the wheels, or not using any extra energy at all and allowing the battery to power the car. Anytime you use the gas engine to charge the battery , you are essentially firing up your own powerplant and that powerplant is less efficient (and will cost you more in the gas you use) than whomever is your electricity provider back home.
Mountain mode makes sense in 2 situations:
1) For it's stated purpose: when climbing a mountain for a long enough time that it will drain your battery
2) for folks who know they will use all their battery anyway during their commute and therefore want the car to use gas where the gas engine is the most efficient (smooth highway driving) and the battery where the gas engine is least efficient (stop and go city driving).

EDIT: I see other posters have already chimed in on this
 

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If you put it in MM on the flats and downhills I think it helps, take it out going up long hills and use the battery.... I do that for long highway runs and notice a difference versus just leaving it in MM the entire highway trip..

MrEnergyCzar
wow, I was doing just the opposite. Using the MM for going up hills and acclerating. I guess it's probably better to drain the battery a bit than to use the gas engine for the least fuel efficient parts of the drive
Sorry, I'm not sure if energyczar is correct, for most cases there should be no difference if not a slight penalty in the described scenario (running mountain mode on the flats/downhills).
Not to pick on the Czar, since I have tremendous respect for him, but in my experience it is more efficient using the ICE going uphill and then switch back into EV going down. I did controlled experiments driving the same route for 1yr testing all 3 approaches including 1) staying in Normal for the entire trip, 2) using MM going up and Normal going down, and 3) using Normal going up and MM going down.

You can read the details in "My MM Games" link in my sig. But the results consistently showed that both the 2nd and 3rd methods performed better than just staying in Normal the entire way. However, method 2 consistently resulted in the best EV range and the least amount of fuel consumed.

This only applies to scenarios where your trip will 1) involve both climbing and descending and 2) exceed the overall battery range. If you can make the entire trip in EV mode (even with the hill), then there is no reason to burn any gas. And if you're driving on a flat, then as others have said, it is less efficient to use MM to recharge the battery. Whenever using MM, it's always best to engage it prior to dropping below the SOC threshold so that you only enter CS mode to maintain SOC and not "charge replenishing" mode as I like to call it where the ICE works harder to rebuild the SOC.
 

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While I admit that I haven't done a strictly scientific comparison, I drive a 120 mile trip two to three times a week. I leave my destination with a full charge which pretty much gets me to the highway. If I let the battery deplete on the highway and just drive the car, I get around 42 mpg for the trip. If I hit MM before my battery is depleted (1 or 2 miles of range left) while I am on the highway, then run for about 12-15 miles and switch back to NM and repeat this process until I exit the highway and run the last 13 miles of my trip in NM I get to my destination with 1-2 miles of range left and an mpg for the trip of about 52. I have repeated this numerous times with the same results.
 

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This is also why the Volt can mechanically connect the wheels to the engine.
I want to clear this statement up. Someone jump on me if I'm wrong but that isn't accurate. At no time is the ICE ever actually driving the wheels via mechanical energy. The ICE is a generator, nothing more. All it does is generate electricity that goes to the electric motor that drives the wheels.

Jeff
 

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I want to clear this statement up. Someone jump on me if I'm wrong but that isn't accurate. At no time is the ICE ever actually driving the wheels via mechanical energy. The ICE is a generator, nothing more. All it does is generate electricity that goes to the electric motor that drives the wheels.

Jeff
Actually, the engine does connect mechanically to the wheels most of the time when the engine is on and the speed is above 35 mph. Here's the powertrain video where GM senior technical folks explained it to the media:

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

Quickly, the car has four drive modes. In all of them, MG B drives the sun gear and pushes power to the wheels on the planet carrier.

The simplest one, single motor EV, draws power from the battery and leaves the ring gear clutched to the case wall, stationary.

Two motor EV clutches MG A to the ring gear, and drives the gear forward, improving efficiency by reducing the speed that MG B is spinning above ~50 mph.

Series extended range is hooked up like single motor, but the engine is linked to MG A, generating power.

Finally, in Power Split extended range, MG A is clutched to the engine on one side and to the ring gear on the other, delivering engine power to the road mechanically (though it still requires electric power on MG B to deliver any torque.)
 
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