Question about two faces of Mountain Mode

[tboult]

As to MM reducing MPG, that may be true under certain transient circumstances; however, I do not believe that any significant amount of energy is actually lost. More likely, percieved "lost" energy has made its way back into the battery as raised SOC.

The first law of thermodynamics makes clear that energy is conserved, one way or another. The second law of thermodynamics says that "lost energy" (no work performed) ends up as heat. If MM really were significantly less efficient, then something should get quite hot to absorb all those "lost" calories. In my experience, after logging many miles in MM, nothing is getting hot. Furthernore, my mileage in MM is every bit as good as in Normal mode.

Not true that is about the same.. looking at the BSFC chart from say http://gm-volt.com/2012/06/25/unders...ed-range-mode/
You'll see that if you get to high RPMS then engine is less efficient. The lost energy can/will be just be wasted as heat. (And charging is always have some loss, so charging via MM is only going to help if you can do it at/near the ideal RPM. You can quickly loose

If you don't know the RPMS, you can definately be less efficient. If you push it to 4500RPM you'll be at about 280 g/kw which is almost 20% less efficient than the 2100RPM.





The first law of thermo is not conservation of energy.. its about heat and work being transfer of energy and in a closed system they interchange .. if its not useful work it will become heat. Second law is increase in entropy (except at thermal equilibrium )

[saghost]

What Terry said.

If the engine is gasping for breath, it spends more energy moving air in and out of the cylinders (this is why best efficiency/lowest bsfc is generally at peak torque at wide open throttle - the engine breathes most easily there, known in technical circles as volumetric efficiency (defined as mass (standardized pressure "volume") of air pulled in for the charge divided by nominal cylinder displacement - some highly tuned engines can approach 110% in narrow operating bands, but most engines are closer to 80% most of the time.)

If power has to be converted from mechanical to electrical and back again, you lose 10-20% of it in the conversions (this is why series hybrid is and will always be less efficient than parallel under most operating conditions - in a series hybrid 100% of the engine power has to make that journey.)

Under the right road load/acceleration in virtual 6th gear, the Volt can approach 1% electric - in normal operating conditions it's more like 40% - and that's going into the battery for later use in electric mode (this I believe is the entire point of v6th - it minimizes conversion losses while allowing the engine to continue to run at WOT at lower power requirements.

(In output split at higher speeds, it's usually about half the power I think. This is one of the Prius's weaknesses I believe - as I understand it, higher speeds force more of their power onto the electric path with it's higher losses.)

Having said all of that, your basic conclusion is mostly right, if not for the reasons you thought. The Volt in MM is as efficient as in NM once it is at the buffer SoC - possibly slightly more so because I think it is somewhat gentler about pushing the engine RPMs when the SoC sinks. Charging up in MM can be more efficient or less so depending on the operating regime at the time of charging.

[Solarwind]

Furthernore, my mileage in MM is every bit as good as in Normal mode.

Have you used MM starting with a depleted battery for a extended distance, over 100 miles???

[Slapshot28]

Have you used MM starting with a depleted battery for a extended distance, over 100 miles???

Yes, I have. Of course I would not recommend using MM to recharge a battery if a plug is available instead.

[Slapshot28]

The first law of thermo is not conservation of energy.. its about heat and work being transfer of energy and in a closed system they interchange .. if its not useful work it will become heat. Second law is increase in entropy (except at thermal equilibrium )

While I do agree with you that it is best to operate the ICE at optimal RPM, I do struggle with your denial that the first law is about conservation of energy. As to the second law, yes it is about entropy; however, entropy manifests itself as heat, dS = dQ/T.

[tboult]

To the OPs questions, While I've never started in MM with an empty battery, its not surprising to see it really drink gas if you recharge it from empty. If MPG_CS was getting say 40MPG. Assume for a second that MM gets similar effective MPG then for the MM mode to recharge while also driving say 20 miles (and building up 15miles of battery charge), then after 20 miles it would have to use gas to justify 20+15 miles, or (20+15)/40 = .875 gallons. So overall after the first 20 miles it would be showing 20/.875= 22.5 MPG after those 20 miles. But if the engine shut of and switched back to CS mode, then it would then run for 15miles without gas and at 35 miles total it would be back to 40MPG overall. But if you delay the switch back to CS mode.. then the 15 miles is held out and if the car was already on gas, you'll never see them unless you stop the car and restart). So say you drove another 40miles at 40MPG, then at 20 miles the car would have used 1.875 gallons and would have only climbed to 32MPG.

However, as I explained above, if you are charging hard you also won't get 40mpg.. that is only when at near ideal RPMs.. if you drop 20% from that it will look bad. Also you drive faster, say 70+mph, you won't be getting 40mpg.

While I do agree with you that it is best to operate the ICE at optimal RPM, I do struggle with your denial that the first law is about conservation of energy. As to the second law, yes it is about entropy; however, entropy manifests itself as heat, dS = dQ/T.

Getting off track, but I see the first law as including conservation of energy as one of its elements, but that conservation of energy is only a part of it. (Maybe I just had a funky thermo-teacher.. who presented stuff that way since we had seen conservation of matter/energy as a principle in earlier physics classes -- and conservation of matter/energy applies in system where no "heat" is being discussed, e.g. atomic interactions, and we can used Einstein's formulas when needed to keep it in balance ). To me the first law of thermo is basically the equation Internal_Energy = heat_applied - work_done. As I learned it, it depends on conservation of energy, so for example does not hold in nuclear reactions, not that it defines it.

But your point about conservation being there is also true -- a specialized form of conservation just for heat/work. The difference in views could all just be a bias of how I learned things (My thermo teacher was a nuclear physicist) . Probably should not have mentioned it or tried to correct folks here, as I don't teach/do thermo and it was long ago. Its great participating in a forum where people can continually teach me.

[rmay635703]

People repeat after me

1. The volt does not accurately record REAL fuel economy when you move in and out of Mountain mode, many times you are getting better FE than what the display shows.

Electric range created from mountain mode needs to be manually added and figured out when you fill the tank, on paper to know what you really got for fuel economy.

That said I wish we had an Economy mode to peg the motor at the best BSFC/charge efficiency rate. Perhaps someone could peruse the code and set a hard RPM limit around the BSFC island?