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I have the greatest respect for the work of the Rocky Mountain Institute, but this recent blog post triggered my general scepticism towards electric cars, or more specifically, their fuel efficiency. This is how the RMI calculates the payback for a Nissan LEAF:

RMI comparison e-cars

I'm assuming their numbers for mpg and kWh/mi as well as the respective fuel prices are sound. Then, an almost 75% cost saving on the fuel indicates a vastly superior fuel efficiency of the electric car. This seems very counterintuitive to me, as I would think producing the electricity, transporting and storing it and then using it to drive the wheels loses a lot of energy along the way, compared to going from thermal directly to kinetic energy.

Now, if we used the same kind of fuel to drive the shaft at the power plant and the one in the car's motor, I guess the large motor at the power plant can be built more efficiently, but I have a hard time imagining that this outweighs all the losses of distributing the power plant's energy via wires and batteries.

Conversely, if we took the electric car and replaced only its motor with the most efficient combustion engine we have, would that show the same superiority over conventional cars, meaning the electrical car's advantage is not the motor technology but that it's generally more efficient?

Intuitively, I would think that distributing the fuel and generating the power in the car makes more thermodynamic sense.

Note that I'm well aware of the fact that there might be other reasons for electric cars (less pollution locally, ability to use things like wind turbines as the energy source etc.). But just focussing on the fuel-to-motion analysis, aren't electric cars much less efficient?

Since the comparison is a financial one, what might be reasons for the great price difference, if the electric really is less efficient thermodynamically?

Hanno Fietz
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    It really sounds more like there's a question for Physics.se in here than Skeptics. – Tacroy Feb 14 '13 at 22:57
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    "...if we used the same kind of fuel to drive the shaft at the power plant and the one in the car's motor..." I think this is the crux of the issue. The cost to generate a given amount of energy from a (generally) coal electric plant is very much lower than the cost of energy from gasoline that's been refined, transported to a station, and sold to the consumer. – Larry OBrien Feb 14 '13 at 23:52
  • The fuel is rarely the same, and internal combustion doesn't use heat to drive its machinations. It uses pressure differentials driven by explosions. If the ICE were significantly more efficient than it is now, you'd be correct. As-is, it loses a lot of energy converting gasoline to kinetic energy. – MCM Feb 15 '13 at 01:35
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    At least here in Germany, about 55% of the price of gasoline is taxes, compared to about 25% on electricity. That would be quite a large factor in this comparison, and totally unrelated to "real" efficiency. – Jens Feb 15 '13 at 08:44
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    Nissan Leaf fuel economy is claimed to be equivalent to 99 MPG - http://www.fueleconomy.gov/feg/noframes/32154.shtml – Tom77 Feb 15 '13 at 10:55
  • See also my [post on sustainability.SE](http://sustainability.stackexchange.com/a/803/486), which also addresses the so-called *well-to-wheels* efficiency issue. – Nate Apr 21 '13 at 12:22
  • It is hard to compare these vehicles because the EV and hybrid cars are heavily subsidized at the Federal and State level. Furthermore, we don't know the margins the manufacturer accepts on these cars, EV's might be sold at low profit or a loss to elevate it's "green" reputation or avoid a CAFE fuel penalty for the marque. It also matters the source of the electricity powering the grid the EV/Hybrid plugs into. – geoO Jun 13 '13 at 20:14
  • @geoO Although on the latter point it's arguable that improving electricity production only requires the construction of a (relative) few new large power plants and all existing electric cars will benefit retroactively; even if a radical new way of improving gasoline efficiency were discovered, all existing cars would still likely need their engines upgraded to take advantage of it... – Shadur Jul 24 '13 at 11:09

2 Answers2

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The short answer is that no, electric cars are most definitely not at a thermodynamic disadvantage compared to a combustion engine. Quite the reverse, they have the advantage.

Electric cars are about 4x as efficient as fossil-fuelled combustion engines, tank-to-wheel: ICE engine efficiency is around 20%. Electric engines tend to be around 80-90%. And the fuel prices in the article are tank prices, so tank-to-wheel is the right measure in this case. If you were interested in the whole-cycle energy efficiency (which is not what your linked claim refers to), then well-to-wheel would be the efficiency you were after, and then it would be very sensitive to how your electricity would be generated.

For the nitty-gritty of the energy consumption of electric, fossil and hydrogen cars, see this paper from George Wallis of the Claverton Energy Group (pdf, 317kb).

Note that pretty much all electric cars benefit from regenerative braking, and very few fossil-cars do.

Efficiencies do depend on the drive cycle: and whereas ICEs tend to be optimised for speeds around 85km/h, the efficiency of electric cars decreases with increasing speed, just as the core physics would lead you to expect:

enter image description here

(source)

For more information on efficiencies power-station to wheel of electric vehicles, which you ask about, but which is not what the claim you've cited refers to, see the US Gov Fuel Economy site.

And please do come over to the new Sustainability Stack Exchange where we take this sort of question too.

Community
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    Useful answer - and thanks for pointing me at the sustainability SE! – Rory Alsop Feb 15 '13 at 09:13
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    Thanks, the efficiency of the two motor types was the missing link in my picture. Also, yes the sustainability SE will be useful to me. – Hanno Fietz Feb 18 '13 at 12:37
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    Wouldn't the short answer be "no", not "yes", given your information? The question is whether electric cars are at an efficiency disadvantage. – Highly Irregular Mar 08 '13 at 07:46
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    It's also worth noting that gasoline production also consumes energy in the form of electricity and natural gas. In states like California that use natural gas to produce electricity, it's almost true that the savings in producing gasoline could power their electric cars. – Ernie Jul 05 '13 at 21:34
  • Why does the answer here not talk about the energy density of petrol being 100 times better than a battery. This makes your stored energy in an electric car much heavier, the trade off designers usually pick is the distance per 'tank'. https://en.wikipedia.org/wiki/Energy_density – daniel May 11 '17 at 11:58
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    @daniel because that's completely irrelevant to the specific question that was asked. We don't do general pointless discussion here - try reddit for that. We do specific answers to specific questions that are about specific notable claims. – 410 gone May 11 '17 at 14:25
  • @EnergyNumberse you are blind to the downsides of lugging around a big battery in a car, a brand new tesla weighs in pretty heafty and does about half the distance as a regular petrol burning car on one tank. From a thermodynamics perspective I'm choosing between 40 kg of petrol of 40 kg of batteries to introduce to my system to solve the problem of traveling, and the petrol let's me travel so much further. – daniel May 11 '17 at 22:50
  • @EnergyNumbers I'll even admit that a hybrid can be 1.5 times as efficient as a normal car on a good day, but you are talking electric cars. Electric motors are great in places we see them used, trams, factories air conditioners. But these all have the advantage of a permanent connection to the grid which a car does not have. – daniel May 11 '17 at 23:06
  • @daniel: An electric car doesn't _need_ anywhere the energy density that a gasoline car does, since an electric car doesn't waste almost all of its stored energy blowing hot air out the radiator and tailpipe. – Vikki Jan 17 '22 at 07:56
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I'm coming into this discussion a few years late, but I wanted to address an issue that others have not:

You speak of the thermodynamics of a gasoline engine over an electric motor. Here are the physics of why electric motors are more efficient than gas engines:

In simplest terms, a gasoline engine takes chemical energy (the gasoline), converts it into heat by burning it, and then it converts that heat into mechanical energy. In general, every time you do a conversion between forms of energy, you're losing some of that energy to entropy. In particular, heat energy is a very disorganized form of energy, and it's very difficult to convert it directly to other forms of energy. You also can't consume all of the energy from heat by converting it to mechanical energy, and a lot of the heat gets wasted as a result. This is most obvious by a gasoline engine's need for a radiator - that's where the waste heat gets dumped into the air.

Electrical energy, by contrast, is very organized. It's easy to convert electricity directly into mechanical energy with an electromagnet pushing against another electromagnet, and very little energy is wasted as heat in the process. There is only one energy conversion - from electricity to mechanical energy, instead of two like in a gasoline engine, and the process does not involve intentionally generating heat (which will need to be removed with a radiator) to be used in a second conversion to mechanical energy.

As a result of these differences, electric motors are about 85-95% efficient at converting electrical energy into mechanical energy, while gas engines are only (at most) 35% efficient, and diesel engines are about 45% efficient, thermodynamically speaking.

If you wanted to look at the rest of the energy supply chain, then obviously there are other factors at play, but even then the production of both electricity and gasoline is highly efficient, although with losses of their own. Anyone who wants to argue in favour of the ultimate efficiency of any fossil fuel though, would be instantly kicked to the curb by the efficiency of directly converting the sun's rays into electricity with solar panels at nearly any efficiency, rather than taking the route of converting the sun's rays into plant matter, letting most of it rot and release its energy back into the soil, adding a few hundred million years to the remaining 1%, and carefully storing it underground for the duration.

The ultimate efficiency of fossil fuel production is gob-smackingly terrible and time consuming.

Ernie
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    The question really is not about the efficiency of an electric motor vs a gasoline engine. It's about generating, transmitting, storing, and using electricity vs gasoline in a car. For the most part, somewhere in that chain on the electric side we're still burning fossil fuel to generate electricity - but it's in a utility-scale power plant that is far more efficient - and uses much cheaper fuel - than the gasoline engine. – Mark Dec 30 '16 at 22:18
  • Please [provide some references](http://meta.skeptics.stackexchange.com/q/5) to support your claims. Your expertise isn't enough to be convincing here. – Oddthinking Dec 31 '16 at 00:32
  • In the question, is asking why it is more efficient to go from fossil fuel -> electricity -> transmission -> battery storage -> electric motor -> wheel, than fossil fuel -> transport -> fuel tank storage -> internal-combustion engine -> wheel. You only address the last conversion in the answer. – Oddthinking Dec 31 '16 at 00:35
  • Heat has nothing to do with how a combustion engine works. Expansive gas does; heat is a byproduct. In fact, over-heating is a serious problem, so some mechanical energy is devoted to pumping coolant, not moving the wheels. That's why a car still runs right when you start it. You don't have to wait for anything to heat first before you can get kinetic energy from it. –  Dec 31 '16 at 06:32
  • @fredsbend: Combining one mole of C7H16 with eleven moles of O2 will yield seven moles of CO2 and eight moles of H2O. Thus, there would be a 20% increase in the PV product due to changes in molecular composition, but that pales in comparison to the difference in temperature between the gas in the piston at each point in the compression stroke and its temperature at the corresponding point in the power stroke. – supercat Jan 16 '22 at 23:01
  • @supercat I don't understand the point you're making. ICEs certainly make a lot of heat, but that's a byproduct of the compression stroke. The ICE is designed to utilize the power stroke, that is, expansion. –  Jan 17 '22 at 00:22
  • @fredsbend: If the reactions that convert C7H16 and 11O2 to 7CO2 and 8H20 were to do so didn't produce any heat, but happened instantaneously at the top of the compression stroke, the maximum energy that could be taken out of the system during the power stroke would be only 20% greater than the amount that was put in via the compression stroke. Most of the energy that is produced in the power stroke is produced as a consequence of the fact that burning fuel produces heat. Less than half of the heat energy gets converted into mechanical energy, but most of the mechanical energy... – supercat Jan 17 '22 at 02:12
  • ...is converted heat energy. The reason cars start quickly is that combustion directly heats the working fluid without having to heat the walls of a heat exchanger first. If mechanical energy wasn't a result of heat convention, by what means would it be produced? – supercat Jan 17 '22 at 02:18