Please explain MPGe

It is only efficiency if the energy sources are the same amount and the same chemistry.

 

??
MPGe converts kWh into the equivalent energy content in gasoline. Resulting in MPGe. A 25mpg car is also a 25MPGe car.
This allows the comparison of gasoline cars with EVs in terms of how efficient they are.
It does not compare cost, or pollution, or CO2. Those require a different 'measuring stick'.

 

I agree with @Zythryn that MPGe is a pretty good way to compare all types of vehicles from an energy efficiency perspective (wall/tank to wheel). This is why the EPA uses it. From the consumer's perspective, it's quite useful.

As for the above comparison, $0.07/kWh is only equal to $2.36/gal of conventional gasoline if you're talking pure energy, but what matters is putting that energy to use. Like @Oniki was getting at, although there are ~33.7 kWh of energy in 1 gallon of conventional gasoline, one must consider that it's much less efficient to combust the gasoline in an ICE for motion than to turn electric motors for motion.

• Say you drive 133 miles in EV mode. This is 133 miles / 133 MPGe = 1 equivalent gallon of gas. With 33.7 kWh/gal and $0.07/kWh, that's $2.36 for your trip.
• Say you drive those 133 miles in HV mode. This is 133 miles / 54 MPG = 2.46 gallons of gas. If you happen to find gas for $2.36/gal (same cost per unit energy as that from your power company), you're actually spending $5.81 on your trip due to the inefficiencies of the hybrid mode.
• For this example, it'd be cheaper to run on gasoline in hybrid mode if you could find gasoline for less than $0.96/gal.

 

I wish my electricity is $0.07/kWh. It is more like $0.21/kWh where I live. Meaning it will be cheaper to run a car strictly on gas only on HV mode, unless gas price hike above $3/gal.

 

Imagine four people:
One eats 2000 kCal a day of fibre
The second eats 2000 kCal a day of a balanced diet
The third eats 2000 kCal a day of sugar
The fourth eats 2000 kCal a day of protein

Per the EPA, these diets are equivalent

 

No, per the EPA they have the same kCal. That does not make them equivalent in any way other than the number of kCal they contain.

 

That is my point -- petrol and electricity are only the same in terms of kCal. In every other way they are different.

 

The problem is that a well-to-wheels number for either electric or gasoline is highly dependent on the time of day and physical location, and varies extremely widely.

Although, you can use averages, and I believe the EPA's CO2 numbers do that, but it's still only tailpipe for ICE/smokestack for EV emissions.

 

Others have already given good answers to this question. I would like to add my two cents, as I think this is a useless figure. A much better figure would be Miles per KWH! With that, you can multiply it by your local utility's rate for electricity and know how much it will cost you per mile. I don't think the car-makers like that figure as it is a pretty low number. 4 to 5 miles per KWH sounds pretty low compared to 60MPG or 130MPGe. It is however roughly equivalent to Cents per mile either way (at my exorbitant utility rate). The US average price for the electrical kind of juice is around 10c/KWH. Mine is 21c/KWH (40c/KWH if I go into tier 2*) and rising steadily. I only use EV mode in town as it is cheaper, and HV on the highway for the same reason. Most folks are better off in EV regarless. Sandy Eggo is even worse in that regard than the rest of California! *I would be in tier 2 every month if I didn't have leased (17c/KWH) solar panels!

 

The stickers also give the number of kWh to drive 100 miles. This can be easily used to calculate your cost to drive 100 miles.
They went with MPGe primarily because the U.S. uses MPG to measure efficiency. So MPGe uses the same ratio.
Many people, myself included, would rather have the more meaningful liters/100 miles many countries use.

So your point is you wish MPGe measured something other than efficiency?
There is lots of data on the stickers, feel free to use the correct measurements that are intended to measure the things you want to measure.

 

My point is that "efficiency" comparisons do not cross fuel types.

EV vs EV -- yes
ICE vs ICE -- yes
EV vs ICE -- NO

 

Sure they can.
Efficiency is a rather basic calculation. (Work out)/(Work in).
Units of energy of various forms of energy can be converted so that they are the same. Typically this is Joules, but the EPA figured a mpg would be less confusing. Thus, making the conversion you get MPGe.

Efficiency Formula

 

I think the point is that a plug/pump to wheels metric isn't actually comparable in terms of environmental impact - you really need well to wheels.

MPGw2wghge makes more sense than MPGp2wenergye.

 

The first part I completely agree with. MPG nor MPGe measure environmental impact.

Other tools are used for that type of measurement.

 

@Oniki, you forgot to finish out your analogy. We are discussing MPG (and MPGe), which are efficiencies. Output/Input. Your analogy has only discussed the inputs and not the outputs. Also, be careful with units: a capital "C" is already a kilocalorie, so a kCal would be a thousand kilocalories and your 2000 kCal is actually 2 million Calories! Anyways, remember the EPA performs tests to get the sticker MPG and MPGe values. The tests have varied over time and the exact test could favor some vehicles over others, but that's not what we're discussing here.

To finish your analogy, each of these people then needs to go for a run to burn those Calories. We'll let them run on flat ground (some might do better in a hilly region, but at least we're having an equal comparison by having them all run the same flat route). Also, let's add in a 5th person who eats a clay brick that has a mass equal to 2000 Calories (remember mass = energy via E=mc^2). Each person will make it a different distance based on their body's efficiency at processing that fuel. The person who ate a brick won't go very far because the body is not good at metabolizing clay bricks into usable energy. That's analogous to filling your fuel tank with, say, water. It might even ruin your body/engine. So yes, the fuels are not equal in that they are processed differently (or not at all), but that's exactly the point. That's a huge reason why electric vehicles are more efficient than gasoline vehicles. An efficient Atkinson cycle engine is doing great if it has a thermal efficiency of 40% (pretty bad). Most of the energy supplied to an engine is lost via heat transfer (thermal convection/conduction/radiation).

When comparing the miles/energy of a EV versus the miles/energy of a gasoline vehicle, you are comparing the energy input (Calories, gasoline, kWh) to the useful work output (miles). @Zythryn is right: it is indeed quite simple. It is a totally valid and fair comparison. This inherently takes into account the differences in processing that "fuel" and other factors. Maybe you don't like hearing the term "gallon equivalent" when it applies to EVs because they have no liquid fuel? You could easily turn the MPGe calculation into a kWhe (kilowatt-hour equivalent) calculation by converting the gasoline energy to kWh and then we'd talk about how an EV might get 3.95 miles/kWh while a gasoline vehicle might get 0.74 miles/kWhe. Now we're using electrical units for a gasoline vehicle?! It's still okay! It's energy! If you still have an issue with these general calculations of efficiency, please be detailed where the issue is. I'm a licensed professional engineer with a masters degree in engineering and I can always make a mistake but these calculations are quite basic.

 

No problem. You can eat paper, I'll eat pasta at a favorite Thai place.
(In case it is not clear, we are both eating a nice carb dish.)

We'll just be sure to consume the same amount of energy.

 

Correct. If it makes no sense to view the inputs as equivalent, the rest of the exercise is nonsensical.

 

Well it's apparent we've reached the end of useful discourse. Hopefully anyone who stumbles upon this thread will find it obvious which responses have more legitimacy than others. As a final example, my commute to work is totally electric in the Prime and at low speed without much traffic and I've been getting 5.1 miles/kWh. That 5.1 mi/kWh*33.7kWh/gal = 172 MPGe. My old vehicle got 22 MPG on the same route. Therefore, for the amount of electrical energy it takes my Prime to get to work, my old car would only make it 12.8% of the distance by combusting an equivalent amount energy in the form of gasoline. Hats off to the Prius Prime.

 

At least we agree that the MPGe metric gives no useful information.

Now tell we why CO2 is not a measure of efficiency ?

 

Generally, when people speak of efficiency, they are talking about utilizing the least amount of an input, while maximizing the output. This applies to anything (an efficient typer might put in the least amount of time to produce a given essay--the output). Context is important. For vehicles, this is generally energy in versus miles out.

Energy conversion efficiency - Wikipedia

CO2 is carbon dioxide. It is a form of air pollution and is a greenhouse gas that is causing climate change. For gasoline vehicles CO2 comes from burning hydrocarbons (gasoline) as well as all the processes that went into making the car and getting that fuel to a gas station, etc. For an electric car, if the electricity used to charge the vehicle came from a coal plant, that contributes to air pollution indirectly, etc. This is all generally discussed as part of an environmental conversation which can easily get complex.

You can have a very efficient machine that puts out a lot of CO2 or an inefficient machine that puts out none. They are not necessarily correlated and CO2 is not a direct measure of efficiency.