Fuel Efficiency

[Tempus]

One of the things that always bugs me when people are comparing the Environmental Impact of different types of fuel is the focus on how many gallons are used.

If you're going to treat Energy as a Resource, then you need to measure the Energy used by each option to get a fair baseline for comparison. Energy is fungible and every unit you don't use is available for other uses.

The first thing to remember is that Fuel Efficiency is not the same thing as Fuel Economy.

We all think in terms of Miles/Gallon when comparing Diesels, Gassers, and Alchohol Burners.

The problem is that Gallons of fuel isn't a fair measure of the Environmental Impact of fuel use, because not all Gallons are created equal.

This is the energy content of different fuel types.

Diesel 147,000 BTU/US Gallon
Gasoline 125,000 BTU/US Gallon
Gasohol (E10) (10% ethanol + 90% gasoline) 120,900 BTU/US Gallon
LPG 95,475 BTU/US Gallon
Ethanol 84,400 BTU/US Gallon
Methanol 62,800 BTU/US Gallon

From this it's clear that if you burn one Gallon of Diesel, you have used more energy than if you burn one gallon of Gasoline.

It also shows why the Mileage (Fuel Economy) of a car running E10 is less than a car running straight Gasoline. There's less energy available in a gallon of E10, so given equal efficiency, it won't move you as far.

If you had a furnace that ran on either Diesel or Gasoline, given equal efficiency in use for each, you could obviously stay warm longer on a gallon of Diesel.

If you assume that energy is a limited resource, and you want to maximize the use of each available unit of energy, then you have to talk in terms of BTUs

So, if you had four vehicles, a Diesel, a Gasser, an Alchol Burner, and an Natural Gas Burner, who could each get 50 miles per gallon, if you phrase it in terms of energy used, you get:

Diesel 2940 BTU/Mile
Gasoline 2500 BTU/Mile
CNG 1910 BTU/Mile
Ethanol 1688 BTU/Mile

Clearly, the Ethanol vehicle is the most efficient and would save the most energy, leaving more of the limited resource available for other uses.

Phrased another way, the mileage of the vehicles could differ, but they could be identical in efficiency and thus in energy impact.

If your Diesel uses 2940 BTU/Mile and gets 50 MPG

Then a Gasser using 2940 BTU/Mile and getting 42.5 MPG would have an identical energy use footprint.

The CNG car would only have to get 32.5 MPG to use the same energy per mile and the Ethanol Burner could get 28.7 MPG and still be equal to the Diesel in efficiency and conservation.

So, while the cost of gallons of various energy storage methods versus the miles per gallon used is the proper way to compute the bottom line effect on your pocketbook, it's not the right way to discuss the bottom line effect on energy conservation.

When someone tells me a Diesel gets X miles per gallon more than the equivalent Gasoline model, my brain always wonders if they are actually saving energy.

Well, here's a comparison.

Take a VW Golf Gasser versus Diesel, both Automatic.

Diesel EPA 37 MPG Combined
Gasser EPA 27 MPG Combined

Diesel is 37% more efficient in mileage (Fuel Economy)

But, to go 50 miles

Diesel uses 198,649 BTU
Gasser uses 231,481 BTU

The Diesel is indeed more efficient in Energy Economy, but only by 16.5%, and with a significant performance tradeoff.

Further, all this really proves is that the VW gasser is a hog by current standards

The Corolla Automatic at 33 MPG combined would use only 189,394 BTU to go 50 miles, and is therefore 1% MORE efficent than the Golf TDI.

Just my rant for the day

[molgrips]

What's important is the CO2 per mile. Diesels are usually much better than the equivalent petrol - have a look on www.vcacarfueldata.org.uk

The Prius has far better CO2 per mile than anything, even though its MPG is only a bit better than a good TDI.

Oh, and there's no performance tradeoff with TDIs - go and drive one, it's a blast.

[Tempus]

<div class='quotetop'>QUOTE(molgrips @ Sep 1 2006, 06:44 AM) [snapback]312653[/snapback]</div>

The Prius has far better CO2 per mile than anything, even though its MPG is only a bit better than a good TDI.[/b]

That's because C02/Mile is not dependent on MPG, which is precisely the point I was trying to make.

It's dependent on the amount of Hydrocarbons burned per mile, which is more proportionally related to the amount of energy in the gallon.

So, you do make a good point, but I'm not sure it's the one you intended.

Not only does a gasoline burner getting identical mileage to a diesel use less energy to move the same distance, it also produces less C02 in roughly the same proportion.

Which reinforces my point.

Measure energy not gallons.