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Diesel Fuel Efficiency Myths August 18, 2008

Posted by federalist in Energy, Transportation.

Do you get better mileage in a vehicle running on diesel fuel instead of gasoline?  Given the increased demand for diesel cars in the United States you would think so.  Congress seems to think so, since its CAFE regulations are based on “Miles Per Gallon” (MPG), regardless of whether those are gallons of diesel or of gasoline.

Any significant difference between diesel and gasoline disappears if we instead talk about “Miles Per Pound” (MPP), or “Miles Per BTU” (MPB).  A gallon of diesel weighs 7 pounds and contains 139kBTU of energy.  A gallon of gasoline weighs 6.2 pounds and contains 124kBTU of energy.  I.e., gasoline and diesel contain the same amount of energy per unit weight.  It’s just that gasoline is less dense, so it takes more volume (gallons) to hold a given amount of energy.

An unbiased measure of fuel economy would quote MPP, not MPG.

(The price per BTU (PPB) of diesel can be different from that of gasoline, but that is primarily a function of refining, storage, and distribution infrastructure.)



1. Tom - September 9, 2008

That’s an interesting thought except it leaves out one very important factor – the engine.

A gasoline engine cannot achieve the compression/power efficiencies available to a diesel engine. For that reason, more power will be derived from the diesel setup than is possible with the gas setup. There’s a reason the most efficient engine in the world is diesel (and it’s not due to the weight of the fuel).

No, changing the way fuel economy is measured doesn’t change the way power is produced. Diesels will always have more power produced and do so more efficiently than a gas engine ever could.

2. federalist - September 9, 2008

It is true that the higher compression of the diesel cycle is more thermodynamically efficient, so all else being equal we would expect diesel engines to rate higher in MPP. But not that much higher — estimates I see give it at most a 10% edge over gasoline cycle engines, not the 30% one might believe looking at MPG. And there are all sorts of practical considerations that come into play and that may still give gasoline engines an edge: How light and durable is the engine, what trade-offs are required to meet various emissions standards or performance objectives, etc.

3. federalist - February 7, 2009

As Larrick and Soll proposed in The MPG Illusion, we should also convert to the reciprocal measure: E.g., Pounds per mile. (Europeans already use these terms, quoting fuel economy in liters per 100km.)

Road and Track explains:

[MPG] is the reciprocal — the inverted version — of what we would actually like to measure. That is, we buy gallons, not miles. Reporting things in “miles per gallon” makes for topsy-turvy calculations and some very bizarre analyses.

4. gruntguru - October 11, 2009

Higher thermal efficiency based on higher compression ratio is only part of the diesel advantage.
1. Higher compression ratio.
2. Leaner than Stoichiometric Air-Fuel-Ratio. Gasoline engines need to run rich at high power levels.
3. Part load throttling reduces efficiency of gasoline engines.
4. Turbocharging further increases diesel efficiency by recovering waste energy from the exhaust.
On the downside, diesel efficiency increases with cylinder size so its advantage is less apparent in cars.

5. DGate - February 26, 2010

Gasoline engines need to run rich at high power levels….
Gee is that why my Honda Insight lean burn runs at 22-25 to 1 air fuel ratio and returns 83-93 mpg imperial.
Its not a diesel or turbo charged.

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