# Homework Help: Determining the Efficiency of a Fuel

1. Feb 2, 2006

Hi,

I'm trying to calculate the efficiency of fuels such as gasoline, diesel, methanol, and ethanol.

I have collected several properties such as energy content per gallon (btu) and densities for each fuel.

I'm a bit confused as to how i can calulate these fuels' efficiencies. Traditionally, i was taught Efficiency=actual/theoretical * 100%

How can i approach this problem??

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2. Feb 2, 2006

### Staff: Mentor

Fuels, alone, don't have efficiencies. If you burn something, 100% of the energy of the reaction is available to you to do something with* - it's what you do with it that has an efficiency. So what are you really looking for here - efficiency of thermodynamic cycles? Cost efficiency? Energy per unit mass?

*Caveat - depending on how you burn it (how much oxygen and other gases, pressure, etc.), you may get more or less than the optimal energy from the reaction.

Last edited: Feb 2, 2006
3. Feb 2, 2006

Sorry i was not clear... I am comparing fuels to power cars/trucks. I want to determine which fuel is the most efficient at powering these vehicles. I guess i am looking for:

-which fuel will provide the most energy per kilogram
-produce the least harmful emissions

4. Feb 3, 2006

### FredGarvin

Again, like Russ already stated, fuels do not have an associated efficiency by themselves. The efficiency measure comes from the engine/machine that is using the fuel.

Emissions are also heavily dependant upon the process that is using the fuel. For example, two cars can have the same fuel, but because one of the cars has a higher combustion temperature, it will, most likely, put put more NOx.

Fuels will have parameters that you are interested in. For example, the energy per unit mass is stated in the fuel's lower heating value. It will be in units of BTU/lbm or MJ/kg. A quick look here: http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/img/500/fig_1-6.gif shows that there is an apparent correlation in fleet activities between a fuel's heating value and fuel efficiency. The rest of the article is here: http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/1_driving-performance/pg4.asp

You might also want to take a look here: http://www.f6rider.com/Valk2/every_thing_you_ever_wanted_to_k.htm [Broken]

Last edited by a moderator: May 2, 2017