Percentage fuel efficiency??

Question

I know that there are a number of factors that affect the efficiency of fuels, but I'm just wanting to know what efficiencies are of these 4 fuels - natural gas, coal, wood, ethanol. Please don't tell me their heats of reactions, but please tell me the efficiency of the energy within each fuel, and the efficiency is calculated by comparing the heat of reaction with effective output (first of all, does this mean to compare the potential energy stored within the fuel with the kinetic energy released when the fuel combusts? but then we all know that the energy content (H) cannot be measured before or after reaction, and only the change in H can be)...so then, how will I determine the efficiency of these fuels (please ignore cost, special designed equipment, environmental impact, etc., because I'm just wanting to know the efficiency of these fuels which are a characteristic of these fuels). Please help me! Thanks!

Answer 1

What you are looking for is Energy Density.  The amount of energy that can be stored, transferred, and then released.

Here are the values.

_______By Mass___By volume

-----------MJ per KG --MJ per Liter

Natural gas-53.6----------10

Coal---------32.5---------72.4

Wood------6-17---------1.8-3.2

Ethanol------30------------24

As you can see these two columns are very different because of the natural state each one exists in.   The gas has the most energy per mass because it has very little mass,  so a small amount has a lot of kick,  but per volume it doesn't show as much energy because gasses try to take up as much volume as they can.  So if you were able to contain that volume into a small space and ignite it you would be able to harness the energy per mass.  

The coal on the other hand is already a solid and compressed as much as it is going to get so it will show a much higher energy per volume but it's actual energy per mass is much lower.

http://en.wikipedia.org/wiki/Energy_dens...

Check out Energy Density on Wiki for more info.

Answer 2

It depends on what kind of energy you are yielding.  If heat is what's desired, natural gas and ethanol can reach over 90% if the combustion products are vented, and 100% if they are not vented.  Coal is about 65% efficient when used for heat and wood about 50%.

If mechanical energy is what's desired the efficiencies are much lower.  Natural gas and ethanol will be about 65% under the best of circumstances.  Coal and wood fall below 50%.

Answer 3

The most practical way is to compare the energy in and out of the entire "Process". For example, a electric power plant burns Coal, Oil, Gas, etc with a BTU energy/hr value. The process produces electric power in the form of Watts/HR energy. This is a good way to take into account the entire process efficiency.

For a car, it would be the BTU/HR in fuel compared to actual energy (HP) to the road.

The last time I looked into power plant process efficiency it was about 20-30% I believe cars are in this range also, but not real sure. I'm old and forget a lot.

Answer 4

efficiency is defined as the change in temperature divided by the hot temperature

e = (Th-Tc)/Th