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Liquid propane vs. gasoline for taxi fuel
Keeping the world's vital statistics

The campaign against breast cancer

Asia's threatened tropical forests

Gasoline vs. LPG (Liquid Propane)
by Robert Jackson 

There have been many criticisms about using Liquid Propane for transportation.  No one I know really seems to know much about it.  I've taken some basic chemistry concepts and applied them to this nonsense to come to the bottom of this once and for all.

 

There can be only one! 

 

• In this article, an Ideal case will be considered. Octane will be considered the main constituent of gasoline.
• Gasoline is a mixture of Hydrocarbons ranging from Pentane (C5H12) to Dodectane (C12H26), and other Isomers of these.

Combustion of Hydrocarbons:

• The above equation means that when you burn any hydrocarbon, Carbon Dioxide and Water are produced. 

Work:

 

• The total amount of energy consumed.

Comparison, Gas Vs Propane

Combustion of Octane:

• Delta H signifies a release of energy, which can be used to do work.

Combustion of Propane:

• Delta H signifies a release of energy, which can be used to do work.
  

From the comparison of the values, we can calculate that it takes 2.461 mol Propane to produce as much energy as 1 mol of Octane:

And from this we can calculate how many mol of Carbon Dioxide are produced:

So by using Propane, Carbon Dioxide output can be dropped a little bit:

Approximately an eight percent drop in CO2 emissions can be seen by using propane in place of octane. On a large scale, this could help with the global warming problem, but not that much.

Economicly speaking, the concentration of the propane gas would need to be measured to provide an explanation as to which of the two is cheaper, but as of this writing:

• cost of gas: $3.18 / gallon
• cost of propane: $1.99 / gallon 

Concentrations

• The Ministry of Commerce and Industry [[1]] website states that all "Liquid Propane should be no less then 90 percent per volume." pg 12 (Spanish) [[2]]

Energy Per Gallon

Propane:

Octane:

Oddly enough, you get more bang for your buck with propane when compared with gasoline.

How can we explain this? Why is it that the forces that be can make something that produces more CO2 and less energy, cost more then it's counterpart?

Even more interestingly, it also should be stated that a can of Diet Coke costs 55 cents for a 345 mL. and thus:

I should have bought stock on Coca-Cola...

Combustion

An example of a four-stroke combustion engine:

• 1. - intake
• 2. - compression
• 3. - ignition (energy released)
• 4. - exhaust

Intake

Entropy goes up as liquid is transformed into a gas and mixed with O2 in the correct proportions, remember:

The ratios of the mixture of Hydrocarbon and Oxygen need to be very close.

In changing from Gasoline to Propane, the injection ratios should be modified accordingly. Otherwise, power can be lost.

Compression

Entropy goes down as the gaseous mixture is compressed.

Also as concentration of O2 and Hydrocarbons goes up, the tendency to make CO2 and H20 goes up.

Preignition from heat of friction, and temperature increase due to compression, are of concern here

Preignition

For Octane:

 

For Propane:

• Simply stated in plain English, as the concentrations of Octane compared to the concentrations of Propane are greatly increased, the Octane will spontaneously combust before the Propane. Thus, propane is a better choice because of it's ability to be more greatly compressed before it self ignites. Propane is resistant to "knocking".

• Preignition isn't a problem with Propane.

Ignition

Energy is released!

The piston moves, converting the chemical energy to mechanical energy.

Exhaust

H2O and CO2 are removed, and the process starts again.

Optimization

• The whole thing turns into a huge optimization problem
• Where do we get the biggest bang for our buck???
  

From the Black Book, page 832:

• Large Electrical Generator.....................Mechanical.....-> Electrical.............Approx Efficiency=99%
• Chemical Battery..................................Chemical........-> Electrical.............Approx Efficiency=90%
• Combustion Engine...............................Chemical........-> Mechanical..........Approx Efficiency=25-30%
  

The efficiency of these devices is of major concern. Over the lifetime of one of these devices, the waste of energy can add up to an incredible amount.

Another concern of these devices are the waste byproducts, the fuel to convert to different form of energy should be a GREEN FUEL to begin with.

Conclusion

Propane has many advantages over Gasoline.  It's cheaper, better for your engine, and better for the environment.  Although Panama gets criticized for a lot, the government taking action over this issue is a great step in the positive direction.  I'd like to congratulate the success of LPG in Panama.

Considering all this however, efficiency is still too low for combustion engines.  CO2 emissions are still a problem with all fossil fuels.  With the environmental problem at hand, the best bet is to push for reform of the automotive industry, while simultaneously changing the consumer mentality that "Bigger is Better," and sway them into buying smaller more efficient cars.

Electric cars are extremely promising, but the automotive industry doesn't want to develop cars that will put them out of the job.  A good documentary on this topic is Who Killed the Electric Car?

Also in this section:
Liquid propane vs. gasoline for taxi fuel
Keeping the world's vital statistics

The campaign against breast cancer

Asia's threatened tropical forests