Expert: Kevin - 11/2/2009

Question
Dear Kevin.
I am currently doing a chemistry assignment on the validity of using LPG or pure Ethanol (not a mix of ethanol and petrol) as an alternative fuel to petrol for powering an internal combustion engine (either modified or unmodified).  I was wondering if you could please provide you expertise with regards to this topic.  The major factors to be considered are:
1.   the combustion energy of the different materials (per mole if possible)
2.   the initiation energy of the materials (to begin combustion)
3.   the chemical equations (polluting by-products etc)
4.   the pressure of the products (in KPa or atmospheres if possible)
5.   the problems/benefits of this substance (e.g. ethanol produces products at a higher pressure then petrol, and is therefore a more suitable substance for use in a combustion engine, as it will cause the piston to drive down with the same force as petrol, with a smaller volume of ethanol)
If you could please tell me what information you know about LPG, Ethanol and regular unleaded petrol, with regards to the above factors, I would greatly appreciate it.
Many thanks
Eddie


Answer
Hi Eddie,

This is too long and involved for me to do so I will supply some brief answers/suggestions.

Q1. You should look this up yourself, try Wikipedia.
Q2. This is the same for all three fuels as they use overkill to ensure ignition.
Q3. They are all hydrocarbons containing C, H and O and they will be burned under pressure in air. The combustion is never pure or complete and the by-products will be CO2, CO, H2O, soot, NOx from the air, and part-burned combustion products. Many of the pollutants will be removed by catalytic converters in the exhaust system.
Q4. You will need to look this up too. Bear in mind that the compression of the pistons means that you are at 8-10 barg before ignition begins.
Q5. The main problem with LPG is it is a gas and thus more difficult to handle than liquids. Other factors are to do with specifics of combustion engines and their settings (Compression ratio, bore, stroke, swirl, fuel/air ratio, etc. etc.) which are outside my experience.

Other issues are that petrol is cheap and easy to get from crude oil whereas ethanol would require more effort. Growing crops to make ethanol in a world that is short of food may not be an option.

I hope this helps
Best wishes
Kevin