Emission: HC's same as Gasoline?

[Raghav_Jagan]

It is well known that, Fuel is not completely burnt during the combustion process. So, the remaining fuel comes out as Hydrocarbons. Do these hydrocarbons posses the same characteristics as the fuel coming in?. For example, do they have the same Calorific value as the former? , will they be still in Atomized state??

[jdmccright]

Gas is a mixture of different lengths of hydrcarbon (HC) chains, more than 200 different hydrocarbon liquids ranging from those containing 4 carbon atoms to those containing 11 or 12 carbon atoms. [Ref: www.eng-tips.com, faq71-1552]

Under ideal stoichiometric combustion, every carbon-carbon and carbon-hydrogen bond is broken, releasing energy, and reforms into carbon mon/dioxide and water.

However, the reaction is usually non-stoichiometric, typically slightly rich to encourage complete combustion of the oxygen. The car's computer maintains this delicate balance between efficiency and power by monitoring the oxygen sensors in the vehicle's exhaust, specifically one located just before the catalytic converter.

So, a slight amount of leftover HCs are still present. These HCs would not be composed of the same mixture, as only some bonds would be broken. They would be composed of much shorter chains that would have less stored chemical energy. They also would ideally be in a gaseous state due to the remnant heat of combustion.

[Raghav_Jagan]

Right.. i understand it much better..
Would it be possible to use generate energy from these HC's??
Like, if i am to compress them and produce a spark. Will it give exothermic reaction,as it does in Fuel?

[jdmccright]

Unlikely, because the remnant HCs will be diluted with the CO/CO2, NOx, and water that makes up the exhaust stream. You would have to separate and concentrate the HCs to get any sustainable combustion.


[Ref: http://www.crypton.co.za/Tto%20know/...emissions.html]

Found this page...if it's true, a ~2% unburned fuel translates to 400ppm HC in the exhaust stream. Sounds like a lot but it isn't.

Could it be extracted to do other work? Maybe...right now all it does is generate heat in the catalytic converter as it is scrubbed from the exhaust. The heat could be harnessed to do work, but you can't pull too much heat from the cat or else it won't be hot enough to properly remove the HCs.

I envision a modern-day Stirling engine to use that waste heat to drive a mini-generator.

[Raghav_Jagan]

Exactly, if i could separate the HC's from NOx,CO,C and CO2..
will it seem possible?
So, i will have HC's alone.. bring stored up. My doubt was, if these HC's can give out energy spontaneously as the Original Fuel itself.. Although less, It is some work right?

[jdmccright]

You could separate the HCs from the waste stream, but remember that will require energy. So, the key would be to have enough net positive energy recovered to do useful work.

The resulting energy density would be alot less than gasoline as well. I imagine it to be a mixture of methane, ethane, propane, and butane, along with other sparse alkenes and cycloalkanes.

Finally, remember that the energy in burning gasoline is released two ways: 1) heat, and 2) gaseous expansion to drive the piston downwards.

1) Heat during combustion causes the pressure of the gases to increase, and can be estimated using the Ideal gas law, heat capacity of the product gases and based on the initial gas temperature and cylinder pressure at detonation.

2) Compare the molar change in gases present before and after combustion:

Gasoline (example octane):
25/2 O2 + C8H18 -> 8 CO2 + 9 H2O --- 13.5 moles of reactant produces 17 moles of product, an increase of 26%.

Ethane (theoretically recovered):
7/2 O2 + C2H6 -> 2 CO2 + 3 H2O ---4.5 moles of reactant produces 5 moles of product, an increase of only 11%

As you can see, gasoline has a clear advantage. More gases are produced and there is more mass to heat up to raise post-combustion pressures to produce the power. This is why that cars made to run on propane or CNG produce less power...there's simply less potential energy to start with.

[Raghav_Jagan]

Yes,yes.. i did have a thought that the energy released would be significantly lesser when compared to gasoline(Octane), although, i didn't think on the basis of Reactants evolved..
I guessed it from the Calorific value..
So,that is why we mix ethanol and Gasoline? Instead of Just Ethanol?
And 1 more question.. I did see that graph, long back, when i was starting to get doubts on IC engines..
I didn't know, how 2% make 400 ppm of HC, i mean.. how did you arrive at that?

[jdmccright]

Energy density is based on the number and type of molecular bonds that are available to be broken...longer chain HCs have more C-C and C-H bonds per molecule than shorter chain HCs. Gas also has more complex HCs, too, with benzene rings and C=C double bonds...more potential energy.

Ethanol is mixed with gasoline purely for emissions and smog reduction, usually in wintertime. Though more recently it has been used simply to cut the fuel, "water it down" so to speak, to make the supplies last a little longer and shift some production to the US, albeit at a great cost...but that's a whole other topic.

The ethanol molecule contains oxygen (C2H5OH), which when present also helps to complete combustion when temperatures are colder. There is no power advantage to it; it has been shown to actually reduce a vehicle's mileage when it is used to oxygenate fuel.

Finally, regarding the %/ppm translation: I pulled that comparison from the source: 1% of unburned fuel translates to 200 ppm HC, so 2% would equate to 400 ppm. That is different from a strict 1% HC concentration in air, which is 10 ppt or 10000 ppm. The source also doesn't note if that is pre- or post-cat but I would assume for the purpose of the article it is pre-cat levels.

[Raghav_Jagan]

I still don't get how 2% converts to 400 ppm.
I saw in another site, that showed the Level of HC's were inbetween 150-50 ppm for different A/F ratios..
On the other hand, i read through a book that says, 6000 ppm of HC are formed in the exhaust that makes around 1-1.5%..

[jdmccright]

I haven't the time to go through a calc iteration, and there are many variables to pin down...cylinder air volume, manifold vacuum, intake air temperature, A/F ratio, exhaust temperature, etc. You'd really need a spreadsheet to set up the formulas, plug in the variables and spit out an answer. In short, those calcs are probably for one assumed set of conditions. If I had the time, the nerd in me would attempt it.

My suggestion is to work on a recovery method that will accept a range of concentrations.