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Old 07-15-2014, 04:24 AM   #1 (permalink)
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how fuel is burnt

Most of you never thought of a cylinder head as a condenser however at top dead center (where most of the burning is done) piston and cylinder head are only millimeters away from each other. That is why the mixture is in close contact with the cylinder head. E.G. a diesel running at lambda=3 with a cylinder head temperature of 90 C. Diesel fuel is a mixture with a mean of C10H22. From Raoults law we know all these different components will give one common vapour pressure. Let take C10H22 as the mean. At lambda =1 it would represent 1.7% of the moles in the mixture wich is also the partial pressure . At lambda =3 it is 0.5%. The vapour pressure at 80C below its boiling point will be 3% of a bar. This means that at 6 bars total pressure it starts condensing. I think a nice way to improve on the mileage is to add steam to process in such a way that the condensed fuel would land on top of the water. This would A. lower the pressure by condensating the H20 formed by the reaction before TDC (less NOx) B facilitate burning off from the cylinder head (giving better mileage and less soot). Most cars have a air weight sensor. so over the OBD you can acces this. I have a full description for gasoline cars but it is to lenghty for this forum


Last edited by peterrr; 07-15-2014 at 05:47 AM.. Reason: slip af a zero
 
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Old 07-15-2014, 05:04 AM   #2 (permalink)
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Diesel is injected into the cylinder under high pressure and the piston is at TDC only for a very short time. There is no time to make the diesel condensate.

Also, when the piston is at TDC most of the diesel will already have been burned.
The combustion reaction starts well before TDC, raising the temperature and rapidly lowering the remaining fuel content.

So I don't think condensation in the cylinder head is much of a problem.
Your block of text was nicely condensed though
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Old 07-15-2014, 05:27 AM   #3 (permalink)
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You could use water injection (search: Aquamist on Google) to run a higher compression ratio higher turbo boost. I doubt if it will be more efficient.
 
Old 07-15-2014, 05:54 AM   #4 (permalink)
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If the diesel was burnt before TCD your engine could ony run backwards
Condensation in a 1cm space with a pesrrue of 30 bars would be surprisingly fast I even looked up the average molecule speed (square root speed)
for c10h22 at 500 C (which is low for temperatures due to adiabatic compression).
Even these fat little buggers go at 9m/s =900cm/s. Let us define TDC as the time the piston has only travelled 1.5 % of the stroke from the top
stroke =0.5*(1-cos(CA)) then you see it lasts from -14 to +14 CA.

6000Rpm= 100Rps. time at TDC 28/360*10msec = 0,7millisecond
The average distance to the to travel 0.5 of a cm. I gues they can reach the cylinder head. And I even did not take into account the fact that there are lighter molecules in the mix which share the same vapour pressure.
You, better believe me ther is condensatio going on.
Or look at a truck on steep hill, Some soot will come out.

Last edited by peterrr; 07-15-2014 at 09:08 AM.. Reason: explanation to short
 
Old 07-15-2014, 05:59 AM   #5 (permalink)
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If you want a better mileage it is better to use steam by attaching a pipe to the exhaust.
You would get the evaporation heat of the water for free.
 
Old 07-15-2014, 07:10 AM   #6 (permalink)
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Quote:
Originally Posted by peterrr View Post
If the diesel was burnt before TCD your engine could ony run backwards
Of course not.

As the engine is turning over the momentum of the crankshaft and flywheel will push the piston to the TDC, then move on to the expansion stroke of the Otto cycle which does all the work.
The Diesel mixture will ignite as soon as the parameters for autoignition are met, which is either before TDC or never. That may happen at about 20 degrees before TDC, depending on a lot of variables.

Modern engines control the moment of combustion by injecting diesel into the cylinder at a precise moment in the cycle.
That will usually be advanced several degrees before TDC.

But don't take it from me, please read Ignition timing - Wikipedia, the free encyclopedia
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Old 07-15-2014, 07:27 AM   #7 (permalink)
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Quote:
Originally Posted by peterrr View Post
If you want a better mileage it is better to use steam by attaching a pipe to the exhaust.
You would get the evaporation heat of the water for free.
That is what Green Turbine is about.
Although it is possible to run a steam powered generator on exhaust gases, the net power gain does not outweigh (literally!) the extra mass, cost and complexity of the system.
Same for the Seebeck generators; you need a big cooling plate and still get just a few % back in power for all the heat that goes through.

As for now, in a moving application like a car, there is no effective way to use exhaust heat except for heating the cabin area with an air cooled engine.
I've seen the cabin heater of an old CitroŽn 2CV just the other day, doing just that; the heater is always on but vents out into the front wheel wells if not needed.
If you come across a 2CV look for the pipe ends right under the front fenders.

btw. there is no such thing as evaporation heat. Evaporation absorbs heat.
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Old 07-15-2014, 07:31 AM   #8 (permalink)
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How fule is burned in an intech document

look at the picture and weep
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Old 07-15-2014, 08:04 AM   #9 (permalink)
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If you have only so much fuel to burn and you want to introduce water vapour in the process it is countereffective to use the fuel to make the water boil and convert it into steam. Air cooled LPG engines also use the exhaust heat to vaporise LPG

Last edited by peterrr; 07-15-2014 at 08:09 AM.. Reason: thick old fingers
 
Old 07-15-2014, 08:05 AM   #10 (permalink)
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