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Author Topic: Engine exhaust temperature versus flow: what are the effects?  (Read 14415 times)

Offline peppercorn

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Several configurations of IC engines (especially marine use) employ exhaust manifolds incorporating a water jacket to cool the exiting gases (a safety feature I believe).

This leads to a loss of efficiency due to a back pressure in the cylinder as the gas cools.

This led me to wonder whether injecting atomised water directly after the exhaust port followed by an expander box would allow a drop in exhaust temp with no loss of gas velocity.

The premise is replacing high temp/low volume with lower temp/higher volume: The pressure remains constant... or at least I think it does!


 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #1 on: 13/08/2008 23:17:02 »
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This leads to a loss of efficiency due to a back pressure in the cylinder as the gas cools.
That doesn't seem right to me; the manifold pressure would be less, if anything, if the gas cools. But, at the rate that an engine works, there is very little time for any significant cooling - it is basically an adiabatic process. Your water injection could speed the cooling, true, but it would be no good for the water to boil; that would increase back pressure. How much water would you propose to carry with you for the cooling and yet avoid boiling? It's heavy stuff for a land transport.
Tuning the exhaust achieves the desired reduction in manifold pressure quite effectively, over a narrow range of revs.

My marine engine doesn't use a water jacket - it mixes the exhaust with the cooling water as most of them seem to. It's nice to have an exhaust pipe made of rubber which never gets hotter than 100C.

 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #2 on: 14/08/2008 10:45:46 »
The standing wave in the exhaust is indeed only tuned to a particular rev.
Could injecting water at a increasing(?) rate through the rev range keep the exhaust tuned dynamically?
Some sports cars have valves that enable two lengths of exhaust depending on high or low rev.

I realise now my intial idea is perhaps the wrong way round.
For instance, would steam condensing into a pipe of moving cold air part way along its length cause the air to move quicker?
 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #3 on: 15/08/2008 12:33:56 »
I've heard this said before - an ICE is an adiabatic process.
This always confuses me: so does the working fluid (mainly Nitrogen) surrounding the bang in a spark ignition engine really just stop the walls of the cylinder melting whilst having enough volume of O2 for stoichiometric ratio to be met? i.e No real heat is transferred to the N2.

But if this was the case the engine wouldn't need cooling....
 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #4 on: 15/08/2008 17:17:24 »
It is called adiabatic because most of the gas, after getting its input of energy, 'keeps that energy' because there is not time to attain equilibrium. There is significant heat loss from contact with the walls, of course. But I think that the majority of the waste heat comes out in the exhaust gases.
The nitrogen doesn't 'surround the bang, does it? Its intimately mixed with the oxygen so it is 'part of' the bang; its temperature is the same as that of the oxygen, fuel and combustion products.

Your idea of condensing water vapour would be along the lines of a steam engine condenser. Unfortunately, you would need to cool the exhaust gases an awful lot before that happened. The steam engine condenser only works with steam at just over 100C and, even then, can only be used on static or marine engines because of the large amount of cooling water needed.
 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #5 on: 16/08/2008 16:58:18 »
SC said "The nitrogen doesn't 'surround the bang, does it?"

The way I picture it (& this may be wrong) is that there is not time for all the O2 throughout all the air in the cylinder to burn with the fuel -here I'm thinking of a fuel injection setup with lean burn ratio. So the actual locale where the bang is (near the spark plug) is surrounded by a charge (ie the working fluid - ~80% N2). It plays no part in the burn but is well suited in protecting the cylinder walls from destructive high temps. Clearly with a compression ignition the situation is more amorphous.

A carb engine is different -with air & fuel premixed, but control of the mixture, being reliant on mechanical tolerance, etc brings other problems.

Now I write this I am aware that injection engines use gas swirl techniques for mixing so my localised explosion view is incorrect: This tells me that the injected fuel must occur well before the air is fully compressed -so mixing can occur.

Assuming I have got analogy right thus far:
How would an engine behave if a plume of O2 was injected just a degree before TDC with a 'seed' of neat fuel at the centre -right on top of the spark plug - ie. heat source & working fluid were (almost) independent (like a external heat engine) -still having the advantages of ICE's?
 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #6 on: 16/08/2008 16:59:36 »
Discount the fact the spark plugs WOULD melt!
 

Offline turnipsock

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Engine exhaust temperature versus flow: what are the effects?
« Reply #7 on: 16/08/2008 21:39:33 »
doesn't mixing carbon monoxide with water produce some sort of acid?

I not even sure a water jacket around the exhast is there as a safety feature. It's more likely that its there as a free heat source to heat water for some other purpose...it's better than chucking out all that free heat out the back of the boat.

Spark Plugs!? in a diesel?
 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #8 on: 17/08/2008 00:21:52 »
In a reasonably efficient ic engine, the fuel is mixed well with the air and it pretty well all burns. With a petrol engine, the flame front is moderated to travel slowly enough (using high octane) to prevent pinking but it burns all the fuel and heats all the air. In a diesel engine, all the fuel ignites at the same time - just as it is injected. The point is that there isn't a cold region and a hot region; there would be no point in not heating up all the cylinder contents. The reason that the metal doesn't melt is that it conducts the heat of gases in contact with it away and its temperature averages out to well below the melting point.

CO2 dissolves in water to form Carbonic Acid.

A simple raw water cooled engine mixes the exhaust with the cooling water - there isn't a 'jacket'.
An indirect cooled engine has a water jacket around the engine, a heat exchanger and seawater also cools the coolant. It's then mixed with the exhaust. In both cases, the exhaust is cool, compared with a vehicle engine.
A calorifier may be used to provide 'domestic' hot water in both cases.
 

Offline turnipsock

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Engine exhaust temperature versus flow: what are the effects?
« Reply #9 on: 17/08/2008 00:35:34 »

CO2 dissolves in water to form Carbonic Acid.

A simple raw water cooled engine mixes the exhaust with the cooling water - there isn't a 'jacket'.


I thought most engines kicked out carbon monoxide, not carbon dioxide.

I still can't understand why cooling the exhast gases from an engine would keep the engine cool.
 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #10 on: 17/08/2008 12:11:43 »
They would be pretty inefficient if they didn't burn the carbon completely! (Traces of CO may be there but that is a contribution to inefficiency.)

I never said that cooling exhaust gases keeps an engine cool. It keeps the exhaust pipe cool (rubber on boats, even) and could help a bit to lower exhaust manifold pressure and help to shift waste gases. There may have been some confusion.
« Last Edit: 17/08/2008 12:14:55 by sophiecentaur »
 

Offline turnipsock

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Engine exhaust temperature versus flow: what are the effects?
« Reply #11 on: 17/08/2008 15:09:52 »
There may have been some confusion.

Well I am well confused.

Most boat engines (including outboards) are cooled with sea water. The cooling water usually comes out the same hole as the exhast. The cooling water runs round a jacket that goes round the block and cyclinder head.

A lot of exhast systems now have a catalyser and these need to be hot to work efficiently, so what is the point of cooling down another part of an exhast pipe? It would seem silly to use power to pump water around an exhast pipe just to cool it down. Even if there was a loss of efficiency, it would not be as great as the power used to pump water around a cooling system for an exhast pipe. Why not just insulate the pipe?

If the pressure in the pipe is inefficent, why not just change the bore of the pipe?
 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #12 on: 17/08/2008 18:50:19 »
Pretty much all marine engines are cooled by seawater - natch. Raw water cooled engines circulate seawater around the block. THEN, the water (which is being pumped anyway) mixes with the exhaust in the manifold, not using any more energy than pumping it over the side.
Freshwater cooling is better as the corrosion is less so that's what most modern engines use. BUT the freshwater is cooled using seawater which is, again, mixed with the exhaust. It is a big advantage to have a cool exhaust system but a luxury you can't have in a car. In an enclosed space a hot pipe is a fire hazard - running alongside cables and, possibly, gas pipes  (and people working in the engine compartment) in a boat.
Note, I said water is mixed with, not pumped round the exhaust system.  It is a total loss system, unlike in a car.
It wasn't my idea to introduce changes in the exhaust system - I was just replying to a question about it with some facts about marine engines.
I expect modern marine engines use catalysers but they still have to get rid of hot gases, afterwards, through a long pipe. It would be unlikely that they don't use the waste cooling water to cool this, too.
The only air cooled marine engine I know of is an old Lister diesel (not marinised), used on some small,  open work boats such as foot passenger ferries. All the others can be seen to shoot water out of their exhaust outlet. Someone may tell me of exceptions, of course. I don't think there's a single one around the Solent.
 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #13 on: 19/08/2008 12:00:51 »
This is not what I had in mind originally (although I've considered such a possibility but dismissed its practicality):

BMW's turbosteamer concept, see -
   http://www.gizmag.com/go/4936

Obviously they are not mixing water with the exhaust, but using a heat exchanger.

I've always wondered why CATs are so far from the engine on cars when they have to get to working temp before having an effect.
 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #14 on: 19/08/2008 19:25:20 »
How much (purified) water would this turbosteamer need to carry to accompany a tankful of petrol, I wonder? On the face of it, it could work. I wonder how the optimum speed of a steam piston engine squares with the 5kRPM of an IC engine?

The spacing of the CAT from the engine could be to allow the gas flow to settle down, rather than the relatively short pulses at the exhaust port(?).
 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #15 on: 24/08/2008 20:47:51 »
How much (purified) water would this turbosteamer need to carry to accompany a tankful of petrol, I wonder?

I'm pretty sure BMW's design condenses & recirculates the water.
It must add quite a lot of weight though. And for only 14hp extra!

There must be a more efficient way to use all that waste heat - A Stirling engine perhaps?

I would have thought more than 14hp is lost in parasitic loads on a modern car. A Stirling engine could drive the alternator, pumps, etc without the need to couple it to the crank.

I'm pretty sure BMW's design condenses & recirculates the water.
It must add quite a lot of weight though. And for only 14hp extra!

There must be a more efficient way to use all that waste heat - A Stirling engine perhaps?

I would have thought more than 14hp is lost in parasitic loads on a modern car. A Stirling engine could drive the alternator, pumps, etc without the need to couple it to the crank.
 

lyner

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Engine exhaust temperature versus flow: what are the effects?
« Reply #16 on: 25/08/2008 00:40:34 »
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I'm pretty sure BMW's design condenses & recirculates the water.
Can it really be possible? I think you would have to shift even more heat than the radiator does, already if you wanted to condense all the steam produced from such a steam engine. The temperature difference which you could work with, if you used a Stirling engine would also be limited by the same consideration.
Why not just drive a turbine with the exhaust gases. A slow response time wouldn't matter if you were using it for battery charging etc,
 

Offline peppercorn

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Engine exhaust temperature versus flow: what are the effects?
« Reply #17 on: 26/08/2008 12:03:45 »
Why not just drive a turbine with the exhaust gases.

Like this?
http://www.integralp.com/SuperGenArticles.aspx

I somehow feel fluid-based energy systems (air/hydraulic) should be inherently more efficient that electric. Plus cheaper & with less manufacture/lifecycle impact.
Conversely, batteries (I believe) offer higher energy densities than an accumulator, so swings & roundabouts...
 

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Engine exhaust temperature versus flow: what are the effects?
« Reply #17 on: 26/08/2008 12:03:45 »

 

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