Some thermodynamics.........

[Jon_in_london]

Without resorting to horribly complicated differential equations..

Are internal combustion engines are intrinsically more efficient than external combustion engines? And if so why.

edit: any ideas on a double-acting internal combustion engine?

 

[Terry]

As I remember it (and first year thermo was like 20 years ago), the maximum efficiency of a heat engine depends only on the temperature difference between the "hot" part and the "cold" part. The specific details of the engine don't affect the theoretical efficiency. Of course practical considerations may mean that a particular configuration doesn't realise it's full potential efficiency.

 

[Jon_in_london]

I was wondering about the possibilities of an external combustion double acting diesel type thingy.

So was wondering why there dont really seem to be any external combustion pistons engines- wondered if it had something to do with thermodynamics.

 

[MRC_Hans]

Double-acting engines have been made. The Danish Burmeister & Wain ship engine builders (and pioneers, btw) have built a few double-action ship diesels. Also, I think the Germans experimented with them for aircraft engines during WW2. I think one of the problems with them is to get the connecting-rod out through what we might call the "lower top". Another, at least for the ship engine, was that it became VERY tall.

What I have wondered about is whethere anybody has tried to make a compound I/C engine. Adding low-pressure cylinders should increade efficiency.

External combustion: Steam engines are external combustion engines. And they're far from obsolete: Modern power stations use steam turbines.

Hans

 

[Jon_in_london]

Double-acting engines have been made. The Danish Burmeister & Wain ship engine builders (and pioneers, btw) have built a few double-action ship diesels. Also, I think the Germans experimented with them for aircraft engines during WW2. I think one of the problems with them is to get the connecting-rod out through what we might call the "lower top". Another, at least for the ship engine, was that it became VERY tall.

If you could gimme a link...
Yes, I know that there might be a problem with achieving a good seal with the con-rod poking out of the cylinder. Mind, they did it with steam and they can use rings etc.. to get a good seal with I/C pistons.... The engine need not become that tall as you could decrease stroke and increase bore to compensate.

What I have wondered about is whethere anybody has tried to make a compound I/C engine. Adding low-pressure cylinders should increade efficiency.

I have never heard of anybody doing this but it might be feasable.... I think though, it would involve using a normal I/C engine as a primary 'high pressure' chamber and then one or two more E/C expansion chambers running on the exhaust. Maybe too complicated......

In a triple expansion engine, is the biggest cylinder the high or low pressure one?

External combustion: Steam engines are external combustion engines. And they're far from obsolete: Modern power stations use steam turbines.

Dont most modern power station use turbines though? turbines are cheating

 

[UnrepentantSinner]

Are internal combustion engines are intrinsically more efficient than external combustion engines? And if so why.

I'm no mechanic, but I'd have to say yes. The combustion of the fuel internally allows the energy from the fuel to power the pistons much more efficiently than igniting it outside the engine. The energy just dissipates into the atmosphere and doesn't power the pistons at all.

some efficiency = more efficient than 0 efficiency.

 

[Jon_in_london]

Re: Re: Some thermodynamics.........

some efficiency = more efficient than 0 efficiency.

yeeeeeeeeeeeeeeees............

I'm no mechanic, but I'd have to say yes. The combustion of the fuel internally allows the energy from the fuel to power the pistons much more efficiently than igniting it outside the engine. The energy just dissipates into the atmosphere and doesn't power the pistons at all.

Sounds like a fairly good reason...
/thinking of ways around this......

 

[MRC_Hans]

If you could gimme a link...

I'll look for one

Yes, I know that there might be a problem with achieving a good seal with the con-rod poking out of the cylinder. Mind, they did it with steam and they can use rings etc.. to get a good seal with I/C pistons.... The engine need not become that tall as you could decrease stroke and increase bore to compensate.

Problem is, pressure and temperature is way higher in an I/C engine (incidentially, that is the reason it is more effective). Well you could also increase bore/stroke ratio on a single action. All else alike, the double action makes for a taller block. There is also the problem of lubricating and cooling the piston. ... I simply don't think the advantages outweigh the disadvantages.

I have never heard of anybody doing this but it might be feasable.... I think though, it would involve using a normal I/C engine as a primary 'high pressure' chamber and then one or two more E/C expansion chambers running on the exhaust. Maybe too complicated......

Well, that would be exactly my idea. One (single-action) low-pressure cylinder could serve two high-pressure, four-stroke cylinders. The low-pressure cylinder would have two-stroke action. I imagine soot deposit in the low-pressure cylinder would be a major problem, plus it would be very bulky.

In a triple expansion engine, is the biggest cylinder the high or low pressure one?

The low one, of course. As the steam expands, it is let to increasingly large cylinders.

Dont most modern power station use turbines though? turbines are cheating


I did saiy turbines. Why is turbines cheating?

And to US: The term "internal combustion" was made to differentiate it from the steam engine, where combustion happens externally to the engine proper, and the energy is transferred into the engine by the steam. Hence, a steam engine is an external combustion engine. There are others, e.g. the Sterling motor.

Hans

 

[Jon_in_london]

There are others, e.g. the Sterling motor.

It was actually the thread on the stirling engine that piqued my interest- that and a visit to the science museum which has a wonderfull gallery of engines from a newcomen engine right through to gas turbines (and a stirling engine too!).


I said
it would involve using a normal I/C engine as a primary 'high pressure' chamber and then one or two more E/C expansion chambers running on the exhaust. Maybe too complicated......

you said
Well, that would be exactly my idea. One (single-action) low-pressure cylinder could serve two high-pressure, four-stroke cylinders. The low-pressure cylinder would have two-stroke action. I imagine soot deposit in the low-pressure cylinder would be a major problem, plus it would be very bulky.

Hmmmmm... Im not sure we are talking about the same thing. Could you elaborate? Do you mean that you would be getting a very incomplete combustion on the low pressure and then scavenge the exhaust gas to charge another 4 stroke cyclinder?

/searches for patent lawyers phone number

 

[MRC_Hans]

OK, I assume there is no big invention here, heheh. Or somebody had already dunnit.

You said one or two L/P cylinders, but I asumed you meant one or two stages. My idea was a two-stage system: Imagine a two-cylinder, four stroke engine. Pistons go up an down syncronously, but have alternating work strokes, one per revolution. This is the traditional way to do it. Now, add a single low-pressure cylinder, of considerally higher volume than the others, where the exhaust from the two other cylinders is led. It will have (at least) three valves; two intake valves (which could be pressure-operated) leading in the exhaust from each H/P cylinder on alternate down-strokes, and one exhaust valve, made to open on each up-stroke.

Thats my two-stage internal combustion engine. Of course, the second stage is not really I/C, it is a pressure engine.

Interestingly, if this, against all odds, should be a novel and genial idea, it can no longer be patented, since I just published it, heheh, hehehh heheheh ... ehrm.

Hans

 

[Jon_in_london]

You said one or two L/P cylinders, but I asumed you meant one or two stages. My idea was a two-stage system: Imagine a two-cylinder, four stroke engine. Pistons go up an down syncronously, but have alternating work strokes, one per revolution. This is the traditional way to do it. Now, add a single low-pressure cylinder, of considerally higher volume than the others, where the exhaust from the two other cylinders is led. It will have (at least) three valves; two intake valves (which could be pressure-operated) leading in the exhaust from each H/P cylinder on alternate down-strokes, and one exhaust valve, made to open on each up-stroke.
Hans

Possible problem with this: The exhasut from the 4 strokes is normally driven out by the upstroke of the cylinder. Adding another 'pressure engine' cylinder onto the exhaust might well just act to apply pressure on the exhaust gases back into the 4 stroke cylinder thus pushing back against the rising piston, reducing power and filling the cyclinder up (partially) with burnt gas that would reduce the volumetric effciency of the engine. (sort of like sticking a banana up your exhaust pipe)

This is also the kind of thing I had in mind originally but then you got me thinking about using a 2-stroke with poor effciency that would only burn say 70% of the fuel/air charge (prolly fairly typical of a 2-stroke) and use the exhaust from this to charge a pair of 4 stroke cyclinders. The 4 stokes would obviously not be that effcient since most of the fuel/air charge would have been burnt already but it may be a good way of reducing the wastefullness of 2 stokes.

Of course, a 2 stroke petrol would be a problem due to the huge amount of gunky oil getting passed into the 4 strokes but to run a 2 stoke diesel.....it.....could.....work.....

 

[malaka]

internal combustion engines

Thought this was related to the fart thread.

 

[MRC_Hans]

Malaka: Heheh, wait for the biogas engine......

Mmm, jon.. of course it would give some back-pressure, but so does a muffler. The piston in the L/P cylinder would be at top then the exhaust valve opened (the H/P cyl. would be at bottom)and would be going down as the H/P piston went up. This would transfer the gasses to the larger L/P cylinder, and assuming appr. equal pressure in both cylinders during the stroke, the large piston would be the one receiving power.

One problem I see is that you get a large and bulky engine (the low-pressure cylinder must be several times the volume og the H/P ones. And at best a moderate efficiency gain.

About the two-stroke idea: Well, you could make a sorta afterburner there, but the idea should be to lower the temperature as much as possible through the system, because (termodynamics here) the amound of energy you get out is, ideally, proportional to the temperature difference from start to end, that is from the moment of combustion till the exhaust gasses leave the engine (not the muffler).

Hans

 

[Jon_in_london]

Mmm, jon.. of course it would give some back-pressure, but so does a muffler. The piston in the L/P cylinder would be at top then the exhaust valve opened (the H/P cyl. would be at bottom)and would be going down as the H/P piston went up. This would transfer the gasses to the larger L/P cylinder, and assuming appr. equal pressure in both cylinders during the stroke, the large piston would be the one receiving power.

That makes my brain hurt. Must think.....must think....Provided the piston is being driven soley by the expansion of the hot exhaust and not the pressue of the rising piston in the HP cylinder, then you are making a gain, Im not sure how big a gain, but better than nothing.

One problem I see is that you get a large and bulky engine (the low-pressure cylinder must be several times the volume og the H/P ones. And at best a moderate efficiency gain.

Yes, it would be getting large and complex, perhaps not to good for a chainsaw motor. But its interesting as an academic excercise to think of ways to improve piston engines.....

Mmm, jon.. of course it would give some back-pressure, but so
About the two-stroke idea: Well, you could make a sorta afterburner there, but the idea should be to lower the temperature as much as possible through the system, because (termodynamics here) the amound of energy you get out is, ideally, proportional to the temperature difference from start to end, that is from the moment of combustion till the exhaust gasses leave the engine (not the muffler).

Hans

Good point. Been getting sidetracked...

I wonder what sort of pressure the exhaust gases of a 4 stroke engine are?

 

[MRC_Hans]

Mmmp, gotta google a bit on that one. I tried on an estimate, but had to make too many assumptions.

Hans

 

[Ziggurat]

At least with an internal combustion engine, would trying to run a second stage cylinder really be any more helpful than using the exhaust to power a turbine?

It was also my impression that cars with turbo did exactly this, but with the turbine used to force more air into the cylinders (rather than output power directly).

 

[ceptimus]

Have you guys heard of the Still Steam-Diesel engine? It is basically a diesel engine and steam engine combined. The waste heat from the diesel exhaust and from cooling the cylinders is used to raise steam, which then acts on the back of diesel pistons for increased efficiency.

Here's a link

Main problems are high complexity and maintenance costs. When fuel is so cheap why bother?

ceptimus.

 

[Jon_in_london]

Here's a link

Main problems are high complexity and maintenance costs. When fuel is so cheap why bother?

ceptimus.

Thanks for the link. Bear in mind though, fuel wont be so cheap for much longer.

At least with an internal combustion engine, would trying to run a second stage cylinder really be any more helpful than using the exhaust to power a turbine? It was also my impression that cars with turbo did exactly this, but with the turbine used to force more air into the cylinders (rather than output power directly).

Running the exhaust through a turbine directly linked to the driveshaft may be the best thing to do... but turbines are cheating

Also, I wonder if the pressure of the exhaust gases are high enough to power a turbine enough make a real difference to the effciency of the engine, cant seem to google it though.....


The problem with a turbo is that there is a 'turbo hole' at low rpm so you dont get much of a boost except when the engine is reving very high. Also you dont get any improved fuel economy, just more powerfrom a smaller engine but use more fuel. You also get turbo-lag when you stamp on the throttle and it takes a moment or two before the exhaust hits the turbo and gives you a boost. You can see this on Kart drivers (turbo) who move onto F1 cars (no turbo), they also stamp on the throttle far too early and spin off the track.

A supercharger is similar but is powered from the drive train rather than through the exhaust. Therorectically this is less efficient but means you dont get the turbo lag or turbo hole.

Either way, the engine isnt any more efficeint except you get more power from a lighter engine.

 

[MRC_Hans]

Right. Turbos (although, I understand cheating) a way to use the energy left in the exhaust. It is generally used for creating higher power, but this also has an economy impact, since, all else alike, a high-yield engine is more economic than a low yield, for the same output. (This is NOT true for two-stroke engines, which tend to get very dirty at high yields)

Hans

 

[Jon_in_london]

Well, we seem to be coming back to the turbine as the most efficient device thermodynaically.. so... now the cheaters........

I know that big things like ships and tanks and aircarft use gas turbines but they arent used in small things like your car or your lawnmower/chainsaw. Apparently this is because they arent fuel efficeint, But if so why use them at all? Apparently they are only effecient when opperating at maximum revs but then why not just use a really small gas turbine to power a car? surely it would be the same as a big turbine in a big vehicle to use a small turbine in a small vehicle?

And I dont buy the 'turbines are more complicated and expensive than piston engines' thing because one reason why the Germans were so keen on jet fighters at the end of WWII is because they were easy to build.