Higher engine efficiency using insulating materials

[harveygp]

(This point came up in a previous thread but thought this deserved a new thread since there were no takers in the original - Mazda rx8 wankel efficiency etc)


GraviMan said:-

"Having seen carbon fibre reinforced aluminium pistons (I think they were cast in an oxygen free environment), I feel that auto engines materials could do with another look. Aluminium and magnesium alloys are just the start. Ford were looking at composite 2-strokes, at one point. Rover (pre BMW) developed an adiabatic ceramic coated engine, which needed no cooling water!"

and patprimer replied

"To be really effective, ceramic coatings need to be thick, to be durable, they need to be thin. If new ceramics are developed that will allow adhesion without de-lamination over a wide temperature range with a thick coating, a lot of problems will be solved, and all engines, wither rotary or piston will have substantial increases in their thermodynamic efficiency."

to which I commented

" 2) If you could raise the thermal efficiency using insulating materials surely you could use atleast say solid ceramic on the piston crown in a conventional engine. I thought I read somewhere that this doesn't work because with a high surface temperature the boudary layer becomes turbulent leading (counter-intuitively) to higher heat transfer in the piston crown. (Analagous perhaps to how snow can keep you relativeley warm if its -40 outside. Or the fact that double glazing is no more effective with separations greater than a certain critical dimension since convection takes over as the dominant heat transport mechanism."

Not sure in retrospect whether the analogies are particularly apposite but you get the idea - not everything in this world is linear and pridictable and counterintuitive effects are everywhere

Actually I've read a number of posts (not necessarily on this site) recently suggesting that if suitable insulating materials were available it would inevitable make higher efficiency engines possible.


I'm not convinced. To react to cyclical changes in temperature (thoughout say 4 stroke cycle) any insulating material would also need to have tiny thermal mass otherwise there's no advantage over an engine that simply runs at a higher average temperature. On the other hand, steel construction engines probably could be run hotter than they are in practice, which kind of suggests that thereis no advantage and probably disadvantage in having higher surface temperature. Also what about detonation. Would this be encouraged be higher surface temperature?

Any takers?

 

[patprimmer]

Higher temperature does lead to detonation, but with the correct fuel, compression ratio and ignition timing, better efficiencies should still be possible.

I think, durability under thermal shock, or from different expansions at a materials interface or limited ductility of the ceramic are problems proving difficult to solve.

Specific heat as well as temperature change is an issue, as is weight of the moving parts, and change of fit of parts at different operating conditions.

Regards
pat pprimmer@acay.com.au
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[harveygp]

Pat

Is it your feeling that (ignoring cost for the moment) that a solid ceramic topped piston, with appropriate fuelling and ignition would lead to higher efficiency?

And if so is would you say that is not possible because solid ceramic would not survive the conditions.

It seems surprising from an uninformed perspective given the relative toughness and heat tolerance of some ceramics.

regards,

geoff

 

[patprimmer]

Yes and Yes, but I am not an expert on ceramics.

As far as I know the problem is attaching the creamic parts to the metal parts with a durable joint and ending up with a piston of suitable weight.

Just a ceramic piston, but a metal head might result in lower efficiency as the compression might be lowered, and the heat losses to the water cooling system might not improve.

Regards
pat pprimmer@acay.com.au
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[patprimmer]

Basically, in principal, if you loose less heat to the cooling systems (oil and water) then you have more to convert to kinetic energy.

You will probably loose some of this to the exhaust, but that increases the potential to extract that with a gas turbine to drive an alternator to charge a battery to either drive the car by electric motor or to use electric motors to drive all ancillaries.

Please note, these thoughts are just speculative ramblings, and are not supported by data.

Regards
pat pprimmer@acay.com.au
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[harveygp]

Isn't the probelem that unless you can find a material with very very low thermal mass, effectively you have overall higher average surface temperature. But even if you go 100C higher that won't have a huge impact on heat input to the walls when most of it surely happens in the turbulant conditions of combustion and at the highest temperatures (1000C?)

On the other hand going 100C higher would start to affect volumetric efficiency in addition to aforementioned issues with detonation etc.

Also as I said before, I've certainly read at least one article/paper saying that raising surface temperature actually increases heat loss to walls due to a hitherto laminar insulating boundary layer being made turbulent by the addtional thermal energy.

Regards,

Geoff

 

[SBBlue]

To take full advantage of an adiabatic engine (ie, one with no heat loss) the expansion ratio would have to be increased -- from say, 8:1 to 18:1. This would lower the exhaust temperature by 500 deg F or so and more of the heat would be converted to mechanical energy.

As far as the compression ratio is concerned -- that couldn't be increased much beyond 10:1 without running into detonation problems. However, the absolute pressure could be increased by using a supercharger followed by a most excellent intercooler. The temperature of the gas mixture at full compression depends upon the compression ratio and the temperature of the gas as it is introduced into the piston. It is not a function of the pressure of the gas introduced into the cylinder.

What I've just described is the "Miller Cycle." And no, the expansion ratio doesn't have to be the inverse of the compression ratio. The compression ratio can be decreased by delaying the closure of the intake valve.

 

[ivymike]

The compression ratio can be decreased by delaying the closure of the intake valve
or by advancing the same, with less impact on pumping work, but a more difficult valvetrain design (short duration = vibration trouble).

 

[schmidtj86]

Goal (of efficiency): Get more power from less fuel
Method: Reduce heat transfer to (and from) engine via insulating combustion chamber (cylinder head, cylinder walls, pistons, valves). More heat remaining in expanding gas to push on piston results in more power from same charge.

Drawback: Increased heat to exhaust, but there is still more power taken from that same dose of fuel, so it still raises efficiency. Only way to get more from that charge would be to somehow allow the piston to travel farther on the expansion stroke, but that won't happen, so the energy would have to be taken from the exhaust temp some other way such as turbine stuff.

I can see some benefit (but not much) from delaying the intake closure and holding the throttle more open (via sophisticated valvetrain control and elec. throttle control (otherwise the throttle would have to be opened by the operator which could be percieved as torque request, or plain sluggishness if always on)) and advancing ignition timing. It's kind of the same logic as EGR, but EGR is better in that the compression still remains high and the ignition wouldn't have to be advanced as far. But if the intake is delayed this requires bigger cylinders which requires more reciprocating mass which reduces efficiency.

 

[GraviMan]

Agreed with all so far! Rover found that the only way to recapture the energy diverted from the cooling system to the exhaust was an exhaust tubine. I don't think the project went this far, though - it was kinda before my time.

The biggest problem with ceramics is the rejection rate from production. There is a good chance of cracking in componentry, and it's difficult to test every one. That said higher temperature resistance would reduce the need for cooling. Shame this line of reasoning was quashed by the "Belligerant Moral Wreckers" (I watched for 5 years as some highly respected engineers became more and more disillusioned - only for us to be collective blamed for a parent company launch cockup).

Anyhow, if BMW carry on insisting that hydrogen is best used in a thermodynamic engine, they are going to find that they need to repeat some of the work that went on in Gaydon (warwickshire). Materials and combustion process will all need the same attention to detail as a gas turbine. Plus ca change...

Mart

 

[SBBlue]

Problem with Exhaust Turbines:

They increase the work of the engine during the exhaust stroke.

Example:

Let's assume an exhaust gas temperature of 1645 deg F.

If we place an exhaust turbine on the engine that works at two atmospheres of pressure, the turbine will recover about 75 BTU/lb of exhaust gas energy. And that's not too shabby -- considering that the overall output of the engine will be about 280 BTU/lb of exhaust gas -- so we're looking at a improved efficiency of 75/280, or 27%, right?

But. . . .the work of the piston pushing out gas at 2 atm of pressure is about 34 BTU/lb, so our net output is really only around 40 BTU/lb -- still not too bad, an improved efficiency of 14%, right?

Not so fast there. These figures are only true if the pressure in the cylinder when the exhaust valve opens is at least two atmospheres. If it's less than that, than the engine will have to do extra work just to compress the exhaust gas to two atmospheres.

So what is the pressure in the cylinder when the exhaust valve opens? Here's some general figures; for a gas engine with a 10:1 compression ratio and the throttle at 100%, the pressure in the cylinder when the exhaust valve opens is about 2.7 atms -- so we should be okay with an exhaust turbine, right?

That is true -- if your engine always runs at 100% throttle. With most automotive applications, the engine will be nowhere near 100% throttle when the car is in cruise mode (60-70%). Instead, depending on the size of the engine and the car, it will be at 25-30%.

And when the intake manifold pressure is 25-30% of atmospheric pressure, than the pressure in the cylinder when the exhaust valve opens is. . . .about 1-1.2 atmospheres.

So although we have plenty of heat in the exhaust, at cruise the there isn't enough exhaust pressure for an exhaust turbine to function. No 27% efficiency improvement, no 14% efficiency improvement. Zero. Naught. None.

There is a rather ingenious, counterintuitive way to retrieve engine heat with a turbine under these conditions sufficient for a 10% increase in efficiency -- but thats another post.

 

[SBBlue]

Uh, in the last post the reference to cruise mode should be 60-70 mph, not 60-70%.

 

[patprimmer]

SBB

In a hybrid with a smallish petrol engine, those exhaust manifold pressures might change, especially if cam timing was designed to optimise a compromise between engine fuel efficiency and exhaust turbine efficiency.

Why don't you start another thread on your method of wasted exhaust energy

Regards
pat pprimmer@acay.com.au
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

Who needs ceramics

http://www.matmod.com/ProdandServ_files/CCComposites.html

 

[schmidtj86]

Doh!
What about atkinson engines that have a longer power stroke than the intake stroke. Does that count? It doesn't have the throttling loss (vs a larger engine taking in less air), it has the same intake and compression as a smaller engine, but benefits from a longer power stroke (to get more expansion work from the gases). I think there's a Ford Escape or some other hybrid with one.
There's a few interesting little websites on it. Check'em out.

 

[GraviMan]

Thanks for the link Unclesyd. Those look very familiar.

The more I learn about Detroit Diesel, the more impressed I am.

Mart

 

[Kiwikid]

You guys need to have a look at the Bourke Engine Project

http://bourke-engine-project.com/4436.html

 

[GraviMan]

Interesting, and well worth a visit. I take it the cycle has compression and expansion seperated?

Mart

 

[Rob45]

SBBlue:
Good analysis; not often found around here...

BTW, someone wanted an "adiabatic engine" running 18:1 compression, and having low exhaust temps - that pretty well describes a Diesel, don't you think?

Robert

 

[carnage1]

do ceramic coatings reduce quench?
it seems like they would. or is quench not a big issue on modern engines?