Improve the efficiency of a gas engine: add insulation?

neutrino_cannon

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neutrino_cannon
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It occurs to me that, gas laws being what they are, loosing heat from the burning fuel-air mixture of an internal combustion engine is loosing potential work, and heating up the cylinders and other engine parts moreover.

Would it be practical to line the cylinder walls with something rather less conductive than metal, both to improve efficiency and to keep the engine from heating as rapidly?
 
neutrino_cannon said:
It occurs to me that, gas laws being what they are, loosing heat from the burning fuel-air mixture of an internal combustion engine is loosing potential work, and heating up the cylinders and other engine parts moreover.

Would it be practical to line the cylinder walls with something rather less conductive than metal, both to improve efficiency and to keep the engine from heating as rapidly?
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There certainly is a lot of heat loss. But, engines do not reach peak efficiency until they are thoroughly warmed up. In fact, there are devices/systems in the car designed to make it warm up quickly and stay hot in order to stay efficient.

So, I suspect the real answer is that the internal combustion engine is just not terribly efficient. It may be better to focus attention on other means of powering vehicles. I like the idea of each wheel being a separate electric motor.
 
They have been working on ceramic engines for years. IIRC, if you drop them, they break :(

In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is unfeasible with current technology.
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http://en.wikipedia.org/wiki/Ceramic
 
Given the tendency of cars (at least in American movies) to explode when struck with a single (non incendiary) bullet, or driven over a bump, I can't help wondering whether engines running at 6000C are such a good idea.
 
madurobob said:
There certainly is a lot of heat loss. But, engines do not reach peak efficiency until they are thoroughly warmed up. In fact, there are devices/systems in the car designed to make it warm up quickly and stay hot in order to stay efficient.

So, I suspect the real answer is that the internal combustion engine is just not terribly efficient. It may be better to focus attention on other means of powering vehicles. I like the idea of each wheel being a separate electric motor.
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And power stations tend to how efficient?
 
You're correct that any heat leaving the cylinder -- whether conducted away through the cylinder walls or exported with exhaust gases hotter than the ambient cold sink -- is energy that might have been spent to do useful work. As so often happens, though, the devil's in the details...

Keeping the engine from heating isn't of itself really helpful. The hotter the engine, the less temperature differential to draw heat away from the gases that we'd rather they keep to do work for us. To at least some extent, a nice hot engine can preheat the reactants (fuel-air charge) for us, which puts that much energy back into the work-generating process. Of course, that heat can't work for you anymore if you let it get away (e.g. dump it out of a radiator).

Keeping the engine from heating "as rapidly" would be a transient solution only; whether the engine heats rapidly or slowly, if we run it long enough it will eventually get as hot as it gets. Call me an industrial pig, but I want my car engine to run all day at my whim; I don't think I'd like a dozen cool-down stops between Houston and Tulsa, especially if super-insulating engine materials required hours to get that heat out. On the other hand, this might be fine if you know you're only gonna run the engine (that way) for a time/times short enough to avoid or tolerate the resulting damage (e.g. wasn't the Mig-25 capable of catching an SR-71, but only *once* because the resulting thermal stresses would ruin the engine?).

While it's true that higher working fluid temperature in the cylinder offers a shot at greater potential thermal efficiency, when the parts quit working together your work output drops to zero (and your overall efficiency goes to 100% or zero, depending on whether you shut down or not). That's not really what you're asking about, though. Bearings, lubricants, and charge need to be held to temperatures at which their performance is tolerable. All the parts need to fit appropriately tight as they expand with temperature; parts with different thermal expansion coefficients (e.g. made of different materials, each appropriate for its function) expand at different rates. Higher temperature inside the engine means somewhere there are larger temperature gradients, which makes parts change shape as the hot side expands more than the cold side.

A less-conductive cylinder lining would reduce heat losses through the cylinder walls. It would also increase the thermal gradient through those walls, raising the "skin" temperature of the inner surface. Whether that's tolerable or doesn't sacrifice other important properties is a materials engineering challenge.

Keeping heat inside the cylinder is only one strategy for improving efficiency. Another important consideration is how much heat is "lost" through the cylinder walls compared to the amount exported with hot exhaust gases.

The important thing is not really to keep the heat in the cylinder, but to keep it "in the engine" until it can be converted to work. From a thermodynamic perspective it would be just as efficient to re-use heat leaking from the cylinder, or recovered from the exhaust stream. One might preheat the input charge (used for some big engines burning heavier fuels), or pre-compress the input charge (e.g. a turbocharger), or reheat working fluids cooled by their initial expansion for a second expansion (some steam turbines).

For a heat engine (ICE, turbine, ramjet, etc.) there's a lower limit to the fraction of input thermal energy that can't be converted to work but must escape as heat; see Carnot's work mentioned above. There's still a lot of room between Carnot's limit and engines sufficiently reliable, flexible, portable, and affordable -- that is, practical -- for consumer use. Most of the "easy" gains to close that gap have been taken; closing it further will involve tradeoffs of one sort or another (cost, performance, reliability, yada yada).
 
A regenerative steam plant is upwards of 80% efficient. Recovery of condensate, waste heat exchangers etc make it so. In diesel or gas turbine plants waste heat exchangers/boilers are also used to improve efficiency.
 
Here's an interesting idea for capturing some of that heat energy for work: a six-stroke engine. There are a number of challenges which make it not so ideal for automotive applications, but it might be useful for things like backup generators.
 
IIRC, the standard decription of the thermodynamic cycles in internal combustion engines assumes that the hot gas is in contact with the cylinder walls for such a short time that very little heat is transfered. Virtually all of the heat that builds up is from friction.

Conduction of heat through the cylinder walls would be a significant problem only for engines that operate at extremly low RPMs. In engine running at 3000 RPM, if I have done the math correctly, the entire thermodynamic cycle takes 0.04 seconds, with hot gases in the chamber for half that time. The amount of heat transfered in 0.02 seconds is negligible.
 
I'm a bit skeptical about the Crower six stroke engine. First, since most of the heat in an engine comes from friction, using the heat to create steam does not extract additional heat from the fuel combustion, it merely recaptures some of the energy lost to friction. Second, at 3000 rpm, the water would have 0.01 sec to vaporize and drive the piston downward. With a combustion chamber temperature of 204 C, as claimed, it may be possible, but I'd need to see some proof. Third, they claim such an engine would not need a radiator, water pump, etc. because the creating steam in the combustion chamber cools the engine sufficiently. But what about the heat from the crankshaft bearings, camshaft, valves, etc? Finally, they claim a 40% reduction in fuel use. Since most of the heat comes from friction, this would require that over 40% of a normal engine's power is wasted as friction. Plus, auto manufacturers would be all over any technology that actually reduced fuel use by 40%.

I would expect only a small increase in efficiency, if any, from such a scheme. But I'm willing to be proven wrong...
 
Patnray- The heat in IC engines largely comes from the combustion of the fuel. Only a very small percentage is frictional, larger amounts from pumping (air, lubricant, coolant).
Unless you know something I don't?

The specific volume of water at 320°F is 0.01765 cubic ft. / lb. of water. When water flashes into steam at 212°F, the specific volume increases to 26.78 cubic ft. / lb. The ratio of the final volume divided by the initial volume is 26.78 / 0.01765, which equates to 1,517 times its former volume. Therefore, when water vaporizes to steam (boils), it expands to 1,517 times its former volume.

Which is why steam engines can be efficient- steam is weird. Think triple expansion engine, of which the six cycle seems to be a variation
I have often wondered why steam powered cars haven't made a comeback. The external combustion can be controlled very well, and boiler technology has advanced as well. In theory, a steam auto could approach your electrical power plant's efficiency.
 
neutrino_cannon said:
It occurs to me that, gas laws being what they are, loosing heat from the burning fuel-air mixture of an internal combustion engine is loosing potential work, and heating up the cylinders and other engine parts moreover.

Would it be practical to line the cylinder walls with something rather less conductive than metal, both to improve efficiency and to keep the engine from heating as rapidly?
Click to expand...

What you suggest could be evaluated by looking at cylinder sleeves progressively using steel, inconel, and tungsten...

In practice there may be two sources of heat (energy loss).

First, the exhaust gas temperature. This can be used to drive a turbocharger, though, and thus is often recaptured.

The second source of heat loss is not the engine block, but the water envelope that cools it. Heat radiating off the block is modulated by the water/antifreeze bath, right? That is utilized for the heating system in the car, but granted, there is some waste of energy there.
 
A great deal of non-engine heat is generated and lost as heat-energy as well --- from the brakes, shock dampeners and internal component friction. But getting back to wasted engine heat, there is a method of recovering a good amount of exhaust gas heat/pressure energy, by means of an exhaust gas turbine supercharger ... otherwise known as a turbocharger. It allows small displacement engines to put out much higher amounts of power than their normally aspirated counterparts when called for, while allowing them to retain the efficiency of smaller displacement engines most of the time.
 
patnray said:
First, since most of the heat in an engine comes from friction, using the heat to create steam does not extract additional heat from the fuel combustion, it merely recaptures some of the energy lost to friction.
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Doesn't matter how you get extra energy, as long as you can get it.

Second, at 3000 rpm, the water would have 0.01 sec to vaporize and drive the piston downward. With a combustion chamber temperature of 204 C, as claimed, it may be possible, but I'd need to see some proof.
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Sure. I believe they have working models, and I'm sure that can be tested easily enough.

Third, they claim such an engine would not need a radiator, water pump, etc. because the creating steam in the combustion chamber cools the engine sufficiently. But what about the heat from the crankshaft bearings, camshaft, valves, etc?
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Much less friction at most of those points: you can use bearings (which role, rather than slide), and you don't need to create a seal at most of those locations. The few places where you can't, you're dealing with much less area and much less pressure differentials you're sealing in, so the heat generation should also be considerably less. As for whether or not it still needs a radiator, that probably depends upon the engine design. The original VW bug didn't need a radiator, and it didn't have any water cooling. Add on some radiator fins directly to this engine, and I wouldn't be surprised if you could get away without a water-pumped radiator, at least for certain sizes (and probably for sizes a fair amount larger than the VW bug's original engine).

Finally, they claim a 40% reduction in fuel use. Since most of the heat comes from friction, this would require that over 40% of a normal engine's power is wasted as friction. Plus, auto manufacturers would be all over any technology that actually reduced fuel use by 40%.
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That's assuming the implementation of this technology doesn't create bigger problems. I suspect it may, at least for cars. For example, you REALLY don't want to be injecting water into a cold engine, and you don't want to leave steam in the engine when you turn it off. Even in an engine where you don't worry about corrosion, you still have to worry about immiscibility with oil, and lubrication problems that could cause. That's probably enough to keep car manufacturers away from this thing - they like technology where you can just turn it on and off without dedicated warm-up and cool-down periods, because that's what customers want. So the failure of car manufacturers to implement this doesn't by itself mean that it doesn't work, or doesn't create significant efficiency improvements (though exactly how much is something I would also want independent verification of), just that it's not worth it for that application.
 
trvlr2 said:
Patnray- The heat in IC engines largely comes from the combustion of the fuel. Only a very small percentage is frictional, larger amounts from pumping (air, lubricant, coolant).
Unless you know something I don't?
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I disagree. Some of the heat from combustion drives the cylinders. Most of the rest is ejected in the exhaust. Only a very small amount is transfered to the engine block. The combustion heat is in contact with the cylinder for about 0.02 secs. All the thermodynamic descriptions of the heat cycles in IC engines treat as adiabiatic. Most of the heat in the engine block is from friction. It is possible, as some have pointed out, to extract additional work from the exhaust heat. You could use glow plugs to retain some heat in the cylinders for use in a steam cycle, but then you're just "stealing" heat from the power stroke.

Furthermore, no where near 40% of the combustion heat goes into the engine block, making the 40% improvement in fuel economy claimed in the link quite suspicious.

Yes, using steam to recover the frictional heat can recapture some lost power, but the frictional heat is a very small percentage of the combustion heat.
 
Combustion engines run at very high temperatures compared with "organic engines". I don't know which organism has the highest operating temperature, but I doubt it's above about 60 deg C. (There may be hyperthermophiles around volcanic vents, let's exclude those).

I wonder if there is some way we might use catalyst chemistry to produce power plants that run at far lower temperatures than combustion engines?
 
Soapy Sam said:
I wonder if there is some way we might use catalyst chemistry to produce power plants that run at far lower temperatures than combustion engines?
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Sure - as in fuel cells. But those are chemical engines. We're talking here about heat engines. As has been mentioned, in a heat engine the efficiency and power extracted depends on the ratio between the upper and lower temperatures in the cycle. For various reasons that means at least one part of the cycle has to be fairly hot.
 
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