/diy/ - Do It Yourself

So build one and put it in a car. what's stopping you?
wtf do you want from me? encouragement or something?
>you can do it anon
>i believe in you
there. now go fuck off to somewhere with no internet access and build your gay car.

>>2642338
Because fuel cost is only one factor in the design of a powertrain. Is coal or wood less expensive per BTU? Yes. But their energy densities are absolute shit compared to petrol. Coal has 1/5 the energy density, so to match the energy stored in the 21 gal / 79 liter fuel tank on my Toyota Tacoma, you'd need to fill the entire cargo area/bed 3/4 of the way up with coal (bituminous, stove sized nuggets). And lets not forget the more complex feed mechanisms for solid fuel, or that the emissions from it would be absurd compared to a petrol engine.

>>2642338
NASA actually tried it and got pretty good results. The fuel economy of the modified Chevrolet Celebrity was improved from 31 mi/gal to 41 mi/gal.
https://ntrs.nasa.gov/citations/19880002196

>>2642387

>>2642338
Because gas is cheaper than those other fuels, and stirling engines are inefficient and bulky.

>>2642388
Note that the Stirling engine used in the NASA test ran on gasoline and took 30 seconds to start.

>>2642338
The limitations that make what you suggest impractical are all well known and documented, going all the way back to when internal combustion engines were invented to overcome those issues and limitations inherent to external combustion / heat engines.

>>2642338
they're very large and heavy and produce very high rpms and very low torque

>>2642338
Various issues. In order to have a high efficiency in a heat engine you need to have a large temperature differential (described by carnot equation). The stirling engine is typically an external combustion engine, a hot side cylinder must be heated. The problem is the cylinder must have a reciprocating piston inside, lubricated, and within a certain tolerance. These material constraints limit your hot side temperature, and material choice, making truly high efficiency and good power density hard. Stirling engine in a car has been done, however not cheaply, and with poor power. Another issue for car applications is throttleability. Also they will never sound as cool as V8

>>2642391
They almost ran the sr71 off of a coal slurry but it didn't cool the surfaces well enough

>>2642338
They suck. You never checked because spoonfeeders don't. KYS.

>>2642338
>anything that burns hot on its hot end
and what, pray tell, of the cold end?
their power output is highly determined by their environment
this is not the kind of thing you can reliably market as a product let alone employ for transportation

>>2642338
I put a lot of thought and research into these a while back when I had the zany idea of building an untaxable home with no grid hookups and whatnot. tl;dr is that in a static application it's almost always better and more versatile to use heat to boil water and generate steam than to try and convert that heat directly into rotational energy with a heat engine. For a small home you can use steam to operate a steam turbine to generate electricity like you normally would, but you can also bleed off excess pressure to run a domestic heat loop, hot water heater, or various other steam powered machinery. Of course, the tradeoff is you have a pressurized boiler that you need to keep safe, which can be hard over a long time.

In small (genset-style) applications it's just basically a shitty ICE replacement with a slower start time and less torque and not usually worth it. Where stirlings have done will industrially is in gas-fired unattended power for things like compressor stations or remote platforms. The one company I know of actively marketing something in that space is Qnergy.

For the home-gamer, the best application would probably be if you don't want to manage the drama of a phase-change / boiler system, where you use it like a generator fired either by nat gas or a small rocket stove as I planned to do. You can run your hot water heater loop over the cold side and harvest the waste heat, and burn wood scraps or other biomass in a super efficient rocket stove to fire the hot side. Still, for my money I'd probably go with the versatility of steam.

>>2642338
While your idea is terrible, stirling engines make total sense for solar concentrator applications.

>>2642338
The most typical use of Stirling engines is for cooling. Primarily of optics.

>>2642391
>30 seconds to start
Where does it say that?

Stirlings can get an efficiency very close to the carnot limit but are heavy for their power output and allegedly have lubrication problems with their hot piston

>>2642445
>They almost ran the sr71 off of a coal slurry but it didn't cool the surfaces well enough
No, it had nothing to do with cooling and everything to do with the coal slurry destroying the engines.

>>2642864
It doesn't have to say it anywhere, it's the nature of a heat engine. First you need to generate some source of heat, then it takes time to heat the 'hot' side sufficiently for it to react, then typically (but not always) the crank will need something to start the first few rotations in order for the heat cycling to begin. Best case scenario, you're using a gas burner and it takes a few seconds just to ignite, then a few more to begin to warm the hot side, and so on.

>>2643080
>it's the nature of a heat engine.
Internal combustion engines are a form of heat engine, so clearly slow startup is not their "nature". Bunch of pseudoscientists in this thread, not that I'm any better...

The major reason Stirling and similar external combustion engines can't compete with ICEs is material limitations. As has been pointed out, the fundamental limit on how efficient a heat engine can be is determined by the absolute temperature difference between the hot and cold sides. In practical application, this almost always means the temperature difference between the working gas at the peak of the cycle and the end of the cycle.

For a given fuel, the stoichiometric flame temperature would be the same for both an internal and external combustion engine. The major difference is that, in an ICE, the fuel/air mixture IS the working gas. This a hugely important, because it means you don't need any additional material to heat up a secondary working gas to 1,500°F+. There are few materials that would survive any length of time in an engine at those temperatures, but the ICE doesn't have to. Because the fuel-air mix need only heat itself, the boundary layer formed at the inner surface of the combustion chamber can protect the engine from the peak temperature of the flame. No part of an ICE (short of maybe the very tip of the spark plug) ever gets anywhere near the temperature of the working gas.

External combustion engines don't have this luxury. Even ignoring the specifics of a given design (friction, pumping losses, heat exchanger efficiency, etc.), the fundamental fact remains that some part or other of an external combustion engine has to get stupid hot for it to be efficient. When using real materials, that kneecaps it before you even get to the losses from the flame>exchanger>working gas heat path that those types of engines necessitate.

They have their niche, but efficiency and power density aren't it.

you don't need a heat exchanger to heat up the expanding gas with fuel inside an ICE, just transferring the heat in a meaningful way with a sterling engine seems like a pain in the ass

cant you have the cold side buried under ground where its colder and the hot side facing the sun?

>>2643546
not really an effective cold sink, is it?

>>2643592
meant heat sink obviously

>>2643088
everything in the universe, including the entire universe itself, is a heat engine

>>2643546
can a sterling like this be built and how many kW of energy would it produce. there's always a difference in temperature on the surface and far below ground...

Several reasons.
A. The weight of the fuel must be considered relative to the weight of the engine, petrol has an incredible amount of energy in it per gram. The only advantage of the petrol engine is that it burns petrol. The petrol is the advantage in the system.
B. External combustion engines require an enormous amount of shielding in most applications, which can be heavy and bulky. The petrol engine is an INTERNAL combustion engine. It you've ever wondered what an external combustion engine is....yea that's the Stirling engine.
C. The engine doesn't produce much torque.i think that's the word. It doesn't handle resistance well. So fine if you're generating power or spinning some kind of enormous wheel but not good to power somthing like a car.
D. You can't really change the engine power very quickly which is limiting

You see these used underwater a lot because of the low ambient temperature and the way they can work in reverse (fun fact) to pump cold air. You also see them in low output low weight applications (space, mirror dishes).

Why aren't they used in RTGs? Well doubtless they are, probably in submarines or nuclear torpedoes. But most RTGs I've seen use far simpler systems where the core itself spins thus removing the need for an external kinetic energy store. Better to run an RTG hydraulicly as a closed system because just like petrol there's huge advantages in keeping the fuel INSIDE the engine

>>2643088
does the spark plug has to ignite the fuel air mix every cycle?

>>2643834
petrol yes, diesel no. petrol is vaporised and compressed and ignited at its peak pressure, with diesel the fuel auto ignites as it is injected into the cylinder
diesels used to have a higher efficiency because you could raise the pressure more before injecting while with petrol the pressure was limited by the auto ignition. if it ignited prematurely it would cause knock and could damage the engine
i'm using past tense here because i'm not sure anymore, i'm not up to date with the latest engine developments. i have no clue how far computer control is involved in current engines

>>2643666
but that's wrong you fucking retard
>In thermodynamics and engineering, a heat engine is a system that converts heat to usable energy, particularly mechanical energy, which can then be used to do mechanical work.[1][2]

>>2642338
You basically have to heat the fuck out of a piston or get shit efficiency. Also, external combustion does not capture the work done by combustion itself. Better off with a Brayton engine.

>>2642388
A Stirling engine is a lot larger than an internal combustion of the same displacement. An inline 4 is also twice as long as an equivalent V4. Why no comparison with a V6?

>>2642391
That's actually impressive economy, but it was built by rocket scientists, nonetheless. Nobody in their garage could make a Stirling engine that efficient.

>>2642396
And kept the limitations inherent to internal combustion.

>>2642419
>>2643710
They don't produce low torque.

>>2642786
No they don't. Growing biomass makes more sense.

>>2642972
>coal slurry destroying the engines
Turbine pitting? Otherwise, powdered coal could act as a lubricant if they removed the ash.

>>2643091
Well, you could develop a internal combustion "Stirling" engine, but at that point, you'd be better off with other engine types.

>>2643546
>>2643592
Maybe with a geothermal loop.

>>2642864
Elsewhere in the linked PDF.

>>2644072
>A Stirling engine is a lot larger than an internal combustion of the same displacement.
NASA's experiment showed that a Stirling engine with 84 horsepower fits in the same space as an Otto engine with 92 horsepower (though the scientists did have to install a bigger radiator designed for a 2.8-L V6 Otto engine), so displacement doesn't matter.

>>2642419
NASA's Stirling engine actually had better torque than the Otto engine.

>>2644101
was scratching my head how tf you'd get that rpm and power out of something that tiny, you still need to heat and cool the working fluid. appears that they compress and expand the gas inside a chamber that's basically needle tubes. apparently it werks, but that also raises the question why it didn't take off

>>2643689
>>2643546
It's possible, at least to run an electric generator. But since you're still dealing with a relatively small difference in temperature, you're limited by your volume, and to my understanding part of the issue is that most LTD engines are gamma designs, but you'd want an alpha or beta design for a geothermal engine. Not actually sure if that has an impact, but I think it does.
You also won't really get much power from it in spring or fall, and it will switch operating direction twice a year, which means you basically have to have two pistons instead of a piston and a displacer. On the bright side the PTFE you'd want to use for those is relatively easy to machine to tight tolerance and should be fairly dimensionally stable within operating temperatures.

Also the second end needs to be below the frost line, obviously

>>2644125
Details from the PDF

>>2644151

>>2644151
>>2644153
yeah so seeing how far you can push heat exchanger technology seems to be the name of the game here.

>>2644101
All I'm saying is it seems deceptive to list their displacements and not their actual sizes. I figured the sizes were roughly equivalent from the videos of it.

>>2644173
Pretty much. Detonation modes and high expansion ratios are the future of piston engines.

>>2644248
Info on size difference