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What contributes to steam engine efficiency?
- Thread starter macardoso
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The stream engine is a marvelous thing, but it needs to be re-imagined. The concept of a large boiler is antiquated. They should use micro boilers and inject minute amounts of water to create the amount of steam needed based upon load and throttle demand. Perhaps even per-cylinder micro boilers. As those boilers are much smaller, and dealing with tiny volumes of water, they can run much hotter, and under higher pressure when water is present. If the cylinders themselves were superheated, then each cycle could be driven by metering a small amount of water in the cylinder which "flash boils" when it comes in contact with the hot cylinder walls. This way, you dispense entirely with a boiler. (similar concept to gas engines which are direct injected).
The slide valve is a great idea whose time has passed. A rotary valve would clearly be the way to go for both intake and exhaust. It would tolerate higher RPMs as well, extending the useful RPM range the steam engine operates at. It would not be reliant upon kluge-fixes like springs to make them work.
Single cylinder designs are certainly nostalgic, (I loved my old Mamod engine as a child), but a two cylinder engine which is dual reciprocating and the cylinders cycle 90 degrees out of phase with each other would eliminate dead spots at start-up time.
The slide valve is a great idea whose time has passed. A rotary valve would clearly be the way to go for both intake and exhaust. It would tolerate higher RPMs as well, extending the useful RPM range the steam engine operates at. It would not be reliant upon kluge-fixes like springs to make them work.
Single cylinder designs are certainly nostalgic, (I loved my old Mamod engine as a child), but a two cylinder engine which is dual reciprocating and the cylinders cycle 90 degrees out of phase with each other would eliminate dead spots at start-up time.
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You should look up the Unaflow engine and the Corliss valve scheme. The last stage of piston steam.
You want a crisp on/off.steam admisission, expansion (how much, I dont know right now), exhaust to the next stage, close, and partially decompress before more steam admission. The bottom stage should exhaust to condensing pressures. There is a limit/optimum in the pressure ratio per stage.
Electric poppet valves sound very promising! There was an article on an electric valved IC engine a few years back. More likely to succeed than pneumatic, in my opinion. Fewer things in the chain. Balance the pressure to cut down on forces and/or pilot a big valve.
Sorry, but three margaritas are, um helping ;-)
Sent from my SM-G892A using Tapatalk
You want a crisp on/off.steam admisission, expansion (how much, I dont know right now), exhaust to the next stage, close, and partially decompress before more steam admission. The bottom stage should exhaust to condensing pressures. There is a limit/optimum in the pressure ratio per stage.
Electric poppet valves sound very promising! There was an article on an electric valved IC engine a few years back. More likely to succeed than pneumatic, in my opinion. Fewer things in the chain. Balance the pressure to cut down on forces and/or pilot a big valve.
Sorry, but three margaritas are, um helping ;-)
Sent from my SM-G892A using Tapatalk
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Was reading about uni-flow engines on Wikipedia and came across a statement which tied back to my first post:
"The exhaust ports are only open for a small fraction of the piston stroke, with the exhaust ports closed just after the piston begins traveling toward the admission end of the cylinder. The steam remaining within the cylinder after the exhaust ports are closed is trapped, and this trapped steam is compressed by the returning piston. This is thermodynamically desirable as it preheats the hot end of the cylinder before the admission of steam. However, the risk of excessive compression often results in small auxiliary exhaust ports being included at the cylinder heads. Such a design is called a semi-uniflow engine."
So it looks like my comment about having the exhaust port open as long as possible is incorrect. It is desirable to allow the gas to be re-compressed on the return stroke (stealing some shaft power) in order to heat the admission end of the cylinder and avoid losses of the hot inlet steam cooling against the cylinder walls and loosing pressure before ever having moved the piston.
So much more reading to do. Thanks guys for having this conversation - it is fun. I'd love to design my own engine to be machined, but I fear it may be above my knowledge level at this point in time.
"The exhaust ports are only open for a small fraction of the piston stroke, with the exhaust ports closed just after the piston begins traveling toward the admission end of the cylinder. The steam remaining within the cylinder after the exhaust ports are closed is trapped, and this trapped steam is compressed by the returning piston. This is thermodynamically desirable as it preheats the hot end of the cylinder before the admission of steam. However, the risk of excessive compression often results in small auxiliary exhaust ports being included at the cylinder heads. Such a design is called a semi-uniflow engine."
So it looks like my comment about having the exhaust port open as long as possible is incorrect. It is desirable to allow the gas to be re-compressed on the return stroke (stealing some shaft power) in order to heat the admission end of the cylinder and avoid losses of the hot inlet steam cooling against the cylinder walls and loosing pressure before ever having moved the piston.
So much more reading to do. Thanks guys for having this conversation - it is fun. I'd love to design my own engine to be machined, but I fear it may be above my knowledge level at this point in time.
Just wondering, is everything lagged, is super heating used, what would be the typical operating pressure and temperature?
I have no direct experience with piston systems just Navy turbines. I think the Navy systems operated at lower pressures and temperature compared to land based. 1200 psi & 1400 F. Vacuum exhaust, Main engine turbines 70,000 shaft HP X four. 25' props @ 166 turns at full speed.
I have no direct experience with piston systems just Navy turbines. I think the Navy systems operated at lower pressures and temperature compared to land based. 1200 psi & 1400 F. Vacuum exhaust, Main engine turbines 70,000 shaft HP X four. 25' props @ 166 turns at full speed.
This thread brings back fond(not really) memories of Thermodynamics classes and the Carnot Cycle(theoretical upper limit of efficiency in a heat/steam system). Forgive me if I don't go into details on enthalpy, entropy and the steam tables. I haven't opened that book in 30 years.