Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines
The Regular Stuff: Chat, Buy, Sell, Off Topic, etc. => General Discussion - Scale Model Gas Engines - Hit & Miss - Throttle Governed - Non-Compression – etc => Topic started by: Jim on October 12, 2020, 05:34:28 pm
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Yes, when idling along under no load a Hit & Miss engine does spin up much higher in RPMs when it fires, so it then must coast down in RPMs until the governor retracts far enough to let the valve close and allow for compression and firing again. However, when they are under load, and hooked up to do work, there is far less spin-up when they fire, and much quicker slow down between firings, so they have far less variation in RPMs while they are doing work, firing much more frequently to maintain a more even speed against load.
Hope that made any sense?!?!
Yes perfect sense. I witnessed this when I ran my steam plant pmr generator with my m90.
Under the load of the generator lighting an incandescent bulb my engine began firing rapidly.
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Thanks Nick!
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Jim, this pair of models was at NAMES, just before 45 seconds, they put their hands on the flywheels, simulating a load and a constant firing. I was too slow with the camera to catch them before, but you can hear the firing slow down when they take their hands off.
https://youtu.be/HcXEaFPLl0Q
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I think everybody films them hitting and missing because it sounds good .
Where as running under load it is not much different to a normal engine that fires every stroke .
Cheers
Dennis
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Thanks Daniel, Dennis & Nick.
I've only ever seen H&M engines once in real life and that was years ago at Steamfest in the Hunter Valley.
I have wondered how they worked with the revs dropping down (because that's how nearly every vid shows them running) thanks guys for clearing that up for me.
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Dennis saved the day :D
Gil probably has videos of his models under load too
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https://www.youtube.com/watch?v=UP8laDwgtcc
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There should be videos on YouTube with one idling and then someone putting a load on it to show it firing more. Not home right now, but I can look later...
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Yes, when idling along under no load a Hit & Miss engine does spin up much higher in RPMs when it fires, so it then must coast down in RPMs until the governor retracts far enough to let the valve close and allow for compression and firing again. However, when they are under load, and hooked up to do work, there is far less spin-up when they fire, and much quicker slow down between firings, so they have far less variation in RPMs while they are doing work, firing much more frequently to maintain a more even speed against load.
Hope that made any sense?!?!
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Full size hit & Miss question.
When I see these running on YT they look like the revs slow down between firings a little...is that right?
If that is correct how did people operate tools, machinery from them if the speed kept alternating? Say you were a shearer and had clippers, would the clipper speed keep constantly changing as you were shearing?