Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

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?!?!

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.

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.

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.