# Antique low-RPM engine: What, and what for? **URL:** \ **Category:** Factual Questions\ **Created:** [January 16, 2024, 9:13pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186 "2024-01-16T21:13:32Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [January 16, 2024, 9:13pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/1 "2024-01-16T21:13:32Z")
The local fair where I used to live had a display of people’s antique engines. One of these was roughly cube-shaped (if I recall correctly – which I might not) and about 24 inches (totally guessing) on a side. Maybe smaller. It seemed to run at about 10 or 12 RPM. That is, the piston would fire and then not fire again for like five or six seconds. The output kept spinning though, so I don’t know if the piston was still going up and down (basically ‘freewheeling’) or if the piston was compressing very slowly via a reduction gear, or something else. (IANA mechanic.) Does this type of engine sound familiar to anyone? Also: I don’t clearly remember what, if anything, the output shaft was attached to. I’m assuming a pulley-flywheel to operate a pump? **ETA:** I used different search criteria, and it looks like I was remembering a [hit-and-miss engine](https://en.wikipedia.org/wiki/Hit-and-miss_engine). I’ll ask a mod to move this thread to MPSIMS if anyone wants to comment, or else close it. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 16, 2024, 10:02pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/2 "2024-01-16T22:02:44Z")
[Moderating] Even though you found your own answer, this thread is still a factual question, so if anyone happens to have something to add, it’s fine remaining here. Or, of course, if nobody has anything to add, then it’ll just drift off the top of the list in whatever forum it’s in. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [January 16, 2024, 10:26pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/3 "2024-01-16T22:26:52Z")
Thank you. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [January 16, 2024, 10:36pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/4 "2024-01-16T22:36:37Z")
The stationary engine article discusses some types: > **[Stationary engine](https://en.m.wikipedia.org/wiki/Stationary_engine)** > > A stationary engine is an engine whose framework does not move. They are used to drive immobile equipment, such as pumps, generators, mills or factory machinery, or cable cars. The term usually refers to large immobile reciprocating engines, principally stationary steam engines and, to some extent, stationary internal combustion engines. Other large immobile power sources, such as steam turbines, gas turbines, and large electric motors, are categorized separately. > Stationary engines were once w... ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [January 16, 2024, 11:07pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/5 "2024-01-16T23:07:03Z")
Yep, a hit or miss engine. They are fascinating to watch while you figure out what is happening. They fire once which speeds them up, then they coast for awhile. During the coast period the intake valve is closed so the engine cycles without compression. A rod cycles back and forth and it has a tab on it. The intake valve operating arm has a hook that rests on the cycling rod. When the rod finally slows down enough the hook is able to grab the tab and the intake opens and the spark plug fires. POP! ON and on. Many of them are huge with 4 foot flywheels. ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [January 16, 2024, 11:16pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/6 "2024-01-16T23:16:00Z")
When they run at fairs and tractor shows, they have no load, so they, well, hit and miss. If you were to put them under load, running something, they’d run like a almost-normal engine. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [January 17, 2024, 2:03am UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/7 "2024-01-17T02:03:32Z")
> [@Johnny\_L.A](#): > > aybe smaller. It seemed to run at about 10 or 12 RPM. That is, the piston would fire and then not fire again for like five or six seconds. The output kept spinning though, so I don’t know if the piston was still going up and down ( hit and miss can also be done by interrupting the spark., I mean its just a protection, the throttle is meant to be controlled for the load. but if the load disappears, it will want to run too fast, so there’s a safety , a centrifugal overspeed system… shorts the spark out. Yeah a simple little engine was called a donkey… general use around a farm or mine or something. pump, agitate, saw, hoist, drive a baler. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [January 17, 2024, 2:26am UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/8 "2024-01-17T02:26:30Z")
> [@mixdenny](#): > > During the coast period the intake valve is closed so the engine cycles without compression. Saying the intake valve is closed so the engine cycles without compression makes no sense - the valves being closed is what causes there _to be_ compression. Is the exhaust valve left open? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [January 17, 2024, 2:46am UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/9 "2024-01-17T02:46:49Z")
Well I suppose what I said isn’t necessarily correct, depending on what is/isn’t in the cylinder during the cycles when there is no fuel. But if the cylinder is fully closed then during the “dead” cycles it must be either compressing, or fighting vacuum. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [January 17, 2024, 4:46pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/10 "2024-01-17T16:46:28Z")
I just read an explanation of the cycle and you are right, the exhaust valve is held open. The intake is only held closed by a light spring and does not a push rod to actuate it. When the mechanism trips, the exhaust valve closes and the engine draws a vacuum on the next stroke which pulls the intake open. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [January 17, 2024, 5:04pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/11 "2024-01-17T17:04:41Z")
They have a lot of old hit-and-miss engines at the apple festival near here (rural PA, near Gettysburg). I asked my son if he knew what a hit-and-miss engine was and he said no. We had a lot of fun with me explaining to him how they worked and the both of us geeking out over antique engines. Some of the engines were actually hooked up to things. A very large one ran a portable sawmill. A smaller one was connected to a corn husking machine. There is a slide show on this page that shows a lot of them (plus other stuff from the festival). > **[National Apple Harvest Festival in Biglerville - Sharing Horizons](https://sharinghorizons.com/national-apple-harvest-festival-in-biglerville/)** > > Tom and I enjoyed being in Harrisburg because it was so easy to get places from Harrisburg.  One Sunday, after church, we headed south a bit to check out ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [January 17, 2024, 10:41pm UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/12 "2024-01-17T22:41:48Z")
It’s not running 10 or 12 RPM, it’s running much faster than that. What’s 10 or 12 is the power strokes per minute. Here’s a place you can see many of these engines running, and other antique engines as well. [https://roughandtumble.org](https://roughandtumble.org) ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [January 18, 2024, 12:35am UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/13 "2024-01-18T00:35:57Z")
I actually had an ad appear on FB two days ago that had a link to purchase a brand new engine of this type. Less than $400 US. [Link](https://www.enginediy.com/products/retrol-engine-hm-01-7cc-engine-4-stroke-horizontal-hit-and-miss-internal-combustion-engine-model?_pos=3&_sid=885148d44&_ss=r&sscid=71k7_8f7wz&) It’s only 7CC displacement, but might be a fun toy. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [January 18, 2024, 7:38am UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/14 "2024-01-18T07:38:20Z")
> [@Princhester](#): > > Is the exhaust valve left open? Yes. It closes on the same cycle that the intake valve opens. The hit-and-miss engines I’ve seen have their intake valves on springs to keep them closed, and their exhaust valves are controlled by ball governors. The exhaust valve is kept open when the engine is running fast enough. When the engine slows down, the governor causes the exhaust valve to close when the piston is near top dead center. Then, when the piston descends, it pulls enough of a vacuum to overcome the force of the spring that holds the intake valve closed. With the intake valve open, the piston draws in the air/fuel mixture, which is ignited when the spark plug fires. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [January 18, 2024, 8:42am UTC](https://boards.straightdope.com/t/antique-low-rpm-engine-what-and-what-for/996186/15 "2024-01-18T08:42:56Z")
Thanks that makes sense now.