Hit and miss RPM question

borna · Mar 24, 2014

Hello all
I am building a small .75” Bore x.75” stroke 4 cycle hit and miss engine. The Governor will be electronic on this engine. Meaning once RPM reaches ~2000 I will leave the exhaust open until RMP drops to XX or is better to keep the exhaust opened for SS seconds?
So I need to find out what XX or SS value should be?
For example once RPM reaches 2000 then leave the exhaust open until PRM dropped to ~1500 or 1000 or even lower?
Or should I just leave the exhaust open for SS seconds meaning 1 sec? 2 secs? Or even more?
Any help on this is appreciated.

Thanks
Borna

Generatorgus · Mar 25, 2014

Borna, just my .02 worth, but a hit and miss engine should usually fire one, maybe two hits and the coast until it nearly stops.
I think at top speed it would be 2 to 4 hundred RPM, unless you have a load it. Then depending on the load it may fire more continually.

GUS

rudydubya · Mar 27, 2014

Borna, here's a plot showing RPM versus time for an Upshur hit and miss model (.75" bore, 1.0" stroke, 3.5" flywheels) I built a few years ago. For this 20 second sample, average maximum speed (immediately after the engine fires) was 860 RPM. Average minimum speed (immediately before the engine fires again) was 498 RPM. Average coast time (between hits) was 3.4 seconds, with an average of 37 flywheel revolutions during those coasts. I wanted a slow runner, and I had to tweak the governor springs and weights to get it to run this slow. I tried to get it to run even slower, but below about 495 RPM the flywheel inertia just wasn't enough to overcome the forces of the compression stroke, and it consistently stalled.

There is a short youtube video of my Upshur running slow (not the same run as the plot above). Let me know if you'd like to see it and I'll try to hunt up the URL.

I hope this is of value. I'm following your build thread and looking forward to seeing your model running.

Regards,
Rudy

Hit and miss RPM question

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Hit and miss RPM question

Help Support Home Model Engine Machinist Forum:

borna

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Generatorgus

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