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Hit & Miss Gas Engine Discussion

Hit n Miss RPM - How is it rated?

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Old 07-22-2010, 08:10:29 PM
Zira Zira is offline
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Question Hit n Miss RPM - How is it rated?

Okay, dumb beginner question: If a hit and Miss governed engine is rated at a certain speed, say 400 rpm, is that the speed at which the governor mechanism "drops out" and it starts not firing, or the speed at which it starts firing again as it slows down, or what?
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Old 07-22-2010, 08:18:24 PM
MotoXkid MotoXkid is offline
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Default Re: Hit n Miss RPM - How is it rated?

Someone correct me if I'm wrong, as I too am very new to this but I believe the RPM rating is average, and not related to the power stroke.
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Old 07-22-2010, 10:18:23 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Hit n Miss RPM - How is it rated?

The horse power is rated at the time the engine hits on every compression stroke, which usually is the point that the rated speed is reached. It is the equivalent to running a throttled engine at full throttle - full load. The purpose of the hit & miss governing is to 1) conserve fuel, wether it be Gas, Kerosene or what ever. 2) it also governs speed within a certain range, but not actually top speed. and 3) it cools the engine cylinder during the 'off cycle (during latch off), when the piston is pumpung air in and out of the cylinder. Top speed on a hit & miss engine (remember, these engines usually run on maximum volumetric effeciency) is determined by: A - type of fuel, B - fuel to air ratio, C - inertia imparted on the mass of the flywheels (heavier wheels accellerate slower, but usually slow at a slower rate), and D - the amount of load (work). Obviously, an unloaded engine will turn faster and longer between cycles, than one that is doing work!

As an example, a 5 HP Hercules is rated at 5 HP @ 425 RPM. This means you will get 5 HP worth of work at 425 RPM, at which point the engine is firing at every compression stroke. The engine, while 'resting' (no load), may hit as much as 600 RPM during the no load cycle. The engine latches off at 425 RPM, but the flywheels and crank accellerate to a higher speed, due to less resistance on the crankshaft. As the engine is loaded, it does not gain the speed it does at no load, therefore the time it takes to reach the 425 RPM latch off RPM is shortened. As more load is placed on the engine, it will gain less speed (less time), and will latch off quicker until, at full load (firing every compression stroke) is reached. Loading beyond this point will slow the engine (overwork).

This is how most hit & miss engines operate:
Governed speed is determined by 1) weight of the governor throw arms, 2) strength of the spring(s) holding the weights in the closed position, and 3) the speed of the engine. It is a fine balancing act between these 3 items. At rest, the springs usually hold the fly weights (governor arms) in the closed or close to the center of the governor assembly position. As engine speed increases, the weights overcome spring temsion, and the arms swing outward. This in turn causes a latch to engage a detent in the exhaust pushrod, holding the exhaust valve open, this the 'miss' cycle. As I stated above, this 'cycle is determined by the amount of time it takes for the springs to overcome the inertial force generated by the flyweights, and that in turn allows the push rod to close the exhaust valve. The return of compression usually means that the flywheels will again accellerate, thus beginning the cycle over again.

Notice I said 'MOST'. There are other systems that will cause a hit and miss cycle. One is a governed ignition interrupt system, like that used in Maytag
upright, model 92 and 82s, and other manufacturers, Another system is that used by a lot of oilfield engines: They are allowed to accellerate until the fuel/air mixture becomes too lean to support combustion. At this point, they slow until the mixture again becomes rich enough to light off, thus beginning the cycle over. Some engines like Otto and the Bachus Water Engine of Newark NJ, use a governor that stops the flow of fuel into the mixer.

The main difference between the hit and miss engines and the throttle governed variety is the amount of speed control needed. Most throttled engines use the same governing principles, but the governor acts on the throttle plate instead of the exhaust arm. Throttle position is determined by load, the more load, the more throttle, until like the hit and miss engines, you are running full bore - full load. Where a hit & miss engine may have as much as a 200 RPM fluctuation in speed, a throttler may only drop 50 RPM and hold the crankshaft to that speed (within a few RPM) during its load cycle.

Different engines for different work. A pump or grinder does not care about turning speeds, but a generator needs a steady speed, with little variation. As an example, a hit and miss engine turning a water pump, hits about once every 20 turns, and uses about a pint of fuel in 4 hours. A throttled engine, running the same pump, at the same rated speed will use about a gallon of fuel. Which would you use? For turning a generator, or running a machine shop off the belt, though, a throttled engine can't be beat. Enough of my preaching, anyone else have a comment?
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Old 07-23-2010, 05:35:36 AM
Ironsides Ironsides is offline
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Default Re: Hit n Miss RPM - How is it rated?

Andrew,thank you for a very clear reply and examples of the differences between the means of engine govenor control.May I have your permission to print your reply in order to share it with other club members.Keep cranking,Norm
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Old 07-23-2010, 04:12:36 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Hit n Miss RPM - How is it rated?

My reply is general knowledge, I learned it by word of mouth about 40 years ago! If you wish to print, I don't mind, but you had better check with Harry about copyright on the Stak first.
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Old 07-23-2010, 04:22:28 PM
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OTTO-Sawyer OTTO-Sawyer is offline
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Thumbs up Re: Hit n Miss RPM - How is it rated?

Darn Good Explanation there, whether it was your own or told to you by others over the years.

Had a guy at work ask me one time a few years ago what does a governor do. He worked on and restored a couple cars, but didn't have a clue what a governor was or did, even though his lawn mower has one on it... even if it is controlled by air movement instead of weights. Told him to take a look at the distributer advance weights, and the governor weights in an automatic transmission sometime. They may be controlling different things than engine speed, but they work the same way.
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