Stationary Engine List

Governing Speed: Part 2

| June/July 2002

Stationary Engine

We started this thread last issue, presenting Part 1 of our discussion on governing speed and what it means when running a hit-and-miss. As we noted then, we had far too much material to use in a single issue, so here, as promised, is Part 2 of our discussion on governing speed.

Suppose you have an Alamo that is rated at, say, 650 rpm with a potential of producing 7 HP. If you reduce the governed speed to 360 rpm then the horsepower output potential is accordingly reduced. One thing to keep in mind is that the horsepower is not being generated simply by maintaining the governed speed. If the engine is not loaded, and is a hit-and-miss, it will fire, coast a while, then fire again, all the while trying to maintain the governed speed. In a throttle-governed engine the throttle will be closed a bit to keep the speed down to the governed speed. With either kind of engine, running at speeds lower than their maximum rated speed will result in reduced potential horsepower output.

Just for grins, sometime you might belt your Alamo up to a little 1 or 2 HP Briggs and set your Alamo exhaust valve open so it doesn't get compression. If the little Briggs can turn your Alamo at its rated speed of 650 or so rpm, you'll see how little horsepower is needed for overcoming engine friction. I suspect a 2 HP Briggs would do the job of spinning the engine. If so, and the maker rated the engine at 7 HP, that means the engine is capable of producing a potential 7 HP on the output pulley, overcoming the approximate 2 HP internal friction loss. Simply put, the engine isn't producing the rated horsepower until you let the clutch out. Then, if the load is equivalent to the rated horsepower, and the engine can maintain the rated rpm, it is.

I found the following information in my 1912 IHC engine operators guide. According to it, the 'Make-and-Break' type is used almost entirely for engines running below 500 rpm. The guide says: 'The governor is designed to hold the speed of an engine uniform. It either regulates the amount of mixture for each charge - as in the case of the throttling-governor - or it acts so as to permit an impulse only when power is required, as in the case of the 'hit-and-miss' type governor.'

Am I right when I say for a constant speed you need a throttle-governed engine and the amount of 'hits' of a hit-and-miss engine depends on the horse power needed at that moment?

I would think that for something like a sawmill you would want a constant speed, but for something like a corn sheller or mill you would want to use a hit-and-miss that you can run all day long if need be on less gas.