Stationary Engine List

Governing Speed: Part 2

Stationary Engine

Content Tools

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.

What about engines where the manufacturer raised horsepower ratings simply by setting the governed speed higher? With such an engine, would running it on the previously rated max load cause damage?

Many engine manufacturers (e.g., Hercules, Witte) claimed that their engines were purposely underrated in horsepower. Thus, the 'upping' in horsepower by simply increasing the speed by 25 or 50 rpms did not hurt the engine in any way. To run the older engine at 25 or 50 rpms faster, or the newer ones at 25 or 50 rpms slower, would not appear to hurt the engine in any way.

It may be running at a constant speed, but it is being load governed as opposed to mechanically governed. I finally found a reference to what I was trying to explain in an international correspondence school book with a copyright date of 1907. It is titled Care and Management of Stationary Engines. I quote section two here:

'In determining the size of an engine for a given amount of work, it should be borne in mind that an engine that is called upon to run at its full capacity during the greater part of the time is actually overtaxed. Working an engine to this extent will result in rapid wearing of the piston and cylinder and consequent loss of power and economy due to leakage. When doing the maximum amount of work possible in a plant, the engine, if governed by the regulation of the number of impulses, should cut off at least once in four or five charging strokes. This will benefit the cylinder through the admission of charges of cool air at more or less regular intervals.'

Here are my thoughts: First why would anyone want to run an antique that you are trying to preserve with a heavy load that will wear it out? Second, it is probably not a good idea to load any old engine much more than 50 percent of it's rated horsepower and probably not much more than 80 percent on brand new engines.

I really have wondered about this for some time now, since I use my 7 HP Alamo to run my cane mill and it is firing under load as long as I am keeping the feedbox full of cane. It does latch up at times when the box isn't full, so it might just run for five minutes solid, with a few misses in between. The mill I am using requires a 6 HP gasoline engine or 4 HP steam engine. It usually takes an hour and a half to make enough juice for one batch.

From what I am hearing this engine shouldn't be run in this manner even though I am using a machine that is rated below the horsepower of the engine. One more excuse to get a larger engine! I didn't think I was overworking the engine because I have never boiled the water in it. I use the engine because it is a great way to use of one of my hobbies to assist in another, syrup making. If you go to shows with steam engines, you will probably notice they give them a pretty good workout, and most of those engines are older than our gas engines.

To summarize, then, so as to make sure I've read ALL the words correctly, whatever the governing system used it is a bad idea to constantly run an engine above 75 to 80 percent of its maximum power output for several reasons:

1) You are operating on the droop of the efficiency curve and therefore below peak fuel efficiency.

2) The engine will be running hot and also probably near the extremes of lubrication. Also perhaps worth noting that most hit-and-miss engines predate NiMoNiC steels and valve warping was common.

3) It's noisy (and in the interest of preservation mostly unnecessary).

I'd add that a hit-and-miss governor is not the most precise governing system (its cyclic variation can be in excess of 20 percent), and I would expect it to latch every time some time before maximum power output is reached. Also, some manufacturers were very conservative in ratings and their machines could be - and were - run for long periods at rated power. Ruston and Petter (pre-war) being two good examples.

Okay, I can't stand it any longer and am compelled to open my mouth (or keyboard). The original premise was that it was harmful to run a hit-and-miss engine under such a load that it fires every time. Because no one has made any compelling arguments regarding exactly WHAT harm would be done to the engine, I must take the position that it does NO harm to an engine to run it at it's rated horsepower.

Certainly, an engine will endure more wear when run under a heavy load, but this is evident, and would hardly be considered 'damaging' by any stretch of the imagination.

A cylinder and piston on a hot running engine wear considerably less than on a cold running engine. The rate of wear climbs dramatically as temperature decreases. We've all seen the effects a cold running engine has on spark plugs; all that carbon buildup is an indicator of inefficient combustion. Those are two good reasons to run an engine hot.

Lubrication is a non-issue if you set your oilers to the manufacturer's specs.

When the work to be performed is a given, then the only other option to using a small engine that 'hits' every time is using a larger engine. The larger engine will have greater losses due to internal friction, the losses due to the inherent inefficiency of combustion will be greater in the larger engine, thermal losses will be greater, and the larger engine just plain costs more. Economically, it's better to get the correct sized engine to perform a given task.

An engine SHOULD be able to run at its rated horsepower. An engine that cannot be run at full power for an extended period is either poorly designed, or in need of repair.

Basically true, BUT, is the rated horsepower the true maximum output of the engine? Full power that the engine can develop may well be above the rating at which the manufacturer intended the engine to operate continuously. If you put 'full RATED power' into that second statement above, no one can reasonably argue against it.

I rather suspect that most of these old engines will in fact run wide open for a long time without incident, but it was often recommended not to do so.

Engine enthusiast Helen French lives in Leicester, England. You can join the Stationary Engine List on the Internet at: Contact Helen French via e-mail at:

'Lubrication is a non-issue if you set your oilers to the manufacturer's specs.'