Governing Speed: Part 1
In the early months of the year, 'cabin fever' often strikes the Stationary Engine Mailing List on the Internet. Subject matter goes wildly off topic and tempers have been known to fray. This year, however, we have enjoyed some of the best discussions seen for a long time, including discussions on the correct way to lift an engine, pouring babbitt bearings and making hopper gaskets.
While 'talking' via e-mail about what people like to see at shows, the subject moved from displays and the engines themselves to how they should be run. Before long there was so much information flying back and forth around cyberspace that even by keeping to the most relevant and informative mails I found myself with an article too long for a single issue of GEM. So, here is Part 1 on the governing of stationary engines. Enjoy!
One thing to keep in mind when belting your show engine up is whether you have a constant or variable load on the engine. If you have a constant load, your hit-and-miss engine is going to be hitting all the time, same as a throttle-governed engine. In other words, your hit-and-miss engine that usually coasts for long periods between firing will not be coasting anymore.
It's my understanding that if a hit-and-miss engine hits all the time it will damage the engine. If it hits all the time you need a bigger engine.
I do know that a larger hit-and-miss engine will latch up on the governor even if it is under a constant load. My 5 HP Economy would hit all the time when we belted it up to several implements. My 9 HP Galloway would not. Perhaps someone with some old literature can shed some light on this discussion.
That is not to say that hit-and-miss engines would not do the work they were rated for or that running them at their rated speed and work load would cause damage. It is simply and correctly saying that any hit-and-miss engine pulling a load that does not allow the Governor to function is an engine that is being asked to work above and beyond it's designed rating, and it will soon suffer from such use.
I have heard this before, and have never seen any documentation to tell me why. In other words, it takes a larger hit-and-miss engine to perform the same work as a throttle-governed one? I haven't run my Alamo under load for more than an hour or so at a time, but it hasn't seemed to hurt anything.
It's not good for lots of throttle-governed engines to run at full power continuously, either. I notice the Wisconsin manuals advise not loading them at over 80 percent of rated horsepower. Even with the optional stellite-faced exhaust valves, seat inserts and valve rotators. A hit-and-miss always 'hits' at full power, so if it's hitting continuously it's wide open.
The first thing to suffer in an overloaded engine is likely to be the exhaust valve. Different materials for valve heads and seats, different designs of valve stems and guides, the cooling around them, exhaust gas temperatures, free oxygen in the exhaust gas, dwell time of the valve off the seat, exhaust backpressure, contact of the valve face with the seat when closed, perhaps the barometric pressure and phase of the moon, can all influence how long a valve can last.
Exhaust valves in gasoline engines running hard can glow cherry red and draw their heads out into a tulip shape as they bump into the seat on closing. That's where the idea to make some exhaust valves in that shape came from. Our engines don't run at the high power densities of things like supercharged aircraft engines, but I'll bet something like my 3 HP IHC M with its uncooled 'hot' head can get the valve temperature right up there under full power. Or a similar hit-and-miss hitting all the time.
It would seem to me that for a hit-and-miss engine to fire every second revolution would be much the same as running a throttle-governed engine with the throttle wide open. In both situations I am assuming the load will keep the rpm at the governed limit. Not many engines like being run flat out for long.
I doubt that any manufacturer of these old engines intended them to be loaded that heavily, but expected they could put out their rated horsepower with less than full throttle or not hitting at every opportunity. To run an engine at full output means that if the load increases a little, the engine will be reduced in speed, which will probably reduce output - and it may even die under load. The effects of excessive heat on the exhaust valve may also, as has been mentioned, lead to premature wear.
why do they put a rated speed on a hit-and-miss engine if it can harm it to run that speed?
The rated speed is the number of revolutions per minute. It doesn't harm it to run at that speed. But, if the load is increased to the point the engine is firing at every opportunity in order to maintain the rated speed, you are getting maximum output from the engine, and the increased heat load may bring about early failure.
You are forgetting that the governed limit can be reduced by quite a bit, at least on the three engines I have. I can set my Alamo, which is rated at 360 rpm, to latch out at probably half that speed. Of course, it isn't producing 7 HP at that speed, either. I am not going to run this engine wide open, but I don't think I have harmed it by running my cane mill or the grist mill, both of which I run under full load much of the time.
The only reference to favoring a throttle-governed engine over a hit-and-miss in my old books is in applications that require a constant speed. There is no mention of avoiding running a hit-and-miss engine under a constant load.
These are books published in 1910 and 1912. Dyke also mentions the same reasons in his short treatise in my 14th edition of Dyke's Motor Manual (1925). The reproduction 'directions for setting up and operating' the IHC engines booklets I have all say basically the same thing, and none of them mention anything about operating speeds.
I've seen discussions of this in some of my early engine books, but can't seem to find them when I want to quote them.
The thinking is that a hit-and-miss engine that is under too much load not to miss never reaches its rated operating speed and is constantly lugging. With a throttle-governed engine you would almost need a tachometer to see if it is running at rated speed, or watch to see if the throttle ever fluctuates. It wouldn't kill the engine in short order, but it would be overworked.
I have seen this also. However, if the hit-and-miss engine is hitting every time, would it not be running at a constant speed?
Faulty logic. The failure of the governor to latch up under a heavy load is an indication that the load is so great the engine is unable to overcome it and attain the set speed point. The speed will not likely be constant.
As the demand for power increases the speed of the engine will drop, and the pressures inside the cylinder will increase until the amount of pressure required to force the piston down the bore is greater than the amount of pressure created by combustion, at which point the engine stalls.
The bearings and cylinder will heat up unless the lubricators are opened up to increase flow to carry away the extra heat generated.
Perhaps so, but perhaps not. The question was concerning a hit-and-miss engine hitting all the time. I've done a little reading (dangerous?) and a little thinking (always dangerous), but try this out.
When a hit-and-miss engine 'hits' it fires with a full charge in the combustion chamber. If it is firing with a full charge all the time (and constantly) it is running at maximum all the time. A throttle-governed engine hitting all the time is not necessarily firing with a full charge in the combustion chamber. There would be a full charge each time it fires only if the governor were set to it's maximum position. Usually this is not the case, and I believe that constant firing in the maximum position would likely damage the engine. I'd have to do some more research, but I believe throttle-governed engine instructions would indicate as much.
If I am correct here, running a hit-and-miss engine in such a way as to not allow it to lock up on the governor, or to run a throttle-governed engine with the governor set wide open for any extended period of time, would have the same results. Either engine can be expected to produce a BANG of the type we would prefer not to hear!
We'll continue this great conversation next month. See you then.
'It's my understanding that if a hit-and-miss engine hits all the time it will damage the engine.'
Why do they put a rated speed on a hit-and-miss engine if it can harm it to run that speed?
Engine enthusiast Helen French lives in Leicester, England. Contact her via e-mail at: Helen@insulate.co.uk You can join the Stationary Engine List on the Internet at: www.atis.net