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Why slow running engines?
Old 07-11-2004 01:26 PM
Smokstak Smokstak is offline
Views: 17,574
I was wondering why most people here talk about slow running engines, If it is slow running then it can not be belted to any thing can it? When I get my engine to run, I think I am going to let it run at its rated speed so I can belt it up to a pump. I do not see the point of having a tool made so that it cannot work as it was intended. Anyone want to explain why slow running is good? -- Mac Leod

Running them slow is a challenge, and all that action happens in SSLLOWWW motion! -- Joe

A belted engine has to run the required rpm's, if not, it's great to see and to hear all the movements in slow motion. John

It takes an engine in good condition and someone who really knows what is going on to make an engine run slow. I see a lot of engines at shows that are running at rated speed but are not belted up to anything and it really doesn't look that great. But an engine at rated speed doing the work it was meant to do is something else. Most people don't have or bring belted equipment to run, so they like to run their engines slow. It really is an art to get an engine to run rrreeeeaaalllllyyy sssllllloooooowwww. Lonnie

If it’s not belted up to anything I will run it slow because everything sounds neat and a lot of people say it died and just before it stops, it hits and kind of makes them jump. Not only that, it is not as loud and you can have more engines running at once and still be able to talk to people instead of talking at the top of your lungs. It is a challenge and an art but if I eventually find things to belt up to them then of course I will run it at the necessary speed. Andrew

You can make anything run fast. The trick is to have a hit and miss governed engine nearly come to a stop before hitting again. And a throttled engine that runs steady at 40 or 50rpm. And besides which is neater to watch run? An engine running its guts out and jumping around, or one running as smooth and slow like a Swiss made clock. Doug

Also by running slow, they use less fuel and less wear on the engine. It is much easier to explain to people what is happening (hit-n-missing) with the engine running very slow. John

Running slow is poetry in motion. You can see what is happening. It's the difference between a lazy Sunday drive in the country compared to morning rush hour traffic on the way to work. It's the difference of a kitten purring as opposed to a Heavy Metal Rock Concert. After a lifetime of running full out, slow is nice. When you get older, you'll understand. Vernon

It's fairly easy to get a Hit and Miss engine to run slow. A throttle governed engine is a little harder. I like to run them at reduced speed, but so they still run free under governor control. A throttle governed engine running with its governor disabled to run slow, takes the personality away from it. But my favorite thing is watching people cranking engines that they tried to get to “Run Too Slow.” Pretty good entertainment sometimes. Ken

Slow running engines on display around a crowd of people are just normally safer if it’s done properly. I like the challenge of mine running slow and easy with all the controls working. All you can hear are the mechanical noises of the engine with the loudest being the magneto tripping. Not much else nicer than watching a Mogul side shaft running slow. Well, not much, except, I say they are retired, and deserve it. Can they be made to run faster, sure, just ask. -- Paul

A nice long stroke engine like a well tuned Mogul or a Titan, or even a 6 HP “M” will coast along with hardly any noticeable exhaust report. I've had people ask me, “Does that engine ever fire?” When a side shaft Mogul in good shape is running all you will hear is a clicketa of the valves and the sproink of the oscillating magneto and maybe a whisper from the crankcase breather. No problem having a conversation when one is running. Now if I only could get a Fuller & Johnson “K” to run like that Ken

Running an engine built to run under a load at low speeds looks and sounds neat! As for less stress, actually the reverse is true. In normal running, the most stress on the engine's parts, is at start up, when the engine is working it's hardest to accelerate and make power to get up to speed.
When the engine is running slow, the amount of time the piston, connecting-rod, flywheels, and crankshaft are under maximum stress from acceleration and deceleration are magnified! Due to low speed, compression ramping (the piston slowing the crankshaft as the fuel-air mixture is compressed) and the firing impulse (the sudden acceleration of the piston, con-rod, crankshaft, and flywheels) are at their greatest. This activity, especially when extreme (near flywheel stop), can actually lead to severe metal fatigue, both in the flywheels, and the crankshaft.
There are several engines that will simply not tolerate running under these conditions without destructive failure. One such engine is the WoodPecker engine, especially the 1&1/2 HP and to the lesser extent, the 3 HP. John Smith (a noted and well known authority on the WoodPecker engines) and I had a conversation about running engines too slow. I have seen many engines at shows running very slowly. I have a 1&1/2 Hercules - Economy that will nearly stop. I had commented to John, that I needed to speed the Herc up a bit, as I thought the stresses from running this slow might lead to torsional stress on the crank. John told me that the Hercules engine was quite durable, and I shouldn't have a problem. However the WoodPeckers were another story! He had a complete set of every one they made. They are quite expensive too!
John had also commissioned Dick Shelly (a museum quality model maker) to make a casting kit and model of the make. Any way, John told me that he had run across 5 or 6 1&1/2 HP WoodPeckers with broken cranks. These engines had set up a torsional stress at low speeds, that broke the crankshaft! When the engine fired, the forged crankshaft actually twisted trying to accelerate the flywheels! This twisting motion (Torque) was at its greatest when starting. When the engine was up to speed, the amount of time the crank was under maximum stress was minimal - thousandths of a second in fact. At start, and in a slow running engine though, the amount of time the crank accelerated and slowed could be measured in multiple hundredths, and in severe cases perhaps in several tenths! At the slow speeds, the crank throw would actually lead the flywheel keys by several thousandths! This doesn't sound like much, but multiply that by how many hours the engine runs at a show, times the RPM, divided by approximately 20 (the average amount of turns of the crank, before firing) and that infinitesimal amount can add up to inches in a hurry!
If you want to see a demonstration of metal fatigue in an accelerated state - get yourself a metal wire coat hanger. Simply bend it in the center of the longest section, back and forth for a minute! Not only does the metal get hot (resistance to bending is friction = metal fatigue), but at some point it will fail completely, and it will break.
The same thing happens to the WoodPecker crankshaft - eventually the torsional stresses that build up actually set up a resonance within the engine crankshaft that will break it! John told me that 90% of the breaks occur behind the timing gear. The metal of the crankshaft at that point receives slightly more stress as the crank drives the timing gear.
It just so happened that about 6 years ago at the Jacktown summer show (Bangor PA), there happened to be 2 1&1/2 HP WoodPeckers in attendance! John had his, along with 3 other WoodPeckers of various sizes, and another gentleman had his 1&1/2. I had talked to the old gent about the hazards of running the engine too slow, and was told where to go and how to do it in no uncertain terms. I am not a glutton for punishment, so I went my way. About 2 hours later, John looked me up and asked if I could help him identify a strange noise in an engine he wanted to buy. As we were walking, he told me he was seriously considering having new cranks made for the 1&1/2 HP WoodPeckers, as he had seen and heard of so many failures of their cranks.
We arrived at the engine he was looking at, and it was my nasty exhibitor's engine that John was looking into. Since he already had one, he was going to buy a second and give it to Dick Shelly, to scale the model - 1/4 or 1/5, he wasn't sure of the scale yet. Anyway, the engine was still running, albeit slowly, and as it was running, every time it fired you heard a strange noise! The engine sounded like this: A slight Hiss (compression), click (igniter trip), KA-CHUCK (unknown), ssSSPOP (exhaust) and about 5 or 6 seconds of relative silence as the engine coasted. I say relative, as the cam gear mesh sounds made the engine noisy, with the gear sounds getting louder and softer as the engine turned. John and I stood there a few minutes and listened to the engine. I then said, "It didn't sound like that two hours ago when I saw it before". The old man just looked at John and me, and asked John who I was. "Just a friend who knows a lot" John said, "What do you think?" he asked, looking at me. I was still looking and listening to the engine. I told John "As slow as it is running, it sounds as if it is actually laboring a bit, It isn't coasting smooth like yours" The old man stated that he had shut the engine off earlier, and it was a little stiff.
Just at that point I happened to be looking at the flywheel rim, from directly behind the engine, when it fired. I actually jumped! John saw me move, and asked "What’s the matter with you?" I pointed to the now coasting engine and said, "It moved!" John then said "Well that’s obvious! It fired!" I then said, "NO you don't understand - It moved!" He looked at the engine for a minute too, and when it fired again, exclaimed, “you're right!” When the engine fired, the left side flywheel actually wobbled for a split second when you heard the KA-CHUCK sound! We then shut the engine off by holding the exhaust valve open, and as the engine spun down I watched the timing gears. I noticed the crankshaft gear was worn oddly - some of the teeth were worn to knife edge sharpness. I pointed this out to John as the engine stopped. We looked at the gears, and I noticed a hairline crack in the gear on the crank. John was quick to say, "Gee - a worn Cam drive gear, I didn't thing it could make so much noise!"
I was still looking at the assembly, and turning the flywheel, when I suddenly noticed that the other flywheel kept moving for a split second after the one I was holding stopped.
A close inspection revealed that the grease had melted out of the crankshaft main bearing, next to the cam drive gear, and there was metallic powder all over the place. I said, "Hey John, you aren't going to believe this, but I know what the noise is!" After a few seconds, I told him. The crank shaft had broken, and the engine was still running! The KACHUCK noise we were hearing, was the left side flywheel catching up to the right side after the firing impulse! At first they didn't believe me, so I told John to hold the right side flywheel, as I tried to turn the left. Sure enough, the left side flywheel turned about 1/4" with the timing gear, and the crank throw, piston and rod, as well as the right side flywheel stood still!
Removing the bearing cap told the story - Instead of a clean break, along with the engine destruction that usually went along with it, this engine decided to go a different route! The crank broke in a spiral fracture, from the edge of the crank throw (where they usually break) all the way to the edge of the flywheel! The only thing keeping the assembly together was the cam drive gear - that itself had split, and was in danger of breaking in half! We all knew that it probably was a matter of minutes from complete failure. As there was a good crowd, there was no telling how many people would have been hurt if the flywheel had broken loose!
The 3 HP engines, John told me, had a different problem. He had seen 2 engines that sheared flywheel spokes, due to torsional stresses from running too slow. I have seen a 3 HP IHC 'M' with broken spokes as well. They were all (4 of 6) sheared in the same direction - the crank had out accelerated the rim, and cracked the spokes in one direction. The breaks were detected after the engine (which was turning at about 50-60 RPM), started making what the owner described as a weird crackling noise. He had stopped the engine to investigate the noise, and wiped off the excess grease with a rag that had kerosene on it. After starting and running a few seconds, the owner then again shut the engine down, and when it stopped, noticed the streaks of kero running away from the breaks, which were now obvious. If you must run the engines slowly, inspect often for defects, and listen for signs of trouble. -- Andrew

I can understand the physics of crankshaft stress but in my not-so humble opinion, I think the slow speed crankshaft breaks are more likely caused by the failure to retard the ignition timing to compensate for slower piston speed.
I've successfully run my ZC-52 at less than 120 RPM and my 2 HP F-M "T" without governor springs and haven't had a hint of trouble because I retard the spark to TDC or a little after.
If the timing is set for 15-20 degrees BTDC and the engine is run really slow, the peak combustion pressure occurs well before TDC and this causes a rapid deceleration as the flywheels give-up kinetic energy to force the piston over the top.
Because of the high pressure, after TDC the flywheel has to soak-up the excess energy generated by combustion.
When the ignition timing is retarded to TDC or after, the flywheel is already accelerating due to expansion of the un-ignited fuel/air charge when ignition occurs, just "boosting" the flywheel.
In my opinion, this causes less stress on the parts than if the engine were running at speed with advanced timing and pulling a heavy load. -- Elden

Elden, you are probably right about the timing, however how many people understand how to make the necessary changes? Most people I see at shows just run their engines without knowing why they run like they do. I just wanted to let people know that there is a danger involved if they don't take care. -- Andrew

Maybe we ought to start a thread about retarding the timing of a fast running engine to make it run slow. Come to think of it, there ought to be hints on mixers, mags and other slow-run issues. -- Elden

I think that a lot of good points have been made here. In addition running the timing very late, TDC or a little after, I think that severely restricting the intake air will also reduce "compression" ramping and the resultant excessive torsional stresses by lowering the compression pressure as well as lowering the firing pressure. I have a 35 HP Superior engine with heavy flywheels. It is possible to get this engine to fire very hard at very low speed. It will lunge from nearly stalled up to about 50 RPM or more in one or two firing impulses. I have no doubt that this is potentially damaging as well as dangerous. But with a late spark and the air choked back, it fires very lightly at low speeds and the compression resistance is so low that the engine hardly "bounces back" off compression when it is shut down. With the throttle opened, it will bounce forward and back several times when it is shut down. "Lugging" any engine is to be avoided, but lugging occurs when an engine is given too much throttle and spark lead at too low a speed against a heavy load. Idling is not lugging. There may be lubrication issues when running an enclosed crank, splash-oiled engine very slowly. Patrick

Engines will run hotter with the timing retarded too, especially throttlers. -- Rob

Your right Patrick, about choking down the air intake on the Superior. In fact on my Superior, I completely shut the air off! Only feed it a tiny amount of propane - just enough to make it run. And on any engine regardless of make, if you retard the timing a little it will hit "softer". For engine collectors new to the hobby, play around with retarding the timing on your engines, if you’re like me and want your engines to fire ever so softly and have them run as slow as possible without worrying about damage. Give it a try! Doug

Mac, I think part of the attraction of running an old engine slow is an attempt to show how different the old is from the new. Many people long for the "old days" real or imagined. You know, when people had time to stop and talk and actually gave a hoot. Just my idea. -- Dave
Getting a engine to run slow and safely is an art form I'd say. When I mean safely, I’m saying having it run at the slowest speed possible while causing minimal or absolutely no (if that is possible) damage to the engine. I've worked and worked and WORKED on my 6hp M to get it slowed down and I’M still not satisfied with it. It's still hitting too hard at the speed I'm running it at, so there will be some more work to be done. Now my Stover is a beauty at running slow. It almost stops before it fires and takes off. But it doesn’t hit hard like it was going to throw the piston out of the engine when it does fire. Plus if you don’t intend to have your engine belted up to something I think it's best to slow it down. Running fast tends to have the engine bouncing around everywhere and it looks uncontrolled. Chase

Because I have my F-M ZC-52 engine running at around 120 RPM, I retard the spark to a little after TDC.
I've also had to do some fiddling with the spring-loaded choke on the mixer. What I ended up with was a way to adjust the spring tension. For best idle, the choke is adjusted. For best acceleration and speed running, the needle is adjusted. Balancing these two adjustments is a little complicated but, when it's done, my engine will go from the slow idle to full speed without missing a lick or smoking.
As far as oiling is concerned, with the closed crankcase engines this could be a problem at low speeds. I've had good luck in using 15W-40 oil along with a viscosity extender like IXL or STP. I also run the engine at rated speed for a while every ten minutes or so to throw some oil around. So far, so good.
With the retarded timing and a good hot spark, I can start my engine with a gentle pull on the flywheels. It will start fine at idle when it's warmed-up.
There are problems with getting enough fire from a rotary magneto to run an engine slow. Most mags run out of steam below a couple hundred RPM. If it's an impulse mag, the engine will run slow (especially on impulse when the spark is retarded) but eventually the impulse mechanism will wear out (not to mention that all that clacking is annoying!).
A slow-running engine is a really nice sight and sound to behold. Sort of like being in the emergency stop lane after living in the fast lane for years.
Oh, yes - I took my 2 HP F-M "T" to a neighborhood "corn roast" on Sunday and managed to get it to where I could start it with only a slow pull on a flywheel. I only had to use the crank to spin it a couple of turns to start it cold. The ignitor is set to fire right at TDC and the "beehive" mixer doesn't even have a choke plate. -- Elden

The IHC M engines are a bugger to slow down, especially the igniter type. The rotary magneto will make more power as it spins faster. Before they came out with electronic ignitions, all the hot cars had a magneto for ignition. The main problem with the IHC M is whether the mag can give enough juice to light the charge when running slow.
I have a 3 HP with the rare under shot igniter. By putting a spring on the throttle to close it, I can get this engine down to about 80 RPM if I am very careful with the mixture adjustments, and only if the engine is hot. It will scuzz up the igniter after a while, as at idle the engine tends to load up.
Slowing F-M Z engines can be a real pain, as well. The F-M engines love to work at speed, but are real lousy idlers, unless some steps are taken. Most of the F-M's problems lie with the mixer. It is not really a true carburetor, as it needs to have a restrictor plate at the front of the intake in order to run properly. Therein lies the main problem - the restrictor plate tends to load the mixer with fuel. When the engine runs at speed, this fuel is sucked up and is burned off as the engine runs. When idled though, this puddled fuel builds up, and either ends up on the ground, or in the engine, causing a stall.
There are things you can do to the restrictor to help alleviate the puddling. First, install a stronger spring behind the plate, just enough to keep the plate from flopping loosely. You have to experiment with different springs. Next, see that the plate is in good condition. I have seen some with the center hole worn to the size of a dime, and others with fractured edges - metal fatigue from 60 years of vibration. If the plate is in good condition, I filed 8 small 1/32" wide notches 1/8" deep around the edge of the restrictor plate disc, at equal distances. These notches allow the air flowing past the plate to draw the puddled fuel into the engine, without affecting the operation of the disc itself.
The best idling F-M Z I have seen was a 1&1/2 HP I owned. I could get this engine down to around 60 RPM! I cheated though - I took a carb off a REO cast iron model 552 engine and bolted it to the Fairbanks! It worked like a charm, going from slow idle to 'full speed' with out missing a beat. Rigging the governor and a fuel tank took a little ingenuity, but I got the job done. I reluctantly had to re-install the original carb though, as my grandpa wanted to use his mower to cut the grass.
Oh Well. All kidding aside, throttling engines can be made to go slower a lot easier than the H & M types, but allowances must still be made. -- Andrew

Another way to slow your throttlers way down in addition to retarding your timing is to run them on propane. A small regulator for a charcoal grill is plenty! The secret to running them slow on propane is to feed the engine ONLY propane, NO air! Completely close off all places where air can enter the engine. In doing so you only need the smallest amount of propane to run the engine. I have seen this done to several IHC "M" engines, and they hit with the nicest "ploooomp" you've ever heard. And only hit hard enough to get through the next round of compression. It's worked for me, Give it a try and tell us your experience with propane. OH, by the way! Propane has many more times the BTU's than natural gas! And it is easy to flood an engine with propane, I always start my propane fueled engines with gasoline first. Then change over to propane once they’re running. And remember, When it's running on propane, close off ALL the air! Hope this is of some help, -- Doug

Well I think I have a better understanding of why people like slow running engines. I think I am going to let mine run at its rated speed so it can be belted up to work. Maybe I will have a slow running engine after this one.
Thanks for explaining why! -- Mac Leod

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