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, con-rod, flywheels, and crankshaft are under maximum stress from accelleration and decelleration are magnafied! Due to low speed, compression ramping (the piston slowing the crankshaft as the fuel-air mixture is compressed) and the firing impulse (the sudden accelleration 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 engines, 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. Even 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 accellerate 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 accellerated 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 and add up to inches in a hurry!!! If you want to see a demonstration of metal fatigue in an accellerated 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 cramkshaft 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 hazzards 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 exhibiter's engine 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. Anyways, 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 2 hours ago when I saw it before". The old man just looked at me and John, 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 "Whats' the matter with you?" I pointed to the now coasting engine, and said "It moved!" John then said "Well thats 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, said Holy S--t - 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 broke) 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 itself! 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 accellerated 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
