Running your gas engines slow

By Staff
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Why do most people here talk about slow-running engines? If it’s slow running, then it can’t be belted to anything, can it? When I get my engine to run, I think I’m going to let it run at its rated speed so I can belt it up to a pump. I don’t see the point of having a tool made so it can’t work as it was intended. Anyone care to explain why slow running is good? – Mac Leod

– Running them slow is a challenge, and all that action happens in s-l-o-w motion! – Joe

– A belted engine has to run at the required rpms. 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’s going on to make an engine run slow. I see a lot of engines at shows running at rated speed, but they’re 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 bring belted equipment to run, so they like to run their engines slow. It really is an art to get an engine to run really slow. – Lonnie

If it’s not belted to anything, I’ll run the engine slow because everything sounds neat. A lot of people think it died, and just before it stops it hits and kind of makes them jump. Not only that, it’s 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’s a challenge and an art, but if I find things to belt to them then of course I will run them 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 50 rpm.

– Besides, which one is neater to watch run? An engine running its guts out and jumping around, or one running as smooth and slow as a Swiss-made clock? – Doug

– Also, by running slow, they use less fuel and there’s less wear on the engine. It’s much easier to explain what’s happening (hitting-and-missing) with the engine running very slowly. – John

Running slow is poetry in motion. You can see what’s happening. It’s the difference between a lazy Sunday drive in the country compared to morning rush hour traffic on the way to work.

– After a lifetime of running full-out, slow is nice. – 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 a reduced speed – but so they still run free under governor control.

– A throttle-governed engine running with a disabled governor to run slow takes the personality away from it. But my favorite thing is watching people cranking engines they tried to get to ‘run too slow.’ Now that’s pretty good entertainment. – 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 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.

– There’s not much else nicer than watching a Mogul sideshaft running slow. 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 sideshaft Mogul in good shape is running, all you’ll hear is a ‘clicketa’ in the valves and the ‘sproink’ in the oscillating magneto and maybe a whisper from the crankcase breather.

– There’s no problem having a conversation when one is running. Now, if I could only get a Fuller & Johnson K to run like that! – Ken

An engine built to run under a load at low speeds looks and sounds neat! As for less stress, the reverse is actually true.

In normal running, the most stress on the engine’s parts is at start-up. When the engine is working, that’s when 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, fly-wheels and crankshaft are under maximum stress from acceleration and deceleration is 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, connecting rod, crankshaft and flywheels), parts stress is at its 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. – Andrew

I can understand the physics of crankshaft stress, but in my not-so-humble opinion, I think 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 Fairbanks-Morse 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 to 20 degrees BTDC, and the engine is running really slow, the peak combustion pressure occurs well before TDC. This causes a rapid deceleration as the flywheels give up kinetic energy to force the piston over the top. Because of the high pressure, the flywheel has to soak up the excess energy generated by combustion after TDC.

When the ignition timing is retarded to TDC or after, the flywheel is already accelerating due to expansion of the unignited fuel-air charge when ignition occurs, which ‘boosts’ the flywheel. In my opinion, this causes less stress than if the engine were running at speed with advanced timing and pulling a heavy load. – Elden

– Elden, you’re probably right about the timing, but 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 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, magnetos and other slow-run issues. – Elden

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

SmokStak ( smokstak.cgi) is an engine conversation bulletin board with over 50,000 messages on file and is part of the Old Engine series of Web sites that started in 1995 as ‘Harry’s Old Engine.’ Harry Matthews is a retired electronic engineer and gas engine collector from Owego, N.Y., now residing in Sarasota, Fla.

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