Q and A: Flywheels and the Cost of a KWH

Flywheel Movie CoverQuestion for George: Hi George, I’m also a big believer in big (or heavy) flywheels. It appears that a weighty rotating mass will smooth the output of a genset. Listeroid engines have such flywheels and are blessed in this regard. However, smaller diesel (and gas) engines have much smaller and lighter flywheels in spite of the fact that they may well produce more hp. While I have no formal training in physics, it seems to me that there is a direct relationship between a large rotating mass and and a slower engine speed. At least where a smoother transition in generator loads is desirable.

I know that all the preceeding may seem obvious. So here’s my question(s):

With your offering of pulleys of varying sizes, why not attach a ring on the pulley that will add weight and increase the rotating mass?

My plan is to duplicate your efforts with a Yanmar clone driving a PMG. I can’t lift as much as I could when I was younger. A listeroid is out. Way too heavy. Listeroid imports are a dicey proposition. But low speed is very attractive.

Changfa type engines elude me. That’s the way I’d prefer to go. Water cooling makes cogen a real consideration. Back to the Yanmar clones. If you look at the power curves, you’ll note that those engines produce their best torque numbers in the mid-range. As do many gas engines. Using a 7? pulley on the engine, and a 5? on the PMG, one could drive the PMG at 3600rpm while running ~2570rpm on the engine. Engine life would be greatly extended at that speed.

My concern is for the output side crank bearing if a heavier rotating mass is added. I’ve read about your efforts with the same engine and PMG using a 5/4.1 pulley set. That means the engine is running at ~83% of PMG speed. Won’t that engine run comfortably at a lower speed and still produce rated output on the PMG? Would more rotating mass make it possible if not? Perhaps a 10? on the engine and a 7? on the PMG?

That means a 70% engine rpm/PMG rpm. While that would reduce the engine speed only 50 rpm from the 7/5 setup, the pulley combination would allow for the addition of more weight. If only in the physical ;”what’s gonna fit?”; sense. I may be overthinkin’ this scenario. But I hope that deliberation and asking questions will better prepare me.

Many thanks, Mike H.

 George’s Answer: Hi Mike, our pulleys have quite a bit of mass. This is a complex topic you ask about, and gets into things we may not understand right away. When we have a big flywheel and the governor calls the fuel rack for more power, that energy needs to power the flywheel to higher RPM along with the load. If we visualize a wide open fuel rack and a lot of opposition to the combustion gases as they push against the piston, we can see more time for gases to try and find away around ring lands and through ring gaps. Maybe a large issue is the increased time the combustion chamber and liner walls have to quench these gases and convert a larger portion of the energy potential into waste heat? What wear and tear do we cause with too much mass and where is the line of diminishing returns? Fortunately, we have the guiding rule… the cost of a KWH, and we can look for that increase or decrease in fuel/kwh production in our specific application.

I’m no Physicist either, but flywheels fascinate me. It allows us to run a smaller power plant, generate superior fuel/kwh figures and >start< loads like induction motors with the stored energy in the flywheel. Yes, we can generate a lot more Power for that short period we need cover for a clean motor start.

But everything is a trade off, When we slow the engine, we have less power output from the engine, and less energy stored in the flywheel. We best define out objective ahead of time and then strike the best compromise. A typical off grid consideration is starting a deep well submersible pump cleanly, and running it. This often dictates what we need to be able to produce out of our generator. (the high mark). From here we can optimize for best fuel/kwh figures, and there are all kinds of things we might try.

As for the flywheel on the generator side, we would of course design this flywheel based on RPM it will run at. The Old Start-O-Matics may be an interesting study if you haven’t looked at them. One owner said his Start-O-Matic had a 50 pound flywheel hanging off the front of it.

An interesting note: We have a lot of really technical people in our DIYer ranks, there are at least four readers, contributors, or customers who are ‘Rocket Scientists’, or retired Rocket Scientists. I remember one who was telling me everything he did at NASA involved complex systems, and he like KISS in hi personal life, he was driving an old ford Pickup with a transplanted German Diesel and manual transmission. Not much of a surprise is his Listeroid powered back up generator for hurricane coverage.

But onto the point, there is more to flywheels than meets the eye, and we have a retired engineer and former Rocket Scientist who is working on a flywheel article to be published here. Most of our articles will be released at 0900 hours, just in time for a cup of coffee. John L’s article may be timely. (Note: enter “John L” into the search box)

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2 Responses to Q and A: Flywheels and the Cost of a KWH

  1. Butch says:

    I like talking about and studying flywheels myself if you dont mind me chiming in.\

    I liked what you said “smoothing out”, that is a good layman discription of what they do and very important to remeber, ALL they do. Flywheels only supply interia thus if inetria wont solve your problem a flywheel wont solve it

    Where to put them? Adding flywheels to the generator shaft was done by Lister on some of the Start-O-Matics. The SOM units also had heavier flywheels. Why both? Two differant problems being tackled, engine pulsations and starting motor loads. Why not just put it all on one shaft for simplicity? Easy, drive stresses. Lets say your after added mass to start your well pump without the lights dimming so you add weight to the crankshaft. when the pump kicks on and the gen shaft trys to come to a stop the extra force that is trying to keep it going is transferred through the drive. Put that added weight on the generator shaft and the drive doesnt know what is going on until the engine governor knows and kicks on some added torque. Lets say flicker is driving you crazy, perfect application for an additional flywheel and it’s handier to put it in the generator head. It will control flicker but the forces again must be transmitted through the drive. If you just had belt chirping under control you just got it back big time.
    I guess the simple of all this is put the weight on the shaft where the problem is. Why not over engineering the drive ? This will take all this out of consideration but at what cost. It’s takes torque (fuel) to spin exta heavy belt section or an extra belt. It also takes fuel to spin flywheels. It will take its toll on the belts. Cutting hairs? Who knows? an interesting study for somebody.

    To another subject,
    To judge what a flywheel is doing you cannot toss out RPMs, where exactly the weight is in relationship to the center and who knows what else. The calculation as for what they are doing for us is summed up in foot Lbs of inertia. It is very possible to have more interia in a much smaller flyhweel at 3600 RPM than a big 24″spoker at 650. I would be curious for someone lots smater than me to do that calculation actualy,,
    Thanks for letting me ramble.

  2. George B. says:


    Somewhere within the old archieves of utterpower are pages of conversation on this topic, the SOM used a vee belt, and in my mind’s eye, I can see that belt taking a beating as it attempts to mitigate some of the problems you mention. Direct drives, and drives that won’t give up even a BTU to heat loss don;t do the same job.

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