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# Flywheel Design for a Diesel Engine

## Flywheel Design for a Diesel Engine

(OP)
My boss has tasked me with designing a better Flywheel for the prototype diesel engine that we are working on (www.buckdiesel.com)

I have a Bachelor's from Ohio State, but I will be the first to tell you that we covered precious little of what I would call "Practical knowledge" in our classes.

I was wondering if anyone could at least point me in the direction of a few formulas to get me started.

### RE: Flywheel Design for a Diesel Engine

(OP)
We think it needs to be heavier.  Comparing numbers with other engines of close to the same size, we've found that our Flywheel is much lighter.  I want to see if this is truly a big problem or if we can keep going with what we have.

### RE: Flywheel Design for a Diesel Engine

Calculations may not do much good since they are based on assumptions about how smooth the engine should run (I don't remember the name of the fudge factor). Find the polar moment of inertia of your current one, then design one say 10% or 20% greater and see if it makes a difference. You just need a quick test before spending any money for a casting. Perhaps there is enough metal on the current flywheel where you could bolt on a ring. By the way, the size of your existing flywheel might have been limited by room inside the flywheel housing, a clutch, or a starter ring gear.

### RE: Flywheel Design for a Diesel Engine

Do you have a vibration roughness or an inertia problem in your applications.

Can you make a one off to test or even bolt extra weights to a current flywheel.

Regards
Pat
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### RE: Flywheel Design for a Diesel Engine

I don't think you can do any damage by going too large (within reason), provided you can get it started and the bearings can take the overhung load. The downside is that the transitory response to load changes will be longer.

Someone with specific experience can chime in, but I wonder if a more massive flywheel would change the harmonics in the crack during use? I think so, but how do other engines take this into account? Is the front damper just tuned to account for this specific resonant frequency?

-------------------
On a different topic - How do you keep the rod cap in place while doing the "upper end replacement"?

ISZ

### RE: Flywheel Design for a Diesel Engine

timbillyosu,

IceStationZebra is correct.  Unless you have some issue with adding weight, then a heavier, higher inertia flywheel is almost always beneficial.

The purpose of a flywheel on a recip piston engine is to increase the polar moment of the rotating assembly.  By it's nature, a recip piston engine's instantaneous output shaft speed varies throughout any given rotation.  This is due to the variations in instantaneous driving torques produced by the engine's intermittent combustion events.  The increased MOI provided by the flywheel mass helps to smooth out these speed variations and the torsional vibrations that result from the angular accelerations/decelerations.

You didn't note how many cylinders your particular engine has.  But in general, an engine with fewer cylinders will require a flywheel with a proportionally greater mass/MOI than an engine with a large number of cylinders.  And turbocharged diesel engines, with their higher peak cycle pressures, will usually want a larger flywheel than something like a N/A spark ignited engine.

Before you run the engine with a bigger flywheel, make sure to do a quick torsional survey to verify that the new (lower) system frequencies are not within a speed range that you will be operating at.

Good luck.
Terry

### RE: Flywheel Design for a Diesel Engine

" I wonder if a more massive flywheel would change the harmonics in the crank during use? I think so, but how do other engines take this into account? Is the front damper just tuned to account for this specific resonant frequency?"

Usually the killer modes are first torsion and first bending. For first torsion the node is practically at the flywheel anyway, so additional inertia there will only have a small effect.

First bending is probably more sensitive to the inertia of the flywheel, but not many engines have durability problems caused by bending, with some exceptions. It does cause a characteristic harshness, typically at slightly higher frequencies than the torsional.

The TV damper is obviously tuned for torsionals, some cars have a separate bending damper, others use a combined damper  - the rubber layer is shaped so that it controls bending as well as torsion.

I have seen a proposal to tune additional dampers for higher modes, I don't know of anyone who has them in production, but it is not a field I've worked in for 8 years.

Cheers

Greg Locock

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### RE: Flywheel Design for a Diesel Engine

What is the operating rpm range? The swept volume? Number of cylinders? Is this an inuse engine or experimental?

### RE: Flywheel Design for a Diesel Engine

(OP)
Wow... thanks for all the responses!

First I guess I should have given more information about the current engine.  Right now, we have a 6-cylinder inline diesel engine with 4.5" pistons.  The engine is currently N/A but will be turbocharged/aftercooled for production (we had an issue with turbocharger bearings recently but we just received the replacement parts yesterday so the turbo will be back on soon.)  We are hoping to get about 650 hp as our max power around 3000 rpm.  If there are other things you would like to know, feel free to ask or check out our website (www.buckdiesel.com)

Now to respond to everyone's responses...

"Perhaps there is enough metal on the current flywheel where you could bolt on a ring. "
- I'm going to talk to my boss about bolting on more material, but the reason all this started is that we found out that some of our competitiors have a Flywheel that is nearly TWICE the weight of our current one.

"On a different topic - How do you keep the rod cap in place while doing the "upper end replacement"?"
-There is a windage tray that is mounted on top of the oil pan, just below the main bearings.  It is there to prevent the oil from sloshing up as the boat goes over waves and making the oil pump suck air.  It is also there as a catch for parts that are dropped during service.  If you don't hold onto the rod cap when you take the bolts out, it will fall onto this windage tray and you can simply rotate the crank around a little and reach your hand past it to remove the part.

"Before you run the engine with a bigger flywheel, make sure to do a quick torsional survey to verify that the new (lower) system frequencies are not within a speed range that you will be operating at."
-This sounds like a great idea... how do I do that?

"What is the operating rpm range? The swept volume? Number of cylinders? Is this an inuse engine or experimental?"
-The rpm range is about 600 to 3500 max rpm. I'm not sure what you mean by "swept volume."  As of now, it is an experimental 6-cylinder.  We are currently seeking funding to take the project into full-scale production.  For now, we are working on tweaking everything so that we will hopefully have all of our bugs worked out when we get into production.

Thanks again for all the feedback and keep things coming!

### RE: Flywheel Design for a Diesel Engine

Have you thought that if a competitor saw your engine, he might be at work trying to figure out if he needs to halve the weight of the flywheel.

Swept volume is displacement. We now know 6 cyl, 4.5 bore. We simply now need a stroke to do the sums.

Marine diesels rarely need to accelerate rapidly, so it is unlikely that a flywheel that is heavier than necessary might be a problem, but lighter than necessary makes the engine inclined to stall when loaded up at low rpm or idled right back.

Regards
Pat
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### RE: Flywheel Design for a Diesel Engine

(OP)
Stroke length is 5"

### RE: Flywheel Design for a Diesel Engine

The website says every engine in development utilizes a 4.625" diameter piston, which would be 504".

Which is it, 4.5 or 4.625?

Also, would you consider running one in an on the road configuration? There would be a pretty good market for the 3 or 4 cylinder in retrofit, depending on the cost.

### RE: Flywheel Design for a Diesel Engine

Sorry I didn't ask this before.
You said the boss whats a better flywheel than what you have.
What are the problems that have come to light with what you have? I guess someone else asked this.
Are you getting harmonics in the prop driveshaft? That would be part of that torsional survey deal.

Rod cap in place! This is a small enough engine, and since hands are in there removing nuts or bolts, just pull it out of there. If your using bolts just have a long one as a tool and slide the cap down it.

### RE: Flywheel Design for a Diesel Engine

(OP)
patdaly, right now we are using 4.5".  We are also currently using a dry-sleeve configuration.  We may be switching to a wet sleeve in the future and in that case, the pistons may grow a little.  As of now, we are only focuding on the marine market.  We figure we'll let all the big boys worry about fighting over 5 and 10 cents to produce an engine.  The inboard marine market is more of a niche with higher profit margins.

dicer, we seem to be having problems with low-rpm idling (<700 rpm).  We would like to be able to idle around 600-650 rpm.  However, this may be due to our small (in my opinion) crankshaft counterweights.  As I said, I'm a little new at this.

Let me give you all a little background on how this was all designed.  My boss, Mike Buck, was basically tired of all the pains that are caused when you have to work on an inboard marine engine.  He had a boat that had a pair of engines.  One blew. About 2 months after fixing that one, the other one went.  Each time he had to cut the entire salon floor out of the boat.  Some of these other guys would have to cut a hole in the hull of the boat to remove an engine!

So he was at the Miami boat show and thinking about what he thought he wanted.  Then he went around to other people at the show with a legal pad and asked them what they did and didn't like and what problems they saw with things.  After the show, he basically came back and started designing and sketching out this engine.  He has had a few people looking over his shoulder throughout the process, a few different people with Phd's in various types of engineering, but for the most part everything you see is what Mike thought would work from his years of working on and around engines.

Basically 5 years later, we have a runing prototype.  We know we still have a little bit of tweaking and some changes that we need to make, but we are closer now than we have ever been to having a finished engine.  We are currently working to secure the capital to move into full-scale production.

So there you have it.  Please feel free to ask any more questions that you have.

### RE: Flywheel Design for a Diesel Engine

If the counterweights are to small to give enough inertia to support a slow idle, add weight to a flywheel to try it out. Don't be shy. Add a lot to see what happens, but there will be a point where increasing weight further gives little improvment.

Regards
Pat
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### RE: Flywheel Design for a Diesel Engine

I watched the videos. Have some ideas.

### RE: Flywheel Design for a Diesel Engine

timbillyosu,

Crank counterweights are mostly sized to improve dynamic balance by partially countering the reciprocating mass inertia forces and balancing the offset rotating mass at the crankpin.  This also helps reduce crank bending between the mains and the radial loads on the mains due to the reciprocating mass inertias.  Sometimes the counterweight masses are also adjusted from one crank web to another, in order to modify the torsional dynamic characteristics of the crankshaft.

If you need more rotational inertia to improve low speed smoothness, and you have the option of adding more counterweight mass, that is definitely a better use of that steel.  So I would highly recommend adding enough counterweight to make sure that your crank is fully balanced for the rotational masses and has enough counterweight to provide the optimum factor for balancing recip masses.

Besides adding rotational inertia to your diesel, low speed smoothness can be helped by careful injector mapping (for BOI, PW, rail pressure, etc.) and by using a turbo with variable geometry that will give more stable performance at off-peak flow conditions.

As I'm sure you're now aware, serious engine development is a very costly endeavor.  My hat is off to Mr. Buck.  I wish I had his money!

Good luck and have fun.
Terry

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