Crankshaft Counterweights on Boxer Engines - Why?

Competition engines and ancillaries - general discussion
plasticbaldy
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Crankshaft Counterweights on Boxer Engines - Why?

Post by plasticbaldy »

Thank you for allowing me access to your Forum.
My interest probably differ from everybody else here but your knowledge and experience will be of great use to me and I hope that you indulge me.
I'm in the process of designing a horizontally opposed twin 40cc gas engine based around two 20cc cylinders and piston assemblies, used in single cylinder petrol (not Methanol/Nitro) engines, that I can buy from China. These engines will be simultaneous firing, rear pull start cooling fan/flywheel and specifically designed with front clutch/pinion assemblies for 'Gasser' conversion of .90 size or 700 size electric Radio Controlled Helicopters. I intend CNC machining crankcases and crankshafts to suit these head/piston assemblies. Induction will be via crankcase reed valve. I'm chosing this style of engine because the current gasser conversions use single cylinder Zenoah engines which have serious vibration repercussions in our helicopters.
My questions are ;
- What purpose do the flywheel counterbalance weights perform on one of these Boxer style engines ?
- If the opposing rod/piston assemblies go close to balancing each other out then why do similar Boxer engines, used in Radio Controlled aircraft, have crankshaft counterbalance weights ?
- Is it to provide the reciprocating momentum without the associated torque stresses of the existing flywheel at the end of the crank ? Or because they don't have such a flywheel ?
- Can I design my engine without crankshaft counterweights ?
- Are there any larger Boxer engines of any type that have very small or no crankshaft counterweights ?
Thank You,
Andy from Australia.
Guy Croft
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by Guy Croft »

Nice one for all the Alfa enthusiasts here!

G
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WhizzMan
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by WhizzMan »

I take it you are talking two stroke engines here? Two strokes have fully filled crank cases so you have less "dead" volume while the piston is moving down and pressing mixture up through the ports. That is one reason why two stroke (boxer) engines have large counter weights. They may not be extremely heavy even, just filling up the space for better pressure during downstroke is important enough.

Boxer engines have counter weights just like any engine to make crank speed differences less extreme during the normal rotation of a crank shaft. You only get pressure on the power stroke, so that would give acceleration. The up stroke (two stroke engine) will slow the engine down. If it wasn't for crank shaft weight, the slowing down would be much more than with counterweights. The second reason is common for every piston engine as well. Secondary vibrations. The movement of the con rod up and down also makes it go side-to-side on the crank shaft side. You need to correct for that, so there's some counter weights added.

With a high rev two-stroke engine, I'd try and get your cranks to fill up the engine as best as possible. You may use a lighter steel to make the cranks from, possibly use roller/ball bearings and a multi-part crank that is assembled with a hydraulic press. Given the high RPM you will be running, you may want to spline the shafts, if you go for the multi part solution. If you have fully round counter weights, you could use "heavy metal" and maybe light filler to balance them out.

There are plenty of articles about how you should balance a crank and what formulas to use on the web. I think there are some postings about it here as well, probably with words like crank lightening in them.

We love pictures here, would you happen to have some of the engine(s) you will be working with?
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plasticbaldy
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by plasticbaldy »

Great reply WhizzMan. Thank you so much.
Yep I see that the racing versions of Zenoahs have a filled crankshaft ('stuffed crank') utilising some sort of light resin to fill the crankcase void.
I hadn't considered the lateral movement of the big end of the conrod needing balancing.
So do you think that the counterweight used on the crankshaft of the single cylinder version (of the Boxer engine I'd build) needs to be much bigger than the counterweight that only needs to balance lateral imbalance of the rod big end on a Boxer version ?
And would a full disc flywheel type crankshaft weight between the two rods have the same effect of making 'crank speed differences less extreme during the normal rotation of a crank shaft' ? I'd obviously have a small part disc counterweight (filled for reducing crank case volume) to bsalance lateral rod oscillation. I guess I'd weigh the rod end, part of the bottom end of the rod and crank pin to calculate a suitable weight ?
I'll attach an image of a similar boxer two stroke used for Radio Controlled aircraft and the crankshaftd you find in these engines. Also an image of the type of cylinder I plan to use and a 'Stuffed Crank' used in a Zenoah.
I hope that I'm not bopring you guys.
Thanks again for indulging me.
Attachments
20cc XYZ Cylinder 001 Email Size Pic.jpg
20cc XYZ Cylinder 001 Email Size Pic.jpg (45.23 KiB) Viewed 14484 times
rcssv50twin #1.jpg
rcssv50twin #1.jpg (35.08 KiB) Viewed 14484 times
Crankshafts edit.jpg
Crankshafts edit.jpg (39.54 KiB) Viewed 14484 times
Stuffed Crank Image email.jpg
Stuffed Crank Image email.jpg (14.93 KiB) Viewed 14484 times
TomLouwrier
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by TomLouwrier »

Hang on second.
The beauty of a boxer layout is that the pairs of pistons cancel out both primary and secondary vibrations. A pair of pistons in a parallel twin setup in 180' phase only takes care of the primaries. Another good thing about boxers is that the bore centres can be closer to each other than with inline engines, reducing the rocking couple resulting from the distance between the centrelines.
(see this discussion as well: viewtopic.php?f=6&t=2799)
Secondary accelerations can not be balanced out by the counter weight on the crank! That would be nice, but sorry it ain't so. They have twice the frequency of the engine speed and that's why balance shafts that cancel them out run at twice the crank speed.

In theory a boxer can run without counter weights altogether and use just a big flywheel at the crank's end to smooth out the running. Problem with that is that on a longish crank the acceleration/deceleration Whizz mentions, in combination with a heavy flywheel, will lead to fatigue and fracture in the crank. Add counter weights to the crank and you solve this breaking, also you can do with a lighter flywheel, reducing the problem even more.
In a very small engine this may never be an issue at all as the stiffness-to-stress relation is better.

There are boxers with balance shafts (BMW twins) but they put them there only to cancel out the effect of the pistons not moving along one bore centreline (crank yaw).

Considering the size and output of your engine I'd say you go for a 2 bearing setup, meaning the web between the two crank journals will be symmetrical. It can very well be a full disc, filling in the volume and acting as flywheel.
The counterweights will be on the outer webs. See the crank marked '100cc fu yong hai' ;-). They should be matched (weight, shape, radius) to the forces resulting from both piston/rod sets going through TDC and BDC and the distance between the two bore centrelines, so that they eliminate the crank's tendency to make a yawing motion. You can 'stuff' them of course.
I think you don't need the complexity (and beauty) of splined parts, but you could dowel them somehow after assembly. That would take away some of the hoop stresses though.

Interesting project. Long time ago I had this idea of building a engine, V6, V8, or even boxer, from scrap 50cc moped bikes. I was about 14 years old and never built it of course. Would love to see you actually do this one.

regards
Tom
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plasticbaldy
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by plasticbaldy »

Lots of information there thanks so much Tom.
Can you please explain how the counterweights reduce the acceleration/deceleration stresses on the crank ? Is it just their rotational inertia ? Why counterweights and not full disc flywheel style weights on a Boxer then ? Or better still why not just one full disc crankshaft weight between the two big-ends that could also function as a flywheel ? Or three if there needs to be more flywheel mass ?
Is the function of the flywheel to allow a lower idle speed ? High idling speed isn't a problem in our engines.
Because of the crankshaft stiffness-to-stress relation in my tiny engine (and I could always add 1mm to the crankshaft diameter anyway) I'm leaning towards eliminating the counterweights altogether and going the three full disc weights option because I need some meat there to fasten the crank pins and it will reduce crankcase volume. The only other consideration I can see with this is total mass.
Rocking couple will be the only remaining source of vibration then - no ?
Has anybody ever built a Boxer engine with the cylinders exactly 180 degrees opposed by having a Z bend in the conrods ?
In a tiny engine like mine could the favourable stiffness-to-stress relationship of a beefed up conrod make this viable ? Any thoughts on this ? I'm aware that I haven't considered the clearance geometry of the rod yet but there are some large induction ports in my heads that could help out here if I rotate the heads a few degrees.
What about offsetting the rod in the piston and retaining a straight conrod ?
Or better still offset bearing journals at both ends of the conrod as well as offsetting the rod in the piston. I'll sketch what I mean. ('journals' - I might be using the wrong jargon here)
This is getting more exciting by the minute ! Sometimes it's good to not know what 'can't be achieved' and do it anyhow.
Attachments
Conrod with offset journals & piston.jpg
Conrod with offset journals & piston.jpg (85.35 KiB) Viewed 14457 times
plasticbaldy
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by plasticbaldy »

In the second image (of the complete engine) in my June 21 posting it looks like the cylinders are offset in both the x and y axis. Why would they be offset in the axis that's 90 degrees to the crankshaft axis or is this an optical illusion ?
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by Guy Croft »

This is not, I think, a 'horizontally opposed' unit but a flat twin, correct?

Forgive me if I inadvertently cut across anything already covered - or something covered better than I could do it. This is no subject for an amateur in the subject which I am to an extent so, armed with what I can remember from my ancient honors degree in automotive I have done some background reading I want to share:

1. An outstanding and highly regarded site with much of relevance to engine vibration/modes and props. I came across this some years back when researching the use of sprag clutches for Bayliss Brands. The dissection of aspects of torsional vibration is first-class:

http://www.epi-eng.com/mechanical_engin ... ntents.htm

2. Regarding the counterweighting of flywheels (in general) on flat engines read:

http://www.geneberg.com/article.php?ArticleID=193

3. On the subject of another two-stroke aero unit with some good cutaways see:

http://www.tn-prop.com/G50-Drawings.html

It is possible to run a flat unit crank without any counterweighting - provided the reciprocating parts (rods/pistons) are well-balanced themselves. Of course the crank itself would still have to be well balanced dynamically.

As for a flywheel, apart from offering a reduction in crank assembly stress, as outlined in (2) above, it's function is to store energy betw firing strokes making a smoother running engine. In general terms the more cylinders the engine has so the lighter the FW can be. The converse is that if you make it too light the engine will stall every time the revs drop below a certain speed. There isn't any way I know of of predicting that speed. On this flat engine the firing strokes don't overlap like an in-line multi-cylinder unit so one has to have some way of keeping it running. I am not competent to judge whether the propellor alone can do that. Use of a FW (I think you suggested locating between the rods) would certainly keep the motor running but if you opted not use a FW for simplicity - then actually crankshaft counterweighting - which is plainly visible on the Zenoah - might be a workable alternative. The one thing I don't like about a FW being used in an application like this is the twisting that will take place along the crank between the rotating mass and the prop. Ring gear starters - bit like a lightweight FW are - I think - usually placed right next to the prop, presumably for that reason.

Hope what I have said is of some use,

G

Edit by GC 4th Jul in blue
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Guy Croft
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by Guy Croft »

oh dear, have I killed the thread? I do hope not...


G
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by timinator »

Guy Croft wrote:oh dear, have I killed the thread? I do hope not...


G
Gosh, no way. I'm still stunned by Gene Berg's comments on oil pressure and temperature gauges. I have been using the same Stewart Warner oil pressure and temperature gauges for my shop standard for 35 years because they were expensive in 1976. Now I find out they are junk. Guess I need to invest $700 in gauges. The other links just take more time to digest. Thank you for the good reading.
TomLouwrier
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by TomLouwrier »

Certainly not, just a couple of busy days here.

Good links, I already knew the one to epi-eng.com and use it to look up / brush up theory every once in a while.
Gene Berg is someone you find whe you look for information on IDF's (IDF > VW Bug > Gene Berg)

Guy, what is the difference between 'horizontally opposed' unit and a flat twin, in your eyes?
As far as I know these are the same thing: 2 cylinders, each with its own crank journal. Both pistons rise and fall in their cylinders at the same time. A boxer like the VW bug and related Porsches to date, BMW bike, the 2CV etc.
If the rods are on one and the same crank journal then I would call it a 180 degree V-engine. The Ferrari 512BB had such a layout, although the BB misleadingly stands for Berlinetta Boxer. You would not do that in just 2 cylinders, it would shake all over like a big single.

I'm looking for some free time to set up a basic calculation for a reasonably accurate counterweight. It will always be a compromise since the inertia forces to be balanced in TDC and BDC are not identical. The balance will be right for the average of the two values.
Considering this is a small twin boxer, the idea of shaping the middle web as a disc and use it for flywheel attracts me as simple to make, compact and nicely integrated.

I'll be back! ;-)

regards
Tom
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by Guy Croft »

Thanks guys.

My understanding is this is a 'flat' or 'boxer' like Chris' Alfa unit.

I think horizontally opposed is more correctly applied to engines like the Napier Deltic or the Junkers aero engines which have no cylinder head. The pistons move toward each other in the same cylinder which would typically have 2-stroke style ports. One piston will be inducting thru its intake port and the other exhausting. These units tend to have more than one crankshaft. Not a big issue, not on this site anyway.

Plenty of good info on Wikipedia - as so many things - about it.

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Brit01
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by Brit01 »

The Alfa and Subaru flat boxers work based on different firing sequences.
The Alfa boxer they say is not a 'true' boxer whatever that means.

The Alfa firing sequence goes from front to back on one side and then front to back again on the other bank. If a mechanic is not familiar with this boxer most get it wrong then wonder why it's not firing correctly!

Front
2-1
4-3

1-3-2-4
plasticbaldy
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by plasticbaldy »

Wow. You gentlemen are so generous with your information. Thank you.
The thread definitely didn't die on my part. I still have to read through Guy's reference links for which I thank him. I had knee surgery on Tuesday (Arthroscopy), stayed in bed Wednesday and Thursday I'm driving and walking unassisted.
I'm keen to hear any of your opinions on my offset piston and conrod journal and other ideas (my July 2 posting) that would maybe give me Zero or little cylinder offset. Remember that this is a tiny 40cc two-stroke and a small increase in rod and journal dimensions will have a large increase in their rigidity.



READERS - I have edited out the word c-r-a-z-y from this post. No offence to the respected member but things which are genuinely in that category (this one isn't ) are not welcome on this forum. Don't use words like that. Always read thru your post and if you see a word replaced by 'unacceptable' it is my word censor at work.

GC
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Re: Crankshaft Counterweights on Boxer Engines - Why?

Post by Guy Croft »

I'm afraid your offset design is a matter for careful stress analysis. Without that it will either be a) overdesigned and too heavy (which brings its own set of problems) or b) too weak and it will fail - likely in bending.

If you open on the subject and are not totally committed to this 2 cyl 2 stroke setup (in style or concept) it would be interesting to hear readers' views on alternative configurations.

G
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