solutions for 2-Stroke vibration

[BillClark]

The vibrations created primarily from the power pulses on single cylinder 2- stroke engines (or any single cylinder) present a problem in my application at idle and low rpm mainly. Soft engine mounts have been created to combat the vibrations with some success. Although the harmonic dampers that are used on automotive engines are designed to deal with a somewhat different set of problems I was wondering if a similar crankshaft mounted device could be employed on the single cylinder engines in an attempt to calm the power pulses mainly at idle and low engine speeds where the frequency is most damaging for the frame in which the engine is mounted. I envision a inner and outer ring with a silicon damping media like this Fluid Damper

http://www.fluidampr.com/IMAGES/CUT_AWAY.jpg

I think the penalty imposed by accelerating the mass would be tolerable if the vibration issue could be significantly improved. Any thoughts?

 

[GregLocock]

I think you'd have more luck with better engine mounts than a crankshaft torsional damper. Unfortunately you need to give a lot more detail before any guidance can be given. A really big flywheel would work, given what you have said so far.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BillClark]

Sorry for the shortness of information. My company is involved in various aspects of radio controlled and UAV aircraft utilizing 1, 2, and 4 cylinder 2-stroke engines in the 30-200cc range. We are trying to come up with fresh ideas for old problems. The single cylinder engines are great because they offer good power and low weight as opposed to multi cylinder engines. The problem is the single cylinders are shakers and the light weight airframes are delicate. A typical single cylinder example would be a airplane with 104" wingspan with a total weight of 25lbs. The controll surfaces bounce up and down from the vibration while on the ground. Its not a pretty sight.
The only place for some form of a crankshaft mounted device is right in front where the air comes in so a diameter of about 4 inches is the practical limit unless it didnt have a solid face in which case 6" may be possible so maybe a skulpted aluminum flywheel with carbide inserts might work? We could also integrate the ignition trigger magnets and timming marks into the assembley. Wondering though what the inertia effects on engine aceleration would be and how to determine the weight required for initial tests. I do have a dyno as well as a whole slew of data aquisition equiptment
A special engine mount would have to be integrated at the beginning of the build so I was dreaming of a bolt on device that would allow installation on a existing platform with relative ease. Thats important from a marketing standpoint.

 

[GregLocock]

Can't be done in any economical fashion. If it could then single cylinder engines would be used in refined applications. A boxer twin is the nicest solution and scarcely more cumbersome than a mechanical balancing shaft to achieve the same ends.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[LadaTrouble]

I recently sold a 2 stroke motorcycle with a balance shaft.(Yamaha DT230)It was a very high tech 2 stroke for a dirt bike,and the balance shaft made it very smooth.I rebuilt the engine,and it was no more complicated inside than any other 2 stroke.

 

[ivymike]

seems that you've described two seperate problems - torque recoil ("power pulses" in the first post) and shaking. The opposed twin is a tried method for the latter. There are also anti-vibration devices which can be mounted within the crankcase and accomplish the same thing (weight moving opposite the piston) without actually involving an extra piston.

how about ditching the piston engine for a rotapower?

http://www.freedom-motors.com/

The might be able to scale it down for you.

 

[MikeHalloran]

I agree with Greg; a boxer twin would cost about the same weight and complexity as a balance shaft, and add some power to boot.

I once saw a beautiful example of such a thing, but I don't think it's in production anymore. It was the starter for an ME262's Jumo 004 jet engine.



Mike Halloran
Pembroke Pines, FL, USA

 

[BillClark]

there is a gentleman developing a rotary for our applications but he is a ways out. weight is critical for sure. the boxers are very popular. an example is of a 85cc single which weights 5.5 lbs, produces 9.5hp and cost $1000 with exhaust. a 100cc opposed twin is about 8lbs, produces about the same peak power and cost $1500. the added weight is of course because of the additional cylinder and also more ignition and exhaust. this also adds up making part of the additional cost. oh, also more fuel capacity is needed. the main drawback to the single is the vibration (understanding power pulse induced "shake" exists in both)and a slight reduction in midrange torque but a single cyl plane will outperform the twin because of reduced weight by a noticable amount. there is a little room for some additional weight and reduced cost is always a factor even if its only in the marketing and thats why I originally asked about a solution. I always try to find new solutions to old problems knowing good and well others have thought about it but in this industry of rc, technology has been known to be lagging. cant blame a guy for trying?

 

[GregLocock]

What vibration are you trying to reduce? presumably the prop doesn't care, and the engine is OK. Have you considered cutting the entire nose of the plane off and mounting it very softly back to the rest of the plane?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PrematureWear]

BillClark,

There are 3 major vibrational modes on a single: Crankshaft & conrod circular vibration, which can be balanced out by crankshaft counterweights; reciprocating piston & conrod little end vibration which requires balance shafts to eliminate, and flywheel/engine rotational vibration (rocking) as a result of engine/flywheel energy exchange during the power and compression strokes.

The only way I can think of to get rid of the last one is to run a counter-rotating jackshaft geared off the crankshaft with an identical flywheel running in the opposite direction to counteract the rotational speed variations of the flywheel. A larger single flywheel will NOT help--the energy exchange must always be there and exists between the engine and flywheel. It can only be cancelled by an identical mass running the other way. Needless to say the two gears which drive this shaft must be capable of absorbing the impact loading that this second flywheel will absorb as the engine stores and retrieves power and compression energy alternately.

The linear vibration of the piston & conrod little end can be offset by balance shafts. Perhaps if the previously mentioned counter-rotating jackshaft with second flywheel is arranged on a line at 90 degrees to the cylinder axis ("beside" the cylinder), you can use these as your balance shafts to reduce weight & complexity. The centerline between the two shafts won't jive with the cylinder axis, but can be made close enough that it will help, and is simpler/cheaper/lighter than two correctly located balance shafts plus one counter-rotating jackshaft for the second flywheel running, say, underneath the crankshaft.

Now that I think about it, perhaps you could have two properly located balance shafts, and use the one that runs opposite to the crankshaft rotation to drive the other flywheel. (Oh yeah, the gears for said multi-purpose shaft must also be rather substantial!)

Alternately, a single-cylinder opposed piston layout can be used (no cylinder head, a crankshaft at either end of the cylinder with two pistons meeting in the middle to form the combustion chamber). With the two crankshafts synchronized by a timing drivetrain to counter-rotate, each having an identical flywheel, plus identical reciprocating masses, and properly balanced crankpin/conrod rotating masses, these engines can be made to run extremely smoothly even as single cylinder units. They also feature piston operated transfer and exhaust ports at opposite ends of the lung for excellent scavenging.

Just some thoughts,

PW