Horizontal twin ic engine

[Dougie SwanParticipant @dougieswan43463]

Hi

Does a horizontally opposed ic engine need

balance/counterweights fitted?

Thanks

Dougie

[duncan webster 1Participant @duncanwebster1]

See previous thread from March this year

https://www.model-engineer.co.uk/forums/topic/panhard-levassor-engine-balancing/#post-840942

[JasonBModerator @jasonb]

looking through a few sets of drawings for Model opposed twins some do and some don’t. I did not fit any to the one I built and it seems to run OK.

[cedric 1Participant @cedric]

A BMW flat twin motorcycle crankshaft. Counterweighted and runs very smoothly.

[cedric 1Participant @cedric]

PS, I did look at drawings for a flat twin model engine a while back that had both con rods on one crank pin so both pistons moved in  the same direction at once, making it as unbalanced as a double sized single cylinder, and the builder didn’t seem to mind the vibration. So perhaps at model size and medium rpm, it’s not so important? (I don’t remember the name of model or designer.)

On the other hand, BMW et al are “boxer” engines with 180 degree crankpin offset so pistons move in opposite directions, cancelling out each other’s primary balance. But each crank throw is still counterbalanced like a single cylinder engine where you balance for the reciprocating weight and half the revolving weight.

[bernard towersParticipant @bernardtowers37738]

Dont all engine crankshafts need balance weights to some degree?

[JasonBModerator @jasonb]

I’ve made several IC engines where no balance weights are shown on the drawings so it would seem not.

Even more with no balance weights on the crankshaft but some form of balancing via the flywheel(s)

As for non IC then plenty with no form of balancing

[cedric 1Participant @cedric]

Probably  a matter of mass x rpm. At model size, mass is very small. So a low revving hit and miss engine etc probably not critical. But a 10,000rpm model airplane engine would need counterbalancing.

[JAParticipant @ja]

A flat twin with two crank pins, such as the BMW engine, has perfect primary balance. However there will be a “rocking” couple due to the offset of the cylinders which could be reduced by balance weights. If the cylinder offset is small the couple will be small. I think the secondary reciprocating forces, which is much smaller, may be unbalanced but it is 70 years since I did such sums (I still have my college notes).

With only one crank pin you cannot conventionally balance the primary reciprocating forces. A Lanchester balance shaft may work.

My old BMW R65 had a vibration period (engine coupled to frame) which meant that it was a bit unpleasant between 70 & 80 mph.

JA

[Dougie SwanParticipant @dougieswan43463]

Thanks for all the replies

They are very informative

Dougie

[JasonBModerator @jasonb]

What’s the actual engine Dougie? and what size. Also what will your conrods and pistons be made of?

As Cedric says mass and speed will come into it though there are quite a few model aero engines without balanced crankshafts so high speeds are not the whole story.

[Dougie SwanParticipant @dougieswan43463]

Hi Jason

It’s my take on the wall wizzard, carved from solid, it’s got a 32 x 30 mm bore and stroke

The Conrods are bronze the piston’s are aluminium, the wizzard had balance weights which I’ve made but not trimmed down yet as I was unsure if they were needed or not

Regards

Dougie

[JasonBModerator @jasonb]

Given the heavier conrod material and the fact they are already half made I would fit them. It is also a reasonably size engine at about 45cc and more likely to be needed as size goes up.

These days a lot of engines being built are unlikely to see sustained running at full speed with a lot just being bench run at tickover with the odd blip of the throttle rather than being put to work so balance at high speed has become less of an issue on some designs.

I’ve made a start on drawing up another opposed twin, whether I will get round to making it or not remains to be seen. Based on the old America Springfield design similar size to the Wall.

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