Big Singles

[Diesel Dave]

Let's start by saying it's impossible to perfectly balance a single cylinder engine.

There are two directions of motion

1) Up and down
2) Forwards and backwards

Now if you set the balance factor to 100% whereby the weight of the piston and top half of the conrod are exactly balanced by the flywheels or crank webs then in theory you cancel out the up and down - as the weight of the piston goes up the weight of the counterbalance goes down - simple!

The fly in the ointment is the forwards/backwards motion caused by the crankshaft and crankwebs/flywheel weight travelling forwards on the piston downstroke and backwards on the up stroke.

So if you were to balance 100% of the piston weight the forwards/backwards out of balance is huge and will soon destroy the motor.

So in a slow reving motor strike a balance at 50% and you will have it about right - there's a whole science behind this and various engine manufacturers state various percentage balance factors depending on config and how high it revs and where you want to shift the vibrations to.

Now balancing shafts allow a much smoother running motor, they are usually set rotating in the opposite direction to the crank so can deal with the forwards/backwards motion as well as SOME of the up and down. Space constraints will usually limit the amount of throw available for the counterweight so it won't be the same a the crank. Overall it will be a lot better than any single without a balancer shaft but compromises and limitations mean it won't be perfect. One day we may get a single that you can balance a coin on whilst reving it up but it's gonna be a long time coming. My solution would be to drive the blower flywheel in the opposite direction to the crank using a set of epicyclic gears but this would be waaaay to expensive.

Just to give you some idea of the weights you are dealing with, a Lombardini 400cc piston weighs about 400 grammes or 0.4 of a kilo (about a pound in old money) - A petrol bullet 500cc piston weighs about 150 grammes and has an alloy conrod too so the forges in motion are exponentially less.

Diesel pistons are heavily constructed because they have to deal with high compressions and have the combustion chamber in the crown - the fuel also burns bloomin quickly; especially at idle when there's a lot of free air (oxygen) available at idle - clatter clatter. Modern car diesels use multiple timed injections and exhaust gas recirculation - all this just to quieten the idle clatter!

Moving away from singles allows better balance however 360 degree twins are exactly the same as a single in terms of balance so I hope the Ruggerini's are 180 degree format!

Triples are better, especially in the 120 degree format and not that stupid 2 up 1 down config. Fours are quite good at 2 up 2 down but the sixes are the best. Don't get me started on V8's, they waste at least one cylinders worth of power fighting their out of balance forces.


Right - lesson over; now what was the question?

[oldbmw]

I wish I had seen this before posting below...

One thing you did not deal with is the torque reaction or rocking caused by the piston accellerating quickly on the power stroke, and slowing through the rest of the time until the next power stroke, with the sharpest de-celeration being on the compression stroke. This really helps it to rock. Moreover it 's strength is more dependant on throttle settings (especially with petrol engines) rather than revs per se. Vibration is also caused by the pistons when they stop. every half rev for them all in singles, parallel twins, boxers and most four cylinder engines. My preference is the 120 ( really a 240 degree) triple. Sadly, the only triple I would like to have is, ermm, inclined to need maintenace

Generally the best you can do is balance it so that at normal cruising speed and throttle it is smoothest, and put up with it being less good everywhere else. Sadly this means the best balance speed changes with gradient, wind, weight and tune of the bike. The industrial diesels are all balanced for 3000-3600 at near full power.

[andrewaust]

Yes balancing is such a fine art, I don't think you'll ever get an engine perfectly balanced "is there such a thing as perfection in a physical world ".

One thing with the single cyl diesel is the high compression etc as well, so even with a balancing shaft/shafts, you'll still get some rocking at certain speeds.

I did hear somewhere that the boxer engine is very smooth running, would be nice to see a 2cyl diesel variant come out .

Id be very cautious to try kick starting a single over 550cc's, wouldn't you get a kick if you got it wrong

I have to say the balancing of the yanclone in the Enfield is quit good, idle is the only time you notice the vibes, but like all engines I guess some of those would be different in some ways. There was an old saying, Monday to Wednesday engines are always better then end of week engines .

Something I found on the net that is interesting, I know it's for twin cylinder engines, if I find a page on singles I'll post it up too.

Shaking forces of twin engines: http://dinamoto.it/DINAMOTO/on-line%20p ... /twin.html

Wikipedia on engine balancing: http://en.wikipedia.org/wiki/Engine_balance

Cheers


Andrew