captinjohn

IS THIS TRUE ?

Here is something to think about, the three or a five cylinder engine would be smoother when you realize that, consider this carefully and leave out preconceived notions. Assuming that the three and the five cylinder engines have the crank throws spaced at 120 degrees and 72 degrees apart respectively, no two components in the engine are moving in the same direction' (by the same amount) at the same time. The four, unless it's a cross plane crank engine, will have four pistons starting and stopping at the ends of their travel at the same time creating shake. For example a four larger than two litres requires some fancy balancing, the 2.3 litre Ford had a very heavy crank with heavy counterweights, my Mitsubishi 2.4 litre four has two balancing shafts

ahicks

My Feedback: (2)

Cap'n,
I don't know about others, but I'm struggling with all the different potential engine configurations? Radial, flat, inline, V? When you speak of 3 and 5cyl., you're narrowing the comparison to radial and inline, and then trying to compare how smooth those are compared to a 4? The inline 4 isn't the smoothest (especially compared to a tripple), but geez, make it flat, and I think Goldwing, where it's hard to tell it's running?

Bottom line, I think I'm missing your point?
-Al

captinjohn

Al, it was not my point. It was what I read and the I was wondering "IS THAT TRUE" was just the question I asked. It would be good to get more replies from several RC fans. Then more knowledge is obtained & shared. Capt,n

jester_s1

Unless I'm missing something, it shouldn't matter. Every crank counterbalance is matched to the piston it goes with (in theory at least), so the engine doesn't shake back and forth. It's not like 4 cylinder car engines are known for excessive vibration after all. The vast majority of vibration in our RC engines comes from the speeding and and slowing down power pulses that gives us that twisting or rocking type of vibration we have to deal with.

av8tor1977

My Feedback: (6)

That crank counter balance never seems to quite properly balance things. An opposed twin, with two equal size and weight pistons, rods, etc. opposing each other runs much smoother than any single.... It probably has more to do with the combustion pulses than actual balance.

AV8TOR

Cyberwolf

My Feedback: (4)

When I made my twin I was very careful to make sure the counter balance matched the weight of the piston assy, it paid off in spads the engine runs very smooth at any RPM. I do the same with my singles but in mid stroke there just plain and simple out of balance and there is nothing a person can do about it. But they don't seem to shake near as bad as some I have seen.

av8tor1977

My Feedback: (6)

Those twin cylinder engines I made with two Echo 24cc engines are amazingly smooth. It still surprises me every time I run them how much difference there really is between a twin and a single.

AV8TOR

Truckracer

My Feedback: (19)

Balance is a subject that can bring on as much discussion as an oil thread when the right people are addressed! Simple fact, no reciprocating engine can be fully balanced throughout it's full operating range. Even flat opposed configuration engines have their issues and some of the early twins shook more than some single cylinder engines even though most flat twins are fairly smooth by nature. What usually happens is that any given engine can have the rotating assembly balanced very well for certain RPM ranges but it may shake at other speeds. Add combustion impulses to the mix and you get even more vibration then add shaft resonance to the mix and you get additional shakes and shudders.

I use to sit behind a 406 racing engine and one of them had a terrible vibration at parade lap speeds (maybe 4000 RPM or so) though it was extremely smooth at high RPM. During a rebuild I told the machine shop of the condition and they said "no problem"! Even though the rotating assembly had been carefully balanced, different people have different ideas how much weight the crank counter weights should have to balance. He changed the bob weights a bit during re-balance and the engine was then quite smooth though yes, it did shake a bit more at just above idle speeds. The point being that any engine regardless of configuration can have its balance altered to move the vibration points around to different RPM ranges.

Some automotive engines have balance shafts separate from the crankshaft and camshafts. The only purpose of these shafts is to counter vibrations that can't be balanced out of the rotating assembly due to design or configuration. These shafts are timed to the rotating assembly just like a camshaft would be.

Some engine designs can't be balanced well due to their basic configuration .... can you say Harley? You simply can't hook two pistons and rods onto a single crankpin of a narrow V configuration engine and expect smooth operation. But tradition is tradition and the shake lives on.

Just barely touched the surface here as there is much to be read on the subject and even more interpretations of how to apply the knowledge.

earlwb

Some twin engines use a single common crankpin and others use separate crankpins. Also the separate crankpins may go up and down together 360 degrees versus some that might be 180 degrees off from each other. With four cylinder engines you can see the same results, with two crankpins and paired cylinders setup in the same manner too. But some fours may have 90 degree separate crankpins though. But they could use 360 or 180 degree crankin setups too.

The three cylinder and five cylinder engines are pretty much forced into using a 120 degree or 72 degree crankpin orientation though. You can't pair up odd numbers of cylinders.

But the three or five cylinder engines would tend to be fairly smooth running, but a four with 90 degree crankpins would be fairly smooth too.

av8tor1977

My Feedback: (6)

Lots of full size airplanes have harmonics ranges that you need to avoid. There will be a placard on the instrument panel stating something like "Avoid continuous operation between 1900 to 2100 rpms". That is the range that they could not tune/adjust/balance that engine, prop, and airframe combination to get rid of harmonic vibrations.

AV8TOR

Truckracer

My Feedback: (19)

I have a 3 cylinder, 2 stroke outboard that has a resonance at about 3/4 throttle. That engine tells you very clearly, both with audible sound and a seat of the pants feel it is not happy in that RPM range. Yet anywhere above or below that power setting it is extremely smooth.

captinjohn

Back in the day when Quadra engines was popular, my cousin bought one. It was a shaker so he sent it to a engine balance person. His name was Dario B. and he did what he called "over balance" on my cousins Quadra 50. It came back & sure enough it was a lot smoother. I seen a very neat engine Dario built from scratch. He is in one of the 4-5 other model engine forums I read once in a while. Capt,n

Cyberwolf

My Feedback: (4)

A 3 cylinder is not exactly one of the easiest engines to balance, Just like the quatro 5 cylinder Audi came out with, although they did a good job it still had it's quirks.
Cadillac made one back in the 30's had pistons about the size of a gallon jug and every time it fired the car went down the road about 50 feet, controlled whiplash.*L*
FYI
I had a 3 cylinder outboard myself 55 HP and one of the best little engine's I ever owned.