How Is A Three Cylinder Engine Balanced?

[bowers baldwin]

Interesting discussion on Reddit about 3 cylinder engines.

http://www.reddit.com/r/askscience/comments/2chs1n/how_is_a_three_cylinder_engine_balanced/

 

[Squier]

Fascinating Read Thanks BB

 

[electroken]

There is no need to read beyond the first response in the link.

I've read a lot of posts where it's assumed that a triple is just an inline 4 with a cylinder lopped off. To paraphrase a TV ad:

That's not how it works.
That's not how any of this works.

 

[wheaters]

There is no need to read beyond the first response in the link.

I've read a lot of posts where it's assumed that a triple is just an inline 4 with a cylinder lopped off. To paraphrase a TV ad:
That's not how it works.
That's not how any of this works.

True. The easiest thing is to think how a single cylinder 4 stroke engine works. It gives out some vibrations but it works well enough, the piston is opposed by a counterweight machined on the crankshaft. There are millions of single cylinder engines working very happily, all over the world, in all sorts of machines.
In reality, a single cylinder engine vibrates fore and aft, not up and down, because the counterweights on the crank aren't opposed by the piston when reaching the horizontal plane.

A three cylinder engine is simply three single cylinder engines tied together at 120 degrees each. By evenly spacing the crank pins around the crankshaft , there are three times as many firing strokes taking place on each engine rotation, but at equal intervals. Obviously, the firing strokes of a three cylinder 4 stroke engine occur every 240 degrees, not at 120 degrees.

A straight six engine, such as the one in my 3 litre Beemer, would halve the firing intervals again (my favourite engine configuration, it's very smooth).

I've had someone who I thought should know better try to tell me that in the Geo (i.e. the Suzuki G10) engine, two crankpins are spaced at 180 degrees like a twin engine and that the third one is tacked on the end of the crankshaft, in opposition. Yes, this would give some very odd vibrations because the firing intervals would be unevenly spaced, you would have two cylinders firing on one crank stroke, then one on the next. But no modern engine is configured like that, they all have "120" degree cranks.

In practice, at idle, on a 3 cylinder engine, compared to a 4 cylinder, it does sound a little strange until you get used to it. You can feel the firing pulses a little more than with a four cylinder engine, simply because there is one quarter fewer firing strokes taking place for the same engine speed and they need to be slightly bigger bangs. But once you get above idle rpm, you no longer notice, as has already been pointed out.

 

[CompTrex]

Obviously, the firing strokes of a three cylinder 4 stroke engine occur every 240 degrees, not at 120 degrees.

Well, Obviously! o_O
Ha! Good read. I'm learning a lot from you guys!

 

[wheaters]

Well, Obviously! o_O
Ha! Good read. I'm learning a lot from you guys!

Well lets just think basics here.

A single cylinder two stroke engine fires every engine rotation, i.e. every 360 degrees of crank rotation.
A four stroke single fires every other stroke, so every 720 degrees of crank rotation.

So a three cylinder four stroke fires three times as often.
720 / 3 = every 240 degrees.

Simple innit? If it's not, don't worry, that's what they pay engine designers for!

 

[CompTrex]

Well lets just think basics here.

A single cylinder two stroke engine fires every engine rotation, i.e. every 360 degrees of crank rotation.
A four stroke single fires every other stroke, so every 720 degrees of crank rotation.

So a three cylinder four stroke fires three times as often.
720 / 3 degrees = every 240 degrees.

Simple innit? If it's not, don't worry, that's what they pay engine designers for!

You lost me at basics.
Probably even at think!

 

[wheaters]

In which case, Youtube is our friend!

 

[skygazer6033]

Most of my experience with 4 stroke 3 cylinder engines has been with my Cushman Turf Truckster yard truck. It has an 850cc sohc Daihatsu 3 cylinder engine laying on its side under the seat. Basically the industrial version of Zs Charade engine. Anyway it will idle down to 600 or so RPM and is still very smooth at that speed.

 

[Ty]

There is no need to read beyond the first response in the link.

I've read a lot of posts where it's assumed that a triple is just an inline 4 with a cylinder lopped off. To paraphrase a TV ad:

That's not how it works.
That's not how any of this works.

"I unfriend you."

... and I hope you realize I'm quoting the same commercial and not really "unfriending" you.