Made me think

[Smudger105e]

Looking on Wikipedia I follwed a link for 4 cylinder engines, and read this paragraph several times, it made me think. Why would the piston speed be greater on the top halg of the crankshaft rotation?

 

The straight-4 engine is much smoother than one, two, and three cylinder engines, and this has resulted in it becoming the engine of choice for most economy cars. However, the straight-4 is not a fully balanced configuration. While it is in primary balance because one pair of pistons is always moving up at the same time as the other pair is moving down, piston speed - as with all internal combustion engines - is higher through the top 180 degrees of the crankshaft rotation than the bottom 180 degrees. Since two pistons are always moving faster in one direction while two others are moving more slowly in the other, this leads to a secondary dynamic imbalance - an up-and-down vibration at twice crankshaft speed. This imbalance is tolerable in a small, low-displacement, low-power configuration, but the vibrations get worse with increasing size and power.

 

Here's the full article if anyone's interested

 

http://en.wikipedia.org/wiki/Straight-4

 

Can anyone explain this to me?

[over_head_cam]

"Since two pistons are always moving faster in one direction while two others are moving more slowly in the other"

 

The pistons are connected together, so whatever velocity the ones going up have, the ones going down will too. I think they might be confused?

[Smudger105e]

That's what I though, but is it something to do with the radius of the crankshaft sweep and the length of the con rods and angles and other poo?

[over_head_cam]

Not unless the con rods are made of rubber

 

As Far as I can remember, the pistons will be at their fastest when the big end bearings are at 3 oclock and 9 oclock and at their slowest when they are at 12 and 6. This assumes though, that the crank is at a 'constant' speed.

[Smudger105e]

I would have thought that the piston acceleration would be the same whether on an upward or downward stroke. Piston speed would never be constant though unless the crank speed was very unconstant.

[over_head_cam]

I would have thought that the piston acceleration would be the same whether on an upward or downward stroke. Piston speed would never be constant though unless the crank speed was very unconstant.

 

Yep