I was wondering if anyone could actually explain why it is generally easier to make a smaller engine rev higher than bigger one, in terms of actual individual cylinder displacement. I would prefer if you could put it in terms of scientific or physic related laws. Pretend I am an idiot if you will. Thanks in advance.

 

As the engine spins, it accelerates the pistons and rods up and down. The faster it spins, the faster these things get accelerated. The faster these things get accelerated, the more they are stressed due to their own inertia as well as the parts that are attached to them. A rod has to accelerate itself as well as the piston, so that takes a beating. I believe the stresses rise as a square function of the rpms.

If these parts weigh more, as in a bigger engine, they have more inertia and therefore higher stresses. Other things come into play, such as the stroke and bore. A larger bore means heavier pistons, but a larger stroke means you have to accelerate them more...

It's like if you spin your arms around, they'll feel like they're getting pulled apart. The faster you spin them or the more weight you hold, the more they'll get pulled apart, until they break.

Anyway, you have to find a balance somewhere. If you want to spin faster, get stronger parts. If they're bigger, they'll be stronger, but they'll also weigh more, so you have to start using expensive materials....

Hope this is somewhat what you were looking for.



[Modified by Lsos, 1:24 AM 2/21/2002]

 

That's all I needed. Thanks alot.

 

Just wanted to add that if you increase the size of an item (ie: a piston, or a valve), the surface area of friction also increases, and (extrpolating on Lsos) the volume also increases substantially over the increase in surface area (by like a factor of two or three times). Larger volume of item = more weight (assuming the same material contruction).

 

doesnt having to compress more air and fuel have a little to do with it?

 

I don't think so. If anything, compressing more air places more compressive stress on the rod, but that just happens to counter the tensile stresses from the inertial load. Other than that, I don't see how this would have much to do with anything concerning revving an engine.....