Poly is right and the detractors haven't thought it through. Picture a crankshaft pointing to 3 o'clock. Now put a connecting rod on it and center the cylinder over the crank centerline. There is no way that the piston will ever be at halfway down when the crank is at 3 o'clock. The connecting rod makes that an impossibility.

There is a mechanism that could produce this, it's called a Scotch Yoke. I know of no automobile engines that have ever been built using a Scotch Yoke instead of a simple connecting rod.

R.