Slow Running Engines

What ever the rpm of an engine the piston comes to a complete STOP each time it is at tdc or at the back of the cycle. Seems like it has to start moving in a hurry to catch up with what it is supposed to be doing.

IMHO, the piston comes at the end of the stroke, but the movement will be the same at every point of the rotation in case of a constant running engine with a constant number of revolutions.

"Sixm" John

Gene, at the beginning of a stroke the piston starts from a standstill and increases speed continually, reaching maximum speed at midpoint of stroke and then decreases speed until stopping at bottom of stroke. Same in opposite direction.

If plotted on a graph it would look just like a sine wave for AC current, Ralph in NC

Not Exactly... Piston speed at midpoint of the stroke is affected by the Angularity of the connecting rod and is Not the Same in both directions of Rotation. This sometimes plays a Big part in the proper timing of Steam Engines and you May have to Fudge Some to get them to run Decent...

Found this page showing a plot of piston position and velocity. Note that max velocity occurs just before 90° and just after 270° resulting in a distorted sin wave for the velocity plot.

Andre' B. Clear Lake, Wi.




Piston velocity and acceleration

To Further add to the Confusion... Some Engines actually had the cylinders offset from the crank to Reduce the connecting rod angularity and eliminate some side thrust on the power stroke. The Rumely Oilpull was one example.

Ken, you're right, I was nappin on that one. Max speed occurs when the connecting rod and crank are 90 degrees. Will be a little off center toward TDC in either direction.

That'll warp the sine wave a little

Ralph in NC

Don't forget that the piston travels a greater distance in the first 90 degrees of the stroke than it does in the last 90 degrees. In other words, if an engine has a 4 inch stroke the piston may travel 2 7/8 inches on the first 90 degrees and only 1 1/8 inches on the last 90 degrees. This will vary depending on the distance between the main bearing centerline and the cyl. (rod length). This is why even though the flywheel is turning at a constant speed, the piston will travel faster on the first 90 than the last 90.

As far as I know, in a gasoline engine anyway, Hart Parr was first to come up with the offset crankshaft/cylinder idea in order to decrease the angularity of the conn rod on the power stroke--straighter push on the crankshaft rod journal=more torque......and it DOES.

Early Rambler automobile engines (around 1910) had a engine with a offset crankshaft.