So here's something interesting I realized while educating myself on engines, their displacement, etc. It seems to be calculated (and indicated) incorrectly. The definition of engine displacement is roughly this: The measurement of the total volume of the cylinders in an engine. Those words, or similar, are used on every website I could find.
But it's wrong.
The indicated displacement of an engine is only the volume that the piston moves, not the total volume of the cylinder.
I calculated a bit, the KTM 690 engine has a bore and stroke of 105mm X 80mm, and a compression ratio of 12.7:1. The volume of the piston stroke works out to 692cc, but this does not take into account the chamber above the piston when at TDC (the combustion chamber). If you take the combustion chamber volume into account (can be calculated if you know the compression ratio) then the total volume of that engine is 751cc.
So why does the definition of engine displacement contradict the way it is calculated? At max air flow the total volume above the piston (at BDC) is completely filled, not only the portion that is taken up by the stroke of the piston.
I'm sure there's a logical (or maybe historical) reason for it, maybe someone can enlighten us?
But it's wrong.
The indicated displacement of an engine is only the volume that the piston moves, not the total volume of the cylinder.
I calculated a bit, the KTM 690 engine has a bore and stroke of 105mm X 80mm, and a compression ratio of 12.7:1. The volume of the piston stroke works out to 692cc, but this does not take into account the chamber above the piston when at TDC (the combustion chamber). If you take the combustion chamber volume into account (can be calculated if you know the compression ratio) then the total volume of that engine is 751cc.
So why does the definition of engine displacement contradict the way it is calculated? At max air flow the total volume above the piston (at BDC) is completely filled, not only the portion that is taken up by the stroke of the piston.
I'm sure there's a logical (or maybe historical) reason for it, maybe someone can enlighten us?
