On this page we present some technical definitions that are used to describe an internal combustion engine. The figure shows a computer animation of one cylinder of the Wright brothers' 1903 aircraft engine. A small section of the crankshaft is shown in red, the piston and piston rod are shown in gray, and the cylinder which contains the piston is shown in blue. We have cut the cylinder so that we can note the movement of the piston.
The crankshaft makes one revolution while the piston moves from the top of the cylinder (lower left in the figure) to the bottom (upper right) and back to the top. Since the piston is connected to the crankshaft, we can note the movement of the piston by the angle of rotation of the crankshaft.
Zero degrees occurs when the piston is at the top of the cylinder. Since there are 360 degrees in one revolution, the piston is at the bottom when the crank angle is 180 degrees. The distance traveled by the piston from zero degrees to 180 degrees is called the stroke - S of the piston. This explains why the Wright engine (and modern automobile engines) is called a four-stroke engine. The piston makes four strokes (and the crankshaft makes two revolutions) between combustion firings. The diameter of the piston, and the inside diameter of the cylinder, is called the bore - B. So the area A of the head of the piston is pi (3.14159) times the diameter squared divided by four.
A = pi * B^2 / 4
The volume swept out during any complete stroke is the piston area times the stroke:
V = pi * S * B^2 / 4
This volume is called the working fluid volume because the work performed by a moving gas under pressure is equal to the pressure of the gas times the volume of gas which is moved. For their 1903 engine, the Wright brothers selected a bore of 4 inches and a stroke of 4 inches. The working fluid volume for one piston is then 50.26 cubic inches. The brothers used four pistons, so the sum of all of the working volumes is 201 cubic inches. For any internal combustion engine, the sum of all of the working volumes of all the cylinders is called the total displacement of the engine.