To have a pretty good idea about what a timing belt does, it's great to take a brief look at the gas engine combustion chamber. The engine cylinders' intake valves are opened to allow a combination of fuel and air to enter the combustion chamber. The valve gets closed, and the piston goes up the cylinder to compact the combination of gasoline and air. A spark plug then ignites this blend, and the ignition forces the piston back down. The exhaust valve then opens, causing the waste gases to be forced out of the engine while the piston moves back to the top of the stroke. The pistons, which are connected to the crankshaft, transform the piston's motions into a rotary output. Camshafts attached here help in opening and closing the intake and exhaust valves. The polyurethane timing belt would be there to ensure that the camshaft's action is aligned with the motion of the crankshaft.
