A piston rod, better known as a connecting rod, connects the piston to the crankshaft in an engine. The piston rod allows power to be transmitted from a controlled explosion in the cylinder to the crankshaft, and to the push rods and lifters, depending on whether the engine has an overhead camshaft or has a standard camshaft. There is one piston rod per cylinder in internal combustion engines.
In a car engine, each cylinder has a piston and two or more intake and exhaust valves. Most front-wheel drive, overhead cam engines have four valves per cylinder: two intake valves and two exhaust valves. Engines with the camshaft in the middle of the engine generally have two valves per cylinder: one exhaust valve and one intake valve. All four-stroke, internal combustion engines, regardless of the type of camshaft, have four strokes: Intake, compression, combustion, and exhaust.
On the intake stroke, the crankshaft pulls the piston rod down. This pulls the piston down and allows a mixture of air and fuel to enter each cylinder. The intake valves on each cylinder are open, allowing the air and fuel to enter the cylinder.
As the crankshaft pushes the piston rod up on the second stroke — the compression stroke — the intake valves begin to close, and the air-fuel mixture is compressed. The compression heats the mixture and makes it more volatile. By the end of the stroke, the intake valves have completely closed, and the exhaust valves are getting ready to open.
The crankshaft starts down again for the combustion stroke, which is the third stroke. The exhaust valves begin to open. The spark plug fires the volatile air-fuel mixture, setting off a controlled explosion and creating power. As the piston rod moves the piston through the stroke, the exhaust valves open further.
On the fourth stroke — the exhaust stroke — the exhaust valves are completely open and the crankshaft starts to push the piston rod up, expelling burned air and fuel into the exhaust system. The exhaust valves begin closing near the end of the stroke, and the intake valves are getting ready to open. At the end of the stroke, the exhaust valves are completely closed, and the piston rod has pushed the piston all the way to the top of the cylinder. The left over air and fuel mixture is completely expelled into the exhaust system, then the process starts all over again.