Let's start with the piston at TDC. The intake valve begins to open as the exhaust valve closes. As the crankshaft turns, the connecting rod starts pulling the piston downward, away from TDC. Keep in mind that the crank is linked to the camshaft by a chain or belt so, as the crank turns, the intake valve opens further until fully open. The downward traveling piston creates suction in the cylinder so air and injected gasoline from the intake manifold are drawn inside. This continues until the piston reaches BDC (bottom dead center). Because of the camshaft's shape, the intake valve is almost totally closed by the time the piston reaches BDC. By the intake stroke's end we're left with a cylinder full of a fresh air/fuel mixture.
The Compression Stroke
By now the piston is beginning its upward trip, being pushed by the crankshaft and connecting rod combo. The intake valve is fully closed and, as the piston's forced upward, the air/fuel mixture is compressed. This compression forces the air and fuel molecules closer together until they become a highly reactive explosive mixture. The closer the molecules' proximity to one another, the easier it is to initiate an explosion. As the piston nears TDC once again, the ignition system fires the spark plug that triggers another explosion inside the cylinder.
The diagram above illustrates...
The diagram above illustrates valve events in relation to crankshaft rotation. Notice that two crankshaft revolutions are required - a total of 720 degrees of rotation - for each full camshaft revolution, or 360 degrees of rotation.
The Power Stroke
By the time the piston is at TDC, the air/fuel explosion inside the tightly contained cylinder is well under way. The explosion's heat and pressure increases rapidly and the piston's pushed back down its cylinder with great force. This is the driving power that spins your wheels and propels you down the track. As the piston is pushed downward and cylinder volume increases, cylinder pressures decrease. Once the piston nears the bottom of the bore, the camshaft begins opening its exhaust valve.
The Exhaust Stroke
There is nothing too exciting going on here. As the piston once again travels from BDC upward, the exhaust valve opens and burnt gases are forced out of the cylinder into the exhaust system. By the time the piston reaches the top of its bore, the exhaust valve is just about closed and the intake valve is beginning to open and the cycle repeats.
Each cylinder of any given four-stroke engine experiences its four strokes for every two crankshaft revolutions. Since camshafts have one bump for each lobe and are driven at half the crankshaft's speed, the valves open with every other crankshaft revolution. Imagine this happening at even a conservative 7,000 rpm where the cycle would repeat itself at a rate of roughly 60 times per second per cylinder. At speeds like this it's easy to imagine a more uninterrupted flow of power coming from this seemingly herky-jerky system.