Ports in the cylinder head or walls carry air-fuel mixture and exhaust gases. In 4-stroke engines, valves open and close the ports. A rocker arm acts on a valve spring to operate the valve.
A cam is a lobe on a camshaft, shaped to control how the valve opens and closes. The camshaft keeps all of the valves working with the correct timing and in the correct sequence.
Power can be transferred from the crankshaft to the camshaft by timing gears, a timing chain running on sprockets, or a timing belt running on toothed pulleys.
The crankcase is the lower part of the cylinder block. In a 2-stroke gasoline engine, air-fuel mixture flows through a transfer port from the crankcase to the combustion chamber.
In crossflow scavenging the inlet or transfer port is opposite the exhaust port. In loop scavenging the inlet or transfer ports are within 90° of the exhaust ports.
The crankshaft is the main rotating component in the engine. The crankshaft rotates in main bearings. The flywheel stores momentum during non-power strokes.
The connecting rod secures the piston to the crankshaft. The piston transfers the force produced by the combustion to the crankshaft. Piston rings make a seal between the piston and the cylinder wall.
