Step 1 Intake valve opens – letting air and fuel into the chamber while the piston is moving DOWN
Step 2 Piston moves UP – this compresses the fuel
Step 3 BOOM – fuel is ignited, forcing the piston DOWN
Step 4 Exhaust valve opens – finally the piston moves back UP which forces the exhaust out
1 - air/fuel intake
2 - compression
3 - ignition/power
4 - exhaust
repeat many times.
Suck
Squeeze
Bang
Blow
And lean burns hotter...
What is depicted here is a 4 stroke (or cycle) engine which uses exhaust and intake valves, and a power (combustion) stroke occurs every other revolution of the crankshaft. Lubricating oil is not mixed with the fuel, the lubricating oil is in the crankcase.
2 stroke engines are less fuel efficient and pollute more, but have fewer parts and produce more power relative to the weight of the engine.