First of all why is it a 4-stroke engine? Because a cycle is done in two piston trips.
The cycle is broken down into 4 steps:
Step 1: Admission
During admission, the exhaust valve is closed and the intake valve is open. The piston goes down to create a depression to suck up the air/petrol mixture from the carburetor.
Step 2: Compression
At this stage, both valves are closed, making the breeese airtight. The piston rises and compresses the air/petrol mixture. This simple compression will raise the carbide mixture to a temperature of about 300oC. If the temperature rises by an additional 100oC, the mixture may ignite spontaneously. This is called self-ignition.
Step 3: Triggering (or exploding)
The piston reached its highest point, a spark gushes between the electrodes of the Spark plug causing the gases to become insanic. The result is an increase in pressure and temperature pushing the piston, which then descends to its lowest point. When the piston reaches this point, both valves are still closed.
Step 4: The exhaust
The exhaust valve opens and the rising piston will push in front of it the burnt gases that escape through this hole alone.
To fully understand how a 4-stroke engine works, need to know the parts that make it up and their functions.
Mobile shutter held in a spring-locked position. It opens momentarily under the pressure of the cam.
It sparks a spark that ignites the air/fuel mixture, creating an explosion.
A moving cylindrical part, which is used to compress gases for an explosion, and which after the explosion converts thermal energy into mechanical energy.
Rigid stem, articulated at both ends. It transforms a linear movement into a rotating motion.
Tree articulated in several off-center tiers. Indirectly transmits mechanical energy to the box.
An airtight chamber where the air/petrol mixture is injected to be compressed, inflamed, and create mechanical energy.
The parts under the piston bathe in oil. This oil is never in contact with the top of the piston. It lubricates: Crankshaft, Connecting rod, Piston, and sometimes it’s the same that lubricates the gearbox. (Unlike the 2 times, where the box is separated from the engine.)