I majored in Automotive technology and was a certified GM technician for a while
basic concept of the 4 stroke engine consists of a single ignition cycle carried out in 4 strokes of the piston. the main components i will be talking about in this concept is the combustion chamber (cylinder), intake valve, exhaust valve, piston, and spark plug (unless it is diesel -which i'll explain more later-)
upon the first stroke the intake valve will open and the piston will move from the top of the combustion chamber to the bottom creating the a vacuum and drawing air and fuel into the combustion camber. this is the end of the first stroke which is named the intake stroke
the second second stroke begins when the piston reaches the bottom of the combustion chamber, once that happens, the intake valves will close and the piston will move up the to the top of the combustion chamber causing the air and fuel mixture to compress withing a small area- this is second stroke known as the compression stroke.
at the transition from the second stroke and third stroke, in a gas engine. a spark plug will create a spark which ignites the air fuel mixture. in a diesel, there is no spark plug, the air fuel mixture is ignited from the heat caused during the compression stoke and happens when the compression stoke ends.
the third stoke is carried out after the air and fuel are ignited, this will cause a controlled combustion of the air and fuel which will force the piston to the bottom of the combustion chamber. -this is the third stoke which is known as the combustion stroke.
the fourth stroke begins when the piston reaches the bottom of the combustion chamber, the engine will use the momentum and energy it has gained from the combustion stroke and will open the exhaust valve and force the piston to the top of the combustion chamber forcing the byproduct gasses from the combustion stoke to flow into the exhaust system and into the atmosphere. this is the fourth and final stroke and is known as the exhaust stroke.