This is a cylinder for a 4-stroke Petrol/Gasoline engine. The first step is to get the air-fuel mixture into the chamber. Mixture enters through an inlet port that is opened and closed by an inlet valve. This is called Intake.
Next is compression. The piston compresses the air-fuel mixture into a smaller volume.
A spark across the electrodes of a spark plug ignites it, and it burns. This burning is called combustion.
The burning gases expand rapidly, and push the piston down the cylinder until it reaches bottom dead center.
The reciprocating action of the piston turns into the rotary motion of the crankshaft.
The crankshaft forces the piston back up the cylinder, pushing leftover gases out past an exhaust valve. And everything is back where it started, ready to repeat the whole process.
The whole process is a cycle. A new mixture enters and is ignited. Combustion occurs, expanding gases drive the piston down and turn the crankshaft which pushes the piston back up the cylinder.
These 5 events occur in all internal combustion engines. How they happen can change but they are always there.
In one 4-stroke cycle, the crankshaft does 2 revolutions. In those 2 revolutions how many strokes does the piston make? It does 4 strokes.
Out of those 4 strokes how many actually produce energy? In one 4-stroke cycle, only 1 stroke out of 4 delivers new energy to turn the crankshaft.
4-stroke engine cycle
What is a stroke? It’s the movement of the piston from TDC (top dead center) to BDC (bottom dead center),
OR BDC to TDC. A 4-stroke engine has the following "strokes", intake, compression, power, and exhaust.
A 4-stroke gasoline engine uses "internal" combustion, meaning that the heat that causes the air in the cylinder to expand is generated "internally". (A steam engine is actually an "external combustion engine" as its heat source is outside the cylinder!)Those 4 strokes must include the 5 key events common to all internal combustion engines - Intake, Compression, IGNITION, Power & Exhaust. Let’s look at a simplified model. Note that the valves are ONLY open during their respective strokes, IE: intake valve open ONLY during the intake stroke, exhaust valve only during the exhaust stroke. Both are CLOSED during compression and power!
The intake stroke starts with the exhaust valve closed, the inlet valve opening, and the piston at its highest point, top dead center.
It starts to move down, increasing the volume above the top of the piston. This makes pressure inside the cylinder lower than the pressure outside. This higher outside air pressure forces the air-fuel mixture into the cylinder. The piston reaches bottom dead center, the inlet valve closes, and the intake stroke ends.
Both intake and exhaust valves stay closed as the piston leaves bottom dead center. The piston moves up, squeezing the air-fuel mixture into a smaller and smaller volume, which compresses it. That causes the air/fuel charges temperature to rise, and that makes ignition easier and combustion (burning of fuel) more complete.
Just before the piston reaches top dead center, the next key event occurs - Ignition. The air expanding in the cylinder pushes the piston down the cylinder. This is the Power stroke that drives the engine.
The piston now moves from bottom dead center to top dead center. The exhaust valve opens, and the piston pushes out the leftover gases.
Let’s look at a complete 4-stroke cycle.
- Intake - takes in air-fuel mixture.
- Compression - squeezes the air-fuel mixture into a smaller and smaller volume.
- Ignition - the mixture under pressure is ignited.
- Power - burning, expanding gases push the piston down creating a power stroke that turns the crankshaft.
- Exhaust - the piston moves upward, forcing burned gases from the chamber.
