A car engine converts the energy stored in fuel into mechanical power that helps move the vehicle.

In a conventional four-stage engine cycle, pistons move inside cylinders while valves control the flow of air and exhaust gases. The crankshaft converts the pistons’ linear movement into rotation that can be transferred through the drivetrain.

Intake: Bringing Air Into the Cylinder

The cycle begins with the intake stage. The piston moves downward while the intake valve opens, allowing air to enter the cylinder.

In spark-ignition engines, fuel is mixed with the incoming air or injected directly into the cylinder, depending on the engine design. In compression-ignition engines, air enters the cylinder first and fuel is injected later. Engine controls regulate fuel delivery and airflow according to operating conditions.

Compression: Preparing the Charge

The intake valve closes as the piston moves upward. During the compression stage, the piston reduces the volume inside the cylinder, increasing the pressure and temperature of the charge.

The compression ratio influences combustion characteristics and engine efficiency. Piston rings help maintain a seal between the piston and cylinder wall while also controlling the movement of engine oil.

Power: Producing Mechanical Energy

Near the top of the compression stage, combustion begins. In a gasoline spark-ignition engine, the spark plug ignites the compressed mixture.

The resulting expansion of gases creates pressure that pushes the piston downward. This power stage converts energy released during combustion into mechanical movement.

The connecting rod transfers the piston's movement to the crankshaft. Because of the crankshaft's rotating geometry, the piston's back-and-forth motion becomes rotational movement.

Exhaust: Removing Combustion Gases

After the power stage, the piston moves upward while the exhaust valve opens. The spent combustion gases are pushed out of the cylinder and into the exhaust system.

As the exhaust stage ends, the exhaust valve closes and the intake valve opens for the next cycle. Depending on the engine's valve timing, the two valve events can overlap slightly to improve the exchange of gases.

How the Four Strokes Work Together

The complete sequence is:

Intake → Compression → Power → Exhaust

A four-stage engine cycle requires two crankshaft rotations to complete one cycle in a single cylinder. In a multi-cylinder engine, the cylinders operate at different points in the cycle, allowing the crankshaft to receive power at regular intervals and maintain smoother rotation.

Why Engine Timing Matters

An engine depends on precise coordination between the pistons, valves, fuel system, ignition system, and crankshaft. Each component must operate at the appropriate point in the cycle.

Modern engine-control systems use sensors and electronic controls to adjust fuel delivery, ignition timing, airflow, and other operating parameters as engine speed and load change. This coordination helps the engine operate efficiently while delivering consistent power.

From the Crankshaft to the Wheels

The crankshaft is the key mechanical link between combustion and vehicle movement. Its rotation is transferred to the drivetrain, where the transmission manages the relationship between engine speed and wheel speed.

The drivetrain then delivers power to the driven wheels. In this way, the movement created inside the cylinders ultimately becomes the rotational force that moves the vehicle.

A conventional engine follows four coordinated stages: intake, compression, power, and exhaust. Air and fuel enter the cylinder, the charge is compressed, combustion produces pressure, and the resulting gases are removed. Pistons, connecting rods, valves, and the crankshaft work together to transform this process into useful rotational power for the vehicle.