Two-Stroke Engines vs. Four-Stroke Engines

For most people, the terms "four-stroke" and "two-stroke" typically equate to preference for mixing oil and gas or keeping both separate.

But have you ever wondered what else makes two-stroke and four-stroke engines different? Or more advantageous in various situations?

The answer: Neither is necessarily better, nor worse. The value of two-stroke and four-stroke engines is rooted in their applications. See an overview of each process below, along with a chart that highlights key differences.

Two-Stroke Engine Process

In a two-stroke engine, the process of converting fuel into motion is accomplished during one up and down movement, or two strokes, of the piston during one crankshaft revolution.

During the Compression stroke, the piston rises and pulls a mixture of air, fuel, and oil into the crankcase through an intake valve. At the same time, an air-fuel-oil mixture is compressed in the combustion chamber.

During the Power stroke, combustion retracts the piston, which closes the intake valve and pulls more air-fuel-oil mixture into the combustion chamber. At the same time, exhaust is expelled from the combustion chamber through an exhaust port.

Four-Stroke Engine Process

In a four-stroke engine, the process of converting fuel into motion is accomplished in four steps—or strokes—that repeat continuously while the engine is running. Each corresponds to one full stroke of the piston; therefore, the complete cycle requires two revolutions of the crankshaft to complete.

During the Intake Stroke, the intake valve opens, and the piston moves down the cylinder to let the air/fuel mixture enter the combustion chamber. As the piston moves down, it draws air and fuel through the intake valve into the cylinder. (This is for a PFI gasoline engine only. Diesel and GDI gasoline engines inject fuel during the compression stroke.)

During the Compression Stroke, the piston travels upward with all valves closed which compresses the air and fuel in the cylinder, raising the temperature inside the cylinder.

During the Power Stroke, when the piston reaches near top dead center, the spark plug fires and ignites the air and fuel mixture causing it to combust and expand, forcing the piston downward.

During the Exhaust Stroke, the exhaust valve opens and the piston travels upward and forces the exhaust gases into the exhaust manifold and out through tail pipe.