How a 4-Stroke Engine Works

The Briggs & Stratton 4-stroke engine, also referred to as a 4-cycle engine, powers an array of outdoor power equipment, including lawn mowers, generators, lawn tractors and tillers. Our 4-stroke engines lead the world in production and quality.

In overhead valve (OHV) engines, the valves are positioned above the piston. The camshaft moves the valves through a tappet, pushrods and rocker arms. 4-stroke OHV engines provide more efficient combustion by allowing the air-fuel mixture to spread more evenly throughout the combustion chamber.

In order to power your equipment, the overhead valve engine completes a repeating 4-step process detailed below.

Element that Allow Internal Combustion Engines to Run

• Air
• Fuel
• Compression
• Spark

Step 1: Intake Stroke

Air and fuel enter the small engine through the carburetor. It’s the job of the carburetor to supply a mixture of air and fuel that will allow for proper combustion. During the intake stroke, the intake valve between the carburetor and combustion chamber opens. This allows atmospheric pressure to force the air-fuel mixture into the cylinder bore as the piston moves downward.

>> Having performance problems? Find out how to troubleshoot carburetor repair and clean / maintain your small engine carburetors.

Step 2: Compression Stroke

Just after the piston moves to the bottom of its travel (bottom dead center), the cylinder bore contains the maximum air-fuel mixture possible. The intake valve closes and the piston returns back up the cylinder bore. This is called the compression stroke of the 4-stroke engine process. The air-fuel mixture is compressed between the piston and cylinder head.

Step 3: Power Stroke

When the piston reaches the top of its travel (top dead center), it will be at its optimum point to ignite the fuel to get maximize power to your outdoor power equipment. A very high voltage is created in the ignition coil. The spark plug enables this high voltage to be discharged into the combustion chamber. The heat created by the spark ignites the gases, creating rapidly expanding, super-heated gases that force the piston back down the cylinder bore. This is called the power stroke.

Step 4: Exhaust Stroke

When the piston reaches bottom dead center again, the exhaust valve opens. As the piston travels back up the cylinder bore, it forces the spent combustion gases through the exhaust valve and out of the exhaust systems. As the piston returns to top dead center, the exhaust valve closes and the intake valve opens and the 4-stroke engine process repeat.

Ever repetition of the cycle requires two full rotations of the crankshaft, while the engine only creates power during one of the four strokes. To keep the machine running, it needs the small engine flywheel. The power stroke creates momentum that pushes the flywheel’s inertia keeps it and the crankshaft turning during the exhaust, intake and compression strokes.