Yamaha Master Technician Practice Exam

The function of the piston in a four-stroke engine is primarily to compress the air-fuel mixture. During the compression stroke, the piston moves upward within the cylinder, reducing the volume of the air-fuel mixture. This compression is crucial because it increases the mixture's temperature and pressure, making it more efficient for ignition. When the spark plug fires, the compressed mixture ignites, resulting in a powerful explosion that drives the piston back down, generating the engine's power.

The piston’s role is critical in both the compression and power strokes of the engine cycle, confirming that its primary responsibility lies in compressing the mixture before ignition occurs. Other functions, such as cooling the engine, igniting the fuel, or filtering exhaust gases, are performed by different components in the engine, highlighting the specific role of the piston as part of the combustion process.