The Four Stroke Combustion Cycle

During the Intake Stroke, air and fuel moves into the low pressure area created by the piston moving down inside the cylinder. The fuel injection system has calculated and delivered the precise amount of fuel to the cylinder to achieve a 14.7 to 1 ratio with the air entering the cylinder.

As the piston moves upward during the Compression Stroke, a rapid pressure increase occurs inside the cylinder, causing the air/fuel mixture to superheat. During this time, the antiknock property or octane rating of the fuel is critical in preventing the fuel from igniting spontaneously (exploding). This precise superheated mixture is now prime for ignition as the piston approaches Top Dead Center.

Just before the piston reaches top dead center to start the Power Stroke, the spark plug ignites the air/fuel mixture in the combustion chamber, causing a flame-front to begin to spread through the mixture. During combustion, hydrocarbons and oxygen react, creating heat and pressure. Ideally, the maximum pressure is created as the piston is about 8 to 12 degrees past top dead center to produce the most force on the top of the piston and transmit the most power through the crankshaft. Combustion by-products will consist primarily of water vapor and carbon dioxide if the mixture and spark timing are precise.

After the mixture has burned and the piston reaches bottom dead center, the Exhaust Stroke begins as the exhaust valve opens and the piston begins its return to top dead center. The water vapor, carbon dioxide, nitrogen, and a certain amount of unwanted pollutants are pushed out of the cylinder into the exhaust system.

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