Four Stroke Engine Operation

Piston Travel

The piston travel in the cylinder is limited in both directions.The upper limit of piston travel called top dead center (TDC), and the lower limit is called bottom dead center (BDC).

Piston Stroke
A stoke is movement of piston from TDC to BDC, or from BDC to TDC. There are four strokes in the engine's four stroke cycle: the intake stroke, compression stroke, power stroke, and exhaust stroke.

Intake Stroke

• Gasoline will not burn unless it is mixed with the correct amount of air.It is very explosive when 1 part is mixed with about 15 parts of air by weight.
• As the crankshaft turns, it pulls the rod and piston down in the cylinder. This action creates alower pressure suction called engine vacuum. Atmospheric presssure pushes the lower pressure mixture of air and fuel through the open intake valve.
• The air-fuel mixture is supplied by the fuel system. The ideal mixture for the combined purposes af engine performance, emission control, and fuel economy is about 15 to 1 by mass. This called a stoichiometric mixture.

SCIENCE NOTE- Air-fuel ratio is measured by weight. A 15:1 air fuel ratio has 15 pounds of air to 1 pound of gasoline. When the air-fuel mixture is measured by volume, there are 10,000 gallons of air for each gallon gasoline.

Compression Stroke

• The compression stroke begins at BDC after the intake stroke is completed. The intake valve closes during the compression stroke as the piston moves up in the cylinder, compressing the air-fuel mixture.
• Compressing the mixture of air and fuel into a smaller area heats it and makes it easier to burn.

Power Stroke

• As the piston approaches TDC on its compression stroke, the compressed air-fuel mixture becomes very flammable. When the ignition system produces a spark at the spark plug, the air-fuel mixture ignites. 
• As the air-fuel mixture burns, it expand, forcing the piston to move down in the cylinder until it reaches BDC. The action of the piston turns the crankshaft to power the car.
• Some Leakage gases past the rings occurs during the power stroke. This leakage, called blowby, causes pressure in the area around the crankshaft.

Exhaust Stroke

• As the piston nears BDC on the power stroke, the exhaust valve opens, allowing the burned gases to escape. Because the burning gases are still expanding, they are forced out through the open valve.
• As the crankshaft continues to turn past BDC, the piston moves up in the cylinder, helping to force the remaining exhaust gases out through the open exhaust valve.
• A few degrees after piston passes TDC, the exhaust valve closes.

The entire four stroke cycle repeats itself, starting again as the piston moves down on the intake stroke.

• One four-stroke cycle require  s two 360-degree revolutions (720 degree) of the crankshaft.
• While the crankshaft turns twice, every cylinder's intake and exhaust valve opens once.
• Ignition occurs once.