The four stroke cycle

The four stroke principle upon which most modern automobile engines work was discovered by a French engineer, Alphonse Beau de Rochas in 1862. The four stroke cycle is often called the Otto cycle after the german Nikolaus August Otto who designed un engine on that principle in 1876.

A stroke is the movement of the piston from TDC (top dead center) to BDC (bottom dead center), or from BDC to TDC. There are four strokes in one four stroke cycle of the engine. They are: the intake stroke, compression stroke, power stroke and exhaust stroke.

• Intake stroke: Gasoline will not burn unless it is mixed with the correct ammount of air. It is very explosive when 1 part is mixed with about 15 parts of air. Shortly before the piston reaches TDC, the intake valve begins to open. As the crankshaft turns, it pulls the rod and piston down in the cylinder toward BDC. The low pressure void created by this action is filled by atmospheric air pressure and fuel through the open intake valve. About 10,000 gallons of air is drawn in for every one gallon of fuel supplied by the fuel system. As the crankshaft continues to turn, the piston begins to move back up in the cylinder and the intake valve closes.
• Compression stroke: The piston moves up in the cylinder, compressing the air – fuel mixture into a smaller area making it easier to burn. The compression stroke begins at BDC after the intake stroke is completed. As the piston moves toward TDC, both of the valves are closed as the mixture is compressed to about 1/8 of the volume it occupied when the piston was at BDC.
• Power stroke: As the piston approaches TDC on its compression stroke, the compressed air – fuel mixture becomes very explosive. When the ignition system generates a spark at the spark plug, the fuel ignites. The air-fuel mixture burns. As the mixture burns it expands, 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. The power stroke is sometimes called the expansion stroke.
• Exhaust stroke: As the piston nears BDC on the power stroke, the exhaust valve opens allowing the spent gases to escape. Because the burning gases are still expanding, they are forced out through the open exhaust valve. As the crankshaft continues to turn past BDC , the piston moves up in the cylinder, helping to push the remaining exhaust gases out through the open exhaust valve. A few degrees after the 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.

The four stroke cycle is considerably more complicated than this simple explanation. When the engine is running, the timing of the opening and closing of the valves actually determines when each stroke effectively begins. Valve timing will be discussed with greater detail in one of the next articles.

Bibliography: – https://www.britannica.com/technology/four-stroke-cycle

             – Automotive Engines – Tim Gilles