A four-stroke engine is an Internal Combustion Engine in which the process of power generation happens in four strokes. A crankshaft in such engines makes two revolutions for a cycle to complete. The four strokes it completes in one cycle include intake, compression, ignition & power generation, and exhaust stroke.
Originated in 1862 by French Engineer Alphonse Beau de Rochas, the principle of four-stroke cycle was used by a German Engineer namely Nikolaus Otto in 1876 to design an engine. Since then, the four-stroke spark ignition engine has revolutionized the world as it could be used in a wide variety of transport vehicles.
A four-stroke engine may be less powerful than a two-stroke engine, but it provides more torque to the engine and are thus more fuel efficient.
Let’s understand the cycles of a four-stroke engine and see how it propels the vehicles and machines.
Intake Stroke
Also known as suction stroke, it is the event wherein a mixture of fuel and air is allowed to get into the combustion chamber/cylinder of an engine. During this stroke, the piston moves in a downward direction (from top-dead center to bottom-dead center) and creates low pressure inside the cylinder. Due to higher atmospheric pressure, the air-fuel mixture enters the cylinder as the intake valve opens. Once the cylinder is filled, the inlet valve closes and the air-fuel mixture is sealed in the chamber.
2. Compression Stroke:
During the compression stroke, the air-fuel mixture trapped inside the combustion chamber is compressed as the piston moves in an upward direction. The volume of air-fuel mixture trapped inside the combustion chamber is called charge. The intake and exhaust valves must remain closed during the compression stroke to avoid any spillage and for maximum efficiency. By the time the compression stroke is complete, the crankshaft completes one revolution.
When the piston compresses the charge, its volume decreases and as the volume decreases, the pressure in the gas is increased (as explained by the Boyle’s Law). While the compression takes place, electrical contact is kept opened and it is closed when the volume is the lowest and pressure is highest.
3. Power Stroke:
The electrical contact which was closed at the end of the compression stroke is now opened and this opening leads to production of spark inside the combustion chamber. The valves during power stroke remain closed and the combustion takes place inside a completely enclosed chamber.
Burning of the air-fuel mixture pushes the piston back to bottom dead center and the force exerted on piston is transferred to the crankshaft where the liner motion of piston is converted to angular motion of the crankshaft. The gasses in the cylinder after combustion expand as the piston moves downward and therefore the pressure and temperature of the gasses sink.
4. Exhaust Stroke:
At the end of the power stroke, the piston reaches the bottom dead center, and soon as it reaches there, the exhaust valve opens making way for the burnt gasses to leave the combustion chamber for the next cycle to begin.
The piston now begins to move in the upward direction forcing the hot gasses out of the cylinder and as it reaches the top-dead center, the exhaust valve is closes and the cycle begins once again.
At the end of the exhaust stroke, the crankshaft has completed a 720-degree turn and the cycle is now complete.
Conclusion: For these four strokes to complete, all the components of the engine must work in tandem. A slight issue with the working or structure of any of the key components of the engine may reduce its efficiency or even cause the engine to cease working.
