Introduction
A four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston make two complete passes in the cylinder to complete one operating cycle. An operating cycle requires two revolutions (720°) of the crankshaft. The four-stroke cycle engine is the most common type of small engine. A four-stroke cycle engine completes five Strokes in one operating cycle, including intake, compression, ignition, power, and exhaust Strokes.
COMPONENTS
Inlet Manifold: It is a passage for the entry of the fuel inside the cylinder.
Exhaust manifold: Passage for the escape of the burnt gases.
Intake valve: It is a cam operated valve through which the fuel or air enter within the cylinder.
Exhaust valve: It is also a cam operated valve through which the burnt gases escapes out.
Spark plug: It is used to generate the spark for the burning of the fuel in spark ignition engine.
Fuel injector: it is used to inject fuel in diesel engine.
Cylinder: It is a part of the engine where piston does reciprocating motion in order to produce all four strokes necessary for the working of an engine.
Piston: It is a part of engine which does reciprocation motion within the cylinder. It performs suction, compression, and exhaust process during the working of the four stroke engine.
Connecting rod: It connects the piston and crankshaft of the engine. It transfers the power generated by the engine from piston to the crankshaft. And the reciprocating motion of the piston changes to the circular motion.
Crankshaft: It is used to convert the reciprocating motion of the piston into circular motion.
Crank case: It holds the crankshaft assembly in it and protects them from damage
TYPES
The four stroke engine are of two types and these are
1. Petrol engine/ gasoline engine:
when petrol is used as a fuel in four stroke engine then it is called as four stroke petrol engine. The construction of petrol engine is slightly different from the diesel engine. In petrol engine there is a spark plug for the combustion of the fuel. And air-fuel mixture is sucked in the cylinder.
2. Diesel engine:
When the fuel used in the four stroke engine is diesel than it is called as diesel engine. In diesel engine there is fuel injector for the injection of the fuel within the cylinder. During suction, only air is sucked within the cylinder. Hot compressed Air is used for the burning of the fuel in this type of four stroke engine.
FOUR STROKE ENGINE CYCLES
1. Intake Stroke
The intake event is when the air-fuel mixture is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC (top dead center) to BDC (bottom dead center) and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air-fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement. The cylinder continues to fill slightly past BDC as the air-fuel mixture continues to flow by its own inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC. Depending on engine design. The intake valve then closes and the air-fuel mixture is sealed inside the cylinder.
Key points :-
Intake valve – OPEN
Exhaust valve – CLOSED
Crankshaft rotation – 180°
2. Compression Stroke
The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. The combustion chamber is sealed to form the charge. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel helps to maintain the momentum necessary to compress the charge.
When the piston of an engine compresses the charge, an increase in compressive force supplied by work being done by the piston causes heat to be generated. The compression and heating of the air-fuel vapor in the charge results in an increase in charge temperature and an increase in fuel vaporization. The increase in charge temperature occurs uniformly throughout the combustion chamber to produce faster combustion (fuel oxidation) after ignition.
The increase in fuel vaporization occurs as small droplets of fuel become vaporized more completely from the heat generated. The increased droplet surface area exposed to the ignition flame allows more complete burning of the charge in the combustion chamber. Only gasoline vapor ignites. An increase in droplet surface area allows gasoline to release more vapor rather than remaining a liquid.
The more the charge vapor molecules are compressed, the more energy obtained from the combustion process. The energy needed to compress the charge is substantially less than the gain in force produced during the combustion process. For example, in a typical small engine, energy required to compress the charge is only one-fourth the amount of energy produced during combustion.
The compression ratio of an engine is a comparison of the volume of the combustion chamber with the piston at BDC to the volume of the combustion chamber with the piston at TDC. This area, combined with the design and style of combustion chamber, determines the compression ratio. Gasoline engines commonly have a compression ratio ranging from 6:1 - 10:1. The higher the compression ratio, the more fuel-efficient the engine. A higher compression ratio normally provides a substantial gain in combustion pressure or force on the piston. However, higher compression ratios increase operator effort required to start the engine. Some small engines feature a system to relieve pressure during the compression stroke to reduce operator effort required when starting the engine.
Key points :-
Intake valve – CLOSED
Exhaust valve – CLOSED
Crankshaft rotation – 180° (total = 360°)
3. Ignition Event
The ignition (combustion) event occurs when the charge is ignited and rapidly oxidized through a chemical reaction to release heat energy. Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat.
Proper combustion involves a short but finite time to spread a flame throughout the combustion chamber. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before TDC (BTDC). The atmospheric oxygen and fuel vapor are consumed by a progressing flame front. A flame front is the boundary wall that separates the charge from the combustion by-products. The flame front progresses across the combustion chamber until the entire charge has burned.
4. Power Stroke
The power stroke is an engine operation Stroke in which hot expanding gases force the piston head away from the cylinder head. Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power Stroke, both valves are closed.
Key points :-
Intake valve – CLOSED
Exhaust valve – CLOSED
Crankshaft rotation – 180° (total = 540°)
5. Exhaust Stroke
The exhaust stroke occurs when spent gases are expelled from the combustion chamber and released to the atmosphere. The exhaust stroke is the final stroke and occurs when the exhaust valve is open and the intake valve is closed. Piston movement evacuates exhaust gases to the atmosphere.
As the piston reaches BDC during the power stroke combustion is complete and the cylinder is filled with exhaust gases. The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.
Key points :-
Intake valve – CLOSED
Exhaust valve – OPEN
Crankshaft rotation – 180°(total = 720°)
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