Short Answer:

An engine cycle is a complete sequence of operations that takes place inside an internal combustion engine to produce power from fuel. It consists of four main stages—intake, compression, power, and exhaust. These stages repeat continuously while the engine runs. Each cycle helps convert the chemical energy of fuel into mechanical energy that drives the vehicle or machine.

The engine cycle is the foundation of engine operation, and it can be either a two-stroke or four-stroke cycle depending on the number of piston strokes required to complete one power cycle. Both types perform the same basic functions but differ in working methods and efficiency.

Detailed Explanation :

Engine Cycle

An engine cycle refers to the complete set of operations that take place within an internal combustion engine to convert fuel into useful mechanical energy. The process involves a systematic repetition of four stages—intake, compression, power (combustion), and exhaust. Each stage occurs in a particular order, and together they form one complete engine cycle. The continuous repetition of these cycles allows the engine to operate smoothly and produce continuous power output.

In an internal combustion engine, air and fuel are mixed and burned inside the cylinder. The high-pressure gases produced from combustion push the piston downward, generating mechanical energy. This energy is then transferred through the connecting rod and crankshaft to produce rotary motion, which drives the wheels or other machinery. The engine cycle ensures the proper timing of these processes for efficient power generation.

Four Stages of Engine Cycle

1. Intake Stroke:
   In the first stage, the piston moves downward inside the cylinder, and the inlet valve opens. This movement allows a mixture of air and fuel (in petrol engines) or only air (in diesel engines) to enter the combustion chamber. The downward motion of the piston creates a vacuum, helping the air-fuel mixture flow into the cylinder.
2. Compression Stroke:
   During this stage, the inlet valve closes, and the piston moves upward. This compresses the air-fuel mixture into a smaller space, increasing its pressure and temperature. Proper compression ensures that the fuel burns efficiently when ignited. In petrol engines, ignition occurs through a spark plug, while in diesel engines, ignition happens due to high compression.
3. Power (Expansion) Stroke:
   In this stage, combustion occurs. The compressed air-fuel mixture is ignited, causing a rapid rise in pressure and temperature. The expanding gases push the piston downward with great force. This movement is the power stroke—the only stroke that produces useful mechanical energy. The force generated is transmitted through the connecting rod to the crankshaft, which converts the reciprocating motion of the piston into rotary motion.
4. Exhaust Stroke:
   After the power stroke, the burnt gases must be expelled from the cylinder. The exhaust valve opens, and the piston moves upward, pushing the exhaust gases out through the exhaust port. This prepares the cylinder for the next intake stroke, and the cycle repeats.

Types of Engine Cycles

1. Four-Stroke Cycle:
   In a four-stroke engine, all four operations—intake, compression, power, and exhaust—are completed in four piston strokes or two revolutions of the crankshaft. Each stroke performs one function, and the engine delivers one power stroke for every two revolutions. This type is commonly used in cars, trucks, and heavy machinery because it provides better fuel efficiency, smoother operation, and lower emissions.
2. Two-Stroke Cycle:
   In a two-stroke engine, the entire cycle is completed in just two piston strokes or one crankshaft revolution. Here, the intake and exhaust processes occur simultaneously, making the engine simpler and lighter. Two-stroke engines are used in scooters, chainsaws, and small boats because of their high power output and compact design, although they consume more fuel and produce more emissions than four-stroke engines.

Importance of Engine Cycle

The engine cycle plays a vital role in ensuring proper engine performance and efficiency. It defines how fuel energy is transformed into useful work. A correctly timed and balanced cycle improves fuel economy, power delivery, and emission control. If any stage in the cycle fails or mistimes, the engine may misfire, lose power, or overheat.

The study of the engine cycle also helps engineers design better engines with improved efficiency and reduced pollution. Modern engines, especially those with fuel injection and computer control, still follow the same basic principles of the engine cycle but with more precision and automation.

Applications of Engine Cycle

Engine cycles are the heart of all internal combustion systems and are used in:

• Automobiles and motorcycles
• Generators and power plants
• Marine engines
• Aircraft piston engines
• Industrial machines and pumps

Each application may vary in design, but the basic four processes of intake, compression, power, and exhaust remain the same.

Conclusion :

An engine cycle is the complete sequence of events that convert fuel into mechanical power in an internal combustion engine. It includes the intake, compression, power, and exhaust stages. Depending on the number of strokes required to complete the cycle, engines are classified as two-stroke or four-stroke. The cycle is essential for the efficient functioning of engines in vehicles and machines. Without it, controlled combustion and power generation would not be possible, making it the foundation of modern mechanical engineering and engine design.