Heat Engine Overview:

A heat engine is a machine designed to convert thermal energy (from the combustion of fuel) into mechanical work. It works on the principle that when fuel burns, heat is produced, and this heat energy increases the pressure of gases, which moves a piston. This movement is transferred to a crankshaft to generate mechanical power.

Classification of Heat Engines:

1. Based on Combustion of Fuel:

• External Combustion Engine:
  • In an external combustion engine, the combustion process occurs outside the engine. For example, in a steam engine, fuel is burned in a boiler, producing steam. This steam is then allowed into a cylinder where it expands and drives a piston. The mechanical work is done by the expansion of steam, not directly by the combustion inside the cylinder.
  • Example: Steam engine used in old locomotives.

• Internal Combustion Engine:
  • In an internal combustion engine, fuel is burned inside the engine cylinder itself. This results in a rapid increase in pressure from the combustion, which pushes the piston down. The movement of the piston is then converted into rotational motion by the crankshaft.
  • Example: Petrol engines in cars, diesel engines in trucks.

2. Based on Fuel Used:

• Diesel Engine: The fuel used is diesel. Diesel engines operate using compression ignition, meaning the air inside the cylinder is compressed so much that it ignites the injected diesel fuel. These engines are typically more fuel-efficient and are used in larger vehicles like trucks, buses, and industrial machines.

• Petrol Engine: The fuel used is petrol. These engines use spark ignition, where a spark plug ignites the air-fuel mixture. Petrol engines are commonly used in cars, motorcycles, and light vehicles.

• Gas Engine: These engines run on gaseous fuels, such as propane, butane, or methane. Gas engines are typically used in industrial applications or as backup generators.

3. Based on Ignition of Fuel:

• Spark Ignition Engine (Carburetor Type Engines):
  • In these engines, a mixture of air and fuel is drawn into the engine cylinder. Ignition occurs via a spark plug which produces a spark to ignite the mixture. This results in constant volume combustion (CVC).
  • Example: Petrol engines.

• Compression Ignition Engine (Injector Type Engines):
  • Here, air is compressed within the cylinder, increasing its temperature to 700-900°C. Diesel is then injected into the compressed air, causing the fuel to ignite spontaneously due to the high temperature. This process results in constant pressure combustion (CPC), where the pressure remains almost constant during combustion.
  • Example: Diesel engines.

4. Based on Working Cycle:

• Four Stroke Cycle Engine:
  • In a four-stroke engine, the cycle is completed in two revolutions of the crankshaft (i.e., the piston moves up and down twice for every cycle). It consists of four stages: intake, compression, power, and exhaust.
  • Example: Most modern cars use four-stroke engines.

• Two Stroke Cycle Engine:
  • In a two-stroke engine, the cycle is completed in one revolution of the crankshaft (i.e., the piston moves up and down once for every cycle). This type of engine is simpler, more compact, and provides more power relative to size.
  • Example: Small equipment like chainsaws, motorcycles, and lawnmowers.

Construction of an Internal Combustion Engine:

The main components of an Internal Combustion (IC) Engine include:

1. Piston: A close-fitting part inside the cylinder that moves up and down.
2. Cylinder: The chamber where the combustion of fuel takes place, surrounded by the piston.
3. Connecting Rod: It connects the piston to the crankshaft, converting the piston’s linear motion to rotary motion.
4. Crankshaft: It rotates as a result of the piston’s movement and transmits rotational energy to power the vehicle or machine.
5. Cylinder Block: It houses the cylinders and other components.
6. Cylinder Head: The top of the engine cylinder, where the combustion space is located.
7. Gasket: Prevents leakage of gases between the cylinder and cylinder head.
8. Gudgeon Pin (Wrist Pin): Connects the piston to the connecting rod at the small end.
9. Flywheel: A heavy wheel attached to the crankshaft that helps smooth out the rotational motion of the engine.
10. Oil Sump: A reservoir for engine oil to lubricate the engine components.

The working principle involves the ignition of a fuel-air mixture inside the cylinder, which results in an increase in pressure, forcing the piston downwards. The connecting rod transmits the piston’s motion to the crankshaft, converting the up-and-down motion into rotary motion.

Working Principle of IC Engine (Four Stroke and Two Stroke Engines):

In both four-stroke and two-stroke engines, the working cycle involves the following stages:

Four Stroke Cycle Engine (2 Revolutions of Crankshaft):

• Intake Stroke (Suction): The intake valve opens, and the piston moves down the cylinder, drawing in a mixture of air and fuel.
• Compression Stroke: The intake valve closes, and the piston moves upwards, compressing the air-fuel mixture. This increases the pressure and temperature inside the cylinder.
• Power Stroke: When the piston reaches the top, the spark plug ignites the compressed air-fuel mixture. This results in a rapid increase in pressure, forcing the piston down. This is where the engine generates power.
• Exhaust Stroke: The exhaust valve opens, and the piston moves upwards again, pushing the burnt gases out of the cylinder to prepare for the next cycle.

Two Stroke Cycle Engine (1 Revolution of Crankshaft):

• Compression Stroke and Exhaust Stroke: As the piston moves up, it compresses the fuel-air mixture. Simultaneously, the exhaust gases are pushed out of the exhaust port.
• Power Stroke and Intake Stroke: When the piston reaches the top, the fuel-air mixture is ignited by a spark plug, and the piston is forced down. This also causes the intake port to open, allowing a fresh fuel-air mixture to enter the cylinder for the next cycle.

Applications of Two Stroke and Four Stroke Engines:

• Four Stroke Engines: These engines are used in applications requiring greater efficiency and lower emissions. Examples include cars, trucks, large machinery, and motorcycles.
• Two Stroke Engines: Due to their compactness and simplicity, two-stroke engines are commonly used in small, portable equipment like lawn mowers, chainsaws, small motorcycles, and generators. These engines are suitable for applications where weight and size are critical factors.

Power and Efficiency:

Power: The power produced by an engine is a result of the combustion of fuel and the movement of the piston. This energy is transferred to the crankshaft, which converts it into rotational motion that powers the vehicle or machinery.

Efficiency:

• Four Stroke Engines are generally more efficient than Two Stroke Engines because they allow for a more complete combustion cycle. The fuel is used more efficiently, and emissions are typically lower.
• Two Stroke Engines tend to be less efficient as they do not have a separate exhaust and intake stroke. This can lead to unburnt fuel and higher emissions, but they are simpler, lighter, and more powerful for their size.