Thermal Machines: Engines, Refrigerators, and Heat Pumps
Thermal Machines
A thermal machine is a device or system that exchanges heat and work with its environment, transforming a substance through a cyclic process. After each cycle, the substance returns to its original state and begins a new cycle. These machines can function as:
- Engines: Convert heat into work.
- Refrigerators or Heat Pumps: Convert work into heat.
Thermal machines convert mechanical energy into heat, and vice versa. Engines convert heat into work, while heat pumps use work to transfer heat.
History
The first thermal machine, the steam engine, emerged in the late 18th century, marking the dawn of the Industrial Revolution.
Classification
Thermal machines are classified as:
- External Combustion: Energy from combustion is generated outside the machine. Work is obtained through a driving device.
- Internal Combustion: The fuel reaction occurs inside the machine, and the generated gases produce work.
Internal and External Combustion Engine Types:
- Reciprocating: Use a piston with linear motion and a crankshaft to convert it into rotary motion.
- Rotary: Directly produce rotary motion.
Specific Engine Types:
- Internal Combustion Engines (Otto Cycle): Utilize a fuel explosion caused by a spark to expand gas and push a piston.
- Diesel Engines: Employ compression ignition to ignite the fuel.
Steam Engines
The steam engine revolutionized industry. James Watt’s improvements in 1769 significantly increased its efficiency. Steam engines use the heat from burning fuel to boil water and create steam, which then drives a piston or turbine.
Reciprocating Steam Engine
High-pressure steam from the boiler is directed to the cylinder through a distributor, pushing the piston. A connecting rod and crankshaft convert the linear motion into rotation. A governor regulates the engine speed.
Rotary Steam Engine (Steam Turbine)
Steam turbines drive generators and ship propellers. Steam is forced through nozzles, causing a wheel with fixed and rotating blades to turn. These are among the most powerful machines, exceeding 4,000,000 kW.
Internal Combustion Engines
Internal combustion engines generate power through combustion within the cylinder. Two main types exist: the Otto cycle and the Diesel cycle. These engines can be either two-stroke (2T) or four-stroke (4T).
Four-Stroke (4T) Engines
Developed by Nikolaus Otto in 1876, the four-stroke engine is the basis for modern gasoline engines. It uses a four-stage process:
- Intake: The piston moves down, drawing in the fuel-air mixture.
- Compression: The intake valve closes, and the piston compresses the mixture.
- Power: A spark plug ignites the mixture, forcing the piston down.
- Exhaust: The exhaust valve opens, and the piston pushes out the burned gases.
Two-Stroke (2T) Engines
Two-stroke engines are simpler and have a power stroke for every revolution of the crankshaft. They typically use a mixture of gasoline and oil for lubrication.
Diesel Engines
Invented by Rudolf Diesel, these engines use compression ignition. Air is compressed, and then fuel is injected. The heat of compression ignites the fuel, causing combustion.
Wankel Engine
The Wankel engine uses a rotating triangular rotor instead of pistons. It produces one power stroke per rotor revolution, leading to a smoother power delivery. However, it has faced challenges with friction and cooling.
Gas Turbines
In an open-cycle gas turbine, air is compressed, mixed with fuel, and ignited. The expanding gases drive a turbine, which produces work.
Refrigerators
Refrigerators use a refrigerant that undergoes phase changes to transfer heat. A compressor compresses the refrigerant, raising its temperature and pressure. The refrigerant then releases heat to the environment in the condenser. Next, the refrigerant expands, lowering its temperature and pressure. It then absorbs heat from the inside of the refrigerator in the evaporator.