Thermodynamics: Laws and Key Figures

• Thermodynamics deals with the study of heat, energy, and their transformation into different forms of work

Count Rumford

• Also known as Sir Benjamin Thompson
• First to study the relationship between work and heat
• Lived March 26, 1753 – August 21, 1814

James Prescott Joule

• First to demonstrate that a definite amount of work is required to produce a given amount of heat
• Lived December 24, 1818 – October 11, 1889

Mechanical Equivalent of Heat (J)

• The amount of work required to produce a unit quantity of heat.
• J = Work/Heat
• J = W/H
• 1 cal = 4.186 [4.19] J

James Watt

• Developed the first steam engine

First Law of Thermodynamics

• When heat is converted into another form of energy, or when other forms of energy are converted into work, there is no loss of energy

Ways of converting heat into work

• By generating steam, the heat comes from burning wood, coal or oil to generate steam from boiling water
• By burning gas, the burning of compressed gas converts the generated heat into work by exerting a force to move a piston of the blades of a turbine

Second Law of Thermodynamics

• Heat flow is always from a body of higher temperature to a body of lower temperature, but this can be reversed.

Ways of reversing the heat flow

• By using heat pumps to move heat from colder areas to warmer areas
• Heat Engines are devices that convert heat energy into mechanical work

Classifications of Heat Engines

• External Combustion occurs when combustion takes place outside the engine
  • A steam turbine is used for ship propulsion and operating in electric power plants
  • A gas turbine involves compressed air, a combustion chamber, heated air, a turbine, and a generator
• Internal Combustion occurs when combustion takes place inside the engine
  • A gasoline engine is for small vehicles like cars and motorcycles

Operation of a gas engine (4-stroke cycle)

• Intake stroke: a mixture of gas and air from a carburetor is introduced to the cylinder
• Compression stroke: the mixture is compressed
• Power stroke: the compressed fuel mixture is ignited by an electric spark from the spark plug
• Exhaust stroke: the burned gases are expelled from the cylinder
• Diesel Engine operates without a spark plug, and has a thicker and heavier cylinder

Differences between 4-Stroke and 2-Stroke Engines

• Number of Strokes: 4-Stroke has 4 (Intake, Compression, Power, Exhaust), while 2-Stroke has 2 (Compression, Power)
• Power Output: 4-Stroke has lower power per cycle, while 2-stroke has higher power per cycle
• Fuel Efficiency: 4-Stroke is more efficient, while 2-Stroke is less efficient
• Maintenance: 4-Stroke requires less maintenance, while 2-Stroke requires frequent maintenance
• Noise: 4-Stroke is quieter, while 2-Stroke is noisier
• Jet engines and rockets are forms of internal combustion engines