Heat engines

• Heat engines 


  1)Heat is supplied to an engine from a source at a high temperature

  2)Some of this heat is transferred to mechanical energy via work done

  3)The rest of the input heat is transferred to a cold reservoir
• Piston


  Heat is supplied to a gas, causing the particles to move around faster and collide with the surface of the piston. The piston is then pushed because of the increased pressure causing the wheel to turn. Heat is then removed causing the pressure to decrease and the piston starts moving back causing the wheel to turn.
• Carnot engine: An ideal heat engine, the most efficient theoretically possible)

  Carnot cycle

  1. 1-2: An isothermal expansion occurs where heat must be transferred in because expansion naturally causes the temp to fall
  2. 2-3: An adiabatic expansion occurs where no heat is being transferred in. The heat already inside continues the expansion.
  3. 3-4: An isothermal compression occurs where heat is pulled out because temperature naturally increases when something is compressed
  4. 4-1: An adiabatic compression occurs where no heat is transferred out and it compresses on its own.
• Carnot Efficiency

  
  

  η = 1 - T(c) ÷ T(h)

  
• Internal combustion in engines (Otto Cycle)

  1. A mixture of air and vaporised gas is injected into a cylinder and compressed adiabatically (with zero heat transfer), by a piston
  2. A spark plug ignites the mixture, raising the temperature and pressure while the volume doesn't change
  3. The high pressure gas pushes the piston, causing it to expand adiabatically and produce mechanical work
  4. The hot gasses are then expelled and replaced by a new mixture. This process is repeated
• Refrigerator cycle (reversed Carnot cycle)


  1. A refrigerant liquid flows into an expansion device causing it to expand exothermically; with the temperature and the pressure dropping. It then flows through the evaporator coil (within the fridge) where heat is pulled in by the cold refrigerant. (to keep temp constant)
  2. The refrigerant then flows to the compressor where it compresses it, causing the temperature and pressure to increase.
  3. It then flows through the condenser coil where it transfers heat to the environment and goes back to the expansion device.
• Refrigerators (reversed Carnot cycle)

  1. A refrigerant liquid flows into an expansion device causing the pressure and temperature to decrease. It then flows through an evaporator coil where the cold refrigerant pulls the heat from inside the fridge
  2. The refrigerant eventually flows to the compressor where it squeezes it and causes temp and pressure to increase. It then passes through to the condenser coil where it radiates heat to its surroundings and goes back to the expansion device, repeating the process