11.2 - Thermodynamics and Engines

• What is the first law of thermodynamics?

  Conservation of energy in systems when cooling, heating and doing work.

  Q = U + W

  Where u is the internal energy, treating it as an ideal gas means that kinetic energy is only taken into account, not potential energy also

  Q = Energy transferred by heating

  W = Work done by the system
• A gas is compressed due to work being done on it, W if therefore positive. True or false?

  False, work is done on a gas so W is negative
• If a gas is cooled, Q is negative. True or false?


  True, energy is removed hence Q is negative
• What assumptions are necessary when applying the 1st law to a closed (non-flow) system?

  That the gas is an ideal gas so the internal energy is only dependent on temperature, work done causes a change in volume and the ideal gas law (pV + nRT) can be applied where n and R will both be constant
• What is an isothermal change?

  A change that occurs at a constant temperature. This means internal energy stays constant so Q = W, meaning any transfer of energy would lead to work being done and vice versa. It obeys Boyle's Law: PV = Constant
• How do you calculate the work done on an isothermal change p-V diagram?

  The work done is the area under the graph, between the two points
• What is an adiabatic change?


  A change in which no heat passes in or out. Q = 0 and W = -U. If work is done by the system, W is positive and the internal energy will decrease.

  
• How can you calculate the work done on a p-V diagram for adiabatic changes?


  The work done is the area under the pressure volume graph.
• What is a constant pressure change?


  If a gas is kept at a constant pressure, it will increase in volume when heated.

  W = pV
• How do you calculate the work done from a constant pressure change pV diagram?

  The work done is the area under the graph
• What is a constant volume change (isovolumetric)?


  When heating gas in an enclosed space, volume stays constant. This leads to an increase in pressure, temperature and internal energy. W = 0 so Q = U. All the heat energy is transferred into internal energy.
• In a cyclic process, the work done per cycle on a graph is the...

  Area of the loop

  (the difference between the work done by the system and the work done on the system)
• What are indicator diagrams?

  a p-V diagram for engines that allows identification of the different processes in the system

  
  

  it may vary depending on speed, timing and load
• What are the key stages of induction in the four stroke petrol engine cycle?

  - The piston moves down the cylinder increasing the volume of gas above it

  - Air and petrol vapour are drawn into the cylinder through the open inlet valve

  - Pressure stays constant, volume increases
• What are the key stages of compression in the four stroke petrol engine cycle?

  - The valves are closed

  - Piston moves back up cylinder, volume decreases, pressure increases (work is done)

  - Near the end of the stroke, the spark plug ignites air/fuel mix causing an increase in temperature and pressure. Minimal volume change
• What are the key stages of expansion in the four stroke petrol engine cycle?

  - Valves are closed, expanded ignited gas moves the piston down

  Work done by the expanding gas is more than the work done to compress the gas, net output of work

  - Near the end of the stroke, the exhaust valve opens to reduce pressure to near atmospheric
• What are the key stages of the exhaust stroke in the four stroke petrol engine cycle?


  - The piston moves back up, getting rid of the burnt air through the exhaust

  - Pressure just above atmospheric
• What assumptions are made for theoretical indicator diagrams of diesel and petrol engines?

  - Pure air is continuously cycled

  - Pressure and temperature changes can be instant

  - The heat source is external

  - The engine is frictionless
• What 4 processes make up the theoretical cycle of a 4 stroke petrol engine?

  1. The gas is adiabatically compressed so no heat is transferred

  2. Volume stays constant as heat is supplied

  3. Gas cools adiabatically

  4. System cools at constant volume
• What 4 processes make up the theoretical cycle of a 4 stroke diesel engine?

  1. Gas is adiabatically compressed

  2. Heat is supplied, pressure stays constant

  3. Gas cools adiabatically

  4. System cools at constant volume
• Describe a 4 stroke diesel engine cycle induction stroke


  - Inlet valve opens

  - Air is drawn into cylinder (no fuel)

  - Volume increases, pressure stays constant
• Describe a 4 stroke diesel engine cycle compression stroke

  - Inlet valve closed

  - Piston moves in, air is compressed

  - Temperature increases, air becomes hot enough to to ignite fuel

  - Diesel is sprayed into cylinder and ignites
• Describe a 4 stroke diesel engine cycle expansion and exhaust stroke

  - Gas expands, moving the piston down

  - Net output of work

  - Exhaust valve opens and removes heat and burnt gas

  - Piston moves in, expelling more gas
• What is the difference between petrol engine and diesel engine indicator diagrams?

  The petrol engine diagram has a sharper peak at the start of the expansion stroke than diesel engines (due to spark)
• Why are corners rounded on actual indicator diagrams?

  It's theoretically assumed that the same air is used constantly but actually the valves need time to open and close
• Why doesn't heating take place at a constant volume in a real engine?

  The increase in pressure and temperature isn't instantaneous in a real engine
• Why don't theoretical indicator diagrams include the negative work done between the exhaust and induction lines?

  It assumes the same air continuously cycles around the system
• Why do theoretical indicator diagrams have a higher peak than actual diagrams?

  The temperature rise from burning the gas isn't as large as predicted as the fuel isn't completely burnt, so maximum pressure isn't achieved (hence lower peak)
• Why is the area within the indicator diagram loop smaller for real engines than theoretical engines?

  The net work done by an actual engine is less than for a theoretical engine as energy is needed to overcome friction in the engine's moving part
• How can you calculate the work done on the gas during the compression phase of an indicator diagram?

  The area underneath the curve during the compression phase
• How can you calculate the work done by the gas during the expansion phase of an indicator diagram?

  The area underneath the curve during the expansion phase
• How can you calculate the net work done during the cycle?

  The area enclosed by the loop, which is work done on the gas during compression minus the work done by the gas during expansion
• How can you predict the engine's indicated power?

  The area enclosed by the loopnumber of cyclesnumber of cylinders

  It is the net work done by the engine cylinder in one second
• Why would the output power be less than the indicated power?

  Some power is used when overcoming frictional forces, this power is called frictional power
• How can you calculate output/brake power?

  Brake power is the power delivered to the crankshaft of the engine.

  
• How can frictional power be calculated?


  Frictional power = Indicated power - brake power
• How do you calculate overall efficiency?

  brake power / input power
• How do you calculate the thermal efficiency?

  indicated power/input power
• How do you calculate the mechanical efficiency?

  brake power/indicated power
• Why can't an engine work using only the first law of thermodynamics?

  Even with the most theoretically efficient engine, some energy is lost to overcoming friction, hence the heat energy can't all be transferred to do work