Engineering Physics

• The first law of thermodynamics describes how energy is conserved in a system through heating, cooling, and doing work.
• The first law of thermodynamics is given by $Q =$$\Delta U +$$W$, where:

  • Q is the energy transferred to the system by heating
  • U is the increase in internal energy of the gas
  • W is the work done on the system
• If a gas does work on a system, the W is positive (expansions). If Work is done on a gas via compression, then W is negative
• The internal energy of a system is the sum of the kinetic and potential energies of the particles in the system.
• A non-flow process is one where gas doesn't escape from a system
• Isothermal changes occur at a constant temperature. For this change, $\Delta U =$$0$ .
• Adiabatic processes are processes where no heat is lost or gained by the system i.e Q = 0. For this process, $pV^\gamma =$$constant$.
• For isobaric changes, $W =$$p\Delta V$.
• For isovolumetric processes, W = 0
• The higher the temperature of a system, the further away the isotherm is from the origin on a p-v graph
• The area under an adiabatic curve is larger than for an isothermal curve at the same initial temperature
• Four stroke petrol engine

  1. Induction

  2. Compression

  3. Expansion

  4. Exhaust
• Induction


  The piston starts at the top of the cylinder and moves down, increasing gas volume above it. This sucks a fuel-air mixture through the inlet valve. Pressure remains constant.
• Compression


  Inlet valve is closed, and the piston moves back up the cylinder, doing work on the gas and increasing pressure. Right before the end of the stroke, the spark plug creates a spark and ignites the mixture. Temperature + pressure suddenly increase at an almost constant volume.
• Expansion


  The hot expanding mixture does work on the piston and moves it downwards. Work done by the gas is more than the work required to compress it (so net work output). Just before the end of the stroke, the exhaust valve opens and pressure reduces.
• Exhaust


  The piston moves back up the cylinder, and burnt gas exits through the exhaust valve. Pressure remains almost constant (just above atmospheric).