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Flashcards in using indicator diagrams Deck (12)
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otto cycle


the theoretical cycle for a four stroke petrol engine



what condition do theoretical indicator diagrams in both petrol and diesel engines assume

  • same gas is taken continuously around the cycle
    • pure air 
    • adiabatic constant of 1.4 (y thing) 
  • pressure and temperature changes are instantaneous
  • heat source is external
  • engine is frictionless





processes in the theoretical cycle for four stroke engines

  • A
    • assumed gas is compressed adiabatically
    • no heat transferred
    • heat is supplied whilst volume is kept constant
  • C
    • the gas is allowed to cool adiabatically
  • D
    • the system is cooled at constant volume


processin the theoretical cycle for a four stroke diesel engine

  • A
    • gas is adiabatically compressed
  • B
    • then heat is supplies
    • this time pressure is kept constant
  • C
    • the gas is allowed to cool adiabatically 
  • D
    • then the system is cooled at a constant volume



What are the main differences between theoretical and real-life diagram

  • corners of theoretical diagrams are not rounded
    • assumed that the same air is used continuously
    • real engines, these corners are rounded as the inlet and exhaust valves take time to open and close
  • in real, heating doesn't take place at a constant volume (process b)
    • increase in pressure and temperature would have to be instantaneous to do this, or the piston would have to pause
  • theoretical model doesnt include small amount of negative work caused by the loop between the exhaust and induction lines because it assumes the same air cycles around the system continuously
  • engines have an internal heat source, not external
    • temperature rise is not as large as in the theoretical model because the fuel used to heat the gas is never completely burned
    • so you can never get the max energy out of it
    • theoretical engines acheive higher pressure
  • energy is needed to overcome friction caused by moving parts
    • net work done always less than a theoeretical engine
    • area inside loop is smaller




types of engien efficiency

  • mechanical efficiency
    • affected by the energy lost through moving parts
    • = brake power / indicated power
  • thermal efficeiny
    • describes how well heat ernergy transferred into work
    • = indicated power / input power
  • ovarall efficeincy
    • = break power / input power



why is all heat not transferred to work

some heat always ends up increasing the temp of th engine 





why must engines objey 2nd law?

  • if engine temp reaches that of the heat source, then no heat flows and no work is done
  • no engine can operate using only 1st law
  • have to obey second law
    • heat engines operate between a heat source and a heat sink 



what would happen if an engine could work from just the first law

  • all heat energy supplies could be transferred to useful work




How are engines never efficient

  • heat energy transferred to the engine from the heat source is QH
  • some of this energy is converted to useful work, W
  • however, some of this energy (QC) must be transferred to a heat sink, which has a lower temp (TC) than the heat source




WHat are the reason the real heat engines effieciencies are lower than their theoretical max

  • frictional forces inside engine
  • fuel does'nt burn entirely 
  • energy needed to move internal components




How is waste heat reused in CHP plants

  • maximise efficiency, as much of the input energy must be trasnferred usefully
  • engines are very ineffeicent
    • waste heat trasnferred to surroudnigs
  • combines heat and power plants try to limit energy waste by using this heat for other purposes
    • e.g. heating houses near yb