using indicator diagrams

Question 1

otto cycle

Answer 1

the theoretical cycle for a four stroke petrol engine

Question 2

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

Answer 2

• 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

Question 3

processes in the theoretical cycle for four stroke engines

Answer 3

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

Question 4

processin the theoretical cycle for a four stroke diesel engine

Answer 4

• 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

Question 5

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

Answer 5

• 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

Question 6

types of engien efficiency

Answer 6

• 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

Question 7

why is all heat not transferred to work

Answer 7

some heat always ends up increasing the temp of th engine

Question 8

why must engines objey 2nd law?

Answer 8

• 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

Question 9

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

Answer 9

• all heat energy supplies could be transferred to useful work

Question 10

How are engines never efficient

Answer 10

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

Question 11

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

Answer 11

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

Question 12

How is waste heat reused in CHP plants

Answer 12

• 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