7) P-V Diagrams

Question 1

( P-V diagram for a real diesel engine ) - Give reasons why the efficiency of a real engine would be less than a theoretical one.

Answer 1

• Work needed for induction and exhaust.
• Area of pumping loop has to be subtracted from the main loop, hence reducing net work and hence the efficiency of a real engine.
• Compression and expansion do not take place infinitely quickly heat is lost; therefore not adiabatic processes, lowering efficiency.
• Area of the loop is smaller for real engine, less work is done per cycle so lower efficiency.

Question 2

( P-V diagram for a real diesel engine ) - What is needed for a real engine that isn’t needed for a theoretical one?

Answer 2

Real engine needs ‘pumping loop’ at near atmospheric pressure for induction and exhaust.
-The theoretical cycle needs no pumping loop - same air continuously taken through repeated cycles.

Question 3

( P-V diagram for a real diesel engine ) - Why are the corners rounded on a real engine diagram?

Answer 3

Because valves are needed and take a finite time to open/close.

Question 4

( P-V diagram for a real diesel engine ) - Why can cooling not occur at constant volume in a real engine?

Answer 4

Because the piston would have to stop.

Question 5

( P-V diagram for a real diesel engine ) - Why does heating does not occur at constant pressure?

Answer 5

Because it’s impossible to control the rate of burning of fuel during injection.

Question 6

Explain why the rapid compression of the gas can be assumed to be an adiabatic
process.

Answer 6

in adiabatic compression there is no heat transfer. If compression is quick there is no time for heat transfer