Thermodynamics

Question 1

Briefly show, using a diagram, how the work done can be calculated by plotting a p,V-diagram.

Answer 1

thermodynamics F7

Question 2

Name five processes that may be used in order to model the real engine process

Answer 2

isobaric process dp=0 p1=p2 v2/v1=T2/T1
isochoric process dV=0 v1=v2 p1/p2=T1/T2
isothermal process dT=0 T1=T2 p1/p2=v2/v1=rc
isentropic process; no heat transfer p1v1^k=p2v2^k k=cp/cv
polytropic process p1v1^n=p2v2^n

Question 3

State the formula of the thermal efficiency in the carnotcycle

Answer 3

max process efficiency

eta=1-Tmin/Tmax

Question 4

State the ideal cycle regarding its thermal efficiency. Why, in practice, is this process impossible to achieve? Draw a p,V-and T,s-diagram. Label the axes!

Answer 4

Carnot-Cycle
1-2: isothermal compression
2-3: isentropic compression
3-4: isothermal expansion
4-1: isentropic expansion

Question 5

Name three common types of cycles that are usually used in order to model idealised engine behaviour.

Answer 5

• Otto cycle (constant volume cycle)
• Diesel cycle (constant pressure cycle)
• Dual combustion cycle ( Seiliinger process)

Question 6

Explain, using p,V-diagrams, Otto-, Diesel-and Dual Combustion Cycle

Answer 6

Otto cycle (thermodynamics F24):
1 → 2: isentropic compression
2 → 3: isochoric combustion
3 → 4:isentropic expansion
4 → 1:isochoric cooling
Diesel cycle (F40):
1 → 2:isentropic compression
2 → 3:isobaric combustion
3 → 4:isentropic expansion
4 → 1:isochoric cooling
Dual-Combustion (F52):
1 → 2 : isentropic compression
2 → 3 : isochoric combustion
3 → 3‘: isobaric combustion
3‘ → 4:isentropic expansion
4 → 1 : isochoric cooling

Question 7

Draw a diagram of the thermal efficiency as a function of the compression ratio for each of the above cycles. Label the axes! Show whether the thermal efficiencies are load dependent!

Answer 7

thermodynamics F65
otto not load dependant
diesel lower load->better efficiency
seilinger ??yes, but not necessarily

Question 8

State the assumptions that are made when describing idealised engine work cycles

Answer 8

• real gases and gas mixture (friction)
• dissociation energy (chemestry)
• real heat supply process (chem. kinetics)
• incomplete combustion
• incomplete gas exchange process
• real heat losses
• blow-by
• friction losses