Gas Power Cycles

Question 1

What assumptions can we make to “idealize” an actual cycle?

Answer 1

• There is no friction
• All expansion and compression processes are quasi-equilibrium processes
• Pipes connecting the components of the cycle are perfectly insulated, so there is no heat transfer to the surroundings
• Kinetic energy and potential energy is negligible

Question 2

What is the reversibility of an ideal cycle?

Answer 2

Ideal cycles are internally reversible but not necessarily externally reversible

Question 3

What are the Air Standard Assumptions?

Answer 3

• The working fluid is air which flows in a closed loop and acts as an ideal gas
• All of the processes that make up the cycle are internally reversible
• The combustion process is replaced by a heat-addition process from an external source
• The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state
• Cold Air Assumption: Specific heat remains constant at their values of T = 25 degrees Celcius

Question 4

What is the compression ratio in a reciprocating engine?

Answer 4

r = Vmax / Vmin

Question 5

What is the mean effective presure?

Answer 5

Wnet / (Vmax - Vmin)
This is an effective way to compare different engines with the same size

Question 6

What are the steps in an Otto Cycle?

Answer 6

• Isentropic Compression (The cycle becomes insulated for this step)
• Constant-Volume Heat Addition (The cycle loses the insulation to add the heat)
• Isentropic Expansion (The insulation returns)
• Heat Rejection (The insulation disappears again)

Question 7

What is the efficiency of an Otto cycle?

Answer 7

1 - 1 / (T^k-1)

Question 8

How does combustion occur in an Otto Cycle?

Answer 8

A spark plug sparks and causes the combustion of a compressed Air-Fuel mixture

Question 9

How does combustion occur in a Diesel Cycle?

Answer 9

Air is taken into the cylinder and compressed. Then fuel is sprayed into the compressed air so we can then combust

Question 10

Why does the compression ratio of an Otto Cycle have to be limited?

Answer 10

To prevent early combustion which will then lead to auto-ignition and engine knock

Question 11

What is the process for an Ideal Diesel Engine?

Answer 11

• Isentropic Compression
• Constant Pressure Heat Addition
• Isentropic Expansion
• Constant Volume Heat Removal

Question 12

What is the process of an ideal Brayton Cycle?

Answer 12

• Isentropic Compression
• Constant Pressure Heat Addition
• Isentropic Expansion
• Constant Pressure Heat Rejection

Question 13

What is the efficiency of a Brayton Cycle?

Answer 13

1 - 1 / (P2 / P1) ^ (k-1 / k)

Question 14

What is the Back Work Ratio?

Answer 14

Work of the compressor / Work of the Turbine

Question 15

What is the effectiveness of a regenerator?

Answer 15

Actual Heat / Maximum Heat

Question 16

How can we decrease the specific volume in a compressor and why should we?

Answer 16

Multistage Compression with Intercooling
We do this to decrease the amount of work done by the compressor

Question 17

What value can n be when considering P * v^n = Constant?

Answer 17

1 < n < k

Question 18

How can we increase the work the turbine produces?

Answer 18

We can maximise the specific volume to increase the work. We can do this by reheating and combustion in multiple stages