Chapter 9

Question 1

Most power-producing devices operate on _____

Answer 1

Cycles

Question 2

A cycles that resembles the actual cycle closely but it made up totally or internally reversible processes

Answer 2

Ideal cycle

Question 3

Have the highest efficiency of all heat engines operating between the same temperature levels

Answer 3

Reversible cycles such as Carnot cycle

Question 4

Reversible cycles vs. ideal cycles

Answer 4

Totally reversible

Unsuitable as a realistic model

Question 5

_____ is a powerful engineering tool that provides great insight and simplicity at the expense of some loss in accuracy

Answer 5

Modeling

Question 6

Ideal cycles

Answer 6

Internally reversible

Not necessarily externally reversible like Carnot Cycles

Question 7

The thermal efficiency of an ideal cycle is ____ than that of a totally reversible cycle operating between the same temperature limits

Answer 7

Less

Question 8

The thermal efficiency of an ideal cycle is considerably ____ than the thermal efficiency of an actual cycle because of the idealization utilized

Answer 8

Higher

Question 9

The idealization and simplifications in the analysis of power cycles

Answer 9

1. The cycle does not involve any friction. Therefore, the working fluid does not experience any pressure drop as it flows in pipes or devices such as heat exchangers
2. All expansion and compression processes take place in a quasi-equilibrium manner
3. The pipes connecting the various components of a system are well insulated, and heat transfer through them is negligible

Question 10

On a T-s diagram, the ratio of the area enclosed by the cyclic curve to the area under the heat-addition process curve represents the ____

Answer 10

Thermal efficiency of the cycle

Increase ration —> increase thermal efficiency

Question 11

The Carnot cycle is composed of four totally reversible processes:

Answer 11

Isothermal heat addition

Isentropic expansion

Isothermal heat rejection

Isentropic compression

Question 12

For both ideal and actual cycles:

Answer 12

Thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or with a decrease in the average temperature at which heat is rejected from the system

Question 13

Air-standard Assumptions

Answer 13

1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas
2. All the processes that make up the cycles are internally reversible
3. The combustion process is replaced by a heat-addition process from an external source
4. The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state

Question 14

When the working fluid is considered to be air with constant specific heats at room temperature (25C)

Answer 14

Cold-air-standard assumptions

Question 15

A cycle for which the air-standard assumptions are applicable

Answer 15

Air-standard cycle

Question 16

The combustion process is replaced by a _____ in ideal cycles

Answer 16

Heat-addition process

Question 17

Braydon Cycle: the ideal cycle for gas-turbine engines

Answer 17

The combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constant-pressure heat-rejection process to the ambient air

Isentropic compression (in a compressor)
Constant-pressure heat addition
Isentropic expansion (in a turbine)
Constant-pressure heat rejection

Question 18

The two major application area of gas-turbine engines are

Answer 18

Aircraft propulsion

Electric power generation

Question 19

The fraction of the turbine work used to drive the compressor

Answer 19

Backwork ratio

Question 20

Gas-tubing engines

Answer 20

The air supplies the necessary oxidant for the combustion of the fuel, and it serves as a coolant to keep the temperature of various components within safe limits. An air-fuel ratio fo 50 or above is not uncommon

The highest temperatures in the cycle is limited by the maximum temperature that the turbine blades can withstand

Question 21

Development of Gas Turbines

Answer 21

1. Increasing the turbine inlet (or firing) temperatures
2. Increasing the efficiencies of turbomachinery components (turbines, compressors)
3. Adding modifications to the basic cycle (intercooling regeneration or recuperation, and reheating)

Question 22

Deviation of actual gas-turbine cycles from idealized ones (Brayton Cycle)

Answer 22

Irreversibilities in turbine and compressors, pressure drops, heat losses

Question 23

Reciprocating engines

Answer 23

Spark ignition (SI) engines

Compression-ignition (CI) engines

Question 24

Displacement volume

Answer 24

Between TDC and BDC

Question 25

Clearance volume

Answer 25

Above TDC

Question 26

TDC

Answer 26

Top dead center

Question 27

BDC

Answer 27

Bottom dead center

Question 28

Can be used as a parameter to compare the performances of reciprocating engines of equal size

Answer 28

Mean effective parameter

Question 29

The engine with a ____ value of MEP delivers more net work per cycle and thus performs better

Answer 29

Larger

Question 30

Wnet —> MEP

Answer 30

MEP x Displacement Volume

Question 31

Four-stroke cycle

Answer 31

SI engine

1 cycle = 4 stroke = 2 revolution

Question 32

Two stroke cycle

Answer 32

1 cycle = 2 stroke = 1 revolution

Less efficient than 4 stroke but simple and inexpensive

High power-to-weight and power-to-volume ratios

Question 33

Ideal cycle for spark ignition engines

Answer 33

Otto cycle

Question 34

Otto cycle

Answer 34

1-2 isentropic compression
2-3 constant-volume heat addition
3-4 isentropic expansion
4-1 constant-volume heat rejection

Question 35

2-stroke engines are commonly used in ____

Answer 35

Motorcycles and lawn mowers

Question 36

Air enters the cylinder through the open intake valve at atmospheric pressure P0 during process 0-1 as the piston moves from

Answer 36

TDC to BDC

Question 37

The intake valve is closed at _____

Answer 37

State 1

Question 38

Work interactions during intake and exhaust _____

Answer 38

Cancel each other

Question 39

In SI engines, the compression ration is limited by ____

Answer 39

Auto ignition or engine knock (combusts on own when too high)

Question 40

The thermal efficiency of the Otto cycle increases with…

Answer 40

The specific heat ratio k of the working fluid