Thermodynamics And Engines

Question 1

Give two ways of changing the internal energy

Answer 1

Use forces to do work or heat transfer

Question 2

What is an adiabatic change?

Answer 2

An energy transfer where no heat transfer occurs

Question 3

Give a way to cause an adiabatic change in a gas

Answer 3

Compress it quickly

Question 4

What does γ stand for

Answer 4

Adiabatic constant

Question 5

Compare adiabatic and isothermal curves on pv graphs.

Answer 5

Adiabatic curves are steeper

Question 6

Describe the pv graphs for:
Change of volume at constant pressure
Change of volume and pressure at constant temp
Change of volume and pressure at change of temp

Answer 6

Constant pressure is horizontal line.
Constant temp is curve
Adiabatic is steeper curve

Question 7

Give the first law of thermodynamics

Answer 7

The heat energy supplied to a system either increases its internal energy or enables it to do work or both

ΔQ=ΔU+ΔW

Question 8

What does the area under a pv graph represent?

Answer 8

Work done

Question 9

Give the equation for work done and Q in and out

Answer 9

Qin =W+Qout

Question 10

Why do you need a low temperature sink?

Answer 10

To draw energy from the source

Question 11

Give the second law of thermodynamics

Answer 11

It is impossible for heat transfer from high temp to source to produce produce an equal amount of work

Question 12

Give the efficiency of an engine

Answer 12

Work done by engine over energy supplied to engine

Question 13

Give the cycle of an engine

Answer 13

Do work, cooled, re heat, do work etc.

Question 14

In a petrol engine what is the working substance?

Answer 14

Air

Question 15

Give an assumption of an indicator diagram

Answer 15

Fixed mass of air in cylinder throughout each cycle obeying gas laws

Question 16

What happens in each stage of an engine cycle?

Answer 16

Compression: air in cylinder trapped and compressed adiabatically by piston.
Ignition: spark ignites mixture to increase pres and temp rapidly.
Power: high pres forces piston outward. Work done.
Cooling: gas cooled rapidly reducing pressure

Question 17

What does the area of the indicator loop give you?

Answer 17

Work done

Question 18

How do you get indicated power?

Answer 18

=Rate of net WD by engine.

=area of loop X number of cylinders X number or cycles per second

Question 19

How do you get input power?

Answer 19

=calorific value of fuel X fuel flow rate

Question 20

Give the thermal efficiency of an engine

Answer 20

WD per second over energy supplied per second

= indicated power over input power from fuel combustion

Question 21

Describe briefly how a Diesel engine works

Answer 21

No spark plug needed.
Diesel oil sprayed into cylinder at max compression.
Air at high temp immediately after compression so fuel ignites, pushing piston down at constant pressure.

Question 22

What is output power referred to as?

Answer 22

Brake power

Question 23

Give mechanical efficiency of an engine

Answer 23

Brake power over indicated power

Question 24

What is the difference in brake power and indicated power referred to as?

Answer 24

Friction power

Question 25

How do you measure the brake power of an engine?

Answer 25

Measure teh engine torque when drive shaft is turning at a constant angular speed.

Question 26

What does brake power equal?

Answer 26

Engine torque X angular speed

Question 27

What is the overall efficiency equation for an engine?

Answer 27

Brake power over input power from fuel combustion

=mechanical efficiency X thermal efficiency

Question 28

What is a reversible engine?

Answer 28

One which can be operated in reverse.

Work on it causes max heat transfer from cold to hot

Question 29

What does efficiency of a reversible engine depend on?

Answer 29

Temp of high temp source and low temp sink

Question 30

What is Th over Tc equal to?

Answer 30

Q in over Qout

Question 31

What does efficiency of a reversible engine equal?

Answer 31

=W over Qin

=Th-Tc over Th

Question 32

What is the only way to make any engine 100% efficient?

Answer 32

Make low temp sink absolute zero which is impossible

Question 33

What are the theoretical values of thermal efficiency in a petrol engine and a Diesel engine?

Answer 33

Petrol engine=58%

Diesel engine=65%

Question 34

What are the actual thermal efficiency percentages for a petrol engine and a Diesel engine?

Answer 34

Petrol engine=30%

Diesel engine=35%

Question 35

Give the four main reasons why thermal efficiency values are lower in practice.

Answer 35

The indicator loop area in practice doesn’t have sharp corners like in the theoretical diagram so less work is actually done.
The maximum temperature is not reached like it is in theory when perfect combustion is used.
The expansion and strokes are not perfectly adiabatic.
Work is done by engine to bring about induction and exhaust strokes.

Question 36

Give the two main reasons why mechanical efficiency is less in real life than theory.

Answer 36

Friction between moving parts.

Oil is used in the engine to lubricate bearings. Oil is viscous so causes some resistance to motion.

Question 37

What is r?

Answer 37

The ratio of max volume over min volume of cylinder. The compression ratio.

Question 38

What is r to the negative 0.4 equal to?

Answer 38

Td-Ta over Tc-Tb

Question 39

Give a typical r value for a petrol engine

Answer 39

9

Question 40

Why are CHP stations better than normal power stations?

Answer 40

The water used to cool the engine block is heated and then used to heat local houses in CHP rather than just being wasted

Question 41

What is a heat pump?

Answer 41

A device that transfers energy from a cold space to a hot space

Question 42

What are heat pumps used for?

Answer 42

Heating buildings economically

Question 43

What does a refrigerator do?

Answer 43

Transfer heat from a hot object to cool it down.