Engineering physics- Thermodynamics and engines

Question 1

What is the first law of thermodynamics?

Answer 1

the first law of thermodynamics is a fundamental principle stating energy cannot be created or destroyed; it can only be transformed or transferred.

Conservation of energy in systems when cooling, heating and doing work.

Q = ΔU + W
Q = Energy transferred by heating
U = Internal energy W = Work done by system

Question 2

A gas is compressed due to work being done on it, W is therefore positive.
True or False?

Answer 2

False, work is done on a gas hence W is
negative.

Question 3

If a gas is cooled, Q is negative. True or False?

Answer 3

True, energy is removed hence Q is
negative.

Question 4

What assumptions are necessary when applying the 1st law to a closed (non-flow) system?

Answer 4

That the gas is an ideal gas so the internal
energy is only dependent on temperature,
work done causes a change in volume and
the ideal gas law (pV = nRT) can be applied
where n and R will both be constant.

Question 5

What is an isothermal change?

Answer 5

A change that occurs at a constant temperature.

This means internal energy stays constant so Q = W, meaning any transfer of energy would lead to work being done and vice versa. It obeys

Boyle’s Law: PV = constant

Question 6

How do you calculate the work done on a isothermal change p-V diagram?

Answer 6

The work done is the area under the graph, between the two points.

Question 7

What is an adiabatic change?

Answer 7

A change in which no heat passes in or out. Q = 0 and W = -ΔU. If work is done by the system, W is positive and the internal energy will decrease.

Obeys pVˠ = constant
(where ˠ is the adiabatic constant that depends on the type of gas)

Question 8

How can you calculate the work done on a p-V diagram for adiabatic changes?

Answer 8

The work done is the area under the pressure volume graph.

Question 9

What is a constant pressure change?

Answer 9

If a gas is kept at a constant pressure, it
will increase in volume when heated.

W = pΔV

Question 10

How do you calculate the work done from a constant pressure change p-V diagram?

Answer 10

The work done is the area under the graph.

Question 11

What is a constant volume change?

Answer 11

When heating gas in an enclosed space,
volume stays constant. This leads to an
increase in pressure, temperature and
internal energy. W = 0 so Q = ΔU. All the heat energy is transferred into internal energy.

Question 12

Draw a constant volume change for heating on a p-V diagram

Answer 12

No work is done as the volume doesn’t
change, the arrow points upwards as
temperature has increased.

Question 13

In a cyclic process, the work done per cycle on a graph is the…

Answer 13

Area of the loop.

(the difference between the work done
by the system and work done on the system).

Question 14

What are indicator diagrams?

Answer 14

A p-V diagram for engines that allows
identification of the different processes in
the system. It may vary depending on speed, timing and load.

Question 15

What are the key stages of induction in the four stroke petrol engine cycle?

Answer 15

● The piston moves down the cylinder
increasing the volume of gas above it.
● Air and petrol vapour are drawn into the
cylinder through the open inlet valve.
● Pressure stays constant, volume increases.

Question 16

Draw the indicator diagram for induction.

Answer 16

Question 17

What are the key stages of compression in the four stroke petrol engine cycle?

Answer 17

● The valves are closed.
● Piston moves back up cylinder, volume decreases, pressure increases (work is done).
● Near the end of the stroke, the spark plug ignites air/fuel mix causing an increase in temperature and pressure. Minimal volume change.

Question 18

Draw the indicator diagram for compression.

Answer 18

Question 19

What are the key stages of expansion in the four stroke petrol engine cycle?

Answer 19

● Valves are closed, expanded ignited gas moves
piston down.
● Work done by the expanding gas is more than the
work done to compress the gas, net output of work.
● Near the end of the stroke, the exhaust valve opens
to reduce pressure to near atmospheric.

Question 20

Draw the indicator diagram for expansion.

Answer 20

The pink line shows expansion, the other lines are the previous stages.

Question 21

What are the key points of the Exhaust stage in the four stroke petrol engine cycle?

Answer 21

● The piston moves back up, getting rid
of the burnt air through the exhaust.
● Pressure just above atmospheric.

Question 22

Draw the indicator diagram for the exhaust stroke

Answer 22

Question 23

What assumptions are made for theoretical indicator diagrams of both diesel and petrol engines?

Answer 23

● Pure air is continuously cycled.
● Pressure and temperature changes can
be instant.
● The heat source is external.
● The engine is frictionless.

Question 24

What 4 processes make up the theoretical cycle of a 4 stroke petrol engine?

Answer 24

1. The gas is adiabatically compressed so no
   heat is transferred.
2. Volume stays constant as heat is supplied.
3. Gas cools adiabatically.
4. System cools at constant volume.

Question 25

What 4 processes make up the theoretical cycle of a 4 stroke diesel engine?

Answer 25

1. Gas is adiabatically compressed. 2. Heat is supplied, pressure stays constant. 3. Gas cools adiabatically. 4. System cools at constant volume.

Question 26

Draw the difference between the theoretical and real four-stroke petrol engine cycle on an indicator diagram.

Answer 26

Question 27

Describe a four stroke diesel engine cycle induction stroke.

Answer 27

- Inlet valve opens. - Air is drawn into cylinder (no fuel). - Volume increases, pressure stays constant.

Question 28

Describe a four stroke diesel engine cycle compression stroke.

Answer 28

- Inlet valve closed. - Piston moves in, air is compressed. - Temperature increases, air becomes hot enough to ignite fuel. - Diesel is sprayed into cylinder and ignites.

Question 29

Describe a four stroke diesel engine cycle expansion and exhaust stroke.

Answer 29

● Gas expands, moving the piston down. ● Net output of work. ● Exhaust valve opens and removes heat and burnt gas. ● Piston moves in, expelling more gas.

Question 30

What is the difference between petrol engine and diesel engine indicator diagrams?

Answer 30

The petrol engine diagram has a sharper peak at the start of the expansion stroke than diesel engines (due to spark).

Question 31

Why are corners rounded on actual indicator diagrams?

Answer 31

It’s theoretically assumed that the same air is used constantly but actually the valves need time to open and close.

Question 32

Why doesn’t heating take place at a constant volume in a real engine?

Answer 32

The increase in pressure and temperature isn’t instantaneous in a real engine.

Question 33

Why don’t theoretical indicator diagrams include the negative work done between the exhaust and induction lines?

Answer 33

It assumes the same air continuously cycles round the system.

Question 34

Why do theoretical indicator diagrams have a higher peak than actual diagrams?

Answer 34

The temperature rise from burning the gas isn’t as large as predicted as the fuel isn’t completely burnt, so maximum pressure isn’t achieved (hence lower peak).

Question 35

Why is the area within the indicator diagram loop smaller for real engines than theoretical engines?

Answer 35

The net work done by an actual engine is less than for a theoretical engine as energy is needed to overcome friction in the engine’s moving parts.

Question 36

What is the input power of an engine?

Answer 36

Input power = calorific value of fuel x fuel flow rate The amount of heat energy per unit time it could gain from fuel.

Question 37

How can you calculate the work done on the gas during the compression phase of an indicator diagram?

Answer 37

The area underneath the curve during the compression phase.

Question 38

How can you calculate the work done by the gas during the expansion phase of an indicator diagram?

Answer 38

The area underneath the curve during the expansion phase.

Question 39

How can you calculate the net work done during the cycle?

Answer 39

The area enclosed by the loop, which is work done on the gas during compression minus the work done by the gas during expansion.

Question 40

How can you predict the engine’s indicated power?

Answer 40

The area enclosed by the loop x number of cycles per second x number of cylinders. It is the net work done by the engine cylinder in one second.

Question 41

Why would the output power be less than the indicated power?

Answer 41

Some power is used when overcoming frictional forces, this power is called frictional power.

Question 42

How can you calculate output/brake power ?

Answer 42

Brake power is the power delivered to the crankshaft of the engine. P = T⍵ In this case P is the output/brake power, T is the engine torque and ⍵ is the angular velocity of the crankshaft.

Question 43

How can frictional power be calculated?

Answer 43

Frictional power = Indicated power - brake power

Question 44

How do you calculate the overall efficiency?

Answer 44

Brake power / input power

Question 45

How do you calculate the thermal efficiency?

Answer 45

Indicated power / input power

Question 46

How do you calculate the mechanical efficiency?

Answer 46

Brake power / indicated power

Question 47

Why can’t an engine work using only the 1st Law of thermodynamics?

Answer 47

Even with the most theoretically efficient engine, some energy is lost to overcoming friction, hence the heat energy can’t all be transferred to work.

Question 48

What is the Second Law of thermodynamics?

Answer 48

The heat energy inputted to an engine is equal to the useful work done by the engine and the heat lost to the sink. Heat cannot be completely converted to work without some being lost in heating a colder body (sink). Heat engines must operate between a heat source and a heat sink.

Question 49

What is the consequence of the second law of thermodynamics for a heat engine?

Answer 49

A heat engine needs to work between a (hot) source and a (cold) sink.

Question 50

Why are CHP plants useful for places such as green houses and schools?

Answer 50

Heat engines have an efficiency of around 35% , they transfer over twice as much energy to the surroundings as is used for useful work. The waste heat can be used for thermal heating in buildings (limiting energy waste).

Question 51

What is a reversed heat engine?

Answer 51

An engine that transfers heat from a colder to a hotter place, these places are reservoir spaces.

Question 52

What is a heat pump?

Answer 52

A heat pump provides maximum energy per joule of work to the hot reservoir. It is an engine used to heat places.

Question 53

What is a refrigerator?

Answer 53

A refrigerator removes the maximum energy per joule of work from a cold reservoir. It is used for air conditioning.

Question 54

For a refrigerator and a heat pump, is work done onto the system or by the systems?

Answer 54

Work needs to be done on the systems, as both systems need inputted energy to operate.

Question 55

Why can we use the term efficiency when considering the effectiveness a heat pump and refrigerator?

Answer 55

The coefficient of performance (COP) is used instead, as it often exceeds 1 greatly, whilst efficiency can never exceed 1.

Question 56

How can you calculate the COP of a refrigerator?

Answer 56

Question 57

How can you calculate the COP of a heat pump?

Answer 57