Thermodynamics and Engines

Question 1

What is the first law of thermodynamics?

Answer 1

Q=ΔU+W
- the energy transferred to a system through heating is equal to the sum of the increase in internal energy and the work done by the system

Question 2

When is Q positive?

Answer 2

when energy is transferred to the system by heating

Question 3

when is ΔU positive?

Answer 3

when there is an increase in temperature

Question 4

when is W positive?

Answer 4

when work is done by the system

Question 5

What is an Open system?
-e.g?

Answer 5

matter can go flow in and out the system
-aerosol can

Question 6

what is a closed system?
-e.g?

Answer 6

no matter can enter or leave the system
-air inside a balloon

Question 7

what is an ideal gas?

Answer 7

-perfectly elastic collisions between molecules
-no intermolecular forces
-no potential energy = internal energy is equal to the sum of the kinetic energies of all its particles

Question 8

what is the ideal gas equation?

Answer 8

pV= nRT

Question 9

what is the equation between pressure, volume and temperature?

Answer 9

p₁V₁/T₁ = p ₂V ₂/T ₂

Question 10

What is an adiabatic process?

Answer 10

no heat leaves or enter the system
- Q=0
- ΔU=-W

Question 11

what is the equation for an adiabatic process?

Answer 11

pVˠ=constant

Question 12

what is an isothermal process?

Answer 12

the temperature of the system remains constant
-ΔU=0
- Q=W

Question 13

what is the equation for an isothermal change?

Answer 13

pV=constant

Question 14

what is a constant pressure change?
(isobaric)

Answer 14

the pressure of the system remains constant

Question 15

what is the equation for a constant pressure change?

Answer 15

W=pΔV

Question 16

what is a constant volume change?
(Isochoric)

Answer 16

the volume of the system remains the same
- W=0
- Q=ΔU

Question 17

what is the area beneath the pV curve?

Answer 17

the work done during the process

Question 18

what does isothermal mean?

Answer 18

constant temperature

Question 19

How do you find the work done per cycle on a pV graph?

Answer 19

the area of the loop

Question 20

What are the Four steps in a four stroke petrol engine?

Answer 20

1. Induction
2. Compression
3. Expansion
4. Exhaust

Question 21

Explain what happens during Induction?
(four stroke petrol engine cycle)

Answer 21

the piston moves down, gas mixture drawn in through inlet valve
-volume of gas increases
-pressure is constant

Question 22

Explain what happens during Compression?
(four stroke petrol engine cycle)

Answer 22

inlet valve is closed and cylinder moves up, doing work on the gas
-volume of gas decreases
-pressure increases
mixture is then ignited and temperature and pressure rapidly increases at almost constant volume

Question 23

Explain what happens during Expansion?
(four stroke petrol engine cycle)

Answer 23

gas mixture expands, doing work on the piston causing it to move down the cylinder and exhaust valve opens.
- pressure drops to slightly above atmospheric pressure

Question 24

Explain what happens during Exhaust?
(four stroke petrol engine cycle)

Answer 24

piston moves up the cylinder, forcing the burnt gas out through the open exhaust valve

Question 25

How is a diesel engine different to a petrol engine?

Answer 25

Diesel only intakes air, not a gas mixture, the diesel is directly inputed into the cylinder as a fine spray using a fuel injector, causing it to ignite

Question 26

Why is the efficiency of a real engine far lower than in theory?

Answer 26

-time taken to open and close valves
-heating does not take place at constant volume

Question 27

How are the indicator graphs for theoretical and actual engine cycles different?

Answer 27

-actual has rounded edges
-theoretical has higher peaks
-actual has a smaller loop area

Question 28

What is indicated power?

Answer 28

the product of the area pf the main pV loop and the number of cycles it undergoes per second and the number of cylinders

Question 29

what is brake power?
-equation?

Answer 29

the power output of the engine
- P=T⍵

Question 30

what is the input power?

Answer 30

the product of the calorific value of the fuel and its flow rate

Question 31

what is the calorific value?

Answer 31

the measure of how much energy the fuel stores per unit volume or mass

Question 32

what are 3 types of engine efficiencies?

Answer 32

-Overall efficiency
-Thermal efficiency
-Mechanical efficiency

Question 33

what is the equation for Overall efficiency?

Answer 33

Overall efficiency = Brake power/input power

Question 34

what is the equation for thermal efficiency?

Answer 34

thermal efficiency=indicated power/input power

Question 35

what is the equation for mechanical efficiency?

Answer 35

mechanical efficiency =brake power/indicated power

Question 36

What is the second law of thermodynamics?

Answer 36

a heat engine must have both a source and a sink to operate

Question 37

why would an engine not abide by the first law of thermodynamics?

Answer 37

if the engine reached the temperature of the source (thermal equilibrium), no more heat would flow and so no work would be done. i.e not 100% efficient