Engineering - Heat Engines

Question 1

Heat engine definition?

Answer 1

A device or system that extracts energy from its environment in the form of heat and converts it into useful work.

Question 2

Working substance definition?

Answer 2

The substance (usually liquid) on which the thermodynamic processes are performed in the engine, by changes of temperature, pressure and volume. Enclosed in cylinder.

Question 3

Egs of heat engines inc (3)

Answer 3

-the internal combustion engine (which uses a petrol-air mixture)
-the diesel engine (which uses a diesel-air mixture)
-the steam engine (which uses water)

Question 4

The thermodynamic processes experienced by the fluid cause a piston in the cylinder to move up and down, and this motion is then converted, via a crankshaft, into…

Answer 4

into a rotational motion.

Question 5

The working substance in each cylinder is forced through a series of thermodynamic processes, eventually returning to its starting state. The processes in each cylinder are…

Answer 5

staggered so that the power output is continuous.

Question 6

The 4-Stroke Petrol Engine Cycle (Otto Cycle):
Idealised cycle, where working substance behaves theoretical way and main processes occur one after the other with no..

Answer 6

time delay.

(FIRST try drawing it. Then bring up picture of first engine cycle on ur phone!!! Don’t look at writing.)

Question 7

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

A’ to A?

Answer 7

Inlet valve opens and exhaust valve closes. Piston moves down. Working substance drawn into each cylinder. This is the INDUCTION STROKE.

Question 8

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

At A?

Answer 8

Inlet valve closes.

Question 9

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

A to B?

Answer 9

Piston moves up, compressing the gas adiabatically. Temp rises. This is the COMPRESSION STROKE.

Question 10

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

B to C?

Answer 10

A spark plug ignites the gas mixture at B, supplying heat Qin and increasing the pressure at a constant volume. Temperature rises.

Question 11

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

C to D?

Answer 11

The increased pressure pushes the piston down as the gas expands adiabatically, decreasing both the pressure and the temperature. This is the POWER STROKE.

Question 12

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

D to A?

Answer 12

The exhaust valve opens at D and most of the burnt gas mixture is released, removing an amount of heat Qout. The pressure and temperature of the gas that remains in the cylinder decrease rapidly.

Question 13

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

A to A’?

Answer 13

A the piston moves up, the remaining gas mixture is expelled. This is the EXHAUST STROKE.

Question 14

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

At A’?

Answer 14

The exhaust valve closes and the inlet valve opens. The cycle repeats.

Question 15

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

Each cycle corresponds to..

Answer 15

Two revolutions of the engine.

Question 16

The 4-Stroke Petrol Engine Cycle (Otto Cycle):

The thermal efficiency of this engine is =

Answer 16

the ratio of max useful work done by engine / energy supplied by fuel.

Question 17

The diesel engine cycle:

Engine that compresses air, raising its temp above the — — of the fuel. No spark plug needed.

Answer 17

ignition point

Question 18

The diesel engine cycle:

Diesel engines require —– —– and produce a higher —- than internal combustion engines.

Answer 18

heavier construction

torque

Question 19

The diesel engine cycle:

Main difference between diesel engine cycle and 4-stroke petrol engine cycle is that, in diesel engine, …

Answer 19

there’s no fuel in the cylinder during compression.

(Ok now first try drawing, then get out ur diesel engine cycle pic. Again - don’t look at writing yet!)

Question 20

The diesel engine cycle:

At A’?

Answer 20

Inlet valve opens and exhaust valve closes.

Question 21

The diesel engine cycle:

A’ to A?

Answer 21

Air drawn into each cylinder at Patm as the piston moves down. This is the INDUCTION STROKE.

Question 22

The diesel engine cycle:

At A?

Answer 22

Inlet valve closes.

Question 23

The diesel engine cycle:

A to B?

Answer 23

Air in cylinder compressed adiabatically, as the piston moves up. Temp rises and becomes hot enough to ignite the fuel. This is called the COMPRESSION STROKE.

Question 24

The diesel engine cycle:

B to C?

Answer 24

Diesel fuel is sprayed into cylinder and is ignited immediately by the hot air, supplying heat Qin. This forces the piston to move down at constant pressure and is the FIRST PART OF THE POWER STROKE.

Question 25

The diesel engine cycle:

C to D?

Answer 25

The fuel supply is cut off at C and the burnt gas expands adiabatically. This forces the piston down and the temperature falls. This is the SECOND PART OF THE POWER STROKE.

Question 26

The diesel engine cycle:

D to A?

Answer 26

The exhaust valve opens at D and releases the exhaust gas, removing an amount of heat Qout. The pressure and temperature of the gas remaining in the cylinder decreases accordingly.

Question 27

The diesel engine cycle:

A to A’?

Answer 27

The remainder of the gas is expelled from the cylinder as the piston moves up. This is the EXHAUST STROKE.

Question 28

The diesel engine cycle:

At A’?

Answer 28

The exhaust valve closes, the inlet valve opens and the cycle is repeated.

Question 29

Comparing Petrol and Diesel Engines:
Give 4 preliminary answers to this.

Answer 29

• Compression ratio
• Torque
• Cost?
• Gases produced?

Question 30

The diesel engine cycle:

Compression ratio (+define) ?

Answer 30

The compression ratio is the ratio of the volume enclosed in the cylinder at the beginning of the compression stroke to the volume enclosed at the end of the stroke.
Diesel engines can achieve much higher compression ratios than petrol - more more efficient!

Question 31

The diesel engine cycle:

Torque?

Answer 31

Diesel delivers greater torque.

Question 32

The diesel engine cycle:

Cost?

Answer 32

Disadvantages of diesel is they operate at higher working pressures than petrol ∴ more expensive to produce bc have to be more robust. Also have lower power-to-weight ratio.

Question 33

The diesel engine cycle:

Gases produced?

Answer 33

Petrol produces more CO, HCs (hydrocarbons), NOx and CO2 than diesel.
However, particulate emission (eg unburnt HCs) that occur in diesel don’t in petrol.
Petrol exhaust emissions improved if catalytic converter installed. This oxidises pollutants, reducing CO, HC and NOx emissions but CO and HCs are still not reduced below levels from a diesel engine.

Question 34

Power of an Engine:

The three types of power - what do they mean?

Answer 34

-Input power - the power derived from the burning of the fuel.
-Indicated power - theoretical power that the engine can deliver, based on the indicator diagram (the area enclosed).
-Output power/brake power - the driving power delivered to the engine’s crankshaft.

Question 35

Power of an Engine:

Input power:
Eq?
What’s CV a measure of? Depends on?

Answer 35

Pinput = calorific value of fuel (JKg-1) x fuel flow rate (Kgs-1)

Calorific value is a measure of fuel’s energy density (larger for petrol). It’s a measure of how fast energy is being consumed ∴ depends on type of engine, speed of car (∴air resistance has an effect), conditions of type etc

Question 36

Power of an Engine:

Indicated power:
Eq?
Assumption?

Answer 36

Pind = area of p-V loop x number of cycles per second x number of cylinders.
This assumes frictionless motion ∴ is the max theoretical power output of an engine.

Question 37

Power of an Engine:

Output power:
What is this?
Eq?
Relate indicated, output and fictional power and make it make sense?

Answer 37

This is a measure of the engines power w/o loss in power caused by the gearbox and other auxiliary components. It’s the power delivered to the engine’s crankshaft (ie the engines flywheel).
Pout = Tω
The indicated power will be greater than the output power bc of frictional processes in the engine and transmission. The difference is the frictional power.
Pfriction = Pind - Pout

Question 38

Efficiency of an Engine:
Mechanical efficiency?
Thermal efficiency?
Overall efficiency?

Answer 38

Mechanical efficiency, η, = Pout/Pind

Thermal efficiency, ε, = Pind/Pinput

Overall efficiency = Pout/Pinput = η x ε