Lecture 12 - Internal Combustion Engines

Question 1

Internal combustion cycles

Answer 1

otto cycle (atkinson cycle), diesel cycle, wankel engines,

Question 2

Otto cycle

Answer 2

4 stroke
1 air intake
2 compression
3 Combustion
4 Exhaust

Question 3

closed system the change in internal energy is

Answer 3

zero

Question 4

efficiency of otto cycle

Answer 4

eta = 1 - 1/rc^n-1

Question 5

Draw otto cycle with location of stroke labelled

Answer 5

1-2 intake stroke

6-1 exhaust stroke

Question 6

What do edges become curved on a real 4 stroke engine

Answer 6

as combustion and exhaust process occur in a finite time - not instant

Question 7

How does compression ratio effect efficiency for otto cycle

Answer 7

higher compression ratio, higher efficiency

Question 8

Ignition in diesel cycle

Answer 8

compression ignition, fuel injector, injects atomized fuel after compression, fuel self ignites due to temp increase from compression

Question 9

What is a glow plug

Answer 9

Used in diesel cycle to artificially heat the air during cold starts

Question 10

Stages in standard air cycles

Answer 10

1-2 isentropic compression
2-3 heat added at const pressure while piston moving downwards
3-4 isentropic expansion
4-1 rejection of heat from air

Question 11

Effieicny of diesel cycle

Answer 11

1 - (1/rc^n-1)*((alpha^n -1)/(n(alpha -1)) alpha = v3/v2

Question 12

For a fixed compression ratio what is more efficient diesel or otto

Answer 12

diesel cycle efficiency is less than the otto

but diesel combustion method allows greater compression ratios to be used increasing the efficiency

Question 13

For a fixed compression ratio what is more efficient diesel or otto

Answer 13

diesel cycle efficiency is less than the otto

but diesel combustion method allows greater compression ratios to be used increasing the efficiency

Question 14

Typical compression ratio in diesel

Answer 14

18 to 24:1

Question 15

Typical compression ratio in otto

Answer 15

10 to 13:1

Question 16

What is the advantage of diesel cylce

Answer 16

higher torque lower in the RPM range

Question 17

Why does diesel potentially have higher emissions

Answer 17

diesel has high energy content per unit mass due to higher density
causes higher firing temp and eventually NOx
Also leads to higher PM as some components in diesel are difficult to burn

Question 18

How can diesel fuel be tuned

Answer 18

to reduce PM emissions (reduce aeromatic composition), cant really do for NOx as cycle is good because of high compression ratios and therefore temps

Question 19

what happens in the atkinson cycle

Answer 19

in the compression stroke - intake valve left open for longer than in otto cycle to allow some air back out of the cylinder and into the intake manifold

Question 20

What is the effect of the atkinson cycle

Answer 20

effective compression ratio is reduced (for short time air escaping from cylinder rather than compressed), expansion ratio is unchanged
therefore compression ratio smaller than expansion ratio

Question 21

What is the atkinson cycle a modified version of

Answer 21

the otto cycle

Question 22

Draw a PV diagram of the atkinson cycle

Answer 22

isentropic compression
isochoric heating
isobaric heating
isentropic expansion
isochoric cooling
isobaric cooling

Question 23

What is the aim of the atkinson cycle

Answer 23

allow pressure in the combustion chamber at the end of the power stroke be equal to atmospheric pressure thus maximum energy has been obtained from combustion fuel

Question 24

What is the issue with the atkinson cycle

Answer 24

lower power density than otto cycle as there is less air in the cylinder under compression (as some allowed to escape)

Question 25

Where is the atkinson cycle used

Answer 25

full electric hybrid car - electric propulsion makes up for power drop when required
With intelligent valve timing allowing the car to switch to otto cycle when more power is required

Question 26

Why should you mistrust “studies”

Answer 26

different drivers
different routes
intended driving route
etc not comparable

Question 27

Types of IC engine arrangments

Answer 27

in line
v
opposed piston
horizontally opposed
Radial

Question 28

Other types of IC engines

Answer 28

Rotary - wankel rotary engine

Question 29

Where are IC engines used apart from cars

Answer 29

old aircraft
for marine
for power