Section 6 - Combustion in IC Engines

Question 1

Flame development angle

Answer 1

Crank angle interval during which flame kernel develops after spark ignition

Question 2

Rapid burning angle

Answer 2

Crank angle interval required to burn most of mixture

Question 3

Overall burning angle

Answer 3

Sum of flame development and rapid burning angles

Question 4

T/F: Turbulent burning velocity is inversely proportional to turbulent intensity

Answer 4

F: just proportional

Question 5

T/F: Turbulent flame velocity is higher at higher engine speeds

Answer 5

T

Question 6

To reduce heat loss during combustion in the engine, we must increase either:

Answer 6

1. Laminar burning velocity
2. Turbulence intensity

Question 7

T/F: Squish promotes in-cylinder turbulence

Answer 7

T

Question 8

Maximum power is obtained for a phi of ____, giving the highest ______ ______ and ______ ______.

Answer 8

1.1

burning velocity

flame temperature

Question 9

Best fuel economy is obtained for a phi that is slightly less than ___

Answer 9

1

Question 10

Spark timing relative to TC affects:

Answer 10

Pressure development (tfr. imep) and power

Question 11

What happens if combustion starts too early/late?

Answer 11

Early: work done against piston

Late: peak pressure reduced

Question 12

MBT timing

Answer 12

Optimum spark timing that gives max brake torque

Question 13

Knock

Answer 13

Pinging noise emitted from engine undergoing abnormal combustion. Shock waves produced in cylinder when unburned F-A mix auto-ignites simultaneously

Question 14

Only way you can burn stuff fast enough at high engine speeds

Answer 14

Turbulence

Question 15

Knock is averted if:

Answer 15

Flame burns all fresh gas before autoignition of end-gas, so knock is a potential problem when the burn time is long

Question 16

Engine parameters affecting knock

Answer 16

1. Spark timing
2. Compression ratio
3. Engine speed
4. Throttle position
5. Turbocharging

Question 17

General rule of thumb to mitigate knock using spark timing

Answer 17

Set actual spark timing to be 1% below max brake torque timing

Question 18

Compression ratio and knock

Answer 18

High ratio = mix is compressed at high P and T = promotes autoignition

Question 19

Engine speed and knock

Answer 19

Low speed = flame velocity is slow = long burn time = more time for autoignition

High speed = less heat loss = unburned gas temp is higher = promotes autoignition

Question 20

Throttle position and knock

Answer 20

Part throttle = residual gas fraction increases (valve overlap) = residual gas lowers laminar burning velocity = lowers turbulent burning rate

Question 21

Turbocharging and knock

Answer 21

Turbochargers compress air which raises initial P and T (biggest issue @ low speeds and high loads (WOT))

Question 22

Engine knock detectors

Answer 22

Knock sensor is a piezoelectric accelerometer that produces an analog electrical signal when engine vibration at a specific frequency is measured (6-15 Hz). Knock signal = ECU retards spark timing as countermeasure and indicates “fault”

Question 23

Fuel knock scale is described by:

Answer 23

Octane number. Determines whether fuel will knock for specified engine at a specified operating condition

Question 24

T/F: Higher octane number = lower resistance to knock

Answer 24

F: opposite is true

Question 25

What compound has an ON of 100?

Answer 25

Iso-octane

Question 26

What compound has an ON of 0?

Answer 26

Heptane

Question 27

How is ON determined?

Answer 27

1. Set compression ratio of engine 2. Start at 100% octane 3. Lower ON until you get onset of knock 4. Whatever percentage of ON that is, is the % of your fuel

Question 28

T/F: A higher octane number indicates the fuel will burn so quickly there will be no more time for autoignition

Answer 28

T

Question 29

Most effective antiknock agents are _____ _____ (list examples)

Answer 29

Lead alkyls 1. Tetraethyl lead 2. Tetramethyl lead

Question 30

T/F: With more dilution, combustion is consistently complete prior to the exhaust valve opening

Answer 30

F: Little or no dilution means combustion will be complete prior to exhaust opening

Question 31

Stable operating limit

Answer 31

Point where engine operation becomes rough and unstable

Question 32

Misfire cycles

Answer 32

Start to occur with too much dilution of F-A. Mixture does not ignite at all.

Question 33

Ignition delay time

Answer 33

Difference between when combustion ends to when it starts again

Question 34

Pre-mixed combustion

Answer 34

Fuel evaporated mixes w/ air so when combustion occurs, it's happening in pre-mixed state

Question 35

4 stages of combustion in CI engines

Answer 35

1. Ignition delay 2. Premixed combustion phase 3. Mixing-controlled combustion phase 4. Late combustion phase

Question 36

2 categories of CI engines

Answer 36

1. Direct-injection 2. Indirect-injection

Question 37

T/F: In CI engines, as the size decreases and engine speed increases, increasing amnts of swirl are used to achieve fuel-air mixing

Answer 37

T

Question 38

Why are indirect injection apparatuses not really used anymore?

Answer 38

Large amnt of surface area in mini chamber which is used for pre-mixing, so the SA to volume ratio is larger

Question 39

Which type of injection and mixing type is used the most in CI engines?

Answer 39

Direct injection multi-hole nozzle swirl in chamber

Question 40

Ignition delay

Answer 40

Time (or CA interval) from when the fuel injection starts to the onset of combustion

Question 41

Good atomization of fuel spray requires:

Answer 41

1. high fuel P 2. small injector hole diameter 3. optimum fuel viscosity 4. high cylinder P

Question 42

Rate of vaporization of the fuel droplets depends on:

Answer 42

1. Droplet diameter 2. Velocity 3. Fuel volatility 4. P and T of air

Question 43

How is ignition quality of fuel defined?

Answer 43

Cetane number (CN)

Question 44

Low cetane fuels:

Answer 44

- Ignition delay is long - Most of fuel is injected before autoignition and rapid combustion - Produces audible knocking sound, "diesel knock"

Question 45

High cetane fuels:

Answer 45

- Ignition delay is short - Very little fuel injected before autoignition - Heat release rate-controlled by rate of fuel injection and F-A mixing - Smoother engine operation

Question 46

Cetane number is defined by the blends of 2 pure H-C reference fuels:

Answer 46

Isocetane, cetane

Question 47

Why is ON and CN inversely correlated?

Answer 47

Gasoline (ON) makes a poor diesel fuel (CN) and vice versa. In SI, the flame is trying to outrace autoignition, but diesel fuel is meant to autoignite early. NEVER MIX THEM UP

Question 48

T/F: Increasing the load (bmep) increases the residual gas and wall temperature, resulting in higher charge temp at injection and a decrease in ignition delay

Answer 48

T

Question 49

Multiple injection diesels

Answer 49

Multiple injections enhance cooling due to vapour evaporation. Reduces rapid pressure rise and engine knock as a result.