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MECH 453 > Section 6 - Combustion in IC Engines > Flashcards

Section 6 - Combustion in IC Engines Flashcards

1
Q

Flame development angle

A

Crank angle interval during which flame kernel develops after spark ignition

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2
Q

Rapid burning angle

A

Crank angle interval required to burn most of mixture

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3
Q

Overall burning angle

A

Sum of flame development and rapid burning angles

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4
Q

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

A

F: just proportional

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5
Q

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

A

T

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6
Q

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

A
  1. Laminar burning velocity
  2. Turbulence intensity
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7
Q

T/F: Squish promotes in-cylinder turbulence

A

T

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8
Q

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

A

1.1

burning velocity

flame temperature

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9
Q

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

A

1

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10
Q

Spark timing relative to TC affects:

A

Pressure development (tfr. imep) and power

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11
Q

What happens if combustion starts too early/late?

A

Early: work done against piston

Late: peak pressure reduced

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12
Q

MBT timing

A

Optimum spark timing that gives max brake torque

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13
Q

Knock

A

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

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14
Q

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

A

Turbulence

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15
Q

Knock is averted if:

A

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

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16
Q

Engine parameters affecting knock

A
  1. Spark timing
  2. Compression ratio
  3. Engine speed
  4. Throttle position
  5. Turbocharging
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17
Q

General rule of thumb to mitigate knock using spark timing

A

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

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18
Q

Compression ratio and knock

A

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

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19
Q

Engine speed and knock

A

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

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20
Q

Throttle position and knock

A

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

21
Q

Turbocharging and knock

A

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

22
Q

Engine knock detectors

A

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”

23
Q

Fuel knock scale is described by:

A

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

24
Q

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

A

F: opposite is true

25
Q

What compound has an ON of 100?

A

Iso-octane

26
Q

What compound has an ON of 0?

A

Heptane

27
Q

How is ON determined?

A
  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
28
Q

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

A

T

29
Q

Most effective antiknock agents are _____ _____ (list examples)

A

Lead alkyls

  1. Tetraethyl lead
  2. Tetramethyl lead
30
Q

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

A

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

31
Q

Stable operating limit

A

Point where engine operation becomes rough and unstable

32
Q

Misfire cycles

A

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

33
Q

Ignition delay time

A

Difference between when combustion ends to when it starts again

34
Q

Pre-mixed combustion

A

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

35
Q

4 stages of combustion in CI engines

A
  1. Ignition delay
  2. Premixed combustion phase
  3. Mixing-controlled combustion phase
  4. Late combustion phase
36
Q

2 categories of CI engines

A
  1. Direct-injection
  2. Indirect-injection
37
Q

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

A

T

38
Q

Why are indirect injection apparatuses not really used anymore?

A

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

39
Q

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

A

Direct injection multi-hole nozzle swirl in chamber

40
Q

Ignition delay

A

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

41
Q

Good atomization of fuel spray requires:

A
  1. high fuel P
  2. small injector hole diameter
  3. optimum fuel viscosity
  4. high cylinder P
42
Q

Rate of vaporization of the fuel droplets depends on:

A
  1. Droplet diameter
  2. Velocity
  3. Fuel volatility
  4. P and T of air
43
Q

How is ignition quality of fuel defined?

A

Cetane number (CN)

44
Q

Low cetane fuels:

A
  • Ignition delay is long
  • Most of fuel is injected before autoignition and rapid combustion
  • Produces audible knocking sound, “diesel knock”
45
Q

High cetane fuels:

A
  • 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
46
Q

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

A

Isocetane, cetane

47
Q

Why is ON and CN inversely correlated?

A

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

48
Q

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

A

T

49
Q

Multiple injection diesels

A

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

MECH 453 (4 decks)

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