Section 6 - Combustion in IC Engines Flashcards
Flame development angle
Crank angle interval during which flame kernel develops after spark ignition
Rapid burning angle
Crank angle interval required to burn most of mixture
Overall burning angle
Sum of flame development and rapid burning angles
T/F: Turbulent burning velocity is inversely proportional to turbulent intensity
F: just proportional
T/F: Turbulent flame velocity is higher at higher engine speeds
T
To reduce heat loss during combustion in the engine, we must increase either:
- Laminar burning velocity
- Turbulence intensity
T/F: Squish promotes in-cylinder turbulence
T
Maximum power is obtained for a phi of ____, giving the highest ______ ______ and ______ ______.
1.1
burning velocity
flame temperature
Best fuel economy is obtained for a phi that is slightly less than ___
1
Spark timing relative to TC affects:
Pressure development (tfr. imep) and power
What happens if combustion starts too early/late?
Early: work done against piston
Late: peak pressure reduced
MBT timing
Optimum spark timing that gives max brake torque
Knock
Pinging noise emitted from engine undergoing abnormal combustion. Shock waves produced in cylinder when unburned F-A mix auto-ignites simultaneously
Only way you can burn stuff fast enough at high engine speeds
Turbulence
Knock is averted if:
Flame burns all fresh gas before autoignition of end-gas, so knock is a potential problem when the burn time is long
Engine parameters affecting knock
- Spark timing
- Compression ratio
- Engine speed
- Throttle position
- Turbocharging
General rule of thumb to mitigate knock using spark timing
Set actual spark timing to be 1% below max brake torque timing
Compression ratio and knock
High ratio = mix is compressed at high P and T = promotes autoignition
Engine speed and knock
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
Throttle position and knock
Part throttle = residual gas fraction increases (valve overlap) = residual gas lowers laminar burning velocity = lowers turbulent burning rate
Turbocharging and knock
Turbochargers compress air which raises initial P and T (biggest issue @ low speeds and high loads (WOT))
Engine knock detectors
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”
Fuel knock scale is described by:
Octane number. Determines whether fuel will knock for specified engine at a specified operating condition
T/F: Higher octane number = lower resistance to knock
F: opposite is true
What compound has an ON of 100?
Iso-octane
What compound has an ON of 0?
Heptane
How is ON determined?
- Set compression ratio of engine
- Start at 100% octane
- Lower ON until you get onset of knock
- Whatever percentage of ON that is, is the % of your fuel
T/F: A higher octane number indicates the fuel will burn so quickly there will be no more time for autoignition
T
Most effective antiknock agents are _____ _____ (list examples)
Lead alkyls
- Tetraethyl lead
- Tetramethyl lead
T/F: With more dilution, combustion is consistently complete prior to the exhaust valve opening
F: Little or no dilution means combustion will be complete prior to exhaust opening
Stable operating limit
Point where engine operation becomes rough and unstable
Misfire cycles
Start to occur with too much dilution of F-A. Mixture does not ignite at all.
Ignition delay time
Difference between when combustion ends to when it starts again
Pre-mixed combustion
Fuel evaporated mixes w/ air so when combustion occurs, it’s happening in pre-mixed state
4 stages of combustion in CI engines
- Ignition delay
- Premixed combustion phase
- Mixing-controlled combustion phase
- Late combustion phase
2 categories of CI engines
- Direct-injection
- Indirect-injection
T/F: In CI engines, as the size decreases and engine speed increases, increasing amnts of swirl are used to achieve fuel-air mixing
T
Why are indirect injection apparatuses not really used anymore?
Large amnt of surface area in mini chamber which is used for pre-mixing, so the SA to volume ratio is larger
Which type of injection and mixing type is used the most in CI engines?
Direct injection multi-hole nozzle swirl in chamber
Ignition delay
Time (or CA interval) from when the fuel injection starts to the onset of combustion
Good atomization of fuel spray requires:
- high fuel P
- small injector hole diameter
- optimum fuel viscosity
- high cylinder P
Rate of vaporization of the fuel droplets depends on:
- Droplet diameter
- Velocity
- Fuel volatility
- P and T of air
How is ignition quality of fuel defined?
Cetane number (CN)
Low cetane fuels:
- Ignition delay is long
- Most of fuel is injected before autoignition and rapid combustion
- Produces audible knocking sound, “diesel knock”
High cetane fuels:
- 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
Cetane number is defined by the blends of 2 pure H-C reference fuels:
Isocetane, cetane
Why is ON and CN inversely correlated?
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
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
T
Multiple injection diesels
Multiple injections enhance cooling due to vapour evaporation. Reduces rapid pressure rise and engine knock as a result.
