ICE_22_Chapter 4.v2.ppt

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Class #12

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Cycle-by-cycle variations
• Mixture preparation with;
• Carburetion
- mixing and vaporization
- simple carburetor
- fixed venturi carburetor
• Fuel injection
-throttle body injection
-port fuel injection (PFI)
-direct injection
• Federal Test Procedure for emissions

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Internal Combustion Engines A. Sobiesiak 2022
Cyclic variation in SI engines
Summary of Non-repeatability of engine cycles; coefficients of variations
Causes:
• Any phenomenon that affects flame development and propagation of the flame leads to
increased Cyclic variations (most can be traced to the air/fuel/EGR mixing and final
mixture homogeneity and thermal state)
• Mixture motion at the spark plug location, varied turbulence characteristics
• Varied mixture composition
• Varied amounts of residual mass (left-over combustion products) and EGR
• Varied amounts of spark energy released
• Different temperature and pressure from cycle to cycle at the spark plug location
Cyclic variations depend on :
• Local equivalence ratio (lean charge leads to higher CPV’s)
• Ignition timing (retarded ignition leads to higher (COV’s)
• Phase state of fuel (liquid vs. gaseous ) impacts COV’s
• Amount of added EGR (more EGR higher COV’s, higher engine speed allows for more
EGR))

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Internal Combustion Engines A. Sobiesiak 2022
Cyclic variation in SI engines and combustion phasing
Non-repeatability of engine cycles
mfb 0-10%
mfb 0-50%
mfb 0-90%
Mass
Fraction
Burned,
MFB

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Internal Combustion Engines A. Sobiesiak 2022
Cyclic variations of Combustion Phasing the 10-80% vs. 0-
10% period (Stone’s text)
Non-repeatability of engine cycles
Combustion phasing, burn duration:
• CA 0-10% mass fraction
burned; flame development
period, laminar flame and
transition to turbulent flame,
• CA10-80% mass fraction
burned; turbulent flame
propagation period,
• CA 0-90% total mass fraction
burned
Note; CA 0-10% and CA 10-80% are (linearly) correlated; the longer CA 0-10% leads to
longer CA 10-80%, and longer entire burn duration. During CA 10-80% period the
piston is on its way down and cylinder temperature is decreasing rapidly. The main
burn, CA 10-80%, is somewhat shorter than the flame development period, CA 0-10%.
20⁰
25⁰
The longest burn CA 0-80% ≈ 53 + 37 = 90 CAD

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Cyclic variations of Combustion Phasing the 10-90% vs. 0-
10% period (experimental split-cycle engine fuelled with
NG, with very high turbulence level. I. Cameron and A.
Sobiesiak, 2015)
Note: (1) Majority of CA 0-10% and CA10-90% are very short (less than ≈13 CAD).
(2) cycles with longer CA 0-10% period tend to have longer CA 10-90% main burn
(3) in this engine the total burn is very short, and as before the main burn period,
CA 10-90%, is somewhat shorter than the flame development period, CA 0-10%.
10̊
13̊
The longest total burn
CA 0-90% ≈ 21.5 +17.5 = 39
CAD

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines ; Mixture preparation
• Fuel needs to be metered (measured)
• Fuel needs to be atomized
• Fuel needs to be vaporized (manifold pressure impact)
• Fuel and air need to be mixed in required proportions (AFR from 12 to 22 )
• Fuel/air mixture needs to be distributed evenly between cylinders
Fuel exist in the inlet manifold
as
• droplets
• vapour
• wall film (puddling, up to 1L of
liquid)
In the inlet manifold there is
• turbulent flow of air (could be
moist)
• fuel vaporization (drops and
wall film)
• fuel condensation

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
Requirements for mixture preparation process;
• The same amount of air/fuel mixture in each cylinder
• The same air/fuel ratio
• The same homogeneity
• The same fuel fractions (% of light and heavy fractions as these
have different Octane number)
Issues;
• Unsteady and transient air flow
• Varying pressure and temperature
• Flow direction changes
• Differences in fuel “wait” time and fuel condensation

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SI Engines fuel delivery systems
• Carburetor (placed well upstream of inlet valves )
- an analog device that dispenses fuel in response to intake air flow
- simple carburetor does not cover entire range of equivalence ratios, extra elements
are needed to cover the entire range
- good mixture homogenetion
• Inlet port fuel injection (PFI)
- fuel injectors are located upstream inlet valve(s) in the inlet port (at pressure
somewhat below atmospheric and temperature somewhat above atmospheric)
- amount of injected fuel is in response to the engine load and rpm
- different injection timing schemes are feasible
- more precise fuel metering, much better equivalence ratio coverage
- at some conditions mixture is not evenly homogenous
• (Gasoline) Direct injection (GDI)
- fuel injectors are located directly in the engine head, squirting fuel directly into cylinders,
- injector operates in higher temperature and pressure (when injected in the compression
stroke), the fuel pump needs to generate higher pressure than in PFI system.
- injector body (the tip in particular) is exposed to very high temperatures and pressures,
- at some conditions mixture is purposely inhomogeneous (stratified).

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
Simple downdraft carburetor (fixed jet), single venturi
nozzle
Analog fuel metering device in response to the air pressure difference
Δpm = Patm – Pm, induces fuel flow in the venturi nozzle at Δpn = patm – pth
Two Δp are of importance
Δpm = patm – pm ; ~ air flow
velocity, where pm < patm
and pth < patm
Δpn = patm – pth; ~fuel flow
velocity
• for each, air and fuel flow,
mass flow rate = ρ ∙ν ∙ A
Fuel is at Patm
pth
Pm, pressure past the
throttle ≈ cylinder pressure
Air inlet at Patm
Mixture flow to cylinders

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Simple carburetor (fixed jet) performance
Trends in Δpn and Δpm with increase of engine load at fixed speed
Δpn = patm – pth; ~fuel flow velocity,
• fuel jet velocity increases and
fuel mass flow rate increases
Δpm = patm – pm ; ~ air flow velocity,
• air flow velocity decreases but
throttle area gets bigger and air
mass flow rate increases
• Rpm increases moves both curves
up (more air and fuel will flow)

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Internal Combustion Engines A. Sobiesiak
2022
Chapter 4. Spark Ignition Engines
• Simple carburetor (fixed jet) performance
Trends in Δpn and Δpm with increase of engine speed at fixed load
Δpn = patm – pth; ~fuel flow velocity,
• fuel jet velocity increases and fuel
mass flow rate increases
Δpm = patm – pm ; ~ air flow velocity,
• air flow velocity increases and air
mass flow rate increases
• Load increases (more throttle opening)
moves the curves in opposite
directions. For given rpm a fuel velocity
will increase, and air will decrease
(although mass flow rate will increase)

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Improvements to a simple carburetor (fixed jet)
performance
- idling system
- choke for start
- compensating air
nozzle
- acceleration pump
- compensating
(enrichment ) jet
- double venturi
increasing rpm
Simple carburetor performance

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Main metering system of downdraft carburetor
Air inlet at Patm
Venturi throat et at Pth
Fuel at Patm
Air/fuel flow into cylinders at
Pm ≈ Pcylinder

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Choke plate (to create very
low pth at the venturi
throat) and automatic
starter for cold start
• Idling system (fuel
injected below the
throttle plate)

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Accelerating pump
(to inject extra fuel
when accelerating)
• Enrichment pipe (to inject
extra fuel at WOT)

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Throttle body injection system by Bosch, and early
port injection system in place of carburetor
Throttle plate
Inlet valve
Oxygen sensor
Injector
• the required
fuel flow rate is
calculated or
looked up based
on a throttle
position sensor
and O2 sensor
(in 7-ECU) and
injected

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• Port Fuel Injection (PFI) system by Bosch, fuel is
injected at the surface of the inlet valve
• Inlet air velocity
is measured
• air flow is
calculated by
ECU (4)
• the required
fuel flow rate is
calculated or
looked up based
measured air
flow, throttle
position sensor
and O2 sensor
(in 4-ECU) and
injected
Oxygen sensor
Air flow meter
Throttle plate
Injector
Inlet valve

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
Summary of Fuel injection systems characteristics
• Fuel flow is driven by the pressure difference (fuel rail and injector tip)
• Fuel metering is more accurate and independent of the air flow
• Atomization independent of the air flow (walls wetting is to be avoided)
• Increased volumetric efficiency (fuel does not displace air as much)
• Improved thermal efficiency
• More torque and power

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
Summary of Fuel injection systems characteristics
• Lower regulated emissions and GHG (Green House Gases)
• Better fuel economy (better matching of mixture strength to operating point
• More fuel tolerant
• Facilitate charge stratification (GDI)
• PFI fuel rail pressure is 0.25 to 0. 36 MPa (atmospheric pressure is 0.1 MPa)
• GDI requires much higher fuel rail pressure, 5 - 7 MPa, with mean droplet
diameter around 20 μm, with qualitative change in injectors design.

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Chapter 4. Spark Ignition Engines
• PFI injection arrangements in a four-cylinder engine
Simultaneous injection
• Two short injections per cycle, all four at the same time
• Cyl. 1 and 4 are fueled the same way (one injection once
the inlet valve is closed, second before it opens)
• Cyl. 2 and 3 are fueled the same way, one injection when
the valve is open, second when it is closed.
• These two groups of cylinders have different levels
of mixture preparation (homogeneity)
• Pulse duration 2 to 4 ms
Group injection
• One longer injection per cycle, two groups of injections
• 1 and 4 are fuelled the same way, injection shortly before
the inlet valve opens
• 2 and 3 are fuelled the same way, injection during
exhaust stroke, the intake valve is closed
• no injection during intake process
• Cyl. 1 and 4 have less time to mix
• Pulse duration 4 to 8 ms
Sequential injection
• Each cylinder is fuelled individually
• All cylinders are fueled the same way, shortly before the
inlet valve opens
One cycle
One cycle
One cycle

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Internal Combustion Engines A. Sobiesiak 2022
Chapter 4. Spark Ignition Engines
• FTP test for emissions (NOx, CO, UBC)
2nd bag
1st bag 3rd bag

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Borghi Diagram of Turbulent
Combustion
Laminar
plane
flames
R
P
1 102 104 106 108
106
104
102
1
u’/SL
l0/δF
u’ = velocity fluctuations
l0 = integral length scale
SL = laminar flame speed
δF = flame thickness
Karlowitz number,
Ka = tc/tη = 1/(tη/tc)
Damkohler number,
Da = tlo/tc = l0 SL/u’δF
Ret = u’l0/ν
tc = δF/SL
tη = √ν/ε
Ka = 1; tc= tη
Da = 1; tlo = tc , l0/δF= u’/SL
R
Laminar
wrinkled
flames P
R
Island’s
formation
P
R
Torn flame
fronts,
flamelets
P
Ideally stirred
reactor/Distributed
reaction zone
Rel = 1
Da > 1
Ka<1
Da < 1
Ka< 1
SI ICE
R
Ka>1

24. A. K. Oppenheim’s text on Combustion in
Piston Engines (2004)
“The main source of evil (pollutants) are
propagating flames where the exothermic
centers are ganged together to form a highly
agitated (and thin) flame front.”

25. A. K. Oppenheim’s text on Combustion in
Piston Engines (2004)
“An alternative way to accomplish the task is illustrated by the following figure, where
the Cal Bears are distributed, each having fun in cultivating his individual flowerpot.
The cultivation, performed before painstakingly in series of propagating fronts of
malcontent bears, is now accomplished in parallel much better and
faster…Therefore, instead of permitting flames to be established beyond any control,
one should not let exothermic centers to agglomerate into delinquent gang-lines.”

26. A. K. Oppenheim’s text on Combustion in
Piston Engines (2004); alternative modes of
combustion
M – charge
mass fraction
Q – fraction of
energy released
a) Flame Traversing the Charge (FTC)
b) Fireball mode of Combustion (FMC),
a controlled explosion (HCCI mode)
c) FTC followed by FMC: end-gas knock
d) FMC followed by FTC; afterburn,
combustion following an explosion
R
P R
P
e) Flamelets in a turbulent flame
d) Distributed reaction combustion