SI Engine Mixture Preparation
1. Requirements
2. Fuel metering systems
3. Fuel transport phenomena
4. Mixture preparation during engine transients
5. The Gasoline Direct Injection engine
MIXTURE PREPARATION
Fuel Air
Fuel Air
EGR
Engine
Metering Metering
EGR ControlMixing
Combustible Mixture
1
MIXTURE PREPARATION
Parameters
-Fuel Properties -Air/Fuel Ratio -Residual/Exhaust Gas Fraction
Impact
- Driveability - Emissions - Fuel Economy
Other issues: Knock, exhaust temperature, starting and warm-up, acceleration/ deceleration transients
Fuel properties (Table D4 of text book)
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2
Tem
pera
ture
(C
)
Gasoline evaporative characteristics
Distillation curve (ASTM D86) of UTG91 (a calibration gasoline)
200200 Cumene (Methyl-ethyl benzene)
160160 (NBP(NBP 152.4152.4oC)
Toluene 120120 (NBP 110oC)
Iso-octane
8080 (NBP 99.2oC)
Iso-hexane
4040 (NBP 60.3oC)
00 2020 4040 6060 8080 100100 % distilled
• Reid Vapor pressure (ASTM D323):
– equilibrium pressure of fuel and air of 4 x liquid fuel volume at 37.8oC
• T10, T50, T90
– Temperature at 10, 50 and 90% distillation points
• Driveability Index (DI) – For hydrocarbon fuels: DI = 1.5 T10 + 3 T50 + T90 (T in oF)
RVP: winter gasoline ~ 11 psi (0.75 bar); Summer gasoline ~ 9 psi (0.61 bar); California Phase 2 fuel = 7 psi (0.48 bar) DI: range from 1100 to 1300; Phase II calibration gasoline has DI=1115; High DI calibration fuel has DI 1275.
Equivalence ratio and EGR strategies (No emissions constrain)
Enrichment to preventEnrichment to knocking at high loadimprove idle stability
Rich
=1
Lean for good fuel economyLean Load
EGR to decrease NO emission and
EGR pumping loss No EGR to
No EGR to maximize airimprove idle flow for powerstability
Load
3
Requirement for the 3-way catalyst
Air/fuel ratio Rich Lean
Cat
alys
t ef
fici
ency
%
Fig 11-57
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FUEL METERING
• Carburetor – A/F not easily controlled
• Fuel Injection – Electronically controlled fuel metering
Throttle body injection
Port fuel injection
Direct injection
4
Injectors
PFI injectors
• Single 2-, 4-,…, up to 12-holes
• Injection pressure 3 to 7 bar
• Droplet size: – Normal injectors: 200 to 80 m
– Flash Boiling Injectors: down to 20 m
– Air-assist injectors: down to 20 m
GDI injectors
• Shaped-spray
• Injection pressure 50 to 250 bar
• Drop size: 10 to 50 m
PFI Injector targeting
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5
INTAKE PORT THERMAL ENVIRONMENT 140
120 Tvalve
100
80
60 Tport
40
20 Tcoolant
0 0 15 30 45 60 75 90 105 120 135 150 165 180 195 210
Time (sec)
Tem
per
atu
re (
deg
C)
Engine management
system
From Bosch Automotive Handbook
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6
Fuel Metering
• A/F ratio measured by sensor (closed loop operation) – feedback on fuel amount to keep =1
• Feed-forward control (transients): – To meter the correct fuel flow for the targeted A/F target, need to
know the air flow • Determination of air flow (need transient correction)
– Air flow sensor (hot film sensor) – Speed density method
Determine air flow rate from MAP (P) and ambient temperature(Ta) using volumetric efficiency (v) calibration
N m V (N,)a D v Displacement vol. VD,2
rev. per second N, P gas constant R RTa
FEATURES OF ELECTRONICALLY CONTROLLED FUEL INJECTION SYSTEM
• Sensors – Air temperature
– Engine Speed
– Manifold air pressure (MAP) / air flow rate
– Exhaust air/ fuel equivalence ratio (): EGO (and UEGO)
– Coolant temperature
– Throttle position and throttle movement rate
– Crank and cam positions
• Controls – Injection duration
– Spark timing
– Other functions
Idle air, carbon canister venting, cold start management, transient compensation, ….
7
ENGINE EVENTS DIAGRAM
0 180 360 540 720 900 1080 1260 1440
Cyl. #1 CA (0 o is BDC compression)
Cyl.#1
Cyl.#2
Cyl.#3
Cyl.#4
BC BC BC BC BCTC TC TC TC
TC TC TC TC TCBC BC BC BC
TC TC TC TC TCBC BC BC BC
BC BC BC BC BCTC TC TC TC
Intake Exhaust Injection
Ign Ign
Ign Ign
Ign Ign
Ign Ign
Effect of Injection Timing on HC
Emissions
SAE Paper 972981
Stache and Alkidas
Engine at 1300 rpm
275 kPa BMEP
Injection timing refers to start of injection
8
Mixture Preparation in PFI engine
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Intake flow phenomena in mixture preparation (At low to moderate speed and load range)
Reverse Blow-down Flow • IVO to EVC:
– Burned gas flows from exhaust port because Pe>Pi
• EVC to Pc = Pi:
– Burned gas flows from cylinder into intake system until cylinder and intake pressure equalize
Forward Flow • Pc = Pi to BC:
– Forward flow from intake system to cylinder induced by downward piston motion
Reverse Displacement Flow • BC to IVC:
– Fuel, air and residual gas mixture flows from cylinder into intake due to upward piston motion
Note that the reverse flow affects the mixture preparation process in engines with port fuel injection
9
Mixture Preparation in Engine Transients
Engine Transients • Throttle Transients
– Accelerations and decelerations
• Starting and warm-up behaviors – Engine under cold conditions
Transients need special compensations because:
• Sensors do not follow actual air delivery into cylinder
• Fuel injected for a cycle is not what constitutes the combustible mixture for that cycle
Manifold pressure charging in throttle transient
Aquino, SAE Paper 810484
m
v d/cyl fire
V
V N
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10
Fuel-Lag in Throttle Transient
The x- Model dM Mf f xm fdt
Mfm (1- x)m c f
m Injected fuel flow ratef
m Fuel delivery ratec
to cylinder
M Fuel mass in puddlef
Fuel transient in throttle opening
mair
cm
Model prediction Observed results
Fig 7-28 Uncompensated A/F behavior in throttle transient
11
sig
nal
1st peak 2nd peakIntegrated HC emissions: 16 mg 55 mg Total: 71 mg (SULEV:
FTP total is < 110 mg) 4
2 x104 ppm C1 Pre-cat HC
0 4
2 x104 ppm C1 Post-cat HC
0 Engine start 2 up behavior Pintake (bar) (e) (f)
x1000 RPM0
50 (d)
2.4 L, 4-cylinder 60 (c) engine
(a) Cylinder 4 pressure
(b) Engine starts with Cyl#2
40 piston in mid stroke of
Ign 2&3 30 compression
Ign 1&4
Firing order 20 Inj 3 1-3-4-2
Inj 1 10
Inj 4
Inj 2 0 0 0.5 1 1.5 2 2.5 3
time(s)
Engine start up behavior
0 2 4 6 8 10
0
1
2
3
time(s)
MA
P(b
ar),
o
r R
PM
/100
RPM
MAP
Crank start
1st round of firing
Speed run up
Speed decay
Engine gradual warm up
Speed Flare
12
Pertinent Features of DISI Engines
1. Precise metering of fuel into cylinder – Engine calibration benefit: better driveability and
emissions
2. Opportunity of running stratified lean at part load – Fuel economy benefit (reduced pumping work; lower
charge temperature, lower heat transfer; better thermodynamic efficiency)
3. Charge cooling by fuel evaporation – Gain in volumetric efficiency – Gain in knock margin (could then raise compression
ratio for better fuel economy) – Both factors increase engine output
DISI technology penetration
• Significant market penetration of DISI – Homogeneous
charge configuration
– As enabler of the boosted-downsizing strategy
0%
10%
20%
30%
40%
Production
Share 2008
2013
DISI Boosted
13
Toyota DISI Engine (SAE Paper 970540)
Straight port
0 1000 30002000 4000 5000
Engine speed (rpm)
Ou
tpu
t to
rqu
e
High pressure injector
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Mitsubishi DISI Engine
End of injection
Piston
Fuel spray impingement
Vaporization and transport to spark plug Reverse tumble Swirling spray
Spherical piston cavity
(SAE 960600)
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14
Wall-guided versus spray-guided injection
Wall-guided injection Spray-guided injection(injector relatively distant (injector relatively close to spark plug)
from spark plug)
SAE 970543 (Ricardo) SAE 970624 (Mercedes-Benz)
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Charge cooling by in-air fuel evaporation
Intake air temperature (oC)Intake air temperature (oC)
Tem
per
atu
re d
iffe
ren
ce (
o C
)
Charge cooling effect Lowering of intake volume
Anderson, Yang, Brehob, Vallance, and Whiteabker, SAE Paper 962018
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15
Full load performance benefit
Torq
ue
Full loadFull load
SAE 970541 (Mitsubishi)
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Part load fuel economy gain
SAE Paper 960600 (Mitsubishi)
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DISI Challenges
1. High cost 2. With the part-load stratified-charge concept :
– High hydrocarbon emissions at light load – Significant NOx emission, and lean exhaust not amenable to
3-way catalyst operation
3. Particulate emissions at high load 4. Liquid gasoline impinging on combustion chamber walls
– Hydrocarbon source – Lubrication problem
5. Injector deposit – Special fuel additive needed for injector cleaning
6. Cold start behavior – Insufficient fuel injection pressure – Wall wetting
Comparison of cold start HC emissions(Koga, Miyashita, Takeda, and Imatake, SAE Paper 2001-01-0969)
Cumulative engine out HC in the first 10 seconds of cold-start
Rel
ativ
e
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17
Significant particle numbers in cold start
SAE 2011-01-1219
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MIT OpenCourseWare https://ocw.mit.edu
2.61 Internal Combustion EnginesSpring 2017
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