ic engines slides
TRANSCRIPT
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Premixed Internal Combustion Engines with Spark Ignition
exhaustintake
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2-StrokeEngines
2-stroke
ReedValve
intake
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Wankel Enginesintakeexhaust
+: No valves neededContinuous motion
less vibration
-: Leaks through seals
low compressionratiopollution (high levels of HCand CO)
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4-Stroke SI Engines Emission characteristicsof air/fuel ratio
Otto cycle
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Topics of Engine Combustion
• Brief introduction: - Spark Ignition (gasoline)- Fuel preparation, Ignition, Power analysis
(efficiency & losses) , Engine Knock, Modeling, Emissions
• Areas of improvement• Emerging new technologies
[Lean Burn, Homogeneous Charge Compression Ignition (HCCI) ]
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SI Engine Performance VS Engine RPM
Engine RPM
Torque
Power
SFC
OutputPowerm
SFC fuel
•
=
Power
Torque
000,60*)(*2)( RPMNMTorquekwPower π
=
Specific Fuel Consumption
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Spark Ignition (SI) Engines:• Control of combustion: ignition
timing• - Low efficiency (low
compression ratio, limited by knocking)
• + Cleanest emission with help of 3-way catalyst
• Advanced Technologies:--New fuel with higher octane number, improving combustion efficiency, reduces losses (pumping, heat, friction, exhaust & intake)
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SI Combustion
-Spark initiates combustion-Turbulent flame propagation-Stoichiometric mixture-High combustion temperature-Compression ratioLimited by autoignition
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Understanding Engine KnockSome physical models in mind:• Unburn gas is compressed by bunred turbulent flame leading
high temperature and pressure• Unburn gas auto-ignites or ignition triggered by hot spot.• Autoignition delay time is of critical importance Pressure waves bounce
back and forward throughout cylinder
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Autoignition delays of large-molecule fuelsExhibit negative temperature behaviors
Low temp.ignition
Mainignition
Negative temperature behaviors:T ↑ delay ↓
n-heptane: C7H16
T ↑ delay ↓
Negative temperature behaviors complicate engine designs
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1
100
10000IsooctaneMethanePRFethanol
PRF
Isoo
EthanoMethane
ition
Del
ay [m
s]Autoignition delays of large-molecule fuels
Exhibit negative temperature behaviors-Methane and ethanoldo not show “negative”temperature in the IC engine application regime•Are oxygenated fuels(biofuels) similar to ethanol?• Predictive models?
Fuel Octane number(RON)CH4 Methane 120C2H5OH Ethanol 107C8H18 Isooctane 100
PRF(80) 80
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To avoid autoignition so that a higher compression ratio can be used higher thermal efficiency
• reduces time available for unburned gas-- higher flame speed (controlled largely by
turbulence)-- multiple ignition sources (limited by space)
• Keep unbuned gas as cool as possible • Fuel additives (small amount of ethanol and
others)…All these are explored by engine tests with test
matrix assisted by CFD simulations
Guidance to SI Engine Design
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Multiple Site Spark IgnitionBenefits:• Combustion is initiated atouter and propagating inward• Fast combustion rate eliminatingend gas auto-ignition• Higher combustion efficiency• Shorter combustion period lessheat loss
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3D-CFD Predictions of Engine Knock
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Combustion Chamber Geometry• Enhances turbulent
combustion speed to decrease burning time
• Minimizes heat transfer & pollution formation
• Needs to be integrated with valve timing and intake manifold designs
• CFD calculations
Candidate Geometries
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Velocity Field
TurbulenceIntensity
CFD of Intake Flows
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Turbulent Premixed Flames
Internal Combustion Engine
FlameDevelopment
BunsenTurbulentFlame
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Turbulent Flames
( )
TDC beforeignition spark pressure motoring
nsfluctuatiovelocity turbulent '
05.01'21.11 4.082.0
==
=
+⎟⎟⎠
⎞⎜⎜⎝
⎛+=
θ
θ
m
mLL
T
pu
pp
Su
SSEmpirical Relation:
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Enhancing turbulence by multiple valve arrangement
Vertical Vortex – generates strongTurbulence as the tumble is brokenup near the top dead center
Fast burn Less time for heat transferPossible higher compression ratio10-20% better fuel economy
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Various Losses from SI EnginesIdeal Otto Cycle
ExhaustBlowdown loss
Heat lossTime loss(slow combustion)
Pumping loss
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Sketch of a Carburetor
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ECU: ElectronicControl Unit
Throttle
Fuel InjectionSystem
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Pressure data from a 1.8 Liter Pontiac Engine at Berkeley
Pumping lossesbecome significant
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Overall Performance from SI Engines with/without Throttling
Current throttled SI engines
Self-ignition unthrottled
Effic
ienc
y
Effective Mean Pressure[bar]
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Importance of valve timing
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Gasoline Direct Injection (GDI)Lean Burn Engines
Advantages:-lean burn -Low (or no) pumping loss-High compression ratio-High efficiency
Disadvantages:-hard to maintain good combustion-high NOx an HC emissions-special NOx absorption catalyst
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• CombustionChamber Geometry
• Combustion ofDiluted Mixtures
• Multiple source ignition
• Exhaust energyretention
Enabling Technologies:
• Friction Reduction
• Optimal control of Spark timing or Injection
•Variable Compression
• Variable Valve Timing (VVT)
Areas offering potentially largeimprovement in engine performancefor current & future fuels
HCCI Technologies
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Modeling of MON, RON and HCCI Number
Complete Model with CFD good for engine designbut too expensive with detailed chemical kinetics
Chemical kinetics base model-- Single zone well-mixed reactor HCCI-- Two zone model (or multizone shell)
SI MON and RON
removed
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Complete CFD with detailed chemistry exceedsCurrent computer capacities
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a) gasoline engine b) diesel engine
Emission as functionof air/fuel ratio
Emission Characteristics from IC Engines
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NOx Emission is highly sensitive to temperature
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Sources of Unburned Hydrocarbon and CO
Unburned HC and CO are stored in crevices and released duringexpansion stroke as wall jets.
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Spark Ignition (SI) Pontiac Engine
Horiba gas analyzers:CO,HC, NOx, O2, CO2
Engine performances:-- Pressure transducer-- torque-- SFC-- emission before and after catalyst
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Comparison:3 Methods of Internal Combustion Engines
Diesel HCCIGasoline
(Premixed Spark) (Direct Injection) (Premixed Autoignitionof lean mixtures)
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What is HCCI ?• Homogeneous-Charged Compression Ignition (HCCI) engine
-- has advantages of both SI and CI engines• Global autoignition of premixed fuel and air
(High compression ratio)• Heat release controlled by chemical kinetics • No flame propagation - operation possible at low Φ*
• Benefits: 1. Low combustion temperature (lean mixture < Φ~0.5)lower NOx emissions
2. Premixed charge no soot3. High compression ratio high thermal efficiency
• Challenges: 1. Control of Start of Combustion (SOC) is the main issue in HCCI (No direct control, i.e. spark or fuel injector)2. High unburned hydrocarbon and CO emissions
• Benefits and Challenges
* equivalence ratio for fuel-air mixture
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