Transcript
Premixed Internal Combustion Engines with Spark Ignition
exhaustintake
2-StrokeEngines
2-stroke
ReedValve
intake
Wankel Enginesintakeexhaust
+: No valves neededContinuous motion
less vibration
-: Leaks through seals
low compressionratiopollution (high levels of HCand CO)
4-Stroke SI Engines Emission characteristicsof air/fuel ratio
Otto cycle
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) ]
SI Engine Performance VS Engine RPM
Engine RPM
Torque
Power
SFC
OutputPowerm
SFC fuel
•
=
Power
Torque
000,60*)(*2)( RPMNMTorquekwPower π
=
Specific Fuel Consumption
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)
SI Combustion
-Spark initiates combustion-Turbulent flame propagation-Stoichiometric mixture-High combustion temperature-Compression ratioLimited by autoignition
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
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
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
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
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
3D-CFD Predictions of Engine Knock
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
Velocity Field
TurbulenceIntensity
CFD of Intake Flows
Turbulent Premixed Flames
Internal Combustion Engine
FlameDevelopment
BunsenTurbulentFlame
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:
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
Various Losses from SI EnginesIdeal Otto Cycle
ExhaustBlowdown loss
Heat lossTime loss(slow combustion)
Pumping loss
Sketch of a Carburetor
ECU: ElectronicControl Unit
Throttle
Fuel InjectionSystem
Pressure data from a 1.8 Liter Pontiac Engine at Berkeley
Pumping lossesbecome significant
Overall Performance from SI Engines with/without Throttling
Current throttled SI engines
Self-ignition unthrottled
Effic
ienc
y
Effective Mean Pressure[bar]
Importance of valve timing
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
• 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
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
Complete CFD with detailed chemistry exceedsCurrent computer capacities
a) gasoline engine b) diesel engine
Emission as functionof air/fuel ratio
Emission Characteristics from IC Engines
NOx Emission is highly sensitive to temperature
Sources of Unburned Hydrocarbon and CO
Unburned HC and CO are stored in crevices and released duringexpansion stroke as wall jets.
Spark Ignition (SI) Pontiac Engine
Horiba gas analyzers:CO,HC, NOx, O2, CO2
Engine performances:-- Pressure transducer-- torque-- SFC-- emission before and after catalyst
Comparison:3 Methods of Internal Combustion Engines
Diesel HCCIGasoline
(Premixed Spark) (Direct Injection) (Premixed Autoignitionof lean mixtures)
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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