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Spark-Ignition-Engine ppt.ppt
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Chapter IV – Spark Ignition Engines (2/27/03)
Overview Combustion process in SI engines
How initiated and constrained Effect of mixtures Ignition Timing Combustion Chamber Design
Conventional and “Compact” lean burn Advanced: VTEC design Direct Ignition Stratified Charge
Catalysts and Emissions Cycle by Cycle Variations and Implications Ignition Systems & Ignition Process Carburetors and Fuel Injection Electronic Controls – DME, Oxygen Sensors,
etc.
Fuel Mixture Strength
wmmp – Weakest Mixture Max PowerLBT – Lean Best TorqueLean Mixture -> Slow Burn -> Lower Pmax, Lower Tmax, Reduced KnockRelationship of sfc & Power Output
SFC & BMEP w.r.t.
Min sfc at 0.9Max BMEP a 1.08What do we do?Why is BMEP at > 1?
Must have > 1 to use all O2
Unburnt gasEfficiency down
Sfc vs. BMEP for various A/FFish Hook Graphs
Power-Fuel Maps for each throttle position
Note A-B B is much more efficient,
more throttle but lower SFC
Exception – WOT 1.1
Why hook: Max efficiency burn as much fuel as possibleToo lean
combustion incomplete - no fuel
Too rich – no O2 left
Controlling Fuel Mixture
Carburetors Fixed Venturi Fixed Jet
Multiple Jets Each different op
range Variable Venturi Variable Jet Multiple Venturi
Old 4BBL, 2 vaccum 2 Mechanical
“Dumper” 4BBl
Fuel Injection Mechanical CIS Electronic Hybrid Systems
Electronic “TBI” – electronic
carb Multiport Port Fuel Injection
3/2/03 Ignition Timing Optimization
Precise timing > Max OutputTiming varies
With RPM With throttle position With output With vacuum or
manifold pressure Combinations?
Electronic, Mechanical,and Vacuum controls
Vacuum advance Vacuum retard Weights
Ignition Timing OptimizationKnock Margin
P up, Knock! Change advance
with load
Note changes in Pmax vs bmepTotal Area is NET of compression loss
Do not confuse PMEP with Compression work!
Part throttle –P down and T down, flame travel slower, so more advance is needed
Combustion Chamber Design
Flathead Optimized Because of design limited
to 6:1 OK, because octane of fuel
was 60-70 in 1920s-30s! Nice turbulent
characteristics – “Squish Area” ejects gasses - Jet
Jet -> Rapid combustion Too much squish – too
rapid, noisy, Pmax up Squish reduces
susceptibility to knock End gas in cooler near
wall, piston and head, small volume
Combustion Chamber Design Goals
Distance traveled by flame front minimized Allows for high engine speeds Reduces time for chain reactions leading to Knock Small DIAMETER can run higher combustion ratio!
Exhaust Valve(s) & Spark Plug(s) close together Very hot (incandescent) and a great source of KNOCK
Is this pre-ignition or self ignition? Far as possible from End Gas
Turbulence is good Mixing and flame propagation, Squish areas or shrouded inlet valves Too much turbulence bad – breaks down boundary laver
Can lead to hot spots, rapid noisy combustion
End gas in cool part of combustion chamber Small clearance creates a cool region Inlet valve should be near end gas region since it is cooled
during induction
Combustion Chamber Considerations (cont’d)
Low surface to volume ratio
Good turbulence Minimize quench areas Minimize heat transfer
Optimum approx 500 cc.Reducing swept volume increases max RPM?
Less time for flame travel 500->200 cc changes
max RPM from 6000 to 8000
Caveats Excellent design
allows for rapid flame travel
High Compression – Maximum Flame Travel
Too rapid travel -> Noisy
Combustion Chamber Design
“Oversquare” higher performance
(HP) Less travel Lower max piston
speeds More piston area Larger valves Poor surface to volume
ratio (Q) So what?
Discuss.
Undersquare – more economy and
higher torque Torque proportional to
stroke Better Surface to
Volume Ratio (Q) More efficient burn Smaller end gas
region Less prone to knock
Examples:350 Chevy
712cc/Cyl 4.0” (102mm) bore 87.2 mm stroke
302 Chevy 625cc/Cyl 102mm bore 79mm stroke
944/928 625cc/Cyl 100mm bore 79 mm stroke
911 EnginesBore (mm)Stroke (mm)Disp (ltr) B/S
80 66 1.99 121%84 66 2.19 127%84 70.4 2.34 119%90 70.4 2.69 128%95 70.4 2.99 135%95 74.4 3.16 128%98 70.4 3.19 139%98 74.4 3.37 132%
100 76.5 3.60 131%102 76.5 3.75 133%104 76.5 3.90 136%
Optimized Chamber Design
Depends on goals! Economics vs Perf.
Wedge ChamberMost popularGood squishGreat for V configGreat for inline May be cross-flow944 and chevy heads both X flowMay use wedge pistons for high CREconomical valve arrangement
Hemispherical HeadEfficient Cross Flow
Great scavenging –w- overlap
Difficult valve gear“Pent Roof” on 4VHemi on 2 V (spherical)Allows for larger valves – why?Spark plug usually offset or dual plug in 2V headsExpensive to machineExpensive to operate valves4V heads in 1920s race cars
Bowl in PistonLow machine costsVery compact Combustion ChamberCan be cross flowAllows for high CRBowls often used in turbo applicationsWhy?
Bath-Tub Head
Compact ChamberCircumferential SquishBetter swirl than wedge
3/6/02 Efficiency Curves
Mechanical Efficiency vs Cycle Efficiency. Is Otto Cycle realistic? Efficiency at Max power vs
Max Economy
3/6/02 High Compression Ratio Fast Burn Designs
High Compression –-w- ordinary fuels?
High turbulence Lean burn Compact
Turbulence Up Leaner burn Why?
Rapid Combustion Less Knock Susceptibility
Compact Q down Concentrated @ Ex Valve Fast burn after spark Eliminate Knock from self
ignitionMay Fireball – 1979
Straight from intake Spark plug at angle Controlled high axial swirl Notre plug location Note piston shape
Design Considerations – Econ & Emissions
Economy Generally good
due to high CR possible, up to 14:1
Good power dues to quick efficient combustion
Good due to lean burn
Emissions Hydrocarbons up Large squish areas Large quench
areas Low temps die to
lean burn May need to
insulate to keep catalyst up to temp (next week)
Other problems Fine mix control Deposits
More CC designs
Straight inlet tracts Not offset
HRCC similar to may fireball but has straight inlet passage
4 Valve Pent Roof
Large Flow Area – why?Do some calculations2V Flat or wedge
Max d=D/2, a= 50%
2V Hemi 30 deg = 66%2V Hemi 45 degrees – 100% (theory)4V flat – 69%4V pent – 90%?
Vf highConstant BMEPBarrel SwirlAs compression occurs, increase in swirl ratio through conservation of momentumAs compression stroke completes, swirl breaks up into random turbulence (example)Enables weak mixture to be fully burn, low emissions and good economyLittle squish ->small quench -> Lower HC
Nissan ZapsZ
Twin PlugHigh Axial SwirlCombustion is at edge, but swirl maintaned and rapid combustionVery little turbulence
Little squish
Rapid comb Allows high CRsCan be 2V or 4V
HRCC
Similar to May FireballSmall combustion chamberRapid CombustionAllows high CR with low mixture strenghtMore efficent than May Fireball because of more efficient inlet tract.Can burn mixtures as low as = 0.6
SWIRL and Knock with optimized combustion chambers
High Swirl Great at low load Kinetic energy used to
create swirl reduces volumetric efficiency
This is OK unless you want to make power! Twin Inlet Tracts –
Can kill swirl when second tract opened
Higher volumetric efficiency
Can select optimum setup Corvette ZR1 Acura NSX VTEC
Compact combustion chambers prone to knock and pre-ignition under high loading (due to proximity of exhaust valve) and need auto transmissions to damp peak loading
Advanced Combustion Systems
Use of EGR Reduces emissions Reduces throttling
loss Only use with fast
burn systems since oxygen level will be lowered, effective decreased
Tumble? Barrel and axial
swirl combined Reduces ignition
delay Reduces burn
duration CoV lowered Greater tolerance
to EGR
How do we optimize a design?
Want All the benefits of Fast 4V Pent Roof Vf UP Valve overlap and
cross flow lead to excellent scavenging
Barrel swirl – Turbulence
Great power
Want All the benefits of ZapZ or other axial swirl designs Tolerance to EGR Lean burn Low emissions Low CoV Quieter slow burn
system –w- lean mix
Solution – Swirl Port?
Economy Mode: Close one inlet
PORT “Swirl control
valve or port” 30% reduction in
burn duration 20% increase in
EGR tolerance Low cyclical
variations (CoV)
Performance Mode Open second port Change axial swirl
to barrel swirl, less KE needed, less restriction, Vf up
Lessen swirl when performance needed so Vf increases
Solution - VTEC Variable Timing and Event
Control
Keeps inlet valve closed, NOT port
Complex flow pattern –w- 2 vortices
Vortices broke up into three or more as compression increased
High velocity due to small valve opening
Votices are prevasive – they do not decay as have tight core
VTEC allows one valve to be diabled in econo mode
as low as 0.66 Low BSFC (12%
lower than stochiometric)
Performance Mode Operates like Pent
Roof
VTEC Control Modes
VTEC Design
Bowl in piston (55mm/75mm bore)Pent Roof DesignAllows AFR to be extended by 2 compared to flat top (I.e.16.7:1 not 14.7.:1) from shape alone – compact combustion chamber!One valve opened doubles flow velocities, increased, swirl strength and momentum increased.
Vtec Swirl Effects
Both -> Pent Roof – High Barrel SwirlInner or Outer – Tumble –
Reduced ignition delay (0-10% Mass Fraction) Reduced Burn Duration Lowe CoV Greater EGR Tolerance
VTEC
Engine Management Strategy3 Modes:
Very Lean 22:1 (Idle – torque – cruise) Stochiometric 14.7 (Below Idle and high Speed) Rich 12.5:1 (Performance)
Faster and more stable –w- one inlet disabled.Fuel consumption down 5.6%EGR tolerance up 10% leading to a BFSC up 2.4%
Stratified Charge /Catalysts - 3/8/01!
Homework
Part 1: Valve configurations and compression ratios
2V, 4V, 5V valve trains
Valve angle and combustion chambers
Part 2: Catalysts and EmissionsChemistry and evolution of catalysts
Part 3: The DISI engine discussion
Chapter 4, Part II Ignition and Fuel systems
The ignition processHow the spark occurs and how it’s generated
Spark Plugs, gaps and temperature
Electrode Needs to run 350-700CToo Hot:
Preignition
Too Cool: Carbon Deposits Form
Hot Plug – Lean CoolCool Plug – PerformanceWhy???
Distributor Ignition Process
Contact Points Capacitor is a
reservoir for charge W/O capacitor charge
would jump points Other Systems:
Magnetic trigger Optical Trigger Etc.
Alternative is CD System –still uses
same trigger and similar coil but no capacitor
Higher voltage for a short period of time
See book for details
Distributor components and Ignition advance
Both Mechanical and Vacuum Advance/RetardWhy is this necessary? Variable RMP Variable Load Boost? Idle? Etc.
Advance Curves
Most systems yse both.Even electronic systems may use mechanical advance to keep cap-pole in proper positionMay be up to 30 degrees!
Distributorless Ignitions
“Crank Fire” (not cam-fire)Wasted SparkDouble Ended CoilMay be self contained or part of a DME systemFires 2 plugs EVERY revolution!Other benefits – easy to install, clean plugsCanned systems available inexpensively
Twin plug distributorless ignition.
Electronic Spark management
Integral –w- fuel management“N” dimensional mapMay integrate knock sensingAs many variable as you have promDone –w- lookup tables and interpolation
Stages of Ignition
Pre-Breakdown Gas is an insulator, but
voltage differential causes electrons to flow toward annode
Breakdown Rapid braekdown of
voltage differential 100A rise in few
nanoseconds Temp 60,000 K and local
P of several HUNDRED bars!
Arc Discharge Game over.
Short duration high amp spark: Better thermal conversion, less CoV of initiation timeLong duration low A spark– more change of masking CoV
Fuel SystemsMixture Prep
CarburatorsMechanical FICISEFI Single Port Multi Port
Manifold Issues –w- Carbs or single port
Sharp corners vaporize fuel where manifold acts as a surface carburetorSurface is wetMay have channels to control fuel flow in startup“Pump the gas!”
ChokeBalancing Multi Carb Setups Multi Choke Setups
Air Fuel Requirements and Load
Fuel Systems need to react to fuel needs for different operating conditions – Saw this with the “Fishhook Curves”
Variable Demands of Engine
This is at constant speedComplete family of curves for many speeds many loads, many pressures, etc.Forms N dimensional surface (Name them)Carbs only react to vaccum and maybe throtte position
Variable Jet Carburetor
Back feed varies both jet and Venturi sizeDo not confuse with piston operated throttle valvesBritish “Stromberg”See p195 for key
Fixed Jet Carburetor
Sonstant venturi and jet(s)Fuel drawn by low PDiscuss
Fuel flow with fixed jet carb
These are the flow characteristics due to vacuum Venturi effects onlyWhat problems does this cause?
Flow through Venturi Incompressible vs, Comp flow
Air correction jet/emulsion tube
Emulsion tube used to “bend” the curve and lean out the engine at high flow.
This changes flow shape only
Usually can get range of “air jets” and emulsion tubes
Carb Idle circuit and mix adjustment
Idle circuit allows for fuel when V too low to draw fuel through main circuitCars usually on this in “cruise mode” as wellExtra prot –w- idle adjustment screw given to fine tune mixture at idleHow would you do this?
Minimum mix for smooth running
Carburetion – 2 & 3 systems combined
Combined flow from Primary and Main, mixed with Idle circuits
Complete carburetion system
Fuel Injection - Basics
Injector –w- “pulse width”Flow also controlled by differential pressureMust compensate fuel pressure for manifold pressure (especially in turbo systems)Pulse 2-8 ms.Flow ratio of 50:1
SFI
Cheap, about 10% less power than multi portAllows for computer controlsBack feed regulator
MFI: Injection in inlet port
Inject to back of valveCools ValveVaporizes FuelMust have multi-channel systemSingle channel would cause pressure fluctuations and require very high fuel pressureEarly 2 channelNow Sequential FI
Sequntial times pulse –w- chargeStabilizes pressureAides in VfCan time it to hit the valve at just the proper moment (when it’s closed)
Schematic (SFI or MFI)
Distribution of droplet size Part Load
Distribution of droplet sizeFull Load
SFC Map
Note BMEP relationship