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