Ch4

Chapter IV – Spark Ignition Engines (2/27/03) Overview 

Combustion process in SI engines  How initiated and constrained

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Effect of mixtures Ignition Timing Combustion Chamber Design  Conventional and “Compact” lean burn  Advanced: VTEC design  Direct Ignition Stratified Charge

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Catalysts and Emissions Cycle by Cycle Variations and Implications Ignition Systems & Ignition Process Carburetors and Fuel Injection Electronic Controls – DME, Oxygen Sensors,

Fuel Mixture Strength wmmp – Weakest Mixture Max Power LBT – Lean Best Torque Lean Mixture -> Slow Burn -> Lower Pmax, Lower Tmax, Reduced Knock Relationship of sfc & Power Output

SFC & BMEP w.r.t. φ

Min sfc at 0.9 Max BMEP a 1.08 What do we do? Why is BMEP at φ > 1?

Must have φ > 1 to use all O2 Unburnt gas Efficiency down

Sfc vs. BMEP for various A/F Fish Hook Graphs 

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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 possible Too lean 

combustion incomplete - no fuel

Too rich – no O2 left

Controlling Fuel Mixture Carburetors  

Fixed Venturi Fixed Jet  Multiple Jets  Each different op

range

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Variable Venturi Variable Jet Multiple Venturi

Fuel Injection    

Electronic 

 Old 4BBL, 2 vaccum 2 Mechanical

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 “Dumper” 4BBl

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Mechanical CIS Electronic Hybrid Systems “TBI” – electronic carb Multiport Port Fuel Injection

Ignition Timing Optimization 3/2/03

Precise timing > Max Output Timing varies    

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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 Optimization Knock Margin  

P up, Knock! Change advance with load

Note changes in Pmax vs bmep Total Area is NET of compression loss 

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Do not confuse PMEP with Compression work! Part throttle –P down and T down, flame travel

Combustion Chamber Design Flathead Optimized 

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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?

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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

Combustion Chamber Considerations (cont’d) Low surface to volume ratio  

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Good turbulence Minimize quench areas Minimize heat transfer

Optimum approx 500 cc. Reducing swept volume increases max RPM? 

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Less time for flame travel 500->200 cc changes

Caveats 

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Excellent design allows for rapid flame travel High Compression – Maximum Flame Travel Too rapid travel -> Noisy

Combustion Chamber Design “Oversquare” 

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higher performance (HP) Less travel Lower max piston speeds More piston area Larger valves Poor surface to volume ratio (Q) So what?  Discuss.

Undersquare – 

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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 Engines Bore (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%

Design

Depends on goals! Economics vs

Wedge Chamber Most popular Good squish Great for V config Great for inline May be cross-flow 944 and chevy heads both X flow May use wedge pistons for high CR Economical valve arrangement

Hemispherical Head Efficient Cross Flow  Great scavenging –woverlap Difficult valve gear “Pent Roof” on 4V Hemi on 2 V (spherical) Allows for larger valves – why? Spark plug usually offset or dual plug in 2V heads Expensive to machine Expensive to operate valves 4V heads in 1920s race cars

Bowl in Piston Low machine costs Very compact Combustion Chamber Can be cross flow Allows for high CR Bowls often used in turbo applications Why?

Bath-Tub Head Compact Chamber Circumferential Squish Better 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 Compact High Compression –-w- ordinary fuels?  High turbulence  Lean burn  Compact Turbulence Up  Leaner burn  Why?  Rapid Combustion  Less Knock Susceptibility

Q down  Concentrated @ Ex Valve  Fast burn after spark  Eliminate Knock from self ignition May Fireball – 1979  Straight from intake  Spark plug at angle  Controlled high axial swirl  Notre plug location  Note piston shape 

Design Considerations – Econ & Emissions Emissions Economy 

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Generally good due to high CR possible, up to 14:1 Good power dues to quick efficient combustion Good due to lean burn

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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 calculations 2V 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 high Constant BMEP Barrel Swirl As compression occurs, increase in swirl ratio through conservation of momentum As compression stroke completes, swirl breaks up into random turbulence (example) Enables weak mixture to be fully burn, low emissions and good economy

Nissan ZapsZ Twin Plug High Axial Swirl Combustion is at edge, but swirl maintaned and rapid combustion Very little turbulence 

Little squish

Rapid comb Allows high CRs Can be 2V or 4V

HRCC Similar to May Fireball Small combustion chamber Rapid Combustion Allows high CR with low mixture strenght More efficent than May Fireball because of more efficient inlet tract. Can burn mixtures as low as φ = 0.6

optimized combustion chambers High Swirl  

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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 

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Higher volumetric efficiency Can select optimum setup Corvette ZR1 Acura NSX

Compact combustion chambers prone to knock and preignition under high loading (due to proximity of exhaust valve) and need auto transmissions to damp peak loading

Advanced Combustion Systems Use of EGR 

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Reduces emissions Reduces throttling loss Only use with fast burn systems since oxygen level will be lowered, effective φ decreased

Tumble? 

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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  

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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: 

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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

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Lessen swirl when performance needed so Vf increases

Solution - VTEC

Variable Timing and Event

Control

Keeps inlet valve closed, NOT port 

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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

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VTEC allows one valve to be diabled in econo mode φ 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 Design Allows 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, ω, increased, swirl strength and momentum increased.

Vtec Swirl Effects

Both -> Pent Roof – High Barrel Swirl Inner or Outer – Tumble –    

Reduced ignition delay (0-10% Mass Fraction) Reduced Burn Duration Lowe CoV Greater EGR Tolerance

VTEC

Engine Management Strategy 3 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

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 Emissions Chemistry and evolution of catalysts Part 3: The DISI engine discussion

Chapter 4, Part II Ignition and Fuel systems The ignition process How the spark occurs and how it’s generated

Spark Plugs, gaps and temperature Electrode Needs to run 350-700C Too Hot: 

Preignition

Too Cool: 

Carbon Deposits Form

Hot Plug – Lean Cool Cool Plug – Performance Why???

Distributor Ignition Process Contact Points 

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Capacitor is a reservoir for charge W/O capacitor charge would jump points Other Systems:  Magnetic trigger  Optical Trigger  Etc.

Alternative is CD 

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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/Retard Why 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 position May be up to 30 degrees!

Distributorless Ignitions

“Crank Fire” (not cam-fire) Wasted Spark Double Ended Coil May be self contained or part of a DME system Fires 2 plugs EVERY revolution! Other benefits – easy to install, clean plugs Canned systems available inexpensively

Twin plug distributorless ignition.

Electronic Spark management Integral –w- fuel management “N” dimensional map May integrate knock sensing As many variable as you have prom Done –w- lookup tables and interpolation

Stages of Ignition Pre-Breakdown 

Gas is an insulator, but voltage differential causes electrons to flow toward annode

Breakdown 

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Rapid braekdown of voltage differential 100A rise in few nanoseconds Temp 60,000 K and local P of several HUNDRED bars!

Arc Discharge

Short duration high amp spark: Better thermal conversion, less CoV of initiation time Long duration low A spark– more change of masking CoV

Fuel Systems Mixture Prep Carburators Mechanical FI CIS EFI  

Single Port Multi Port

Manifold Issues –w- Carbs or single port Sharp corners vaporize fuel where manifold acts as a surface carburetor Surface is wet May have channels to control fuel flow in startup “Pump the gas!”

Choke Balancing  

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 speed Complete 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 size Do not confuse with piston operated throttle valves British “Stromberg” See p195 for key

Fixed Jet Carburetor Sonstan t venturi and jet(s) Fuel drawn by low P Discuss

Fuel flow with fixed jet carb These are the flow characteristics due to vacuum Venturi effects only What problems does this cause?

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 circuit Cars usually on this in “cruise mode” as well Extra prot –w- idle adjustment screw given to fine tune mixture at idle How would you do this? 

Minimum mix for

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 pressure Must 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 port Allows for computer controls Back feed regulator

MFI: Injection in inlet port Inject to back of valve Cools Valve Vaporizes Fuel Must have multichannel system Single channel would cause pressure fluctuations and require very high fuel pressure Early 2 channel Now Sequential FI

Sequntial times pulse –w- charge Stabilizes pressure Aides in Vf Can 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 size Full Load

SFC Map Note BMEP relationsh ip