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

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

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

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  1. 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.

  2. 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

  3. Min sfc at 0.9 Max BMEP a 1.08 What do we do? Why is BMEP at f > 1? Must have f > 1 to use all O2 Unburnt gas Efficiency down SFC & BMEP w.r.t. f

  4. Fish 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 possible Too lean combustion incomplete - no fuel Too rich – no O2 left Sfc vs. BMEP for various A/F

  5. 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 Controlling Fuel Mixture

  6. Precise timing > Max Output Timing varies With RPM With throttle position With output With vacuum or manifold pressure Combinations? Electronic, Mechanical,and Vacuum controls Vacuum advance Vacuum retard Weights 3/2/03 Ignition Timing Optimization

  7. Knock Margin P up, Knock! Change advance with load Note changes in Pmax vs bmep Total 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 Ignition Timing Optimization

  8. 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

  9. 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 Design Goals

  10. 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 Considerations (cont’d)

  11. “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 Combustion Chamber Design

  12. 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 Examples:

  13. Optimized Chamber Design • Depends on goals! Economics vs Perf.

  14. 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 Wedge Chamber

  15. Efficient Cross Flow Great scavenging –w- overlap 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 Hemispherical Head

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

  17. Compact Chamber Circumferential Squish Better swirl than wedge Bath-Tub Head

  18. Mechanical Efficiency vs Cycle Efficiency. Is Otto Cycle realistic? Efficiency at Max power vs Max Economy 3/6/02 Efficiency Curves

  19. 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 ignition May Fireball – 1979 Straight from intake Spark plug at angle Controlled high axial swirl Notre plug location Note piston shape 3/6/02 High Compression Ratio Fast Burn Designs

  20. 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 Design Considerations – Econ & Emissions

  21. Straight inlet tracts Not offset HRCC similar to may fireball but has straight inlet passage More CC designs

  22. 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 Little squish ->small quench -> Lower HC 4 Valve Pent Roof

  23. 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 Nissan ZapsZ

  24. 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 f = 0.6 HRCC

  25. 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 SWIRL and Knock with optimized combustion chambers

  26. Use of EGR Reduces emissions Reduces throttling loss Only use with fast burn systems since oxygen level will be lowered, effective f decreased Tumble? Barrel and axial swirl combined Reduces ignition delay Reduces burn duration CoV lowered Greater tolerance to EGR Advanced Combustion Systems

  27. 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 How do we optimize a design?

  28. 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 – Swirl Port?

  29. 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 f as low as 0.66 Low BSFC (12% lower than stochiometric) Performance Mode Operates like Pent Roof Solution - VTEC Variable Timing and Event Control

  30. VTEC Control Modes

  31. 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, w, increased, swirl strength and momentum increased. VTEC Design

  32. 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 Swirl Effects

  33. 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 2.4% VTEC

  34. 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 Stratified Charge /Catalysts - 3/8/01!

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

  36. Electrode Needs to run 350-700C Too Hot: Preignition Too Cool: Carbon Deposits Form Hot Plug – Lean Cool Cool Plug – Performance Why??? Spark Plugs, gaps and temperature

  37. 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 Ignition Process

  38. Both Mechanical and Vacuum Advance/Retard Why is this necessary? Variable RMP Variable Load Boost? Idle? Etc. Distributor components and Ignition advance

  39. Most systems yse both. Even electronic systems may use mechanical advance to keep cap-pole in proper position May be up to 30 degrees! Advance Curves

  40. “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 Distributorless Ignitions

  41. Twin plug distributorless ignition.

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

  43. 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 time Long duration low A spark– more change of masking CoV Stages of Ignition

  44. Carburators Mechanical FI CIS EFI Single Port Multi Port Fuel SystemsMixture Prep

  45. 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 Manifold Issues –w- Carbs or single port

  46. Fuel Systems need to react to fuel needs for different operating conditions – Saw this with the “Fishhook Curves” Air Fuel Requirements and Load

  47. 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 Demands of Engine

  48. Back feed varies both jet and Venturi size Do not confuse with piston operated throttle valves British “Stromberg” See p195 for key Variable Jet Carburetor

  49. Sonstant venturi and jet(s) Fuel drawn by low P Discuss Fixed Jet Carburetor

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

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