Chapter IV Spark Ignition Engines 22703

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
SFC BMEP w.r.t. f

• Must have f 1 to use all O2
• Unburnt gas
• Efficiency down

• Min sfc at 0.9
• Max BMEP a 1.08
• What do we do?
• Why is BMEP at f 1?

4
Sfc vs. BMEP for various A/F

• 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

5
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

6
3/2/03 Ignition Timing Optimization

• 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

7
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
• Do not confuse PMEP with Compression work!
• Part throttle P down and T down, flame travel
  slower, so more advance is needed

8
Combustion Chamber Design

• Flathead Optimized
• Because of design limited to 61
• 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

9
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

10
Combustion Chamber Considerations (contd)

• 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

11
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

12
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

13
Optimized Chamber Design

• Depends on goals! Economics vs Perf.

14
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

15
Hemispherical Head

• 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

16
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?

17
Bath-Tub Head

• Compact Chamber
• Circumferential Squish
• Better swirl than wedge

18
3/6/02 Efficiency Curves

• Mechanical Efficiency vs Cycle Efficiency. Is
  Otto Cycle realistic?

• Efficiency at Max power vs Max Economy

19
3/6/02 High Compression Ratio Fast Burn Designs

• Compact
• Q down
• Concentrated _at_ 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

• High Compression -w- ordinary fuels?
• High turbulence
• Lean burn
• Compact
• Turbulence Up
• Leaner burn
• Why?
• Rapid Combustion
• Less Knock Susceptibility

20
Design Considerations Econ Emissions

• 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

• Economy
• Generally good due to high CR possible, up to
  141
• Good power dues to quick efficient combustion
• Good due to lean burn

21
More CC designs

• Straight inlet tracts
• Not offset

• HRCC similar to may fireball but has straight
  inlet passage

22
4 Valve Pent Roof

• Large Flow Area why?
• Do some calculations
• 2V Flat or wedge
• Max dD/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

23
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

24
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 f 0.6

25
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

26
Advanced Combustion Systems

• 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

27
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

28
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

29
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
• f as low as 0.66
• Low BSFC (12 lower than stochiometric)
• Performance Mode
• Operates like Pent Roof

30
VTEC Control Modes
31
VTEC Design

• Bowl in piston (55mm/75mm bore)
• Pent Roof Design
• Allows AFR to be extended by 2 compared to flat
  top (I.e.16.71 not 14.7.1) from shape alone
  compact combustion chamber!
• One valve opened doubles flow velocities, w,
  increased, swirl strength and momentum increased.

32
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

33
VTEC

• Engine Management Strategy
• 3 Modes
• Very Lean 221 (Idle torque cruise)
• Stochiometric 14.7 (Below Idle and high Speed)
• Rich 12.51 (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

34
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

35
Chapter 4, Part II Ignition and Fuel systems

• The ignition process
• How the spark occurs and how its generated

36
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???

37
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

38
Distributor components and Ignition advance

• Both Mechanical and Vacuum Advance/Retard
• Why is this necessary?
• Variable RMP
• Variable Load
• Boost? Idle? Etc.

39
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!

40
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

41
Twin plug distributorless ignition.
42
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

43
Stages of Ignition

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

• 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.

44
Fuel SystemsMixture Prep

• Carburators
• Mechanical FI
• CIS
• EFI
• Single Port
• Multi Port

45
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

46
Air Fuel Requirements and Load

• Fuel Systems need to react to fuel needs for
  different operating conditions Saw this with
  the Fishhook Curves

47
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

48
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

49
Fixed Jet Carburetor

• Sonstant venturi and jet(s)
• Fuel drawn by low P
• Discuss

50
Fuel flow with fixed jet carb

• These are the flow characteristics due to vacuum
  Venturi effects only
• What problems does this cause?

51
Flow through Venturi Incompressible vs, Comp flow
52
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

53
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 smooth running

54
Carburetion 2 3 systems combined

• Combined flow from Primary and Main, mixed with
  Idle circuits

55
Complete carburetion system
56
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 501

57
SFI

• Cheap, about 10 less power than multi port
• Allows for computer controls
• Back feed regulator

58
MFI Injection in inlet port

• Inject to back of valve
• Cools Valve
• Vaporizes Fuel
• Must have multi-channel 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 its closed)

59
Schematic (SFI or MFI)
60
Distribution of droplet size Part Load
61
Distribution of droplet sizeFull Load
62
SFC Map

• Note BMEP relationship