Chapter5 Compression Ignition Engines

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Chapter5 Compression Ignition Engines

• Two Types
• Direct Injection
• Indirect Injection
• DI Less air motion
• Less Turbulence
• Higher Injection Pressure
• Pressure-gtVelocity-gtTurbulence
• 1500 bar 22,000 PSI!
• Multi-Hole Nozzels
• Lower Speed Range
• WHY?

• ID
• Lower Pressure
• Single Hole Injector
• More complicated CC
• Smaller Engine Size
• Only 300Bar (1500 PSI)

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Injection Types

• Pretty much all we care about is fuel delivery!
• 2 Styles Pumps/Rails
• Injector Pumps
• In-line (expensive)
• Rotary (cheap, lowP)

• Alternatives
• Unit Pumps
• Have pump built into injector
• Common Rail
• Similar to SI MFI
• Can control both pressure and timing/duration in
  rate of injection

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Other CI engine Characteristics

• Turbocharging
• High Efficiency
• High Compression
• Variable Output Range
• Weight Reduction
• Discuss
• Reduces CR to 121
• NA up tp 241

• Min Volume 400CC, otherwise surface to volume
  wont work
• Slow combustion lower RPM limit
• Output increased by displacement or turbocharging
• Highly efficient
• CR often driven by cold start ability
• Better economy would be possible with lower CR
  but need higher to start engine

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Other charactoristics

• Automotive may use high RPM to keep size down
• TDI Beetle 1.9 ltr.
• 4500 RPM
• 150 Ft-lbs Torque
• 95 HP
• 60 MPG!
• Why is low RPM range inadvisable for automotive
  apps?

• Marine-Industrial large size and high efficiency
• Narrow Operating Range
• Huge Weight!
• Excellent fuel economy with minimal heat transfer
• Adiabatic Engine?

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Efficiency Comparison (2000 RPM)
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DI Systems

• Flat Head
• Symmetric
• CC completely in head
• Swil very important.
• Why?

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Swirl and CC design

• Swirl Ratio
• Swirl Speed/RPM
• Read about how to measure in book
• Swirl robs engine of KE Effective pumping loss,
  and Vefficiency! Trade off
• Swirl increases heat transfer (bad!)
• Ususally 2V, but now using 4V w- throttle plate
• Squish can be employed and CCs are usually
  coaxial I.e. symmetric
• Coaxial CC allows for cons.angular momentum
• Compact CC reduces heat transfer
• Undersquare engines-easy to get higher CR and
  more torque at low RPMs

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Swirl Ratio and SFC

• Note peak at about 10
• SFC Down, BMEP up.
• More efficient combustion vs. Voilumetric
  Efficiency loss and increase in KE

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Comparison of Piston Speed

• Why problem? Slow flame propogation!

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Indirect Injection Systems

• Divided Combustion chambers speed combustion
  process
• This increases engine output by increasing max
  speed
• Also creates turbulence with dense charge
  propagating into cylinder area
• Swirl chambers rely on air flowing readily into
  swirl chamber to promote quick combustion
• Swirl Chamber has NO CC in cylinder!

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Precombustion chambers

• Precombustion chambers cause turbulence
• Used by Mercedes Benz
• Better economy as dense charge expands through
  nozzle into main CC.
• These have CC in cylender

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Comet Combustion Chamber

• Most advanced is recardo swirl chamber
• Combination of swirl chamber and precombustion
• Note controlled flow into cylinder
• High CRs needed for cold start why? Swirl is
  very low at low RPMs

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Performance

• Note loss of efficency as RPMs diverge from
  optimal

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Air Cell Who Cares

• Discussed oin book for no apparent reason.
• Says it stinks so ignore it.

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Cold Starting CI Engines

• Speed critical as piston reaches top of stroke
• DANG hard for a starter to drive at constant
  speed
• Several solutions
• May use large flywheel
• Pressure Release (decompression)
• May use extra injection
• Glow Plugs
• Volatile Starter Agent (ether)
• Heated Air
• Real pin in the butt
• BUT Once one fires the rest catch quickly

• May require higher CR to start cold
• Variable CR pistons?
• In your dreams
• Hard to start
• Low turbulence
• Low pressure (leakage)
• Low temp (both because of ambient block temp and
  lower pressure)
• More time for heat transfer P down

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Starter Mechanisms

• Glow Plugs
• Single filament
• Wired In Series
• Unshelded
• Nightmare if one fails
• Ignition occurs by spraying fuel DIRECTLY on the
  heater cores
• Surface rather than bulk ignition as source much
  more efficient

• Excess Fuel Enrichment
• Easier to fire
• Raises CR (fuel is incompressible)
• Seals piston rings by wetting cylinders

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Summary DI vs ID (4/6/01)

• ID
• Higher RPM Rapid Combustion
• Only works 400-800cc/cyl (1.4 4 cyl to 6.4 ltr
  V8)
• Reduced ignition delay
• More swirl
• 5-15 fuel efficiency penalty
• More complicated combustion chamber design
• May require ceramic liner in pre chambers to
  limit heat transfer

• DI
• Lower RPM limited by piston speed (flame front
  must keep up with piston)
• Longer ignition Delay
• More efficient
• Unlimited size
• Injectors exposed directly to cylinder pressures
• More exotic injectors required

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DI Engines Governed by fuel injection systems
DI lower SFC
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Fuel Injection Systems
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5.5 CI Fuel Injection Systems

• Traditional
• Plunger In Barrel (like Mech FI)
• Connected with thick walled lines
• Account for delay
• Mechanical Controls
• Rotary Pumps
• Similar to oil pump
• Modern Systems
• Eliminate high pressure fuel lines and associated
  delays
• Unit Injectors
• Pump and injector in one unit (may use engine oil
  as pumping medium)
• Common Rail Injectors
• Similar to EFI
• Steady high pressure
• Electronically controlled injectors

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Requirements of CI FI system

• Ability to change power at each RPM
• no throttle!
• Ability to change volume with RPM
• RPM dependant at constant loading
• Ability to change advance with RPM
• Because of ignition delay
• Ability to change advance with loading

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Pump Delivery Rate Variance with RPM/Throttle

• Max fuel limit is smoking
• 100 limit is pumping capacity
• Note this is PER STROKE
• Efficiency peak for engine and pump

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General Layout of CI FI System (5.5.1)

• Injection Pump usually mechanical drive
• Belts and rollers not good, use gears and chains
• Note spill line from injector, pump, separator

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General Characteristics (5.5.1)

• Pump runs at ½ engine speed
• Controls Quantity AND timing of injection
• Max fuel limited by smoke limit
• Timing varies with load and speed
• Timing accurate to 1o crank angle

• How does timing vary with load?
• Ignition delay is SHORTER (higher density) BUT
• Although ignition delay is shorted, still need
  more advance to ensure all fuel is burnt during
  stroke
• At max load fuel variance among cylinders should
  be less than 3 otherwise power limited by smoky
  exhaust of richest cyl.

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Fuel Injectors (5.5.2)

• Nozzle type dictates performance
• Single Hole
• Good for ID
• 1mm hard to clog
• Multi hole
• Better misting
• Easy clog as size -gt 0.1mm
• Clogs caused by decomp of leaked fuel
• Differential pressures cause opening
• Note needle design pressure OPENS nozzle
• Differential pressures
• f(needle diameter vs. seat diameter)
• Spring closing
• Harder to open than to keep open
• Smaller seat contact area and strong spring
  enhance sealing, eliminate dribble
• Dribble leads to emissions and deposits

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Operation of needle

• This is why its easier to keep a needle open
  than to open it initially
• Good idea to provide pressure release mechanism
  to fast and accurate closing

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Pintle Nozzle

• Excellent disbursement, provides conical spray
  pattern
• Looks Similar to that used in CIS systems
• Opens UPWARD
• Excellent clog resistance

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More Injector Considerations

• Aux hole to bleed excess fuel and prevent
  deposits
• 4V Heads
• Upside
• Vf Up
• Central injector position
• Downside
• Less swirl
• More nozzle holes for good disbursion/combustion,
  as small as 0.1 mm
• Nozzles cooled by fuel
• Cooling important to maintain tolerances and
  sealing
• Spray Pattern Critical!
• Aspect Ratio of 2-8
• Larger Aspect Ratio more penetration
• Larger Aspect ratio Smaller cone
• Atomization up w- velocity, but restricts
  penetration as well

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Components Notes
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Pilot Injection

• Small Amount of fuel early to initiate flame
  front
• Allows for large advance
• Eliminates knock and corresponding problems
  associated with high peak pressures and wave
  impingement
• 2 Spring Special injector needed for 2 mode
  operation

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In-Line Pumps

• Driven from crank ½ speed
• Multi-lobe cam
• This example uses rack, not lever (see next
  slide)
• Rack rotates plunger assy and controls flow
• Governor and advance coupling driven by rotating
  weights acting against a spring (like mechanical
  advance on distributor)

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Plunger Design Traditional Injection Pump

• Plunger forces fuel through fitting
• Rotating Lever controls how much spills back
  lever controls fuel flow (no throttle)
• All run by cam driven by crank

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Plungers

• Operation
• Plunger moves up and blocks inlet
• Fuel is allowed to escape through spill port
  (notice helical grove)
• Reminder of fuel forced out outlet port
• Stroke is constant by delivery varied by rotation

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Leakage Pressure

• Leakage effects accurate metering
• Proportional to
• Fuel Density
• Pressure Differential
• Diameter
• Clearance (cyl/barrel)
• Reciprocal of viscocity
• Pressure increases with
• Load (more fuel through outlet)
• RPM (viscous effects)
• Both conspire against accurate fuel metering

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Delivery valve operation

• Delivery Valve used to shift pressure curve
  allowing for more accurate opening and closing of
  injector

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A pump aint so simple!
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Layout of conventional fuel system
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Rotary Pump

• Much less complicated but lower pressures
• Few moving parts
• Fed by transfer pump
• Metering through governor mechanism rotor
  slides
• Pressurization via sliding pistons

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Typical Rotary Pump
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Schematic of diesel control 4/12/01

• A bunch of sensors
• Engine Temp
• Boost
• Air Temp
• Speed/TDC
• Pedal Position
• Not throttle!
• Output
• Fuel Volume
• Timing

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Pressure Comparison
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Electronic Unit Injection

• Electronic Unit Injection
• Solenoid Controlled
• So fast pilot injection can be used
• Expensive to produce
• Widely used in heavy truck where emissions and
  economy are critical
• Controlled just like SI EFI

• Variation is HEUI

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HEUI

• Hydraulic Electronic Unit Injection
• Uses Hydraulic pressure from engine oil
• Pumps oil to high pressure (like common rail)
• Intensifier mulAtiplies hydraulic pressure using
  piston
• Selinoid controls oil flow
• Used on
• Caterpillar
• VW TDI (Golf/Bug)
• Navistar (Ford Powerstroke)

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Common Rail Systems

• Like SI EFI
• Very high pressures
• Uses EMS
• Engine
• Management
• System

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CR System - High Pressure Pump

• 150-1600 bar
• Inlet flow is controlled
• Higher efficiency
• Only pumps pressure it needs
• Similar to rotary pump in that it uses cam/roller
  to provide positive displacement

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Electro hydraulic Injector
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EMS contrrols flow

• Inputs w- sensors
• Engine Temp
• Boost
• Air Temp
• Speed/TDC
• Pedal Position
• Not throttle!
• Output
• Fuel Volume
• Timing

• Can operate in up to four phases
• 2 pilot injections
• Main injection
• Post Injection
• Post injection can add fuel to provide HC needed
  to reduce Nox

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Emissions Read about it

• There will be homework on emissions