Calibration 101

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Calibration 101

• John Bucknell
• DaimlerChrysler
• Powertrain Systems Engineering
• September 30, 2006

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Calibration

• What is it?
• Optimizing the control system (once hardware is
  finalized) for drivability, durability
  emissions
• Its just spark and fuel how hard could it be?
• Knowledge of Thermodynamics, Combustion and
  Control Theory all play in
• Fortunately race engines have no emissions
  constraints and use race fuel (usually eliminates
  any knock) therefore are relatively easy to
  calibrate

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Calibration Terms

• Stoichiometry Chemically correct ratio of fuel
  to air for combustion
• F/A Fuel/Air Ratio Mass ratio of mixture,
  a determination of richness or leanness.
  Stoichiometry 0.0688-0.0696 FA
• Lambda Excess Air Ratio Stoichiometry 1.0
  Lambda
• Rich F/A F/A greater than Stoichiometry Rich lt
  1.0 Lambda
• Lean F/A F/A less than Stoichiometry Lean gt
  1.0 Lambda

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Calibration Terms continued

• Brake Power Power measured by the absorber
  (brake) at the crankshaft
• BSFC - Brake Specific Fuel Consumption Fuel
  Mass Flow Rate / Brake Power grams/kW-h or
  lbs/hp-h
• LBT Fuelling Lean Best Torque
  Leanest Fuel/Air to Achieve
  Best Torque LBT 0.0780-0.0800 FA or 0.85-0.9
  Lambda
• Thermal Enrichment Fuel added for cooling due
  to exhaust component temperature limit
• Injector Pulse Width - Time Injector is Open

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Calibration Terms continued

• Spark Advance Timing in crank degrees prior to
  TDC for start of combustion event (ignition)
  
• MBT Spark - Maximum Brake Torque
  Minimum Spark Advance to Achieve
  Best Torque
• Burn Rate Speed of Combustion Expressed as
  a fraction of total heat released versus crank
  degrees
• MAP - Manifold Absolute Pressure Absolute
  not Gauge (which references barometer)

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Control System Types

• Alpha-N
• Engine Speed Throttle Angle
• Speed-Density
• Engine Speed and MAP/ACT
• MAF
• Engine Speed and MAF

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Alpha-N

• Fuel and spark maps are based on throttle angle
  which is very non-linear and requires complete
  mapping of engine
• Good throttle response once dialed in
• Density compensation (altitude and temperature)
  is usually absent needs to be recalibrated
  every time car goes out

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Speed-Density

• Fuel and spark maps are based on MAP density of
  charge is a strong function of pressure,
  corrected by air temp and coolant temp therefore
  air flow is simple to calculate
• Less time-intensive than Alpha-N, once calibrated
  is good most common type of control
• Needs less mapping can do WOT line and mid-map
  then curve-fit air flow (spark needs a little
  more in-depth for optimal control)

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MAF

• Fuel and spark maps are based on MAF airflow
  measured directly
• MAF sensor isnt the most robust device
• Pressure pulses confuse signal, each application
  has to be mapped with secondary damped MAF sensor
  (usually a 55 gallon drum inline)
• Least noisy signal is usually at air cleaner so
  separate transport delay controls need to be
  calibrated for transients and leaks need to be
  absolutely eliminated
• Boosted applications usually add a MAP as well

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Control System Components

• Fuel System
• Injectors, Fuel pump Regulator
• Basic Sensors
• Manifold Absolute Pressure (MAP) or Mass Air Flow
  (MAF)
• Crank Position (Rpm TDC)
• Cam Position (Sync)
• Air Charge Temp (ACT)
• Engine Coolant Temp (ECT)
• Knock Sensor
• Lamda Sensor

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Fuel System

• Injectors
• Volumetric flow rate solenoids, linear
  relationship between pulsewidth and flow for
  given pressure delta
• Battery offset is time necessary to open and
  close solenoid time is fixed for any voltage
• Duty cycle is injector on time itll go static
  above 95
• Bernoulli relationship for different pressure
  deltas allowing differing flow rates for a
  given injector
• High impedance injectors have lower dynamic range
  and lower amperage and thus less heat in
  controller
• Fuel Pump Regulator
• Pressure needs to be sufficiently high to prevent
  vapour lock (gt4bar) and low enough that engine
  can idle
• In-tank regulation adds least heat but has
  line-loss as flow rate increases, ie fuel
  pressure changes with flow
• Manifold-referenced regulation can help injectors
  achieve higher flow rates at elevated boost or
  lower flows at low vacuum making calibration
  more complicated

Bernoulli Effect of Fuel Pressure
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Sensors

• Manifold Absolute Pressure (MAP)
• A variable-resistance diaphragm with perfect
  vacuum on one side and manifold pressure on other
• Mass Air Flow (MAF)
• A heating element followed by a
  temperature-sensitive element. Heated element is
  maintained at a constant temperature and based
  upon the measured downstream temperature the mass
  flow rate can be determined.
• Crank Position
• High resolution for spark advance, less-so for
  crank speed and with once-per-rev can indicate
  TDC
• Cam Position
• Low resolution for syncronization for sequential
  fuel injection and individual cylinder spark
• Air Charge Temp and Engine Coolant Temp
• Thermistors used for air density correction and
  startup enrichment

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Sensors, cont

• Knock Sensor
• A piezoelectric load cell that measures
  structural vibration. Knock is a pressure wave
  that travels at local sonic velocity and rings
  at a frequency that is a function of bore
  diameter (typically between 14-18kHz). When the
  structure of the engine (typically the block) is
  hit with this pressure wave it rings as well, but
  at a frequency that is a function of the
  structure (ie materials and geometry). A FFT
  analysis of different mounting positions (nodes
  not anti-nodes) is necessary to determine the
  center frequency to listen for knock (which is
  measured via in-cylinder pressure measurements)
  without picking up other structure-borne noise.

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Sensors, cont

• Lamda Sensor (EGO)
• Compares ambient air to exhaust oxygen content
  (partial pressure of oxygen). Sensor output is
  essentially binary (only indicates rich or lean
  of stoichiometry).
• Wide-band Lamda Sensor (UEGO)
• Compares partial pressure of oxygen (lean) and
  partial pressure of HmCn, H2 CO (rich) with
  ambient. Gives output from 0.6 to 2 Lamda.

EGO Schematic
UEGO Schematic
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Calibration Goals

• Combustion Thermodynamics
• Work, Power Mean Effective Pressures
• Knock, Pre-Ignition
• Burn Rate
• Transients
• Wall film
• Thermal Enrichment
• Drivability

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• Combustion Terms
• Work
• Power Work/Unit Time
• Specific Power Power per unit displacement or
  weight
• Pressure/Volume Diagram Engineering tool to
  graph cylinder pressure

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• Combustion terms continued
• MEP Mean Effective Pressure
• Torque Normalized to Engine Displacement (VD)
• Average cylinder pressure over measuring period
• BMEP Brake Mean Effective Pressure
• IMEP Indicated Mean Effective Pressure
• MEP of Compression and Expansion Strokes
• PMEP Pumping Mean Effective Pressure
• MEP of Exhaust and Intake Strokes
• FFMEP Firing Friction Mean Effective Pressure
• BMEP IMEP PMEP FFMEP

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Combustion Terms continued

• Knock Autoignition of end-gasses in combustion
  chamber, causing extreme rates of pressure rise
• Knock Limit Spark - Maximum Spark Allowed due to
  Knock can be higher or lower than MBT
• Pre-Ignition Autoignition of mixture prior to
  spark timing, typically due to high temperatures
  of components (especially spark plug ground
  electrode)
• Combustion Stability Cycle to cycle variation
  in burn rate, trapped mass, location of peak
  pressure, etc. The lower the variation the
  better the stability.

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Knock

• Causes of Knock
• Knock f(Time,Temperature,Pressure,Octane)
• Time Higher engine speeds or faster burn rates
  reduce knock tendency. Burn rate can come from
  multiple spark sources, more compact combustion
  chambers or increased turbulence
• Temperature Reduced combustion temperatures
  reduce knock through reduced charge temperatures
  (cooler incoming charge or reduced residual
  burned gases), increased evaporative cooling from
  richer F/A mixtures and increased combustion
  chamber cooling
• Pressure Lower cylinder pressures reduce knock
  tendency through lower compression ratio or MAP
  pressure
• Octane Different fuel types have higher or
  lower autoignition tendencies. Octane value is
  directly related to knocking tendency

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Knock continued

• Effects of Knock
• Disrupts stagnant gases that form boundary layer
  at edge of combustion chamber, increasing heat
  transfer to components and raising mean
  combustion chamber temp that can lead to
  pre-ignition
• Scours oil film off cylinder wall, leading to dry
  friction and increased wear of piston rings
• Shockwave can induce vibratory loads into piston
  pin, piston pin bore and top land - reducing oil
  film thickness and accelerating wear
• Shockwave can be strong enough to stress
  components to failure

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In-cylinder Pressure Measurement

• Piezoelectric pressure transducers develop charge
  with changes in pressure
• Installed in combustion chamber wall or spark
  plug to measure full-cycle pressures

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Typical pressure probe installation
Passage drilled through deck face (avoiding
coolant jacket)
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Cylinder Pressure Trace No Knock
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Cylinder Pressure Trace Knock Limit or Trace
Knock - Best Power
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Cylinder Pressure TraceSevere Damaging Knock
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Pre-Ignition

• Effects of Pre-Ignition
• Increases peak cylinder pressure by beginning
  heat release too soon
• Increased cylinder pressure also increases heat
  load to combustion chamber walls, sustaining the
  pre-ignition
• Increases loads on piston crown and piston pin
• Sustained pre-ignition will typically put a hole
  in the center of the piston crown

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Burn Rate

• Burn Rate f(Spark, Dilution Rate/FA Ratio,
  Chamber Volume Distribution, Engine Speed/Mixture
  Motion/Turbulent Intensity)
• Spark
• Closer to MBT the faster the burn with trace
  knock the fastest
• Dilution Rate/FA Ratio
• Least dilution (exhaust residual or anything
  unburnable) fastest
• FA Ratio best rate around LBT
• Chamber Volume Distribution
• Smallest chamber with shortest flame path best
  (multiple ignition sources shorten flame path)
• Engine Speed/Mixture Motion/Turbulent Intensity
• Crank angle time for complete burn nearly
  constant with increasing engine speed indicating
  other factors speeding burn rate
• Mixture motion-contributed angular momentum
  conserved as cylinder volume decreases during
  compression stroke, eventually breaking down into
  vortices around TDC increasing kinetic energy in
  charge
• Turbulent Intensity a measure of total kinetic
  energy available to move flame front faster than
  laminar flame speed. More Turbulent Intensity
  equals faster burn.

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Combustion Thermodynamics Summary

• Peak Specific Power
• LBT fuelling for best compromise between
  available oxygen and charge density
• MBT spark if possible, fast burn rate assumed at
  peak load
• Highest engine speed to allow highest compression
  ratio
• Highest octane
• Peak Thermal Efficiency at desired load
• Highest compression ratio will have best
  combustion, usually with highest expansion ratio
  for best use of thermal energy
• MBT spark with fastest burn rate
• 10 lean of stoichiometry will provide best
  compromise between heat losses and pumping work,
  but not used because of catalyst performance
  impacts in pass cars

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Wall Film

• Liquid fuel does not burn, only fuel vapour
• Heat from somewhere must be used to make vapour
  which is why up to 500 more fuel must be used on
  a cold start to provide sufficient vapour for
  engine to run (relationship between temperature
  and partial pressure)
• Most of heat during fully warm operation comes
  from back side of intake valve and port walls
• Because of geometry a large portion of fuel wets
  wall this film travels at some fraction of free
  stream. Therefore some fuel from every pulse
  goes into engine and some onto port wall.
• On a fast acceleration, additional fuel must be
  added to offset the slowly moving wall film.
  Opposite true on decels.
• If injector is positioned far upstream volumetric
  efficiency increases due fuel heat of
  vapourization cooling incoming charge, but a
  large amount of wall is wetted leading to poor
  transient fuel control

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Injector Targeting
Bad Tip Location
Better Tip Location
Targets Valve
Targets Port Wall
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Thermal Enrichment

• Durability
• Combustion temperatures can reach 4000 deg K and
  drop to 1800 deg K before Exhaust Valve Opening
  (EVO)
• Materials must operate at sufficiently low
  temperature to maintain strength, so Exhaust Gas
  Temperature (EGT) limits must be adhered to for
  sufficient durability
• Usually 950 deg C runner temperature is
  acceptable for a developed package, as low as 800
  deg C for undeveloped components may be necessary
• Primary path for cooling is additional fuel
  beyond LBT, as heat of vapourization cools the
  charge before ignition (pressure-charged engines
  primarily)

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Drivability

• Throttle Response
• Drivers expect some repeatability of thrust
  versus pedal position some degree of spark
  mapping (retard) and pedal to throttle cam can
  help a drivers confidence
• Usually least developed and of most importance is
  tip-in (throttle closed to small opening) where
  torque can come in as a step change

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Closing Remarks

• Calibration is compromise
• Best spark for drivability may not produce
  sufficient combustion stability or fuel
  consumption
• Best fuelling for drivability is voracious fuel
  consumer - decel fuel shut off can improve
  economy by 20 but has tip-in torque bumps
  without careful calibration

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Q A