Powertrain & Calibration 101

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Powertrain Calibration 101

• John Bucknell
• DaimlerChrysler
• Powertrain Systems Engineering
• December 4, 2006

2
Powertrain Calibration Topics

• Background
• Powertrain terms
• Thermodynamics
• Mechanical Design
• Combustion
• Architecture
• Cylinder Filling Emptying
• Aerodynamics
• Calibration
• Spark Fuel
• Transients Drivability

3
What is a Powertrain?

• Engine that converts thermal energy to mechanical
  work
• Particularly, the architecture comprising all the
  subsystems required to convert this energy to
  work
• Sometimes extends to drivetrain, which connects
  powertrain to end-user of power

4
Characteristics of Internal Combustion Heat
Engines

• High energy density of fuel leads to high power
  to weight ratio, especially when combusting with
  atmospheric oxygen
• External combustion has losses due to multiple
  inefficiencies (primarily heat loss from
  condensing of working fluid), internal combustion
  has less inefficiencies
• Heat engines use working fluids which is the
  simplest of all energy conversion methods

5
Reciprocating Internal Combustion Heat Engines

• Characteristics
• Slider-crank mechanism has high mechanical
  efficiency (piston skirt rubbing is source of
  50-60 of all firing friction)
• Piston-cylinder mechanism has high single-stage
  compression ratio capability leads to high
  thermal efficiency capability
• Fair to poor air pump, limiting power potential
  without additional mechanisms

6

• Reciprocating Engine Terms
• Vc Clearance Volume
• Vd Displacement or Swept Volume
• Vt Total Volume
• TC or TDC
• Top or Top Dead Center Position
• BC or BDC
• Bottom or Bottom Dead Center Position
• Compression Ratio (CR)

7
Further explanation of aspects of Compression
Ratio
8

• Reciprocating Engines
• Most layouts created during second World War as
  aircraft manufacturers struggled to make the
  least-compromised installation

9
Thermodynamics

• Otto Cycle
• Diesel Cycle
• Throttled Cycle
• Supercharged Cycle

Source Internal Comb. Engine Fund.
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• Thermodynamic Terms
• MEP Mean Effective Pressure
• Average cylinder pressure over measuring period
• Torque Normalized to Engine Displacement (VD)
• 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

11

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

12
Indicated Work
TDC
BDC
Source Design and Sim of Four Strokes
13
Pumping Work
TDC
BDC
Source Design and Sim of Four Strokes
14
History of Internal Combustion

• 1878 Niklaus Otto built first successful four
  stroke engine
• 1885 Gottlieb Daimler built first high-speed four
  stroke engine
• 1878 saw Sir Dougald Clerk complete first
  two-stroke engine (simplified by Joseph Day in
  1891)

1891 Panhard-Levassor vehicle with front engine
built under Daimler license
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Energy Distribution in Passenger Car Engines
Source SAE 2000-01-2902 (Ricardo)
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Using Exhaust Energy

• Highest expansion ratio recovers most thermal
  energy
• Turbines can recover heat energy left over from
  gas exchange
• Energy can be used to drive turbo-compressor or
  fed back into crank train

Source Advanced Engine Technology
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Supercharging

• Increases specific output by increasing charge
  density into reciprocator
• Many methods of implementation, cost usually only
  limiting factor

Source Internal Comb. Engine Fund.
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Mechanical Design
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Two Valve Valvetrain

• Pushrod OHV (Type 5)

• HEMI 2-Valve (Type 5)

• SOHC 2-Valve (Type 2)

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Four Valve Valvetrain

• DOHC 4-Valve (Type 2)

• SOHC 4-Valve (Type 3)

• DOHC 4-Valve (Type 1)

• Desmodromic

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Valvetrain

• Specific Power f(Air Flow, Thermal
  Efficiency)
• Air flow is an easier variable to change than
  thermal efficiency
• 90 of restriction of induction system occurs in
  cylinder head
• Cylinder head layouts that allow the greatest
  airflow will have highest specific power
  potential
• Peak flow from poppet valve engines primarily a
  function of total valve area
• More/larger valves equals greater valve area

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Combustion Terms

• 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 component temperature limit
• Injector Pulse Width - Time Injector is Open

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Combustion Terms continued

• Spark Advance Timing in crank degrees prior to
  TDC for start of combustion event (ignition)
  
• MBT Spark Maximum Brake Torque Spark
  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 (does not reference barometer)

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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
• 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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Engine Architecture Influence on Performance

• Intake Exhaust Manifold Tuning
• Cylinder Filling Emptying
• Momentum
• Pressure Wave
• Aerodynamics
• Flow Separation
• Wall Friction
• Junctions Bends
• Induction Restriction
• Exhaust Restriction (Backpressure)
• Compression Ratio
• Valve Events

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Intake Tuning for WOT Performance

• Intake manifolds have ducts (runners) that tune
  at frequencies corresponding to engine speed,
  like an organ pipe
• Longer runners tune at lower frequencies
• Shorter runners tune at higher frequencies
• Tuning increases local pressure at intake valve
  thereby increasing flow rate
• Duct diameter is a trade-off between velocity and
  wall friction of passing charge

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Exhaust Tuning for WOT Performance

• Exhaust manifolds tune just as intake manifolds
  do, but since no fresh charge is being introduced
  as a result not as much impact on volumetric
  efficiency (8 maximum for headers)
• Catalyst performance usually limits production
  exhaust systems that flow acceptably with little
  to no tuning

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Tuned Headers
Tuned Headers generally do not appear on
production engines due to the impairment to
catalyst light-off performance (usually a minimum
of 150 additional distance for cold-start
exhaust heat to be lost). Performance can be
enhanced by 3-8 across 60 of the operating
range.
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Momentum Effects

• Pressure loss influences dictate that duct
  diameter be as large as possible for minimum
  friction
• Increasing charge momentum enhances cylinder
  filling by extending induction process past
  unsteady direct energy transfer of induction
  stroke (ie piston motion)
• Decreasing duct diameter increases available
  kinetic energy for a given mass flux
• Therefore duct diameter is a trade-off between
  velocity and wall friction of passing charge

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Pressure Wave Effects

• Induction process and exhaust blowdown both cause
  pressure pulsations
• Abrupt changes of increased cross-section in the
  path of a pressure wave will reflect a wave of
  opposite magnitude back down the path of the wave
• Closed-ended ducts reflect pressure waves
  directly, therefore a wave will echo with same
  amplitude

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Pressure Wave Effects cont

• Friction decreases energy of pressure waves,
  therefore the 1st order reflection is the
  strongest but up to 5th order have been
  utilized to good effect in high speed engines
  (thus active runners in F1 in Y2K)
• Plenums also resonate and through superposition
  increase the amplitude of pressure waves in
  runners small impact relative to runner
  geometry

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Effects of Intake Runner Geometry
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Tuning in Production I4 Engine
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Aerodynamics

• Losses due to poor aerodynamics can be equal in
  magnitude to the gains from pressure wave tuning
• Often the dominant factory in poorly performing
  OE components
• If properly designed, flow of a single-entry
  intake manifold can approach 98 of an ideal
  entrance on a cylinder head port (steady state on
  a flow bench)

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Aerodynamics cont

• Flow Separation
• Literally same phenomenon as stall in wing
  elements pressure in free stream insufficient
  to push flow along wall of short side radius
• Recirculation pushes flow away from wall, thereby
  reducing effective cross-section so-called vena
  contracta
• Simple guidelines can prevent flow separation in
  ducts studies performed by NACA in the 1930s
  empirically established the best duct
  configurations

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Aerodynamics cont

• Wall Friction
• Surface finish of ducts need to be as smooth as
  possible to prevent tripping of flow on a macro
  level
• Junctions Bends
• Everything from your fluid dynamics textbook
  applies
• Radiused inlets and free-standing pipe outlets
• Minimize number of bends
• Avoid S bends if at all possible

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Induction Restriction

• Air cleaner and intake manifolds provide some
  resistance to incoming charge
• Power loss related to restriction almost directly
  a function of ratio between manifold pressure
  (plenum pressure upstream of runners) and
  atmospheric

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Exhaust Restriction
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Compression Ratio

• The highest possible compression ratio is always
  the design point, as higher will always be more
  thermally efficient with better idle quality
• Knock limits compression ratio because of
  combustion stability issues at low engine speed
  due to necessary spark retard
• Most engines are designed with higher compression
  than is best for low speed combustion stability
  because of the associated part-load BSFC benefits
  and high speed power

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Valve Events

• Valve events define how an engine breathes all
  the time, and so are an important aspect of low
  load as well as high load performance
• Valve events also effectively define compression
  expansion ratio, as compression will not
  begin until the piston-cylinder mechanism is
  sealed same with expansion

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Valve Event Timing Diagram

• Spider Plot - Describes timing points for valve
  events with respect to Crank Position
• Cam Centerline - Peak Valve Lift with respect to
  TDC in Crank Degrees

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Valve Events for Power

• Maximize Trapping Efficiency
• Intake closing that is best compromise between
  compression stroke back flow and induction
  momentum (retard with increasing engine speed)
• Early intake closing usefulness limited at low
  engine speed due to knock limit
• Early intake opening will impart some exhaust
  blowdown or pressure wave tuning momentum to
  intake charge
• Maximize Thermal Efficiency
• Earliest intake closing to maximize compression
  ratio for best burn rate (optimum is
  instantaneous after TDC)
• Latest exhaust opening to maximize expansion
  ratio for best use of heat energy and lowest EGT
  (least thermal protection enrichment beyond LBT)

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Valve Events for Power

• Minimize Flow Loss
• Achieve maximum valve lift (max flow usually at
  L/D gt 0.25-0.3) as long as possible (square lift
  curves are optimum for poppet valves)
• Minimize Exhaust Pumping Work
• Earliest exhaust opening that blows down cylinder
  pressure to backpressure levels before exhaust
  stroke (advance with increasing engine speed)
• Earliest exhaust closing that avoids
  recompression spike (retard with increasing
  engine speed)

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Engine Power and BSFC vs Engine Speed
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Summary

• Components Relative Impact on Performance
• Cylinder Head Ports Valve Area
• Valve Events
• Intake Manifold Runner Geometry
• Compression Ratio
• Exhaust Header Geometry
• Exhaust Restriction
• Air Cleaner Restriction

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Powertrain Closing Remarks

• Powertrain is compromise
• Four-stroke engines are volumetric flow rate
  devices the only route to more power is
  increased engine speed, more valve area or
  increased charge density
• More speed, charge density or valve area are
  expensive or difficult to develop therefore
  minimizing losses is the most efficient path
  within existing engine architectures
• Highest average power during a vehicle
  acceleration is fastest peak power values dont
  win races

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Break
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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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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 components, sustaining
  the pre-ignition (leading to run-away
  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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Transient Fuelling

• 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 of fuel fractions)
• 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 and resolution
  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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References

• Internal Combustion Engine Fundamentals, John B
  Heywood, 1988 McGraw-Hill
• The Design and Tuning of Competition Engines
  Sixth Edition, Philip H Smith, 1977 Robert
  Bentley
• The Development of Piston Aero Engines, Bill
  Gunston, 1993 Haynes Publishing
• Design and Simulation of Four-Stroke Engines,
  Gordon P. Blair, 1999 SAE
• Advanced Engine Technology, Heinz Heisler, 1995
  SAE
• Vehicle and Engine Technology, Heinz Heisler,
  1999 SAE

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