Powertrain 101

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Powertrain 101

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

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Powertrain Topics

• Background
• Powertrain terms
• Thermodynamics
• Mechanical Design
• Combustion
• Architecture
• Cylinder Filling Emptying
• Momentum
• Pressure Wave
• Aerodynamics
• Flow Separation
• Wall Friction
• Junctions Bends

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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

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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, internal combustion has less
  inefficiencies
• Heat engines using working fluids which is the
  simplest of all energy conversion methods

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Reciprocating Internal Combustion Heat Engines

• Characteristics
• Slider-crank mechanism has high mechanical
  efficiency
• 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

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• 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)

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Further explanation of aspects of Compression
Ratio
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• Reciprocating Engines
• Most layouts created during second World War as
  aircraft manufacturers struggled to make the
  least-compromised installation

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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

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• 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

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Indicated Work
TDC
BDC
Source Design and Sim of Four Strokes
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Pumping Work
TDC
BDC
Source Design and Sim of Four Strokes
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Spark Ignition

• 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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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 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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Combustion Summary

• 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
• 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

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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
• 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)
• 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 (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
  are best
• 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
• Most engines are designed with higher compression
  than is best for combustion stability because of
  the associated part-load BSFC benefits

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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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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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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 with
  existing engine architectures

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Q A