power train c

7/29/2019 Power Train c

1/85

Powertrain &

Calibration 101

John BucknellDaimlerChrysler

Powertrain Systems Engineering

December 4, 2006


2/85

Powertrain & Calibration Topics Background

Powertrain terms Thermodynamics Mechanical

Design Combustion

Architecture Cylinder Filling &

Emptying Aerodynamics

Calibration Spark & Fuel Transients &

Drivability


3/85

What is a Powertrain?

Engine that converts thermal energy tomechanical work

Particularly, the architecture comprising allthe subsystems required to convert thisenergy to work

Sometimes extends to drivetrain, whichconnects powertrain to end-user of power


4/85

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 fromcondensing of working fluid), internal

combustion has less inefficiencies

Heat engines use working fluids which is thesimplest of all energy conversion methods


5/85

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


6/85

Reciprocating Engine TermsVc = Clearance Volume

Vd = Displacement or Swept Volume

Vt = Total Volume

TC or TDC =

Top or Top Dead Center PositionBC or BDC =

Bottom or Bottom Dead CenterPosition

Compression Ratio (CR)

c

cd

V

VVCR


7/85

Further explanation of aspects of Compression Ratio


8/85

ReciprocatingEngines

Most layouts createdduring second WorldWar as aircraft

manufacturersstruggled to make theleast-compromisedinstallation


9/85

Thermodynamics

Otto Cycle

Diesel Cycle

Throttled Cycle

Supercharged Cycle

Source: Internal Comb. Engine Fund.


10/85

Thermodynamic TermsMEP 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 StrokesFFMEP Firing Friction Mean Effective Pressure

BMEP = IMEP PMEP FFMEP

)liter(V

)Nm(Torque4)kPa(BMEP

D

.)in.cu(V

)ftlb(Torque48)psi(BMEP

D


11/85

Thermodynamic Terms continued

Work=

Power = Work/Unit Time

Specific Power Power per unit, typically

displacement or weightPressure/Volume Diagram Engineering

tool to graph cylinder pressure

dVP

Cycle/volutionsRe

Second/CyclesWorkPower


12/85

Indicated Work

TDC BDC

Source: Design and Sim of Four Strokes


13/85

TDC BDC

Source: Design and Sim of Four Strokes

Pumping Work


14/85

History of Internal Combustion

1878 Niklaus Otto builtfirst successful fourstroke engine

1885 Gottlieb Daimlerbuilt first high-speedfour stroke engine

1878 saw Sir DougaldClerk complete first two-stroke engine (simplifiedby Joseph Day in 1891) 1891 Panhard-Levassor vehicle

with front engine built underDaimler license


15/85

Energy Distributionin Passenger Car Engines

Source: SAE 2000-01-2902 (Ricardo)


16/85


17/85

Source: Advanced Engine Technology

Using Exhaust Energy

Highest expansionratio recovers mostthermal energy

Turbines can recoverheat energy left overfrom gas exchange Energy can be used to

drive turbo-compressor or fedback into crank train


18/85

Source: Internal Comb. Engine Fund.

Supercharging Increases specific

output by increasingcharge density intoreciprocator

Many methods ofimplementation, costusually only limiting

factor


19/85

Mechanical Design


20/85


21/85

Two Valve Valvetrain

Pushrod OHV (Type 5) HEMI 2-Valve (Type 5) SOHC 2-Valve (Type 2)


22/85

Four Valve Valvetrain

SOHC 4-Valve (Type 3) DOHC 4-Valve (Type 2)

DOHC 4-Valve (Type 1)

Desmodromic


23/85

Specific Power =f(Air Flow, Thermal Efficiency)

Air flow is an easier variable tochange than thermal efficiency

90% of restriction of inductionsystem occurs in cylinder head

Cylinder head layouts thatallow the greatest airflow willhave highest specific powerpotential

Peak flow from poppet valveengines primarily a function oftotal valve area

More/larger valves equalsgreater valve area

Valvetrain


24/85

Combustion Terms

Brake Power Power measured by the absorber(brake) at the crankshaft

BSFC - Brake Specific Fuel Consumption

Fuel Mass Flow Rate / Brake Powergrams/kW-h or lbs/hp-h

LBT Fuelling - Lean Best TorqueLeanest 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


25/85

Combustion Terms continued

Spark AdvanceTiming in crank degrees prior toTDC for start of combustion event (ignition)

MBT Spark Maximum Brake Torque SparkMinimum Spark Advance to Achieve Best Torque

Burn Rate Speed of CombustionExpressed as a fraction of total heat released versus

crank degrees

MAP - Manifold Absolute PressureAbsolute not Gauge (does not reference barometer)


26/85

Combustion Terms continued

Knock Autoignition of end-gasses in combustionchamber, causing extreme rates of pressure rise.

Knock Limit Spark - Maximum Spark Allowed due toKnock can be higher or lower than MBT

Pre-IgnitionAutoignition of mixture prior to sparktiming, typically due to high temperatures ofcomponents

Combustion StabilityCycle to cycle variation inburn rate, trapped mass, location of peak pressure, etc.The lower the variation the better the stability.


27/85

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


28/85

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 intakevalve thereby increasing flow rate

Duct diameter is a trade-off betweenvelocity and wall friction of passing charge


29/85

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 muchimpact on volumetric efficiency (~8%maximum for headers)

Catalyst performance usually limitsproduction exhaust systems that flowacceptably with little to no tuning


30/85

Tuned Headers

Tuned Headersgenerally do notappear on productionengines due to theimpairment to catalyst

light-off performance(usually a minimum of150% additionaldistance for cold-startexhaust heat to belost). Performancecan be enhanced by3-8% across 60% ofthe operating range.

WOT IMEP Exhaust Manifold Comparison4-2-1 Tubular Header vs 4-1 Close Coupled Cast

1000

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

Engine Speed (rpm)

IMEP(kPa)/PMEP(

kPa)

-150

-135

-120

-105

-90

-75

-60

-45

-30

-15

0

IMEP 4-2-1 1044.1 1122.8 1188.5 1226.6 1269.2 1290.5 1337.9 1390.1 1445.7 1427 1445.8 1435.4 1411.7 1337.9

IMEP 4-1 Cast 1102.5 1162.2 1225.5 1252.3 1248 1262.4 1320.9 1403.6 1403.5 1406.3 1398 1367.2 1294.6

PMEP 4-2-1 -5.3 -9.7 -14.2 -19.7 -23.0 -29.9 -38.4 -52.3 -64.0 -78.5 -90.8 -107.9 -122.8 -136.2

PMEP 4-1 Cast -12.5 -16.8 -20.8 -26.1 -32.0 -40.3 -54.0 -68.6 -81.0 -89.0 -99.8 -111.5 -119.5

1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200 5600 6000 6400


31/85

Momentum Effects Pressure loss influences dictate that duct

diameter be as large as possible for minimumfriction

Increasing charge momentum enhances cylinder

filling by extending induction process pastunsteady direct energy transfer of inductionstroke (ie piston motion)

Decreasing duct diameter increases availablekinetic energy for a given mass flux

Therefore duct diameter is a trade-off betweenvelocity and wall friction of passing charge


32/85

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 reflecta wave of opposite magnitude back downthe path of the wave

Closed-ended ducts reflect pressure wavesdirectly, therefore a wave will echo withsame amplitude


33/85

Pressure Wave Effects cont Friction decreases energy of pressure

waves, therefore the 1st order reflection isthe strongest but up to 5th order have

been utilized to good effect in high speedengines (thus active runners in F1 in Y2K)

Plenums also resonate and throughsuperposition increase the amplitude of

pressure waves in runners small impactrelative to runner geometry


34/85

Effects of Intake Runner Geometry


35/85

Tuning in Production I4 Engine

350

370

390

410

430

450

470

Engine Speed (rpm)

AirMassperCylinder(mg)

Trapped Mass 372 381 373 421 428 402 397 430 454 453 458 460 431 401

1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200 5600 6000 6400


36/85

Aerodynamics Losses due to poor aerodynamics can be

equal in magnitude to the gains frompressure wave tuning

Often the dominant factory in poorlyperforming OE components

If properly designed, flow of a single-entryintake manifold can approach 98% of anideal entrance on a cylinder head port(steady state on a flow bench)


37/85

Aerodynamics cont Flow Separation

Literally same phenomenon as stall in wingelements pressure in free stream insufficientto 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 separationin ducts studies performed by NACA in the1930s empirically established the best ductconfigurations


38/85

Aerodynamics cont Wall Friction

Surface finish of ducts need to be as smoothas possible to prevent tripping of flow on amacro 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


39/85

Induction Restriction

Air cleaner and intake manifolds providesome resistance to incoming charge

Power loss related to restriction almostdirectly a function of ratio betweenmanifold pressure (plenum pressureupstream of runners) and atmospheric


40/85

Exhaust RestrictionBack Pressure Effects on Peak Power - 2.0L SOHC R/T

145

146

147

148

149

150

151

152

0 2 4 6 8 10 12 14 16

Back Pressure (in-Hg)

CorrectedPower(cBhp)

Peak Power Back Bhp


41/85

Compression Ratio

The highest possible compression ratio is alwaysthe design point, as higher will always be morethermally efficient with better idle quality

Knock limits compression ratio because ofcombustion stability issues at low engine speeddue to necessary spark retard

Most engines are designed with highercompression than is best for low speedcombustion stability because of the associatedpart-load BSFC benefits and high speed power


42/85

Valve Events

Valve events define how an engine breathes allthe 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 notbegin until the piston-cylinder mechanism is

sealed same with expansion


43/85

Valve Event

Timing Diagram Spider Plot -

Describes timing pointsfor valve events withrespect to CrankPosition

Cam Centerline -

Peak Valve Lift withrespect to TDC inCrank Degrees


44/85

Valve Events for Power Maximize Trapping Efficiency

Intake closing that is best compromise between compressionstroke back flow and induction momentum (retard withincreasing 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 protectionenrichment beyond LBT)


45/85

Valve Events for Power

Minimize Flow Loss

Achieve maximum valve lift (max flow usually atL/D > 0.25-0.3) as long as possible (square liftcurves are optimum for poppet valves)

Minimize Exhaust Pumping Work

Earliest exhaust opening that blows down cylinderpressure to backpressure levels before exhaust

stroke (advance with increasing engine speed) Earliest exhaust closing that avoids recompression

spike (retard with increasing engine speed)


46/85

Centerline Effects On Torque

420

430

440

450

460

470

480

490

500

510

520

530

540

550

560

570

1600 2000 2400 2800 3200 3600 4000 4400 4800 5200 5600

Engine Speed (rpm)

Torque(ft-lbs

)

115 degree centerline 120 degree centerline 124 degree centerline


47/85

10

20

30

40

50

60

70

80

90

100

110120

240

250

250

275

275

300

300

350

400450500

600700

1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200 5600 6000 6400

d Speed [rpm]

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

2006 2.4L WE BSFC MAP (g/kW-h) Engine Power and BSFC vs Engine Speed


48/85

Summary Components Relative Impact on

Performance1. Cylinder Head Ports & Valve Area

2. Valve Events3. Intake Manifold Runner Geometry

4. Compression Ratio

5. Exhaust Header Geometry

6. Exhaust Restriction

7. Air Cleaner Restriction


49/85

Powertrain Closing Remarks

Powertrain is compromise Four-stroke engines are volumetric flow rate

devices the only route to more power isincreased 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 withinexisting engine architectures

Highest average power during a vehicleacceleration is fastestpeak power values dont

win races


50/85

Break


51/85

Calibration What is it?

Optimizing the control system (once hardware isfinalized) 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 emissionsconstraints and use race fuel (usually eliminates anyknock) therefore are relatively easy to calibrate


52/85

Calibration Terms

Stoichiometry Chemically correct ratio of fuel to airfor combustion

F/A Fuel/Air Ratio

Mass ratio of mixture, a determination of richness orleanness. Stoichiometry = 0.0688-0.0696 FA

Lambda Excess Air RatioStoichiometry = 1.0 Lambda

Rich F/A F/A greater than StoichiometryRich < 1.0 Lambda

Lean F/A F/A less than StoichiometryLean > 1.0 Lambda


53/85

Calibration Terms continued

Brake Power Power measured by the absorber(brake) at the crankshaft

BSFC - Brake Specific Fuel Consumption

Fuel Mass Flow Rate / Brake Powergrams/kW-h or lbs/hp-h

LBT Fuelling Lean Best TorqueLeanest 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


54/85

Calibration Terms continued

Spark AdvanceTiming in crank degrees prior toTDC for start of combustion event (ignition)

MBT Spark- Maximum Brake TorqueMinimum Spark Advance to Achieve Best Torque

Burn Rate Speed of CombustionExpressed as a fraction of total heat released versus

crank degrees

MAP - Manifold Absolute PressureAbsolute not Gauge (which references barometer)


55/85

Lean Best Torque Fuel Air Sweeps

76%

78%

80%

82%

84%

86%

88%

90%

92%

94%

96%

98%

100%

102%

0.0660 0.0690 0.0720 0.0750 0.0780 0.0810 0.0840 0.0870 0.0900 0.0930 0.0960 0.0990 0.1020 0.1050 0.1080 0.1110

F/A FN

TorqueDelta

FactorFromL

BT

1856 RPM, 70 kPa MAP 3296 RPM, 98 kPa MAP 3296 RPM, 56 kPa MAP 3296 RPM, 84 kPa MAP

4544 RPM, 70 kPa MAP 3296 RPM, 98 kPa MAP 2688 RPM, 70 kPa MAP

Spark Held Constant During Fuel Air Sw eep

Spark Advance vs Torque


56/85

Spark Advance vs Torque

84%

86%

88%

90%

92%

94%

96%

98%

100%

102%

-22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Delta Spark Advance From MBT

TorqueDeltafromM

BT


57/85

Control System Types

Alpha-N

Engine Speed & Throttle Angle

Speed-Density Engine Speed and MAP/ACT

MAF

Engine Speed and MAF


58/85

Alpha-N

Fuel and spark maps are based onthrottle angle which is very non-linear

and requires complete mapping ofengine

Good throttle response once dialed in

Density compensation (altitude andtemperature) is usually absent needs tobe recalibrated every time car goes out


59/85

Speed-Density

Fuel and spark maps are based on MAPdensity of charge is a strong function ofpressure, corrected by air temp and coolanttemp therefore air flow is simple to calculate

Less time-intensive than Alpha-N, once calibratedis good most common type of control

Needs less mapping can do WOT line and mid-map then curve-fit air flow (spark needs a littlemore in-depth for optimal control)


60/85

MAF

Fuel and spark maps are based on MAFairflow measured directly

MAF sensor isnt the most robust device

Pressure pulses confuse signal, each application has tobe mapped with secondary damped MAF sensor (usuallya 55 gallon drum inline)

Least noisy signal is usually at air cleaner so separate

transport delay controls need to be calibrated fortransients and leaks need to be absolutely eliminated

Boosted applications usually add a MAP as well


61/85

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


62/85

Fuel System Injectors

Volumetric flow rate solenoids, linear relationshipbetween pulsewidth and flow for given pressure delta

Battery offset is time necessary to open and closesolenoid time is fixed for any voltage

Duty cycle is injector on timeitll 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 (>4bar) 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 lowerflows at low vacuum making calibration morecomplicated

1

2

1

2

P

P

V

V

Bernoulli Effect of Fuel Pressure

Pulsewidth

Pulsewidth + Battery Offset

Pintle

Height

S


63/85

Sensors Manifold Absolute Pressure (MAP)

A variable-resistance diaphragm with perfect vacuum on one sideand 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 andbased upon the measured downstream temperature the mass flow

rate can be determined Crank Position

High resolution for spark advance, less-so for crank speed andwith once-per-rev can indicate TDC

Cam Position

Low resolution for syncronization for sequential fuel injection andindividual cylinder spark

Air Charge Temp and Engine Coolant Temp Thermistors used for air density correction and startup enrichment


64/85

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 theengine (typically the block) is hit with this pressure wave it ringsas well, but at a frequency that is a function of the structure (iematerials and geometry). A FFT analysis of different mountingpositions (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 otherstructure-borne noise.


65/85

Sensors, cont

Lamda Sensor (EGO) Compares ambient air to

exhaust oxygen content(partial pressure of oxygen).

Sensor output is essentiallybinary (only indicates rich orlean of stoichiometry).

Wide-band Lamda Sensor(UEGO) Compares partial pressure of

oxygen (lean) and partialpressure of HmCn, H2 & CO(rich) with ambient. Givesoutput from ~0.6 to 2 Lamda.

UEGO Schematic

EGO Schematic


66/85

Calibration Goals

Combustion & Thermodynamics

Work, Power & Mean Effective Pressures

Knock, Pre-Ignition

Burn Rate

Transients

Wall film

Thermal Enrichment

Drivability


67/85

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 knockthrough reduced charge temperatures (cooler incoming chargeor reduced residual burned gases), increased evaporative coolingfrom richer F/A mixtures and increased combustion chambercooling

Pressure Lower cylinder pressures reduce knock tendencythrough lower compression ratio or MAP pressure

Octane Different fuel types have higher or lower autoignitiontendencies. Octane value is directly related to knockingtendency


68/85

Knock continued Effects of Knock

Disrupts stagnant gases that form boundary layer atedge of combustion chamber, increasing heat transferto 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 filmthickness and accelerating wear

Shockwave can be strong enough to stresscomponents to failure


69/85

In-cylinder Pressure Measurement

Piezoelectric pressuretransducers developcharge with changesin pressure

Installed incombustion chamber

wall or spark plug tomeasure full-cyclepressures

l b ll


70/85

Typical pressure probe installation

Passage drilled through deck face (avoiding coolant jacket)

Cylinder Pressure TraceNo Knock


71/85

No Knock

Cylinder Pressure TraceKnock Limit or Trace Knock - Best Power


72/85

Knock Limit or Trace Knock Best Power

Cylinder Pressure TraceSevere Damaging Knock


73/85

Severe Damaging Knock


74/85

Pre-Ignition

Effects of Pre-Ignition Increases peak cylinder pressure by beginning heat

release too soon

Increased cylinder pressure also increases heat loadto combustion chamber components, sustaining thepre-ignition (leading to run-away pre-ignition)

Increases loads on piston crown and piston pin

Sustained pre-ignition will typically put a hole in thecenter of the piston crown


75/85

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 shortenflame 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 vorticesaround TDC increasing kinetic energy in charge

Turbulent Intensity a measure of total kinetic energy available to move flame frontfaster than laminar flame speed. More Turbulent Intensity equals faster burn.

C b ti & Th d i


76/85

Combustion & ThermodynamicsSummary

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 usedbecause of catalyst performance impacts in pass cars


77/85

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 toprovide sufficient vapour for engine to run (relationshipbetween temperature and partial pressure of fuel fractions)

Most of heat during fully warm operation comes from backside 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 fromevery pulse goes into engine and some onto port wall.

On a fast acceleration, additional fuel must be added to offset theslowly moving wall film. Opposite true on decels.

If injector is positioned far upstream volumetric efficiency increasesdue fuel heat of vapourization cooling incoming charge, but a largeamount of wall is wetted leading to poor transient fuel control


78/85

Injector Targeting

Bad Tip Location

Targets Valve

Targets Port Wall

Better Tip Location


79/85


80/85

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 sufficientdurability

Usually 950 deg C runner temperature is acceptablefor 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 beforeignition (pressure-charged engines primarily)


81/85

Drivability Throttle Response

Drivers expect some repeatability andresolution of thrust versus pedal positionsome degree of spark mapping (retard) andpedal to throttle cam can help a driversconfidence

Usually least developed and of most

importance is tip-in (throttle closed to smallopening) where torque can come in as a stepchange


82/85


83/85

Closing Remarks Calibration is compromise

Best spark for drivability may not producesufficient combustion stability or fuelconsumption

Best fuelling for drivability is voracious fuelconsumer - decel fuel shut off can improveeconomy by 20% but has tip-in torque bumps

without careful calibration


84/85

References

Internal Combustion Engine Fundamentals, John BHeywood, 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


85/85

Q & A

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