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发动机及标定 powertrain & calibration 101.ppt

Powertrain & Calibration 101,John Powertrain Systems EngineeringDecember 4, 2008,Powertrain & Calibration Topics,BackgroundPowertrain termsThermodynamicsMechanical DesignCombustionArchitectureCylinder Filling & EmptyingAerodynamicsCalibrationSpark & FuelTransients & Drivability,What is a Powertrain?,Engine that converts thermal energy to mechanical workParticularly, the architecture comprising all the subsystems required to convert this energy to workSometimes extends to drivetrain, which connects powertrain to end-user of power,Characteristics of Internal Combustion Heat Engines,High energy density of fuel leads to high power to weight ratio, especially when combusting with atmospheric oxygenExternal combustion has losses due to multiple inefficiencies (primarily heat loss from condensing of working fluid), internal combustion has less inefficienciesHeat engines use working fluids which is the simplest of all energy conversion methods,Reciprocating Internal Combustion Heat Engines,CharacteristicsSlider-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 capabilityFair to poor air pump, limiting power potential without additional mechanisms,Reciprocating Engine TermsVc = Clearance VolumeVd = Displacement or Swept VolumeVt = Total VolumeTC or TDC =Top or Top Dead Center PositionBC or BDC =Bottom or Bottom Dead Center PositionCompression Ratio (CR),Further explanation of aspects of Compression Ratio,Reciprocating EnginesMost layouts created during second World War as aircraft manufacturers struggled to make the least-compromised installation,Thermodynamics,Otto CycleDiesel CycleThrottled CycleSupercharged Cycle,Source: Internal Comb. Engine Fund.,Thermodynamic TermsMEP – Mean Effective PressureAverage cylinder pressure over measuring periodTorque Normalized to Engine Displacement (VD)BMEP – Brake Mean Effective PressureIMEP – Indicated Mean Effective PressureMEP of Compression and Expansion StrokesPMEP – Pumping Mean Effective PressureMEP of Exhaust and Intake StrokesFFMEP – Firing Friction Mean Effective PressureBMEP = IMEP – PMEP – FFMEP,Thermodynamic Terms continuedWork = Power = Work/Unit TimeSpecific Power – Power per unit, typically displacement or weightPressure/Volume Diagram – Engineering tool to graph cylinder pressure,Indicated Work,TDC,BDC,Source: Design and Sim of Four Strokes,TDC,BDC,Source: Design and Sim of Four Strokes,Pumping Work,History of Internal Combustion,1878 Niklaus Otto built first successful four stroke engine1885 Gottlieb Daimler built first high-speed four stroke engine1878 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,Energy Distribution in Passenger Car Engines,Source: SAE 2000-01-2902 (Ricardo),Source: Advanced Engine Technology,Using Exhaust Energy,Highest expansion ratio recovers most thermal energyTurbines can recover heat energy left over from gas exchangeEnergy can be used to drive turbo-compressor or fed back into crank train,Source: Internal Comb. Engine Fund.,Supercharging,Increases specific output by increasing charge density into reciprocatorMany methods of implementation, cost usually only limiting factor,Mechanical Design,Two Valve Valvetrain,Pushrod OHV (Type 5),HEMI 2-Valve (Type 5),SOHC 2-Valve (Type 2),Four Valve Valvetrain,SOHC 4-Valve (Type 3),DOHC 4-Valve (Type 2),DOHC 4-Valve (Type 1),Desmodromic,Specific Power = f(Air Flow, Thermal Efficiency)Air flow is an easier variable to change than thermal efficiency90% of restriction of induction system occurs in cylinder headCylinder head layouts that allow the greatest airflow will have highest specific power potentialPeak flow from poppet valve engines primarily a function of total valve areaMore/larger valves equals greater valve area,Valvetrain,Combustion Terms,Brake Power – Power measured by the absorber (brake) at the crankshaftBSFC - Brake Specific Fuel Consumption Fuel Mass Flow Rate / Brake Powergrams/kW-h or lbs/hp-hLBT Fuelling - Lean Best Torque Leanest Fuel/Air to Achieve Best TorqueLBT = 0.0780-0.0800 FA or 0.85-0.9 LambdaThermal Enrichment – Fuel added for cooling due to component temperature limitInjector Pulse Width - Time Injector is Open,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 TorqueBurn 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),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 MBTPre-Ignition – Autoignition of mixture prior to spark timing, typically due to high temperatures of componentsCombustion Stability – Cycle to cycle variation in burn rate, trapped mass, location of peak pressure, etc. The lower the variation the better the stability.,Engine Architecture Influence on Performance,Intake & Exhaust Manifold TuningCylinder Filling & EmptyingMomentumPressure WaveAerodynamicsFlow SeparationWall FrictionJunctions & BendsInduction RestrictionExhaust Restriction (Backpressure)Compression RatioValve Events,Intake Tuning for WOT Performance,Intake manifolds have ducts (“runners”) that tune at frequencies corresponding to engine speed, like an organ pipeLonger runners tune at lower frequenciesShorter runners tune at higher frequenciesTuning increases local pressure at intake valve thereby increasing flow rateDuct diameter is a trade-off between velocity and wall friction of passing charge,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,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.,Momentum Effects,Pressure loss influences dictate that duct diameter be as large as possible for minimum frictionIncreasing 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 fluxTherefore duct diameter is a trade-off between velocity and wall friction of passing charge,Pressure Wave Effects,Induction process and exhaust blowdown both cause pressure pulsationsAbrupt 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 waveClosed-ended ducts reflect pressure waves directly, therefore a wave will echo with same amplitude,Pressure Wave Effects con’t,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,Effects of Intake Runner Geometry,Tuning in Production I4 Engine,Aerodynamics,Losses due to poor aerodynamics can be equal in magnitude to the gains from pressure wave tuningOften the dominant factory in poorly performing OE componentsIf 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),Aerodynamics con’t,Flow SeparationLiterally same phenomenon as stall in wing elements – pressure in free stream insufficient to ‘push’ flow along wall of short side radiusRecirculation 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,Aerodynamics con’t,Wall FrictionSurface finish of ducts need to be as smooth as possible to prevent ‘tripping’ of flow on a macro levelJunctions & BendsEverything from your fluid dynamics textbook applies Radiused inlets and free-standing pipe outletsMinimize number of bendsAvoid ‘S’ bends if at all possible,Induction Restriction,Air cleaner and intake manifolds provide some resistance to incoming chargePower loss related to restriction almost directly a function of ratio between manifold pressure (plenum pressure upstream of runners) and atmospheric,Exhaust Restriction,Compression Ratio,The highest possible compression ratio is always the design point, as higher will always be more thermally efficient with better idle qualityKnock limits compression ratio because of combustion stability issues at low engine speed due to necessary spark retardMost 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,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 performanceValve events also effectively define compression & expansion ratio, as “compression” will not begin until the piston-cylinder mechanism is sealed – same with expansion,Valve Event Timing Diagram,Spider Plot - Describes timing points for valve events with respect to Crank PositionCam Centerline - Peak Valve Lift with respect to TDC in Crank Degrees,Valve Events for Power,Maximize Trapping EfficiencyIntake 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 limitEarly intake opening will impart some exhaust blowdown or pressure wave tuning momentum to intake charge Maximize Thermal EfficiencyEarliest 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),Valve Events for Power,Minimize Flow LossAchieve maximum valve lift (max flow usually at L/D > 0.25-0.3) as long as possible (square lift curves are optimum for poppet valves)Minimize Exhaust Pumping WorkEarliest 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),Engine Power and BSFC vs Engine Speed,Summary,Component’s Relative Impact on PerformanceCylinder Head Ports & Valve AreaValve EventsIntake Manifold Runner GeometryCompression RatioExhaust Header GeometryExhaust RestrictionAir Cleaner Restriction,Powertrain Closing Remarks,Powertrain is compromiseFour-stroke engines are volumetric flow rate devices – the only route to more power is increased engine speed, more valve area or increased charge densityMore speed, charge density or valve area are expensive or difficult to develop – therefore minimizing losses is the most efficient path within existing engine architecturesHighest average power during a vehicle acceleration is fastest – peak power values don’t win races,Break,Calibration,What is it?Optimizing the control system (once hardware is finalized) for drivability, durability & emissionsIt’s just spark and fuel – how hard could it be?Knowledge of Thermodynamics, Combustion and Control Theory all play inFortunately race engines have no emissions constraints and use race fuel (usually eliminates any knock) – therefore are relatively easy to calibrate,Calibration Terms,Stoichiometry – Chemically correct ratio of fuel to air for combustionF/A – Fuel/Air RatioMass ratio of mixture, a determination of richness or leanness. Stoichiometry = 0.0688-0.0696 FALambda – Excess Air RatioStoichiometry = 1.0 LambdaRich F/A – F/A greater than StoichiometryRich 1.0 Lambda,Calibration Terms continued,Brake Power – Power measured by the absorber (brake) at the crankshaftBSFC - Brake Specific Fuel Consumption Fuel Mass Flow Rate / Brake Powergrams/kW-h or lbs/hp-hLBT Fuelling – Lean Best Torque Leanest Fuel/Air to Achieve Best TorqueLBT = 0.0780-0.0800 FA or 0.85-0.9 LambdaThermal Enrichment – Fuel added for cooling due to exhaust component temperature limitInjector Pulse Width - Time Injector is Open,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 TorqueBurn 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),Control System Types,Alpha-NEngine Speed & Throttle AngleSpeed-DensityEngine Speed and MAP/ACTMAFEngine Speed and MAF,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 inDensity compensation (altitude and temperature) is usually absent – needs to be recalibrated every time car goes out,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 calculateLess time-intensive than Alpha-N, once calibrated is good – most common type of controlNeeds less mapping – can do WOT line and mid-map then curve-fit air flow (spark needs a little more in-depth for optimal control),MAF,Fuel and spark maps are based on MAF – airflow measured directlyMAF sensor isn’t the most robust devicePressure 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 eliminatedBoosted applications usually add a MAP as well,
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本文标题:发动机及标定 powertrain & calibration 101.ppt
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