Diesel EnginesEngine Systems and Performance

TopicsEngine power - description and measurementEngine thermodynamicsCycles and efficiencyDiesel enginesDesign variationsCombustion chambersFuel injectionFuels

You Know How an Engine WorksHow a basic 4-stroke, compression ignition engine worksIntake, compression, power, and exhaust strokesWhen exhaust and intake valves open and closeWhen fuel is injected

How Powerful is an Engine?Depends on where you measure itGross PowerFuel equivalent - Power available if you can capture all energy in fuel consumed

How Powerful is an Engine?Depends on where you measure itNet PowerBrake power - at the flywheel

How Powerful is an Engine?Depends on where its measuredAt the piston

Indicated mean effective pressure - an averageFriction power = Pi - Pb

EfficienciesThermodynamic efficiency for an engine (really any thermodynamic process) is ratio of work out to heat put in:

Also can be defined as power out divided by power in

Engine EfficiencyEfficienciesIndicated thermal

Mechanical

Brake thermal

Example Torque, HP CurvesCat 525 skidderCAT 3126 engineRated 230 bhp at 2200 rpm

Torque and HP CurvesFor the CAT 525 skidder (3126 engine)

Timberjack 460DA John Deere 6068H engineIntermittent 211 bhpContinuous 189 bhpBoth at 2000 rpm

Torque and HP CurvesJohn Deere 6068H engine

Torque ReservePercent increase in torque available compared to that at rated speedRated engine speed = 2000 rpmTorque at rated speed = 554 ft-lbMax torque = 664 ft-lbTorque reserve = (664 - 554) / 554 = 20%

Gas VS DieselSame (nearly) displacement, 2.5LGasDiesel

Measuring Engine PowerDynamometerAbsorption Dyno

Load Cell

Measuring Engine PowerChassis DynoMeasure angular acceleration of spinning weight

Engine EfficiencyEfficiency is relative amount of work we get out of an engine compared to amount of heat energy we put in.Thermal efficiencyCompare gas and diesel enginesTalk about fuel injection and combustion in diesel engines

Pressure - Volume DiagramsGraph of pressure as a function of cylinder volume above the pistonBottom Dead Center - volume is maximumTop Dead Center - volume is minimum

Cylinder VolumeChange in volume as piston goes up and downVolume goingVolume goingTDC

Change in VolumeBDCTDCBDCVolume

P-V Diagram - Gas EngineBottom Dead CenterStart of compression stroke - volume above piston filled with fuel/air mixture

P-V Diagram - Gas EnginePiston travels up, fuel/air compressed and pressure rises

P-V Diagram - Gas EngineTop Dead CenterEnd of compression stroke - volume in cylinder at a minimum

P-V Diagram - Gas EngineThen have an instantaneous introduction of heat, which increases pressure againCOMBUSTION

P-V Diagram - Gas EnginePower StrokePressure forces piston down, creating torque on crank shaft

P-V Diagram - Gas EnginePiston reaches bottom dead center again, exhaust valve opens, burned fuel/air expelled

Gas Engine EfficiencyRemember, efficiency is work out divided by heat inP-V diagram tells us both thingsHeat in: temperature change from burning fuelWork out: area between curvesAnd, make a few assumptions, can calculate efficiency

Gas Engine EfficiencyDefine compression ratio as:

Then efficiency can be shown to be:

Higher efficiency means higher compression ratio.

Diesel Engine EfficiencySame approach, slightly different cycle: constant pressure heat injection

Efficiency works out to be: Constant pressure heat injection - happens as piston moves down

Gas vs DieselWhich is more efficient?

C is always bigger than 1, so gas is more efficient at the same compression ratio

Comparison of FordsExpedition: 4.6L SOHC 2V V8 engineCompression ratio = 9.4:1

F-250 SD, 6.0L Power Stroke diesel 32V V8Compression ratio = 18:1

Diesel CombustionA better thermodynamic model: Dual cycleConstant volume and constant pressure heat injection

Diesel CombustionControl of a diesel engine is through injection duration Low or Partial LoadMaximum Load

Diesel EfficiencyA diesel is at peak efficiency at partial load Low or Partial LoadMost Efficient: SI Engine

Fuel Injection In CI EnginesThere are three distinct phases of fuel injection:Delay periodInjection starts, but not combustionRapid combustionLarge pressure spikeDiffusion combustion

Fuel Injection In CI EnginesDelay PeriodRapid Combustion - knockDiffusion Combustion - smokeCylinder Pressure

Fuel Injection In CI EnginesLess air means less fuel requiredFuel injection system has to compensate for air density to minimize smoke output7% decrease in air density per 1000m altitude

CI Engine Combustion ChambersTwo types:Indirect (divided chamber) injection - into a pre-combustion chamber

CI Engine Combustion ChambersTwo types:Direct injection - into cylinder

Combustion Chamber ComparisonIndirectGoodExcellent mixing, turbulence characteristicsCan burn lower quality fuelLower injection pressureLess pronounced knock

Combustion Chamber ComparisonIndirectBadVery high temperature/pressure in injection chamberHigher emissions, especially NOxHarder to start - glow plugsLess efficient

Combustion Chamber ComparisonDirectBadPressure rise can be great, knockHigh injection pressure, high quality fuel

Combustion Chamber ComparisonDirectGoodLower specific fuel consumption - 20%Lower emissionsBigger valves, higher volumetric efficiency

Direct InjectionAir-fuel mixing importantControlled by:Use of swirl-inducing designsIntake port geometryPiston cap geometry

Injection system designHigh pressureMany nozzle holesPositioningBad: high-tech injection systemGood: Higher volumetric efficiency

Injection SystemsControl of injection amount, timing, pressure are criticalCAT 525 engine:Tier 1 compliantMechanical control of injectionTJ 460D engineTier 2 compliantElectronic control of injection

Diesel Fuel CharacteristicsCetane numberMeasure of relative ease to initiate combustionHigher number: easier to igniteOctane number for gas: oppositeHigher number: less tendency to ignite

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