Diesel Engines - PowerPoint by PIjPmjW7

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									     Diesel Engines


Engine Systems and Performance
                   Topics

• Engine power - description and
  measurement
• Engine thermodynamics
   Cycles and efficiency
• Diesel engines
   Design variations
     Combustion chambers
     Fuel injection
• Fuels
   You Know How an Engine
           Works

• How a basic 4-stroke, compression
  ignition engine works
   Intake, compression, power, and exhaust
    strokes
   When exhaust and intake valves open and
    close
   When fuel is injected
 How Powerful is an Engine?

• Depends on where you measure it
   Gross Power
     ‘Fuel equivalent’ - Power available if you can
      capture all energy in fuel consumed
                  Pfe  m f H g
                        Ý
                               BTU
                  H g  19,000
                                lb
 How Powerful is an Engine?

• Depends on where you measure it
   Net Power
     ‘Brake power’ - at the flywheel

                      Pb  2 Tb Nc



       
 How Powerful is an Engine?

• Depends on where it’s measured
   At the piston
                            pime DN
                       Pi 
                               2
     Indicated mean effective pressure - an ‘average’
• Friction power = Pi - Pb

        
           Efficiencies

• Thermodynamic efficiency for an engine
  (really any thermodynamic process) is
  ratio of work out to heat put in:
               W
            e
               Q
• Also can be defined as power out divided
  by power in
     
          Engine Efficiency

• Efficiencies
                                        Pi
   Indicated thermal           eit 
                                        Pfe

                           P
   Mechanical         em  b
                           Pi
                      
                                Pb
   Brake thermal         ebt 
                                Pfe
            


                 
 Example Torque, HP Curves

• Cat 525 skidder
   CAT 3126 engine




                      Rated 230 bhp at 2200 rpm
      Torque and HP Curves

• For the CAT
  525 skidder
  (3126 engine)
        Timberjack 460D

• A John Deere
  6068H engine




                 Intermittent 211 bhp
                 Continuous 189 bhp
                 Both at 2000 rpm
     Torque and HP Curves

• John Deere
  6068H
  engine
          Torque Reserve

• Percent increase in torque available
  compared to that at rated speed
                        Torque at rated speed =
                        554 ft-lb
                        Max torque = 664 ft-lb
                        Torque reserve =
                        (664 - 554) / 554 = 20%


                         Rated engine
                         speed = 2000
                         rpm
            Gas VS Diesel

 • Same (nearly) displacement, 2.5L

Diesel                                Gas
   Measuring Engine Power

• Dynamometer
   Absorption Dyno
      Load Cell


                  Pb  2 T N



        
 Measuring Engine Power

 Chassis Dyno


                 Measure angular
                 acceleration of
                 spinning weight
          Engine Efficiency

• Efficiency is relative amount of work we
  get out of an engine compared to amount
  of heat energy we put in.
• Thermal efficiency
   Compare gas and diesel engines
   Talk about fuel injection and combustion in
    diesel engines
 Pressure - Volume Diagrams

• Graph of pressure as a function of cylinder
  volume above the piston


           Bottom Dead           Top Dead Center
           Center - volume       - volume is
           is maximum            minimum
          Cylinder Volume

• Change in volume as piston goes up and
  down




                        TDC
      Volume going             Volume going
          Change in Volume
Volume




         BDC     TDC     BDC
P-V Diagram - Gas Engine




                                      Pressure
     Bottom Dead Center
    Start of compression stroke -
    volume above piston filled with
    fuel/air mixture

                                                 TDC            BDC

                                                       Volume
P-V Diagram - Gas Engine

     Piston travels up, fuel/air
     compressed and
     pressure rises




                                   Pressure
                                              TDC            BDC

                                                    Volume
P-V Diagram - Gas Engine


     Top Dead Center
    End of compression stroke -
    volume in cylinder at a minimum




                                      Pressure
                                                 TDC            BDC

                                                       Volume
P-V Diagram - Gas Engine


    Then have an
    instantaneous
    introduction of heat,




                            Pressure
    which increases
    pressure again




    COMBUSTION                         TDC            BDC

                                             Volume
P-V Diagram - Gas Engine


                                   Power Stroke



         Pressure


                    TDC                 BDC


                          Volume
       Pressure forces piston down,
       creating torque on crank shaft
P-V Diagram - Gas Engine

                                   Piston reaches bottom
                                   dead center again,
                                   exhaust valve opens,


       Pressure
                                   burned fuel/air expelled




                  TDC            BDC


                        Volume
               Gas Engine Efficiency

  • Remember, efficiency is work out divided
    by heat in
  • P-V diagram tells us both things
                                            Work out: area
                                            between curves
                Pressure




                                                    And, make a few assumptions,
Heat in:                                            can calculate efficiency
temperature                TDC            BDC
change from
                                 Volume
burning fuel
      Gas Engine Efficiency

• Define compression ratio as:
                                                          VBDC
                                                     r




                                    Pressure
                                                          VTDC
• Then efficiency can be shown
  to be:               1
                e  1       0.4   
                         r                     TDC               BDC

                                                     Volume

• Higher efficiency means higher
  compression ratio.
          
    Diesel Engine Efficiency

• Same approach, slightly
  different cycle: constant   Constant pressure heat
                              injection - happens as piston
  pressure heat injection     moves down




                               Pressure
• Efficiency works out to
  be:                                     TDC            BDC
                   C
            e  1 0.4                          Volume
                  r
            Gas vs Diesel

• Which is more efficient?      Gas     e  1
                                              1
                                             r 0.4
                                              C
                                Diesel e  1 0.4
                                             r

• ‘C’ is always bigger than 1, so gas is more
                           
  efficient at the same compression ratio
      Comparison of Fords

• Expedition: 4.6L SOHC 2V
  V8 engine
   Compression ratio = 9.4:1



• F-250 SD, 6.0L Power
  Stroke diesel 32V V8
   Compression ratio = 18:1
        Diesel Combustion

• A better thermodynamic model: Dual cycle
   Constant volume and constant pressure heat
    injection

                  Pressure




                             TDC            BDC

                                   Volume
                     Diesel Combustion

• Control of a diesel engine is through
  injection duration
  Low or Partial Load               Maximum Load
    Pressure




               TDC            BDC
                                    Pressure   TDC            BDC

                     Volume                          Volume
                       Diesel Efficiency

• A diesel is at peak efficiency at partial load
                                    Most Efficient: SI Engine
    Pressure




                                         Pressure
               TDC            BDC
                                                    TDC            BDC
                     Volume
                                                          Volume
 Low or Partial Load
  Fuel Injection In CI Engines

• There are three distinct phases of fuel
  injection:
   Delay period
     Injection starts, but not combustion
   Rapid combustion
     Large pressure spike
   Diffusion combustion
   Fuel Injection In CI Engines

Cylinder Pressure
                             Diffusion Combustion -
                             smoke

        Rapid Combustion -
        knock


 Delay Period
   Fuel Injection In CI Engines

7% decrease in air density
                           Less air means less fuel required
per 1000m altitude


     Fuel injection system has to
     compensate for air density to
     minimize smoke output
     CI Engine Combustion
           Chambers

• Two types:
   Indirect (divided
    chamber) injection -
    into a pre-
    combustion chamber
      CI Engine Combustion
            Chambers

• Two types:
   Direct injection - into
    cylinder
       Combustion Chamber
          Comparison

• Indirect
   Good
      Excellent mixing, turbulence characteristics
       • Can burn lower quality fuel
      Lower injection pressure
      Less pronounced knock
       Combustion Chamber
          Comparison
• Indirect
   Bad
      Very high temperature/pressure in injection
       chamber
       • Higher emissions, especially NOx
      Harder to start - glow plugs
      Less efficient
      Combustion Chamber
         Comparison
• Direct
   Bad
     Pressure rise can be great,
      knock
     High injection pressure,
      high quality fuel
      Combustion Chamber
         Comparison
• Direct
   Good
     Lower specific fuel consumption -
      20%
     Lower emissions
     Bigger valves, higher volumetric
      efficiency
             Direct Injection

• Air-fuel mixing important
   Controlled by:
     Use of swirl-inducing designs
      • Intake port geometry
      • Piston cap geometry



     Injection system design
      • High pressure           Bad: high-tech injection system
      • Many nozzle holes
                                Good: Higher volumetric efficiency
      • Positioning
          Injection Systems

• Control of injection amount, timing,
  pressure are critical
   CAT 525 engine:
     Tier 1 compliant
     Mechanical control of injection
   TJ 460D engine
     Tier 2 compliant
     Electronic control of injection
  Diesel Fuel Characteristics

• Cetane number
   Measure of relative ease to initiate
    combustion
     Higher number: easier to ignite
   Octane number for gas: opposite
     Higher number: less tendency to ignite

								
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