Gas Power Cycle - Internal Combustion Engine by pharmphresh28

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									Gas Power Cycle - Internal Combustion Engine




                  Otto Cycle
                                           Otto Cycle
P    3                                T                     3      • 1-2 isentropic compression
                                                                   • 2-3 constant volume heat
                                                                   transfer
                                  4       2
    2                                                              • 3-4 isentropic expansion
                                                           4
                                                                   • 4-1 constant volume heat
                                  1       1                        rejection
                                      v                          s
    Thermal efficiency of the system:
         Wcycle       W34 + W12 m[( u3 − u4 ) + ( u1 − u2 )]    (u − u1 )
    η=            =            =                             =1− 4
          Qin            Q23           m( u3 − u2 )             ( u3 − u2 )
                                               (u4 − u1 )      C (T − T1 )      T     T4 / T1 − 1 
    For an ideal gas, u=C v T , η =1 −                     = 1− v 4          =1− 1                
                                               ( u3 − u2 )     Cv (T3 − T2 )    T2    T3 / T2 − 1 
    Since T4 / T1 = T3 / T2 (why?)
             T1
    η = 1−      . From isentropic compression relation for an ideal gas
             T2
                  γ −1
    T1  V2                  1                V                                        cp
      =                  =          , where r=  1  is the volume compression ratio, γ =
    T2  V1 
                           r γ −1             V2                                      cv
                                                            Otto Cycle-2
                              100
                                                                          Thermal efficiency of an Otto cycle,
                               80
thermal efficiency




                                                                                   1
                               60                                         η = 1 − γ −1
                     η( r )                                                      r
                               40
                                                                            Typical value of r for a real engine:
                               20                                           between 7 and 10
                                0
                                    0   3       6       9       12   15
                                                   r
                                            compression ratio



                     • The higher the compression ratio, the higher the thermal
                     efficiency.
                     • Higher r will led to engine knock (spontaneous ignition)
                     problem.
               Improvement of Performance
• Increase the compression ratio

• Increase the engine displacement: more power

• Compress more air into the cylinder during intake: using
supercharger and turbocharger.

• Cool the air before allowing it to enter the cylinder: cooler
air can expand more, thus, increase the work output.

• Reduce resistance during intake and exhaust stages: multiple
valve configuration: 4 cylinders/16 valves engine

• Fuel injection: do away with the carburetor and provide
precise metering of fuel into the cylinders.
                                Diesel Cycle
                                                         2-3: a constant pressure
P     2        3            T                            process (instead of a
                                                3        constant volume process)
                                                         and is the only difference
                                2                        between an idealized
                        4
                                                4        Diesel cycle and an
                                                         idealized Otto cycle.
                        1        1
                            v                        s
    • Fuel injection for an extended period during the power stroke and therefore
    maintaining a relatively constant pressure.
    • Diesel cycle has a lower thermal efficiency as compared to an Otto cycle
    under the same compression ratio.
    • In general, Diesel engine has a higher thermal efficiency than spark-ignition
    engine because the Diesel engine has a much higher compression ratio.
    • Compression-ignition: very high compression ratio 10 to 20 or even higher.

          Diesel Cycle                  Internal Combustion Engine
     Thermal Efficiency of Diesel Cycle

• Introduce parameter β=V3/V2
• Show that the efficiency of an ideal Diesel cycle is:

                            1        βγ −1
         η Diesel = 1 −
                          r γ −1   [γ ( β − 1)]

• It can also be shown that ηOtto > ηDiesel for the same
compression ratio. However, Diesel engines can usually
operate at higher compression ratio (Why?).
• If the maximum pressure is the same, the Diesel engine
has a higher efficiency than the Otto engine.
                             Dual Cycle
P 2.5   3
                               • Some heat is added at constant
                               volume (2 è 2.5)
   2
                     4         • The remaining heat is added at
                               constant pressure (2.5 è 3)
                     1
                         v
 • Define β=V3/V2.5, α=P3/P2

                                γ −1
                         1                  αβ γ − 1        
  Show that: ηdual   =1−              (α − 1) + γα ( β − 1) 
                         r                                  

• It has an efficiency falling between the Otto and Diesel limits.

								
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