Page 331 - Compression Machinery for Oil and Gas
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316 SECTION    II Types of Equipment


            older than most people realize, yet its use in the automobile is so routine that
            most take it for granted as part of their daily lives. It is a proven technology that
            serves society effectively every day.
               The reciprocating engine most commonly used in gas compression is a spark-
            ignited internal combustion design fueled by the natural gas itself. The thermo-
            dynamic theory involved is, at the basic level, no different than that for the tur-
            bine:fuelandairarecombined,compressed,andignited,andtheexpansionofthe
            burning gas is tapped to extract usable shaft work to power other processes, such
            as the gas compressor at the back of the compressor package.
               Reciprocating internal combustion engines can be sorted at the highest level
            into two basic designs: the two-stroke cycle and the four-stroke cycle (Fig. 7.7).
            Both use a piston moving inside an enclosed cylinder to draw fuel and air in,
            compress the mixture, ignite with a spark, then use the expansion of the burning
            gases to force the piston to turn a crank. But while the four-stroke cycle uses
            separate movements of the piston to do each step (intake-compression-
            power-exhaust), the two-stroke design accomplishes those activities with just
            two piston movements.
               The two-stroke design claims the advantages of relative simplicity and
            power density. The two-stroke designs commonly use open ports for intake
            and exhaust, with flow being regulated by the position of the piston instead
            of actuated valves as are found in a four-stroke engine. Also, in the two-stroke
            each cylinder has a power stroke on every rotation of the crankshaft, giving
            twice as many firing impulses as a four-stroke engine. The four-stroke design
            enables more precise control over the air/fuel mixture and the intake and
            exhaust valve timing, factors which can be manipulated to achieve high-
            efficiency and low exhaust emissions. When speaking of drivers for high-speed
            separable compressors, most engines today are four-stroke designs.


              Four-stroke cycle
                                                             Intake  Exhaust
                       Spark plug  Valves closed  Valves closed  valves closed  valve open
              Intake valve
              open           Exhaust valve
                             closed
              Air-fuel
              mixture
                                                                      Exhaust
                                                         Spark plug
                                                                      gases
                                                           firing
             Combustion
             chamber
             Piston
             Connecting
             rod
             Crankshaft
                       Intake      Compression     Power        Exhaust
                     Air-fuel mixture  Air-fuel mixture  Explosion forces  Piston pushes out
                      is drawn in  is compressed  piston down  burned gases
             © 2007 Encyclopædia Britannica, Inc.
            FIG. 7.7 Schematic representation comparing two- and four-stroke cycles.
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