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Construction and Working Principle of Indicator

  • An engine indicator consists of a small bore cylinder containing a short stroke piston which is subjected to the same varying pressure that takes place inside the engine cylinder during one cycle of operations.
  •  This is done by connecting the indicator cylinder to the top of the engine cylinder in the case of single-acting engines, or through change over cocks and pipes leading to the top and bottom ends of the engine cylinder in the case of double-acting engines.
  • The gas pressure pushes the indicator piston up against the resistance of a spring, a choice of specially scaled springs of different stiffness being available to suit the operating pressures within the cylinder and a reasonable height of diagram.
  • A spindle connects the indicator piston to a system of small levers designed to produce a vertical straight-line motion at the pencil on the end of the pencil lever, parallel (but magnified about six times) to the motion of the indicator piston.
  • The “pencil” is often a brass point, or stylus, this is brought to press lightly on specially prepared indicator paper which is scrapped around a cylindrical drum and clipped to it.
  • The drum, which has a built-in recoil spring, is actuated in a semi-rotary manner by a cord wrapped around a groove in the bottom of it; a hook at its lower end to a reduction lever system from the engine crosshead attaches the cord, passing over a guide pulley.
  • Instead of the lever system from the crosshead, many engines are fitted with a special cam and tappet gear to reproduce the stroke of the engine piston to a small scale.
  • The drum therefore turns part of a revolution when the engine piston moves down, and turns back again when the engine piston moves up, thus the pencil or stylus on the end of the indicator lever draws a diagram which is a record of the pressure in the engine cylinder during one complete cycle.


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