Steam Engine Design/Free Piston Steam Hydraulic

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Free Piston Steam-Hydraulic Engine

Main > Energy > Steam Engine


Intake and exhaust poppet valves operated electrohydraulically.

This engine's pulsating hydraulic output would be directed to a hydraulic accumulator in a typical stationary hydraulic power installation.

See some other free-piston references here and here.


Some features of this design concept

  • This is a variable stroke, variable intake cutoff, variable exhaust cutoff machine, so it should be able to operate efficiently over a fairly wide range of operating conditions
  • The intake, exhaust, and hydraulic valve timings are controlled by an "intelligent" algorithm in a microcontroller
  • There is no flywheel required or desirable
  • The piston returns to TDC (ironically, Top Dead Center is downwards because of the inverted design) by gravity
  • Gravity return limits the maximum operating speed to a few hundred rpm
  • The amount of energy the falling piston mass carries as it approaches TDC is directly proportional to the stroke length; the stroke length is determined on a cycle-by-cycle basis by the hydraulic valve timing
  • The energy of the falling piston mass is delivered to two places
    • Recompressing the steam
      Recompressing the steam to match the inlet pressure eliminates one type of energy loss at the intake valve
    • Compressing the rebound springs
      The rebound springs are not theoretically essential but provide some extra flexibility in engine control timing
      Energy stored in the rebound springs is returned to the piston's upstroke so this is not an energy loss
  • The piston will typically accelerate upwards ballistically (with no hydraulic load, because the free-flow hydraulic valve is open) during the beginning of the stroke
  • At a certain point in the piston up-stroke the hydraulic valve will close, decelerating the piston rapidly and delivering its energy into the hydraulic system
  • At very light loads, the engine can operate on an intermittent basis, doing one or two cycles then waiting at TDC.

Simulation Model

Simulation model using freely available LTSpice IV simulation program. LTSpice is intended for electrical schematics, so it labels everything as volts and amps. However the simulation math doesn't care whether we are voltage or pressure or mass.

This code is a "first cut" and is missing several friction/loss mechanisms. It also lacks a lookup table for wet steam behavior. Feel free to exercise the model with different parameter values (for many you can just edit the .param lines) and to add improvements. Steam_Engine_Design/Free_Piston_Steam_Hydraulic/SpiceCode