18 February 2016

Free-Piston Engine

free-piston engine is a linear crank less internal combustion engine.

Combination of a reciprocating engine and a rotary turbine.

It consists of a gasifier and a turbine. Compressor piston are directly attached to diesel piston.

The piston is free because its motion is not controlled by the position of a rotating crankshaft Hence, It is called a free piston engine.

Important features of the free-piston engine are potential reductions in frictional losses.

Operating principle :

The free-piston engine is restricted to the two-stroke operating principle, as a power stroke is required on every cycle. Although two-stroke engines suffer from poorer performance compared to four strokes, this performance gap is declining and recent years have seen an increased interest in small scale two-stroke engines.

Important steps in this direction are :
  • Improvements in the fuel properties 
  • Component design changes for higher efficiency
  • Lower cost and weight
Reasons for the interest in developing the free piston engine :
  • Higher part load efficiency
  • Possible multi-fuel operation
  • Reduced frictional loss
  • Simple design
  • few moving parts
  • variable compression ratio
Free piston engine are usually divided into three categories :
  • Single Piston
  • Dual Piston
  • Opposed Piston
Advantages of free-piston engine :
  • Mechanical simplicity
  • Flexibility and reliability
  • Power to weight ratio
  • Multi fuel capability
  • Low turbine operating temperature
  • Less vibration and noise
  • Less maintenance
  • Easy to start piston 
  • Highly suitable for waste heat recovery
  • Give high degree of maneuverability 
  • Less manufacturing cost
Disadvantages of free-piston engine :
  • Poor fuel economy
  • Stability of fuel supply
  • Poor part load efficiency
  • High combustion rate
  • Reduction gearing is require
Application of free-piston engine :
  • Submarine air compressor units
  • Suitable power generation 
  • Ship propulsion
  • Road and rail traction
  • Aircraft
  • High speed centrifugal air compressor
  • Mixed gas-steam cycle
  • For pumping oil 
  • Free-piston gas generators
  • Hydraulic free-piston engines