Hydraulic Free Piston Engine Pump


Student: Ke Li , Ali Sadighi, Michael Koester


Project introduction:
The free piston engine is one of the alternatives of the conventional internal combustion engines. Instead of crank shaft, the pistons are synchronized by the hydraulic fluids and are able to vary the compression ratio. This technology provides a compact engine with high efficiency, while in the meantime, imposes control challenges in terms of precise and robust engine operation.


The main advantages of this engine include:
-Variable compression ratio
-Better fuel economy
-Multi-fuel operation
-Higher power density
-Internally balanced


The engine starts at its bottom dead center (BDC) where the distance between inner piston and outer piston are at the furthest Then the pistons move toward each other while the gas in the cylinder is being compressed towards the top dead center (TDC) where the gas undergoes an auto-ignition process. Force generated by combustion would then push the piston away from each other while the gas inside the other cylinder is being compressed to auto-ignition. The two chambers fire alternately to keep the piston pair moving linearly. Fig. 1 also shows the diagram of the hydraulic circuit. There are three linear hydraulic pistons attached to the engine piston. Two of them are located on the side push rods of the outer piston pair; a larger piston is mounted on the inner piston pair, with its plunger area equal to the total plunger area of the two small ones. During the piston oscillation, fluids are pushed from the left chamber of the hydraulic pumps into the high pressure accumulator, while fluids are drawn into the left chamber of the other hydraulic pistons from the low pressure accumulator. In other words, the kinetic energy of the pistons is converted into hydraulic energy stored in the accumulator. The right chambers of the hydraulic pumps are interconnected as a synchronizing volume; a pair of Lee valves is added to the circuit to control the synchronization. Together they form a synchronizing mechanism that keeps the inner and outer piston aligning with each other during the engine operation. Figures 2 and 3 show the schematic and picture of the test cell setup.



Figure 1 Hydraulic Free Piston Engine



Figure 2 Schematic of test cell setup


test cell

Figure 3 Picture of test cell setup



  1. Li, K. and Sun, Z., “Modeling and Control of a Hydraulic Free Piston Engine with HCCI Combustion”, Proceedings of the 2011 International Fluid Power Exposition, pp.567-576, March, 2011.
  2. Li, K. and Sun, Z., “Stability Analysis of a Hydraulic Free Piston Engine with HCCI Combustion”, to appear, Proceedings of the 2011 Dynamic Systems and Control Conference.
  3. Sadighi, A., Li, K. and Sun, Z., “A Comparative Study of Permanent Magnet Linear Alternator and Hydraulic Free Piston Engines”, to appear, Proceedings of the 2011 Dynamic Systems and Control Conference.