Design of a hydraulic free-piston engine

Dutch companies Noax and Innas have developed an engine in which nearly all combustion energy is directly converted into hydraulic energy.

Complexity is never a goal in designing a new product, and that includes the off-highway industry. Although simple solutions are feasible, manufacturers continue to produce more complex products, and customers buy these products due to the lack of alternatives. Engines are designed to have low fuel consumption, reduced emissions, reliability, and, of course, to satisfy the customer.

An example of a redundant complexity is the motor/pump combination found in some off-highway equipment. In these combinations, the energy of the combustion process is converted into mechanical energy by means of a piston. At the pump side, the mechanical energy is converted into hydraulic energy, again by means of a translating piston. It seems logical to connect the combustion piston direcdy to the hydraulic piston and eliminate the mechanisms in between. This would then result in a “free-piston” configuration (Figure 1). Nevertheless, this is not the case in current motor/pump units. Instead, the engine and the pump are defined as two separate machines, both having a rotating shaft to connect to each other.