The invention provides a 
free piston engine, which comprises a 
cylinder block, a compression accumulator, a pump 
station and a 
piston assembly, and also comprises a first proportional overflow valve, a proportional pressure reducing valve, a proportional velocity regulating valve and a 
pressure sensor; a 
piston chamber in the 
cylinder block is divided into a 
combustion chamber, an air intake chamber, a balance chamber, a 
pump chamber and a compression chamber by the 
piston assembly. As a pipeline connected with two inlets of a compression main port and a compression starting port of the compression chamber and the compression accumulator is provided with the proportional pressure reducing valve, the proportional velocity regulating valve, a 
reversing valve, the proportional overflow valve and the 
pressure sensor, according to the detection 
signal of the 
pressure sensor connected with the compression accumulator, the 
reversing valve is switched between the compression accumulator and an 
oil supply accumulator, and therefore 
oil supply work on high and low-pressure two oil lines is completed by utilizing one 
hydraulic pump station; moreover, after hydraulic oil in the compression accumulator flows through the proportional pressure reducing valve, stable pressure can be obtained and supplied for the compression chamber, so that the pressure of the inlet of the compression chamber is stable.