The velocity of the free-piston assembly (FPA) of the hydraulic free-piston engine (HFPE) is very high. which is up to 10 m/s. In order to avoid the collision of the FPA to the cylinder head when the FPA moves towards the dead centres. the cushioning device must be involved. https://thegreensjunglebeautyshops.shop/product-category/oil-serum/