Optical internal combustion engine for tests with piston cooling

The engine has an extension placed on a lower piston (20) to carry an active piston (26). An oil pump (54) acquires cooling fluid e.g. oil, present in a base of a case (14) for distributing it to distributing ducts. A supplying unit (58) supplies the fluid to the active piston. A circulating unit circulates the fluid inside the active piston and a return unit returns the fluid into the case to obtain a cooling circuit in closed loop.