Abstract: A two-stroke cycle nonscavenged internal combustion engine operates under controlled detonating conditions without damage to the engine and with greatly enhanced fuel economy. The engine may have any desired number of cylinders operating initially on a two-stroke cycle with spark and then under auto ignition conditions. A variable compression device is provided in each cylinder to store excess energy liberated at peak pressures and to release it back to the piston during its power stroke. A Scotch yoke connection between the piston rod and crank shaft is laterally guided to eliminate side thrust. A bounce piston shock absorber is provided to assist piston reversal and dampen inertia forces.