The invention relates to a method capable of realizing unloading when an engine is braked or the engine is started or stopped, which is composed of a servovalve mechanism and an oil circuit controlling mechanism and is characterized in that the servovalve mechanism comprises a
locknut, a stop collar, a stretching screw, a
piston, a check ring and a
piston spring; and the oil circuit controlling mechanism comprises a
solenoid valve, a control valve, a control valve clamp spring, a control valve return spring and a
braced frame. The stop collar, the
piston, the check ring and the piston spring move downward under the function of
oil pressure to push a slide valve on an
engine valve bridge in order to open an
exhaust valve about 1 mm before the engine is started. Under the two conditions that the engine is started or stopped, the initial resistance of the compression
stroke can be reduced when the engine is started, the resisting moment is decreased when a starting engine rotates a
crankshaft and the service life of the starting engine is prolonged; under the condition of braking, the exhaust value is opened at an angle before a
top dead center is compressed, and the engine which is equivalent to an
air compressor consumes
kinetic energy to slow a vehicle in order to achieve the braking effect. The invention has the advantages of simple structure and
control system and high response speed; and the engine consumes the least energy in the whole starting process, which improves the economy of the whole engine.