Engines of the type in which a rocker arm is provided in a valve mechanism draw in and discharge a fuel gas and an exhaust gas by pressing a valve end, so as to open the valve with an adjustment screw on the distal end of a rocker arm that is actuated by a cam. When the rocker arm returns to an original position, the valve is closed again under a resilient force of a spring.
A clearance (hereinafter referred to as a tappet clearance) is provided between the valve end and the adjustment screw, for allowing the valve to be fully closed when the rocker arm returns to the original position. If the tappet clearance is too small, then the clearance may possibly be eliminated due to thermal expansion at high temperatures. If the tappet clearance is too large, then the valve end and the adjustment screw produce large sounds as noise when they contact each other. Therefore, the tappet clearance has to be adjusted accurately to an appropriate value (or within an appropriate range) that is preset in design. Particularly, a process for manufacturing a large quantity of engines in a wide variety of types needs to have a reduced adjustment time per engine, while maintaining a high adjustment accuracy level. It is preferable to be able to adjust the tappet clearance automatically in order to prevent adjustment fluctuations.
Processes for adjusting tappet clearances are described in Japanese Patent Publication No. 5-35243 and Japanese Laid-Open Patent Publication No. 11-153007. According to the valve clearance setting process described in Japanese Patent Publication No. 5-35243, an intake passage and an exhaust passage, which are opened and closed by valves, are sealed, and compressed air of a predetermined pressure is introduced into the intake passage and the exhaust passage. Then, adjustment screws engaging rocker arms are threaded in, so as to press and open the valves, until the pressure in the intake passage and the exhaust passage is lowered to a given pressure. Thereafter, the adjustment screws are rotated a predetermined angular interval in a direction opposite to the direction in which they were threaded in, for thereby closing the valves and providing a predetermined clearance between the valves and the adjustment screws. This method is capable of appropriately setting the valve clearance with considerable accuracy.
According to the process described in Japanese Laid-Open Patent Publication No. 11-153007, the tappet clearance is adjusted while the pressure in the combustion chamber that is supplied with air under high pressure is being monitored. The tappet clearance can be adjusted accurately almost without requiring any skill.
According to the process described in Japanese Laid-Open Patent Publication No. 11-153007, because the pressure inside the combustion chamber is relatively high, air flow tends to be disturbed, and hence accurate measurements cannot be made until the pressure in the combustion chamber becomes stabilized. Accordingly, it may be difficult to adjust the tappet clearance quickly. In particular, this process is not appropriate for adjustment of tappet clearances of a large quantity of engines having a wide variety of engine types.
According to the process, since the worker uses a screwdriver to adjust the distance that the adjustment screw is threaded in, it would be desirable to make the process automatic in order to reduce the burden on the worker as well as to adjust the tappet clearance with higher accuracy in a shorter period of time.
According to the process of pressurizing the combustion chamber in the processes disclosed in Japanese Patent Publication No. 5-35243 and Japanese Laid-Open Patent Publication No. 11-153007, the supplied pressure cannot catch up with an air leakage from the gap in the piston ring. Since different amounts of air leak through the ring gaps in various engine types, an adjusting apparatus needs to be prepared respectively for each of the engine types.