Mechanism of direct-actuated type variable valve control system, HYPER VTEC


Aiming at reconciling engine power and environmental protection measures for sports type motorcycle engines of high revolution and high power, the Development Team of Asaka R&D Center challenged to the development of "direct-actuated type" variable valve control. As the result, an epoch-making variable valve system came into the world, which has a valve operation switchover system built inside valve lifters.

Goal of Development of Valve Control System for Sports Type Motorcycle Engines
A variety of technological goals were set for the development of HYPER VTEC. The first was to guarantee the operation of kinetic valve system up to the high revolution range of engines over 10,000rpm. The next was to minimize the increase of mass around the valves, the most serious impediment to high revolution engines. Moreover, to make the mechanism as compact and simple as possible, let it have high reliability and durability and enable to be mass-produce at reasonable costs. The development of "direct-actuated type" valve control system started, aiming at these goals.
Details of HYPER VTEC Mechanism
HYPER VTEC is designed for four valve engines. A cylinder is equipped with two intake valves and two exhaust valves. Set to each of them were normal valves working over the whole ranges from low speed revolution to high speed revolution, and paused valves which shut down in low and medium revolution range and are actuated at time of high revolution operation only. The system is made up of, in addition to them, a hydraulic control system including the valve operation switchover system and hydraulic circuit built inside the valve lifter of paused valves. The valve shutdown and actuation are controlled by the positions of the valve switchover pins built inside the valve lifters (hereinafter called the slide pins). Needless to say, cam profile giving the optimum valve timing is set on each normal use and shutdown valve.

Details of HYPER VTEC Mechanism

State of Valve Shutdown
While the valves are in the state of shutdown, no hydraulic pressure works on the slide pins so that the slide pins are pushed against the stopper pins by the return springs. Since the valve stems then crawl into the through holes provided in the slide pins, the valves will remain in the state of shutdown even if the valve lifters and switchover mechanism are driven by the cams. The valve lifters and switchover mechanism follow the motion of the cam profiles with the outer valve springs.

State of Valve Shutdown

Conditions of Valve Operation
Hydraulic pressure passes through the hydraulic passage inside the cylinder head and acts on the slide pins through the outer peripheral orifices of the lifter from the hydraulic grooves inside the lifter hole, to slide the slide pins. Since the slide pins slide, when the valve lifters are driven, the flat section of slide pins will push the valve stems to move the valves and valve lifters in one body. Valves, valve retainers and cotters follow up with the motion of the cam profiles by inner springs.

Conditions of Valve Operation

Comparative Examination of Valve Shutdown Mechanism
With respect to the valve shutdown mechanism built inside the valve lifters, various mechanisms were examined in an effort to make it more compact and simple. What to do with the shim used for adjusting tappet clearance? How to regulate the movements of valve lifters and valve stems while the motion is idle, i.e., in the state of shutdown? We will introduce here four plans of valve shutdown mechanism, which were compared and examined at the stage of development.

Comparative Examination of Valve Shutdown Mechanism

The Plan A is characterized by the adjustment of tappet clearance with outer shims and slide pin installing holes were formed directly on the valve lifters. The plan grows large in the mass of mechanism because of outer shims. Furthermore, machining of slide pin installing holes on the valve lifters is difficult.

The plan B is featured with the adjustment of tappet clearance with inner shims, with the switchover mechanism being separated from the valve lifters and with projection provided in the middle of valve lifters, and the spring for idle motion of valve lifter was arranged between the valve lifter and switchover mechanism. The shutdown mechanism and valve lifter were placed on top of the conventional kinetic valve system. Detents were needed for the valve lifters and shutdown mechanism, and space was insufficient for the installation of springs for idle motion of valve lifters, so that spring load could not be set to follow up to high revolution range.

For the plan C, the slide pin was located downward to reduce the idle motion space for the valve lifters. In the plan, the mass of mechanism for outer shims grew large and the machining of slide pin installing holes on the valve lifters was difficult.

For the plan D, the tappet clearance was adjusted with inner shims and the switchover mechanism was made a separate body. The switchover mechanism could be manufactured by cold forging to make it lightweight.

As the result of comparison and examination of these plans, the plan D was adopted to produce very compact and lightweight valve shutdown mechanism suitable for HYPER VTEC.


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