Internal-combustion engines – Poppet valve operating mechanism – Hydraulic system
Reexamination Certificate
2000-09-29
2001-12-04
Lo, Weilun (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
Hydraulic system
C123S090490
Reexamination Certificate
active
06325028
ABSTRACT:
The present invention relates to internal combustion engines of the type comprising:
at least one intake valve and at least one exhaust valve for each cylinder, each provided with respective spring means biasing the valve to a closed position, for controlling communication between respective intake and exhaust conduits and a combustion chamber,
a cam shaft for actuating the intake and exhaust valves of the engine cylinders by means of respective tappets, each intake valve and each exhaust valve being driven by a cam of said cam shaft,
wherein at least one of said tappets drives the respective intake or exhaust valve, against the action of said biasing spring means, with the interposition of hydraulic means including a pressure fluid chamber,
said pressure fluid chamber being able to be connected through a solenoid valve to an outlet channel, for uncoupling the valve from the respective tappet and cause a quick closing of the valve, under the action of the respective biasing spring means,
said hydraulic means further comprising a piston associated with a stem of the valve and slidably mounted within a guiding bush, said piston facing a variable volume chamber defined thereby inside the guiding bush, said variable volume chamber being in communication with the pressure fluid chamber by means of an end aperture of said guiding bush, said piston having an end appendage adapted to be inserted into said end aperture during the final portion of the travel of the piston corresponding to closing of the valve, in order to reduce the communication passage between said variable volume chamber and said pressure fluid chamber, so as to slow down the travel of the valve in proximity of its closed position.
An engine of the above indicated type is disclosed for example in European patent application EP-A0 803 642 of the same Applicant.
The above-described system provides a variable control of the opening of the intake and/or exhaust valves without altering the mechanical parts which drive the displacement of the valves. As a matter of fact, while in a conventional timing system the movement of each intake or exhaust valve is only determined by the geometry of the mechanical parts which drive the valve (cam, tappet, and rocker arm, if any) in the above described known system the solenoid valve controlling the pressure chamber associated with a given valve can be driven so as to open at any moment this is desired (typically this solenoid valve is controlled by electronic control means depending upon one or more parameters of operation of the engine), so as to empty the above mentioned chamber from fluid under pressure (usually the lubrication oil of the engine) and cause quick closing of the intake or exhaust valve, under the action of the respective biasing spring means, even during a stage in which the respective cam would tend to keep the valve opened.
As already indicated above, the known solution provides that with the valve there is associated a piston slidably mounted within a guiding bush. The piston faces a variable volume chamber defined thereby inside the guiding bush and communicating with the pressure fluid chamber through an end aperture of the guiding bush. In order to slow down the travel of the valve in proximity of its closed position, so as to avoid damages due to an impact of the valve against its seat at a too great speed when the pressure chamber is emptied in order to uncouple the valve from the respective tappet, the above mentioned piston has a tubular end appendage adapted to be inserted into the above mentioned end aperture during the final portion of the closing travel of the valve, in order to restrict the communication passage between the variable volume chamber and the pressure fluid chamber, so as to brake the valve in proximity of its closed position.
Studies and tests conducted by the Applicant however have shown that the braking effect obtained thereby can become excessive if fluid under pressure (typically the lubrication oil of the engine) has a high viscosity due to a low value of its temperature. Thus, for example, when the ambient temperature is low, such as in the order of −10° C., and the engine has not reached a condition of normal operation following a cold start, the viscosity of the oil may be such as to render the time required for closing the valve too long. For example, lubrication oil which in normal operative conditions may have a kinematic viscosity in the order of 15 centistokes, may arrive to have a viscosity of 4,000 centistokes at a temperature of −20° C.
In order to overcome this drawback, the invention provides an engine of the type indicated at the beginning of the present description, characterised in that said tubular end appendage has at its front a diametrical slot intercepting the inner cavity of said tubular end appendage and opening both on the front end and on the two opposite sides of this end tubular appendage, and in that the wall of said end tubular appendage has a radial hole opening on one side on the inner cavity of the tubular appendage and on the other side on the lateral wall of the appendage, this hole including a radially inner portion having a predetermined reduced diameter.
The provision of the above mentioned diametrical slot causes a gradual reduction of the cross-section of the oil flowing area during the closing travel of the valve and hance a smooth and progressive braking action. Furthermore, the above-mentioned portion of reduced diameter of the above mentioned radial hole is precisely chosen in such a way as to determine the required approaching speed of the valve to its seat. As a matter of fact, once the above mentioned diametrical slot comes out of communication with the variable volume chamber during the final portion of the valve travel, the above mentioned hole of predetermined diameter constitutes the prevailing path for the oil to escape from the variable volume chamber (apart from the very narrow coupling clearance between the piston and the bush).
The above-described measures dramatically reduce the sensitivity of the device to variations of temperature of the fluid. All the other conditions being the same, for a determined geometry of the hole of reduced diameter, there is a specific viscosity value of the fluid above which the fluid flow from turbulent becomes laminar. In the former case the viscosity does not affect the flow rate, whereas in the latter case the viscosity strongly affects the fluid flow rate. The geometrical shape for which the above mentioned transition occurs at the highest values of viscosity is a circular hole of short length. It is just to reduce the length of the hole as much as possible that in the invention the portion of the hole with reduced diameter is followed by a hole portion of a sufficiently enlarged diameter.
Therefore, the present invention keeps the approaching speed of the valve to its seat as much uniform as possible when the temperature, and hence the fluid viscosity, varies, since by the above-mentioned measures, the fluid motion is always substantially of turbulent type.
Therefore, the present invention efficiently solves the above mentioned problems, while insuring a narrow variability of the closing timing of the valve within a large thermal range, so that the engine can be controlled efficiently in any condition of operation.
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Albanello Stefano
Macor Lorentino
Vattaneo Francesco
C.R.F. Societa Consortile per Azioni
Lo Weilun
Sughrue Mion Zinn Macpeak & Seas, PLLC
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