Internal-combustion engines – Starting device – Compression relieving type
Reexamination Certificate
1999-02-08
2002-02-05
Yuen, Henry C. (Department: 3747)
Internal-combustion engines
Starting device
Compression relieving type
Reexamination Certificate
active
06343582
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a decompression device for a four-stroke engine, and more particularly, to a decompression device which temporarily opens the exhaust valve before the piston reaches the top dead center (TDC) in the compression stroke, when starting a hand-held four-stoke engine.
DESCRIPTION OF THE RELATED ART
The engines are frequently used in mowers, chain-sawing machines, pumps and exhaust fans etc. As a two-stroke engine is apt to cause air pollution, it is gradually substituted by a four-stroke engine which serves as the power source of the afore-mentioned machines.
An inlet valve and an exhaust valve are used to control the intaking and exhausting of the engine, respectively. The inlet valve and the exhaust valve are opened and closed by using a cam-follower mechanism, thereby realizing an inlet stroke, a compression stroke, a power stroke (or combustion stroke) and an exhaust stroke.
FIG. 1
is a front view illustrating a cam-follower mechanism of a conventional four-stroke engine. An arrow indicates the rotating direction of the cam.
FIG. 2A
is a front view illustrating the cam-gear portion shown in FIG.
1
.
FIG. 2B
is a side view illustrating the cam-gear portion shown in FIG.
1
.
As shown in
FIG. 1
, the cam-follower mechanism comprises a cam
101
having a profile surface and a central hole
107
, a gear
102
having the same central hole
107
as that of cam
101
and is fixed to cam
101
, a first follower
103
and a second follower
105
being biased, respectively, by a spring (not shown) so as to keep in contact with cam
101
and swingable around a pin
108
, and a first link
104
and a second link
106
keeping in contact with first follower
103
and second follower
105
at a point M and a point N, respectively.
Hereinbelow, the operation of the cam-follower mechanism will be described.
Referring to
FIG. 1
, gear
102
is driven to drive cam
101
to rotate clockwise. First follower
103
and cam
101
are constantly kept in touch with each other as described above. When first follower
103
is in contact with the ascending profile of cam
101
, first follower
103
is lifted by cam
101
and a counterclockwise pivoting moment around pin
108
is generated. Thus, the point M rises and pushes up first link
104
and, in turn, further opens an inlet valve (not shown) through another mechanism (not shown).
Next, when first follower
103
is in contact with the descending profile of cam
101
, the inlet valve is closed by the restoring force of an inlet valve spring (not shown).
Likewise, when second follower
105
is kept in contact with the ascending profile of cam
101
, second follower
105
is lifted by cam
101
and a clockwise pivoting moment around pin
108
is generated. Thus, the point N rises and pushes up second link
106
and, in turn, further open an exhaust valve (not shown) through still another mechanism (not shown).
Next, when second follower
105
is in contact with the descending profile of cam
101
, the exhaust valve is closed by the restoring force of an exhaust valve spring (not shown).
It should be understood that first follower
103
and second follower
105
are located on different two planes parallel to the paper plane of FIG.
1
.
FIG. 3
is a chart showing the relationship between the opening &PSgr; of the inlet (exhaust) valve and the crank angle &thgr; in a conventional four-stroke engine. As shown in
FIG. 3
, the horizontal and vertical axis denote the crank angle &thgr; and the opening &PSgr; of the valves respectively, the symbols E, I, C and P denote the exhaust stroke, the inlet stroke, the compression stroke and the power stroke respectively, and the symbols TDC and BDC denote the top dead center and the bottom dead center of the engine piston respectively.
Referring to
FIG. 3
, the curve shown by dotted lines designate the relationship between the opening &PSgr; of an inlet valve IV and the crank angle &thgr;, while the curve shown by solid lines designate the relationship between the opening &PSgr; of an exhaust valve EV and the crank angle &thgr;. When the crank angle &thgr; is from 0 to 360°, the piston moves from a bottom dead center (BDC) to a top dead center (TDC), and then return to the BDC.
Exhaust stoke E is realized when the crank angle &thgr; is approximately between 0° and 180°. In this duration, exhaust valve EV is gradually opened to a maximum opening and then gradually closed. On the other hand, inlet stroke I is realized when the crank angle &thgr; is approximately between 180° and 360°. In this duration, inlet valve IV is gradually opened to a maximum opening and then gradually closed. Thereafter, a compression stroke C is realized when the crank angle &thgr; is approximately between 360° and 540°, and a power stroke P is realized when the crank angle &thgr; is approximately between 540° to 720°. Among these four strokes, the pressure in the cylinder is the maximum when the piston reaches the TDC in the compression stroke C. Therefore, the resistance force of the engine shaft is the maximum.
When starting a conventional four-stroke engine, an external force (such as electric force or human force) is required to rotate the engine shaft so as to provide a moment of inertia for the engine to begin self-running after ignition in the combustion chamber. Due to the maximum resistance force when the piston reaches the TDC in the compression stroke, a larger force is required to overcome it so as to start the engine.
As a result, if the exhaust valve is opened slightly and temporarily before the piston reaching the TDC in the compression stroke, it is helpful for decreasing the pressure in the cylinder and reducing the starting force. However, this function has to be disabled when the engine is operated normally so that the efficiency is not affected. It is therefore a problem to be solved by the present invention.
SUMMARY OF THE INVENTION
In order to solve the above problem, it is therefore an object for the present invention to provide a decompression device for a four-stroke engine which can decompress the pressure in the cylinder by opening the exhaust valve before the TDC in the compression stroke, thereby reducing the starting force. Alternatively, this device can also disable the decompression function when the engine is operated normally.
In accordance with the present invention, there is provided a decompression device for a four-stroke engine, wherein the operating cycle of the engine includes an inlet stroke, a compression stroke, a power stroke and an exhaust stroke, and the engine includes a piston which reaches a top dead center (TDC) and a bottom dead center (BDC) at the two ends of each stroke, the decompression device comprising: an inlet valve and an exhaust valve for controlling the inlet stroke and the exhaust stroke respectively, a first rocker and a second rocker for driving the inlet valve and the exhaust valve respectively, a first link and a second link for driving the first and the second rocker respectively, a first follower and a second follower for driving the first link and the second link respectively, and a cam having a cam profile surface for driving the first and the second followers, characterized in: that the cam further comprises a guiding recess provided on the side surface thereof and a slot provided on the cam profile surface communicating with the guiding recess; that the decompression device further comprises a centrifugal member, received within the guiding recess, having a projecting portion and being rotatable and movable within the guiding recess, and a spring having two ends fixed to the cam and the centrifugal member; and that through the cooperation with the spring and the guiding recess, when the cam is rotated at a lower speed, the projecting portion of the centrifugal member projects through the slot to the outside of the cam profile surface under the action of a weaker centrifugal force, thereby lifting the second follower so as to open the exhaust valve before the TDC of the compression stroke, and when the ca
Castro Arnold
Industrial Technology Research Institute
Yuen Henry C.
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