Air scavenging two-stroke cycle engine

Details Air scavenging two-stroke cycle engine Air scavenging two-stroke cycle engine

2000-05-15

2002-03-05

Wolfe, Willis R. (Department: 3747)

Internal-combustion engines

Two-cycle

Rear compression

C123S0650WA

Reexamination Certificate

active

06352058

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present Invention relates to an air scavenging two-stroke cycle engine used as a driving source of a small machine such as a brush cutter.
2. Description of the Related Art
The conventional air scavenging two-stroke cycle engine is shown in
FIG. 6
(see Japanese Patent Application Publication No. Hei. 10-121973)
FIG. 6
shows a state in which a piston
57
is positioned at a top dead center. This engine is provided with a scavenging block
53
between a cylinder
51
and a crank case
52
, in which a plurality of scavenging passages
54
are formed. In a state in which the piston
57
is positioned in the vicinity of a bottom dead center, the plurality of scavenging passages
54
make a combustion chamber
50
in the cylinder
51
and a cylinder head
61
communicate with a crank chamber
52
a
in the crank case
52
. These scavenging passages
54
are connected to air passages
55
for introducing air from a portion of an intake passage(not shown) that is situated downstream from an air cleaner. Check valves
56
comprising reed valves for opening or closing openings
55
a
of the air passages
55
that are opened to the scavenging passages
54
are provided on inner surfaces of the scavenging passages
54
.
In the engine so configured, a piston
57
in the cylinder
51
moves upward from a bottom dead center, which sequentially closes a scavenging port
54
a
of the scavenging passage
54
that is opened in the combustion chamber
50
and an exhaust port (not shown) to thereby cause an internal pressure of the combustion chamber
50
to be increased and an inside of the crank chamber
52
a
and the scavenging passage
54
to have negative pressures. Thereby, the intake passage (not shown) connected to the crank chamber
52
a
is opened and fuel-air mixture is introduced into the crank chamber
52
a
. Simultaneously, the check valve
56
is opened and air from the air passage
55
is Introduced into the scavenging passage
54
.
Just before a top dead center of the piston
57
, the fuel-air mixture in the combustion chamber
50
is ignited by an ignition plug
58
and explodes, and then the piston
57
begins to move downward. Along with this downward movement of the piston
57
, the exhaust port is first opened and combustion gas in the combustion chamber
50
is then discharged externally to thereby cause the internal pressure of the combustion chamber
50
to be reduced. Then, the scavenging port
54
a
of the scavenging passage
54
is opened. Prior to the fuel-air mixture, the air introduced into the scavenging passage
54
is ejected into the combustion chamber
50
where the internal pressure has been reduced and expels the residual combustion gas inside thereof externally from the exhaust port, to thereby perform initial scavenging operation in the combustion chamber
50
by using the air. In this case, the scavenging air blowing by from the exhaust port is air, and therefore blow-by of the fuel-air mixture hardly occurs. Following this, the fuel-air mixture in the crank chamber
52
a
is introduced through the scavenging passage
54
into the combustion chamber
50
, whereby scavenging operation is completed. Thereafter, the above cycle is repeated.
However, in the engine so configured, since the check valve
56
is provided in the scavenging passage
54
, there is a possibility that the scavenging air or the fuel-air mixture does not smoothly flow through the scavenging passage
54
into the combustion chamber
50
due to an obstacle such as the check valve
56
. In addition, it is necessary to provide the check valves
56
as many as the scavenging passages
54
. Accordingly, the man-hour of assemblies is increased with an increase in the number of the check valves
56
or stoppers thereof and fitting screws, leading to high cost. Further, since the scavenging block
53
is formed separately from the cylinder
51
to allow the check valve
56
to be provided in the scavenging passage
54
, the number of parts such as screws or gaskets with which the scavenging block
53
is mounted is increased, also leading to high cost.
In the above configuration, the initial scavenging operation is not necessarily performed in the combustion chamber
50
only by using air. More specifically, although the residual fuel-air mixture in the scavenging passage
54
after a previous scavenging stroke is returned to the crank chamber
52
a
due to the air introduced through the air passage
55
, the fuel-air mixture and the air tend to be mixed in the scavenging passage
54
because of small length and large cross-sectional area of the scavenging passage
54
. In addition when the air is introduced into the scavenging passage
54
, the fuel-air mixture remains in an end portion of the scavenging passage
54
, i.e., between the scavenging port
54
a
and the opening
55
a
of the air passage
55
. For this reason, the initial scavenging operation is performed in the combustion chamber
50
by using the air including the fuel-air mixture, and the fuel-air mixture blows by from the exhaust port.
Another example of the air scavenging two-stroke cycle engine is described in Japanese Patent Application Publication No. Hel. 9-268918. In this engine, an air passage is connected to a scavenging passage through which the fuel-air mixture is introduce into the combustion chamber, and the air is ejected into the combustion chamber from the scavenging port. In this case, it is also necessary to provide the check valves as many as the scavenging passages at connected portions of the scavenging passages and the air passages. Therefore, the number of parts is increased. In addition, the air and the fuel-air mixture are mixed in the scavenging passages, and therefore the initial scavenging operation cannot be performed only by using air.
SUMMARY OF THE INVENTION
In consideration of such circumstances, it is an object of the present invention to provide an air scavenging two-stroke cycle engine which is capable of smoothly supplying fuel-air mixture from a crank case into a combustion chamber, reducing the number of parts to thereby reduce the man-hour of assemblies and cost, and performing scavenging operation only by using air in the initial stage of the scavenging stroke to suppress blow-by of the fuel-air mixture from an exhaust port, in order to realize high fuel efficiency and reduction of concentration of HC emission.
In an aspect of the present invention, an air scavenging two-stroke cycle engine that introduces fuel-air mixture introduced into a crank case provided on a lower portion of a cylinder through an intake port into a combustion chamber in an upper portion of the cylinder from a first scavenging port and discharges a combustion gas externally from a discharge port, by up and down movement of a piston in the cylinder, comprises: an air passage; a communicating passage; and a second scavenging port, wherein the second scavenging port is formed in the cylinder such that it is disposed above the intake port and has an upper end higher than an upper end of the first scavenging port and lower than an upper end of the discharge port, the air passage is connected to the second scavenging port for introducing air through a check valve, and a portion of the air passage that is situated downstream from the check valve communicates with an inside of the crank case through a communicating passage.
In the engine so configured, in a scavenging stroke, scavenging operation is firstly performed by using air such that the second scavenging port is opened, scavenging air filled in the communicating passage is ejected into the combustion chamber, and then combustion gas in the combustion chamber is expelled externally from the exhaust port. Following this, the first scavenging port is opened and fuel-air mixture in the crank case is thereby introduced into the combustion chamber, whereby scavenging operation is completed.
When the scavenging operation performed in the combustion chamber by using the air is completed, the fuel-air mixture is tille

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