Cooling system for spark-ignition two-cycle engine

In a spark-ignition two-cycle engine, the inlet end of a cylinder cooling water passage is connected to the discharge port of a cooling water pump by a cooling water pipe. The inlet end of a cylinder head cooling water passage is connected to the discharge port of a cooling water pump by a cooling water pipe. The outlet end of the cylinder cooling water passage is connected through a thermostat to the upper end of a cylinder cooling radiator, and the outlet end of the cylinder head cooling water passage is connected through a three-way valve to the upper end of a cylinder head cooling radiator and a pipe fitting. Upon the increase of the temperature of the cooling water for cooling the cylinder head to a predetermined temperature, a CPU provides a control signal to a servomotor to close a port of the three-way valve connected to the pipe fitting and to open a port of the three-way valve connected to the cylinder head cooling radiator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a cooling system for a spark-ignition 
two-cycle engine that makes a fresh charge charged into its combustion 
chamber self-ignite at least in a low-load operation mode during a cold 
condition of the engine such as at the start of operation of the two-cycle 
engine. 
2. Description of Background Art 
A conventional automotive spark-ignition two-cycle engine to be mounted on 
a motorcycle has a cylinder provided with an exhaust port and a scavenging 
port disposed so as to be opened and closed by a piston fitted in the 
cylinder bore thereof. New charge compressed in a crank chamber is 
supplied through the scavenging port into the cylinder bore while the 
exhaust gas is discharged through the exhaust port, and the fresh charge 
compressed in the combustion chamber is ignited by an ignition plug. Since 
some quantity of fresh charge must be supplied into the cylinder during 
idling operation, a restrictor valve provided on an intake passage must be 
opened at a certain opening, for example, at an opening equal to 10% or 
more of the full opening. 
When a large exhaust port is formed in the cylinder of such a conventional 
spark-ignition two-cycle engine to increase the output and efficiency of 
the spark-ignition two-cycle engine in a high-speed high-load operation 
mode, the blow-by and unstable combustion of the fresh charge occur while 
the spark-ignition two-cycle engine is operating in a low-load operation 
mode, which increases the unburnt hydrocarbon concentration of the exhaust 
gas and fuel consumption. 
To solve such problems, the applicants of the present patent application 
developed a spark-ignition two-cycle engine and proposed the same in 
Japanese Patent Application No. 5-187488. This previously proposed 
spark-ignition two-cycle engine regulates the valve opening ratio of an 
exhaust control valve according to the engine speed and the opening of the 
throttle valve to regulate the pressure in the cylinder at an appropriate 
pressure in a state where the exhaust port is closed by the piston at 
least in a low-load operation mode to make the fresh charge supplied into 
the combustion chamber self-ignite at an ignition time suitable for the 
operation of the engine. 
Combustion initiated in an activated thermal atmosphere by positively 
controlling ignition timing suitable for the operation of the engine will 
be called "AR combustion" hereinafter. 
The spark-ignition two-cycle engine capable of causing AR combustion in 
such a low-load operation mode activates the fresh charge by the thermal 
energy of the combustion gas. Therefore, it is difficult to cause AR 
combustion at the start of the engine because the temperature of the wall 
of the combustion chamber is low. 
Particularly, a conventional water-cooled two-cycle engine 01 is provided 
on a cooling water passage with a thermostat 02 that allows cooling water 
to flow at a low flow rate even when the temperature of the engine is low. 
A cooling water pump 03 is directly coupled with the crankshaft, not 
shown, and a water jacket forms cooling water passages around a wall 04 
defining a cylinder bore and a wall 05 defining a combustion chamber. 
Therefore, the cooling water flows through the cooling water passage 
around the wall of the combustion chamber to cool the wall of the 
combustion chamber even immediately after the engine has been started and, 
consequently, increase in the temperature of the wall of the combustion 
chamber is suppressed and hence it is difficult to start AR combustion. 
OBJECTS AND SUMMARY OF THE INVENTION 
The present invention relates to improvements in a spark-ignition two-cycle 
engine to overcome such disadvantages and it is therefore an object of the 
present invention to provide a cooling system for a spark-ignition 
two-cycle engine that causes a fresh charge charged into its combustion 
chamber to self-ignite at least in a lowload operation mode, the cooling 
system comprising a cylinder cooling system, and a cylinder head cooling 
system combined in parallel with the cylinder cooling system and having a 
cooling capacity lower than that of the cylinder cooling system at the 
start of the two-cycle engine, wherein a cooling water temperature 
regulating means for increasing the cooling capacity of the cylinder head 
cooling system after the temperature of the cooling water circulating 
through the cylinder head cooling system has reached a predetermined 
temperature. 
According to the present invention, the temperature of the cooling water 
circulating through the cylinder head cooling system rises at a rate far 
higher than the temperature of the cooling water circulating through the 
cylinder cooling system immediately after the start of the two-cycle 
engine. Therefore, the temperature of the wall of the combustion chamber 
reaches a temperature at which AR combustion is possible in a short period 
of time, so that the unburnt hydrocarbon concentration of the exhaust gas 
is reduced and fuel consumption is improved. 
Upon the increase of the temperature of the cooling water circulating 
through the cylinder head cooling system to a predetermined temperature, 
the cooling water temperature regulating means operates to increase the 
cooling capacity of the cylinder head cooling system to maintain the wall 
of the combustion chamber at an appropriate temperature even during 
high-load operation to ensure the smooth operation of the spark-ignition 
two-cycle engine. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

PREFERRED EMBODIMENTS OF THE INVENTION 
A cooling system in a preferred embodiment according to the present 
invention will be described hereinafter with reference to FIGS. 1 to 5. 
A spark-ignition two-cycle engine 1 incorporating the present invention and 
provided with a restrictor valve controller is mounted on a motorcycle, 
not shown. The spark-ignition two-cycle engine 1 has a crankcase 2, a 
cylinder block 3 fixedly mounted on the crankcase 2, and a cylinder head 4 
fixed to the upper end of the cylinder block 3. 
A piston 6 axially slidably fitted in a cylinder bore 5 formed in the 
cylinder block 3 is connected to a crank 8 by a connecting rod 7. As the 
piston moves axially in the cylinder bore 5, the crank 8 is driven for 
rotation. 
An intake pipe 10 is connected to a crank chamber 9 formed in the crankcase 
2, and a carburetor 11 and a reed valve 13 are arranged one after the 
other on the intake pipe 10. The piston type throttle valve 12 of the 
carburetor 11 is connected through a rod 14 and a lever 15 to a throttle 
drum 16 which in turn is connected to a throttle grip, not shown, by a 
wire, not shown. When the throttle grip is turned in one direction the 
throttle valve 12 is raised to increase throttle opening. 
The intake pipe 10 is connected to the crank chamber 9 of the crankcase 2. 
A scavenging port 17 and an exhaust port 18 are formed in the wall of the 
cylinder bore 5. The scavenging port 17 communicates with the crank 
chamber 9 by means of a scavenging passage 19, and the exhaust port 18 is 
connected to an exhaust passage 20. 
A recess is formed in the cylinder head defining a combustion chamber 21 
over the cylinder bore 5 on the side of the exhaust port 18, and a spark 
plug 22 is placed in the recess. An air-fuel mixture i.e., a fresh charge, 
is taken through the reed valve 13 into the crank chamber 9 in which a 
negative pressure prevails while the piston 6 is in an up stroke. The 
fresh charge is compressed in the crank chamber 9 while the piston 6 is in 
a down stroke, and the compressed fresh charge is caused to flow into the 
combustion chamber 21 when the scavenging port 17 is opened. Then, part of 
the combustion gas is discharged from the combustion chamber 21 through 
the scavenging port 17 into the scavenging passage 19. As the piston 6 
moves upward, first the scavenging port 17 is closed, then the exhaust 
port 18 is closed, and then the fresh charge is compressed in the 
combustion chamber 21. Upon the arrival of the piston 6 at a position near 
the top dead center, the fresh charge is ignited by the spark plug 22 or 
is caused to self-ignite by the thermal energy of the residual combustion 
gas. 
An exhaust control valve 23 is disposed near the exhaust port 18. The 
exhaust control valve 23 is fitted in a space 26 of a uniform width formed 
between a recess 24 formed in the cylinder block having a longitudinal 
cross section having the shape of a circular arc and an exhaust passage 
member 25 having a longitudinal cross section substantially the same as 
that of the recess 24, and is pivotally supported for turning in a 
vertical plane. As shown in FIG. 2, a driving lever 28 is fixedly mounted 
on a driving shaft 27 fixed to the exhaust control valve 23, and the 
driving lever 28 is connected to a pulley 31 fixed to the output shaft of 
an exhaust control servomotor 30 by a driving cable 29. The exhaust 
control servomotor 30 drives the exhaust control valve 23 for vertical 
turning to set the exhaust control valve 23 at an exhaust opening ratio 
.theta..sub.e in the range of 0 to 100%. 
The side arms 23b of the exhaust control valve 23 having a U-shaped 
horizontal cross section are fitted in spaces 32 extending on the opposite 
sides of the exhaust passage 20, so that only the circular valve portion 
23a of the exhaust control valve 23 for closing the exhaust port 18 is 
exposed to the exhaust gas. The side arms 23b do not adversely interfere 
with the flow of the exhaust gas at all. 
As shown in FIG. 4, the spark-ignition two-cycle engine 1 is provided with 
cooling water pumps 33 and 34, which are driven by the crank 8. 
The inlet end of a cylinder cooling water passage 35 is connected to the 
discharge port of the cooling water pump 33 by a cooling water pipe 37. 
The inlet end of a cylinder head cooling water passage 36 is connected to 
the discharge port of the cooling water pump 34 by a cooling water pipe 
38. The outlet end of the cylinder cooling water passage 35 is connected 
through a thermostat 39 to the upper end of a cylinder cooling radiator 
40, and the lower end of the cylinder cooling radiator 40 is connected to 
the suction port of the cooling water pump 33. 
The outlet end of the cylinder head cooling water passage 36 is connected 
through a three-way valve 41 to the upper end of a cylinder head cooling 
radiator 42, and the lower end of the cylinder head cooling radiator 42 is 
connected through a pipe fitting 43 to the suction port of the cooling 
water pump 34. The cylinder head cooling system is operated at a lower 
cooling capacity than that of the cylinder cooling system when the engine 
is in a cold condition such as at the start of the engine. Upon the rise 
of the temperature of the cylinder head cooling water beyond a 
predetermined temperature, a CPU 45 provides a control signal to operate a 
servomotor 44 so that the three-way valve 41 is changed from a position 
which connects the cylinder head cooling water passage 36 to the pipe 
fitting 43 into a position to connect the cylinder head cooling water 
passage 36 to the cylinder head cooling radiator 42 and to disconnect the 
cylinder head cooling water passage 36 from the pipe fitting 43. 
Referring to FIG. 5 typically showing an essential portion of the 
spark-ignition two-cycle engine 1, the valve opening .theta.th of the 
manually operated restrictor valve 12 is detected by a restrictor valve 
opening detector 46, such as a potentiometer, and a signal representing 
the valve opening .theta.th is given to the CPU 45. 
The CPU 45 also receives signals representing an engine speed Ne detected 
by an engine speed detector 47, an intake pressure P.sub.i detected by an 
intake pressure sensor 48, a cooling water temperature T.sub.w measured by 
a thermometer 49, an indicator pressure, a maximum indicator pressure 
generating time or ignition time detected by an optical sensor 50 or a 
compression starting pressure P.sub.EC, the condition of the clutch, and 
the speed of the transmission. 
The CPU 45 judges the operating condition of the spark-ignition two-cycle 
engine 1 from those input signals and provides control signals. In an 
operation mode for AR combustion, the CPU 45 operates on the basis of a 
control map shown in FIG. 6 specifying exhaust opening ratio .theta..sub.e 
according to engine speed Ne and restrictor valve opening .theta.th and 
sends a driving signal .DELTA..theta..sub.e to select an exhaust opening 
ratio .theta..sub.e specified in the control map to the servomotor 30. 
In the cooling system shown in FIGS. 1 to 5 as described, a port of the 
three-way valve 41 connected to the pipe fitting 43 is opened to return 
the cooling water to the suction port of the cooling water pump 34 without 
passing the cooling water through the cylinder head cooling radiator 42. 
Therefore, the cooling water flowing through the cylinder head cooling 
water passage 36 is not cooled in the cylinder head cooling radiator 42 
and hence the wall of the combustion chamber 21 is not cooled excessively. 
Consequently, the combustion chamber 21 can be quickly heated to a 
temperature at which AR combustion is possible. 
Upon the increase of the temperature of the cooling water flowing through 
the cylinder head cooling water passage 36 to the predetermined 
temperature, the port of the three-way valve 41 connected to the pipe 
fitting 43 is closed and a port of the three-way valve 41 connected to the 
cylinder head cooling radiator 42 is opened to allow the cooling water 
flowing through the cylinder head cooling water passage 36 to flow into 
the cylinder head cooling radiator 42. Consequently, the cooling water 
cooled properly in the cylinder head cooling radiator 42 is returned into 
the cylinder head cooling water passage 36 by the cooling water pump 34 to 
keep the temperature of the combustion chamber 21 in an appropriate 
temperature range, so that the spark-ignition two-cycle engine 1 is able 
to continue operation in a normal combustion mode or the AR combustion 
mode. 
Referring to FIG. 7, as the temperature of the cooling water flowing 
through the cylinder head cooling water passage 36 rises from, for 
example, 50.degree. C. toward 90.degree. C., the indicated average 
effective pressure suitable for AR combustion decreases and the lower 
limit traveling speed decreases as well. Accordingly, AR combustion is 
possible even if the traveling speed further decreases, the discharge of 
unburnt hydrocarbons can be suppressed and the fuel consumption can be 
improved. 
Although the embodiment shown in FIGS. 1 to 5 is provided with the 
three-way valve 41 provided on the line connecting the cylinder head 
cooling water passage 36 to the cylinder head cooling radiator 42, in 
another embodiment, it is also possible to connect the cylinder head 
cooling water passage 36 through a thermostat 51 to the cylinder head 
cooling radiator 42 as shown in FIG. 8. In FIG. 8, the three-way valve 41 
and the bypass line connecting the three-way valve 41 to the pipe fitting 
43 are omitted. Although the embodiment shown in FIG. 8 is unable to raise 
the temperature of the cooling water flowing through the cylinder head 
cooling water passage 36 as quickly as the embodiment shown in FIGS. 1 to 
5, the restrictive effect of the thermostat 51 and the resistance against 
the flow of the cooling water higher than that of the cylinder cooling 
water passage 35 make the temperature of the cooling water flowing through 
the cylinder head cooling water passage 36 rise at a rate higher than that 
at which the temperature of the cooling water flowing through the cylinder 
cooling water passage 35 to raise the temperature of the wall of the 
combustion chamber as quickly as possible to a temperature at which AR 
combustion is possible. 
It is also possible, as shown in FIG. 9, to form the cylinder cooling water 
passage 35 and the cylinder head cooling water passage 36 in a parallel 
combination. In this arrangement, the cooling water flowing through the 
cylinder cooling water passage 35 and the cylinder head cooling water 
passage 36 flow through a single radiator 53 by a single cooling water 
pump 52 while the temperature of the cooling water is high. A shut-off 
valve 54 provided in the cylinder head cooling water passage 36 is closed 
by an actuator 55 operated by a control signal provided by the CPU 45, 
which receives a temperature signal from the cylinder head 4, to raise the 
temperature of the cooling water in the cylinder head cooling water 
passage 36 quickly while the temperature of the cooling water is low. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.