Fuel supply system for internal combustion engine

A fuel supply system for an internal combustion engine comprising: a suction pump provided in a fuel supply passage, a ventilation passage for allowing interior space of a fuel tank to be communicated with the atmosphere, a normally closed ventilation valve disposed in the ventilation passage, and a normally-closed release valve disposed in the fuel return passage and adapted for opening the fuel return passage at the time of start-up of the engine. The release valve and the ventilation valve being operatively connected to each other such that they are opened and closed simultaneously.

BACKGROUND OF THE INVENTION 
The present invention relates to a fuel supply system for an internal 
combustion engine and more particularly, to a fuel supply system having a 
fuel passage for supplying a fuel from a fuel tank to a carburetor of an 
internal combustion engine, a suction pump which is operated at the time 
of start of the engine and adapted for sucking fuel into the carburetor 
through the fuel supply passage, a fuel return passage for allowing 
excessive fuel in the carburetor to be returned to the fuel tank, and a 
normally closed release valve provided in the fuel return passage and 
adapted to open the fuel return passage when the engine is started. Still 
more particularly, the present invention is concerned with a fuel supply 
system of the type mentioned above, suitable for use in a device 
incorporated in various types of working machines. 
Fuel supply systems of the type stated above have been used in internal 
combustion engines used as the power sources of various types of working 
machine, for the purpose of supply fuel from fuel tanks to carburetors, 
especially diaphragm-type carburetors with release valves. In this type of 
known fuel supply system, the suction pump serving as the start-up pump is 
usually provided in the fuel return passage. 
This known fuel supply system, however, suffers from a problem in that air 
tends to flow into the suction pump through the fuel return passage. In 
particular, when the air in the fuel tank is heated to develop a pressure 
higher than the atmospheric pressure as in the case of the use at high 
ambient air temperature, air flows into the suction pump through the fuel 
return passage, so that the fuel pump fails to suck the fuel 
satisfactorily. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a fuel supply 
system which is improved in such a way as to overcome the above-described 
problem of the prior art. 
More specifically, the present invention aims as its object at providing a 
fuel supply system in which a suction pump is provided in the fuel supply 
passage for supplying fuel to a carburetor so as to prevent air from 
flowing backward through the fuel return passage and being sucked by the 
suction pump, thus ensuring a high efficiency of operation of the suction 
pump, while simplifying the construction and facilitating the operation of 
the fuel supply system. 
Namely, according to the present invention, there is provided a fuel supply 
system for an internal combustion engine having a fuel supply passage for 
supplying a fuel from a fuel tank to a carburetor of an internal 
combustion engine, a suction pump adapted to be operated at the time of 
start-up of the engine and adapted to suck the fuel into the carburetor 
through the fuel supply passage, a fuel return passage for allowing the 
excessive fuel in the carburetor to be returned to the fuel tank, and a 
normally closed release valve disposed in the fuel returning passage and 
adapted for opening the fuel returning passage when the engine is started 
up, said fuel supply system characterized in that said suction pump is 
disposed in the fuel supply passage and in that a normally closed 
communication valve is disposed in the communication passage and is 
operatively connected to a release valve so that the communication valve 
and the release valve are opened and closed simultaneously. 
In the fuel supply system of the present invention, a suction pump serving 
as a start-up pump is provided in the fuel supply passage, and a 
communication passage is provided to allow the interior of the fuel tank 
to be communicated with the atmosphere, the communication passage having a 
normally-closed communication valve which is operatively connected to the 
release valve such that the communication valve and the release valve are 
opened and close simultaneously. With this arrangement, the suction pump 
is disposed apart from the fuel return passage and the pressure of the 
interior of the fuel tank is maintained at the same level as the 
atmospheric pressure when the engine is started, so that the back flow air 
through the fuel return passage into the suction pump is avoided to 
maintain the pumping performance of the suction pump. In addition, since 
the communication valve opens and closes in accordance with the operation 
of the release valve, it is possible to realize quick start-up of the 
engine and to facilitate the start-up of the engine, without causing any 
unfavorable effect on the normal operation of the engine. 
The fuel supply system of the invention having the described features can 
operate efficiently even under hot atmosphere. In addition, the pressure 
rise in the fuel tank which may be caused by the initial boiling gas at 
the time of filling of the tank is restrained by suitably opening the 
communication valve. 
The above and other objects, features and advantages of the present 
invention will become clear from the following description of the 
preferred embodiment when the same is read in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention will be described 
hereinunder with reference to the accompanying drawing. 
The attached sole FIGURE is a diagrammatic illustration of an embodiment of 
the fuel supply system of the invention designed for supplying a fuel to 
an internal combustion engine. Referring to this Figure, an internal 
combustion engine (not shown) of a working machine has a fuel tank 1 
attached to the frame of the machine and capable of storing fuel, and a 
carburetor through which air-fuel mixture is fed to the engine. The 
carburetor 2 has a fuel supply passage 4 constituted by, for example, a 
conduit. An automatic needle valve 3 adapted to be controlled by a 
diaphragm 18 is provided at the downstream end 5 of the fuel supply 
passage 4. The upstream end 6 of the fuel supply passage 4 opens into the 
fuel of the fuel tank 1 through a strainer. A check valve 7 such as a ball 
valve is disposed at an intermediate portion of the fuel supply passage 4 
and a suction pump 8 serving as a start-up pump is disposed in the portion 
of the fuel supply passage 4 downstream from the check valve. The check 
valve 7 allows the fuel to flow from the fuel tank 1 to the suction pump 8 
but prevents the same from flowing from the suction pump 8 to the fuel 
tank 1. The suction pump 8 is made of a hollow bulb made from a synthetic 
resin or a rubber. Prior to the start-up of the internal combustion 
engine, the operator compresses the bulb and then allows it to expand and 
this operation is repeated so that the fuel is sucked from the fuel tank 1 
and fills a fuel chamber 19 of the carburetor 2 by forcibly opening the 
needle valve 3. 
The carburetor 2 is communicated with an upstream end 11 of a fuel outlet 9 
of a fuel return passage 10 constituted by, for example, a fuel conduit, 
while a downstream end 12 of the fuel return passage 10 is opened to the 
interior space 17 of the fuel tank 1. The fuel return passage 10 is 
provided at its intermediate portion with a release valve 13 which is 
provided with a release lever 14 which is adapted to be manually operated 
so as to activate a valve member adapted for opening and closing the flow 
passage therein. The release valve 13 is normally closed but the operator 
can operate the release valve 13 by manipulating the release lever 14 such 
that the release valve 13 is opened to allow excessive fuel to return to 
the fuel tank 1 from the fuel outlet 9 of the fuel chamber 19. 
In addition, the fuel supply system of the present invention has a 
ventilation passage 15 having one end opened to the interior space 17 in 
the fuel tank 1 above the level of the fuel therein. The ventilation 
passage 15 is opened to the atmosphere. A ventilation valve 16 is normally 
closed and is disposed in the vicinity of the release valve 13. A valve 
manipulating lever 20 is operatively connected to the release lever 14 of 
the release valve 13 in such a manner that, when the release valve 13 
operates so as to open the fuel return passage 10 at the time of start-up 
of the engine, the valve manipulating lever 20 also opens the ventilation 
valve 16 so as to provide a communication between the interior space 17 of 
the fuel tank 1 and the atmospheric air. The ventilation valve 16, 
however, is normally closed so as to interrupt the interior space 17 of 
the fuel tank 1 from the ambient air. 
In operation, the operator manipulates the release lever 14 so as to 
simultaneously open the release valve 13 and the ventilation valve 16 so 
as to provide a communication between the fuel chamber 19 of the 
carburetor 2 and the interior space 17 of the fuel tank 1 so as to bring 
the interior space 17 of the fuel tank 1 into communication with the 
atmosphere through the ventilation passage 15, thus maintaining the 
pressure of the interior space 17 of the fuel tank 1 at the same level as 
the atmospheric pressure. In this state, the bulb consisting the suction 
pump 8 is compressed and allowed to expand, thus causing the fuel in the 
fuel tank 1 to be fed through the upstream end 5 of the fuel supply 
passage 4 via the needle valve 3, thus filling the fuel chamber 19 of the 
carburetor 2. In this state, the excessive fuel overflowing the fuel 
chamber 19 of the carburetory 2 is returned to the fuel tank 1 through the 
fuel return passage 10 via the fuel outlet 9. The carburetory 2 is now 
ready to operate. The release lever 14 is then manipulated so as to 
simultaneously open the release valve 13 and the ventilation valve 16. In 
this state, the fuel supply passage 4 is completely filled with the fuel 
so that the carburetor 2 is continuously supplied with the fuel from the 
fuel tank 1 through the fuel supply passage 4 at a rate corresponding to 
the rate of supply of the fuel from the carburetor 2 to the internal 
combustion engine.