Choke for internal combustion engine

A choke for use with a carburetor on an internal combustion engine comprises a housing having an air inlet and an air outlet the air outlet being adapted for communication with the air intake of a carburetor. A valve means within the housing includes a valve member and means biassing the valve member in a direction to block the passage of air between the air inlet and the air outlet of the housing. The valve means is responsive to engine vacuum during starting of the engine to move the valve member in a direction against the biassing means to permit air to enter the carburetor from outside the housing via the air inlet and air outlet thereof.

The present invention relates to chokes or starting aids for small internal 
combustion engines and more particularly to a choke or starting aid which 
is responsive to the vacuum developed during start up of an internal 
combustion engine. 
When starting small internal combustion engines, it is usually necessary to 
pull on the starter rope several times before the engine kicks over and 
begins to run. Generally, after a couple of pulls on the starter rope the 
engine starts and runs for a short period of time and then stops. This is 
what is commonly known in the field as a "false start". This "false start" 
phenomenon has been present in the chainsaw art for several years and has 
come to be accepted by the users of such saws as an acceptable starting 
method. The user generally has knowledge of the fuel system procedure and 
understands why the system is not starting. 
The difficulty in starting a cold internal combustion engine centres around 
the choke system of these particular engines. When the choke system is in 
a closed position, the fuel line system of the cold engine has a very high 
restriction in the air intake. The restriction of the air intake forms a 
vacuum in the fuel line, sucking fuel into the engine via the carburetor 
from the fuel tank. As the starting rope is pulled, the engine sucks fuel 
into the carburetor by the vacuum created in the system. As the engine 
begins to fire, a certain amount of air is necessary to keep the engine 
running. With a manual choke butterfly type, the user must open the choke 
quickly after the engine begins running or the user will experience the 
false start phenomenon. The reason for the "false start" is that as the 
speed of the engine increases, the engine sucks more fuel. With the choke 
in the closed position, however, the amount of air flow entering the 
engine is not increased. Thus a proper mixture of air and fuel is not 
achieved and the engine dies instantly. Also, if the engine does not start 
up a substantial amount of fuel is sucked into the engine, via the 
carburetor causing the engine to become flooded, further hampering the 
starting procedure of the engine. 
If the user is not familiar with the start up procedure, misses the false 
start and keeps pulling the starter rope with the choke on, the engine 
becomes so flooded that the spark plug, crankcase and cylinder must be 
allowed to dry out before start up. 
Accordingly, it is an object of the present invention to mitigate the above 
mentioned problems. 
According to the present invention there is provided a choke for use with a 
carburetor on an internal combustion engine, the choke comprising a 
housing having an air inlet and an air outlet, the air outlet being 
adapted for communication with the air intake of a carburetor, and valve 
means within the housing including a valve member and means biassing the 
valve member in a direction to block the passage of air between the air 
inlet and the air outlet of the housing, the valve means being responsive 
to engine vacuum to move the valve member in a direction against the said 
biassing means to permit air to enter the carburetor from outside the said 
housing via the air inlet and air outlet thereof during starting of the 
engine.

Referring now to the drawings wherein similar numerals have been used to 
indicate like parts, there is shown therein a choke for use on a 
carburetor generally indicated at 10 according to the invention. The choke 
10 comprises a housing 11 having an air inlet 12 and an air outlet 13. 
The housing 11 is secured to a carburetor 14 so that the air outlet 13 is 
in communication with the air inlet (not shown) of the carburetor 14. The 
housing contains a valve means 15 disposed between the air inlet 12 and 
the air outlet 13. The valve means 15 comprises a shallow cylindrical 
valve chamber 16 integrally formed with a hollow shaft 17. A coil spring 
18 acts in compression between the internal sidewall 19 of the housing 11 
and the chamber 16 to bias the chamber 16 against the opposite internal 
sidewall 20 of the housing 11. A portion 21 of the chamber 16 is of 
reduced diameter and forms an air outlet for the chamber 16 which projects 
slightly into the air outlet 13 of the housing 11. An `O` ring seal 34 is 
secured around the portion 21 of the chamber 16 so that it abuts the 
adjacent region 23 of the sidewall 20 to provide a substantially airtight 
seal. 
A flat valve member 24 is located within the valve chamber 16 and is 
arranged to open and close apertures 25 in a flat valve seat region 26 of 
the valve chamber 16, the apertures 25 constituting an air inlet for the 
chamber 16. The valve member 24 is integrally formed with a shaft 27 which 
projects within the hollow shaft 17 through an aperture 28 in the chamber 
16. The end 29 of the shaft 27 is threaded and carries a nut 30 and washer 
31. A coil spring 32 is located on the shaft 27 and acts in compression 
between the nut and washer and the portion 33 of the valve chamber 16 
within the region of the hollow shaft 17, so as to urge the valve member 
24 against the valve seat region 26. The end 35 of the hollow shaft 17 
projects out of the housing 11 through an aperture 36 in the sidewall 19 
and is secured to a control knob 37. 
The sidewall 19 of the housing 11 has an outwardly projecting cam surface 
40 and the control knob 37 has a cam follower surface 41, so that rotation 
of the knob through approximately 90.degree. causes the hollow shaft 17 
and thus the valve means 15 to be moved to the left as shown in FIG. 3, 
thus exposing the air outlet 13. The cam surface 40 has formed thereon 
suitable recesses 60 to enable the knob 37 to be retained in its rotated 
position with the hollow shaft 17 in the leftmost position. Clearly, as 
the knob is rotated and valve means 15 is moved to the left, the coil 
spring 18 is compressed, and when the knob is rotated in the opposite 
direction, the coil spring 18 acts to urge the valve means 15 into 
abutment with the sidewall 20 again. The housing 11 also has an air filter 
42 secured adjacent to the air inlet 12 to filter air entering the housing 
11. 
As shown, the carburetor 14 is attached to an internal combustion engine 
50. As the piston 51 of the engine 50 reciprocates up and down, it causes 
corresponding periodic vacuum signals in the engine crankcase 52 which are 
transmitted to the air outlet 13 via a reed valve 53. The combustion 
mixture enters the engine crankcase 52 and it is transferred to the 
cylinder 55 via conduit 56 as the piston moves downwards to bottom dead 
centre. 
Thus, as the piston 51 approaches top dead centre, the vacuum signal 
produced in the crankcase opens the reed valve 53, thus transmitting the 
vacuum signal to the air inlet 13. The vacuum signal then acts to draw the 
valve member 24 off the valve seat 26 against the bias of the coil spring 
32, thus allowing air to enter the carburetor 14 via the air inlet 12, 
apertures 25 and air outlet 13. Thus, sufficient air is allowed to enter 
the carburetor to allow a start and run condition of the engine. The 
position of the valve member 24 in the starting mode of the engine is 
illustrated in FIG. 2. The user does not therefore have to open the choke 
quickly after the engine starts, thus mitigating the problem of the prior 
art. 
In FIG. 1, the piston 51 is at bottom dead centre and there exists a high 
pressure pulse signal in the crankcase 52 which acts to close reed valve 
53 resulting in a decrease in the vacuum signal in the carburetor 14, 
enabling the valve member 24 to return to its initial position on the 
valve seat 26, thus closing the apertures 25. 
As shown in FIG. 3, once the engine has been started, the control knob 37 
is rotated to move the valve means 15 bodily away from the outlet aperture 
13, against the bias of the spring 18 to enable continuous running of the 
engine. It will be appreciated that the size of the air outlet 13, the 
area of the valve member 24, and the strength of the coil spring 32 will 
determine the number of pulls to start the engine, the speed at which the 
engine runs with the choke on and the time period for which the engine 
will run with the choke on. 
The parameters are selected such that: 
(A) The engine will start and run in the same number of pulls or an 
acceptable one or two extra pulls as the conventional butterfly choke and 
will remain running with the choke on, or run for a sufficient period of 
time that would enable the operator to knock the choke "off" manually. 
(B) If in the starting mode the ignition switch was switched "off", choke 
"on", engine cranked ten times, the engine must start in no more than a 
further 5/7 pulls with the choke in the "off" position, and the ignition 
switch in the "on" position. If the same occurred with the butterfly 
choke, the engine would be so flooded that the spark plug, crankcase and 
cylinder must be allowed to dry out before start up. 
TEST RESULTS 
Tests were carried out on the Stihl FS-60 strimmer. 
TEST NO. 1 
Vacuum Test 
The vacuum developed at the fuel inlet during cranking was recorded with 
the conventional butterfly choke and the disc choke. The disc choke 
designs parameters were selected to fulfill the afore mentioned criteria. 
Butterfly Choke 
5 pulls--5" H.sub.2 O 
Disc Choke 
5 pulls--5" H.sub.2 O 
TEST NO. 2 
Start Test 
The number of pulls required to start and run a completely dry cold engine 
with both types of choking systems was recorded. 
BUTTERFLY CHOKE 
6 pulls for a false start with the choke "on" and one extra pull for a 
start and run with the choke "off". 
DISC CHOKE 
7 pulls for a start and run with the choke "on", and remains running at 
4500/5000 rpm with the choke on. 
NOTE: In previous two tests the engine settings were: 
IDLE--2400/2500 rpm 
W.O.T.--7300/7500 rpm 
Also, when starting an engine with the choke according to the invention, 
the throttle must be held in the fully open position, instead of the 
partially open position with the butterfly choke. Although the invention 
has been described for use on a reed valve engine it will also clearly 
have application on other engines for example, piston ported engines.