Low oil sensor for an internal combustion engine

A low oil sensor especially for application with an internal combustion engine is provided with an over-center spring and diaphragm structure to maintain the sensor in the inactivated mode whenever there is an adequate oil level in an engine sump. Crankcase pulse pressures provide an adequate pressure differential across the diaphragm to move the switch to activate an indicator circuit and to stop the engine. Inadvertent switch operation is prevented by utilization of a check valve and bleed port arrangement. The switch can be reset by an externally operable reset plunger.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention discloses a low oil sensor for use with internal 
combustion engines. More particularly, such an oil sensor finds 
application in small, four-cycle engines. A low oil level signal is 
sensed, generally in an oil sump, and communicated to a switch assembly 
coupled to an indicator circuit. 
2. Prior Art 
Oil level sensors are known in the art. However, they are characterized by 
cumbersome, difficult to assemble structures, which are susceptible to 
malfunction by failure of one or more of their complicated structural 
members. Further, the earlier apparatus are relatively insensitive to 
signal pressures, many do not provide positive disconnects, some are 
difficult to reset, and/or they are prohibitively expensive to 
manufacture. 
U.S. Pat. No. 3,577,121--Wing discloses a brake fluid level switch 
assembly. The switch is illustrated in the open or reference condition 
with the fluid level in reservoir 34 in the figures. A low level oil 
condition is illustrated in reservoir 32 wherein diaphragm 60 has expanded 
allowing conductor switch 56 to contact housing 48 to close the contact 
circuit. An attendant is required to disassemble and reset the switch 
apparatus after the contact and circuit closure. The circuit in this 
particular disclosure has an electrically grounded cover 48 to energize 
signal light 78. 
U.S. Pat. No. 3,964,079--Katagiri discloses a brake fluid reservoir with a 
fluid level indicator. The fluid reservoir is partitioned by a diaphragm 
into a liquid phase and a vapor phase portion with the diaphragm 
therebetween and movable in response to changes in the liquid level of the 
liquid phase portion. An operating rod is movably connected to the 
diaphragm and contacts a sensing means. The rod is movable to actuate the 
sensing means to open or close a circuit in response to movement of the 
diaphragm as a function of changes in the brake fluid level. The signal 
circuit may be either normally open or normally closed. 
U.S. Pat. No. 3,333,259--Carothers, Jr., illustrates a vacuum operated oil 
level indicator. The indicator utilizes an indicator circuit with two 
series aligned switches. The first switch is manually closed by an 
operator to introduce a vacuum to control switch 24. Manual actuation of 
the first switch closes switch contacts 52, 54 and introduces a vacuum at 
a normal or adequate oil level to switch 24 which open contacts 44, 46 of 
the second switch. The second switch maintains indicator lamp 68 in a 
non-indicating mode. Inadequate oil in the oil reservoir communicates air 
to the vacuum reservoir maintaining diaphragm operator 40 in its reference 
mode with contacts 44, 46 closed. The air dissipates the vacuum, thus 
moving diaphragm 24 and closing the circuit to actuate indicator lamp 68. 
U.S. Pat. No. 3,022,497--Tyner teaches a variable orifice dipstick assembly 
in cooperation with a signal circuit which is responsive to a low level 
liquid in a sump or reservoir. The device is particularly taught for use 
in an automobile engine with the dipstick inserted into the oil sump. The 
assembly includes a diaphragm actuator and a guide member in a housing. 
The diaphragm operator separates the cavity of the housing into two 
distinct chambers. Contacts are provided that are spring-biased and 
diaphragm-operable or responsive to close a circuit contact. The housing 
serves as a ground member within the electric circuit and must be 
insulated from at least the contact means extending through the housing 
assembly. In operation, manifold vacuum from the engine is communicated to 
a first chamber on one side of the diaphragm and atmosphere is provided to 
the second chamber on the opposite side of said diaphragm. A plunger-like 
apparatus and valve assembly extends through the housing and diaphragm to 
contact a spade plug externally of the housing and is also movable with a 
valve guide of the diaphragm operator. This assembly includes two biasing 
springs, one biases the diaphragm operator assembly and the second spring 
biases a pin with a valve head attached thereto and operable through the 
diaphragm operator. As described, the dipstick at full or adequate oil 
level provides a stop for communication of atmosphere or crankcase 
pressure to the first chamber, which is connected to manifold vacuum. At 
low oil levels, atmosphere (crankcase pressure) is communicated to the 
first chamber and reduces the vacuum. The diaphragm operator is biased to 
close the circuit thereby energizing a signal lamp through the network of 
pins and contacts within the housing. 
A low oil sensor known in the art, is coupled to the oil sump of an 
internal combustion engine and is operable in response to a crankcase 
pressure signal to move a reset switch. The sensor and switch are a 
spring-biased diaphragm operated mechanism. The over-center spring and 
reset switch are displaced from the diaphragm and switch cavity. 
Although it is known to utilize oil or fluid sensors in internal combustion 
engines, the known apparatus generally provide variable or fixed-rate 
springs and diaphragms with connecting rod-type apparatus to close 
circuits or short circuit operating means to prevent engine damage. 
SUMMARY OF THE INVENTION 
The present invention discloses a fluid level sensor coupled to an internal 
combustion engine crankcase and having a diaphragm biased by a reference 
pressure. The sensor is operable at a predetermined crankcase pressure 
signal to engage and move a switch means against an interference-fit 
flexible arrangement to activate a signal circuit. The switch means is 
baised by an over-center, spring device with an interference fit, which 
arrangement permits utilization of a sensitive switch means in an 
environment that includes high frequency vibration and sudden, high 
magnitude mechanical jolts. This switch responds to a detected 
low-pressure signal in a crankcase above a predetermined value. The 
interference-fit, actuator-switch means coupled with a calibrated check 
valve and a properly sized diaphragm produce a control valve very 
sensitive to low pressure signals that is inexpensive, easily assembled, 
relatively insensitive to vibration and dirt, and which may be reset with 
only the push of a button.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates an internal combustion engine 10 in an elevational view 
having a vertical axis 12, a bottom portion 14, a drive shaft 16 
protruding from bottom portion 14 along axis 12 and a crankcase, oil 
reservoir or sump 18 with oil at a reference level 20. An oil level sensor 
22 is mounted on engine 10 and communicates with sump 18 through a conduit 
24. Conduit 24 may be of any material commonly utilized for connecting 
hoses in engine environments, such as neoprene or nylon. As shown in FIG. 
1, conduit 24 includes a flexible material portion 26, a metallic section 
28, a flexible coupling 30 and a metal tube 32 extending into the oil bath 
in crankcase 18. Tube 32 is shown in FIGS. 2 and 3 in plan view and in 
cross-section. This preferred embodiment with tube 32 of a connecting 
means is utilized to demonstrate a secure member not subject to relative 
or random movement thereby securing a constant oil level reference point 
for fluid pressure signal (crankcase pulses) communication to oil level 
sensor 22. The reciprocating pistons of engine 10 produce the crankcase 
pulses which are especially noticeable in an engine with an odd number of 
pistons not shown, but known in the art. 
Oil level sensor 22, for example, is illustrated in FIGS. 4a, 4b and FIG. 5 
in cross-section. Sensor 22 includes a housing 34 having a top segment 36 
with slots 39, a first wall annulus 41, a second wall annulus 43 and a 
shoulder 45 at their junction; and, a lower segment 38 with posts 47 and 
annular segment 49. Posts 47 are matable with slots 39 and annular segment 
49 contacts shoulder 45 when upper segment 36 and lower segment 38 are 
coupled. Segments 36 and 38 may be secured by means known in the art such 
as rivets, glue or, as illustrated in FIGS. 4a and 4b with snap fits. 
Segments 36 and 38 cooperate to define cavity 40 with a diaphragm 42 
secured in cavity 40 and retained on lower segment 38. Diaphragm operator 
42 cooperates with housing 34 to define a first or reference chamber 44 
and a second or actuation chamber 46. Diaphragm operator 42 includes a 
plate 48 secured thereon. 
Switch means 50 in cavity 44 is a thin structure which includes a switch 
actuation means 52, an over-center spring actuation device 54 with movable 
second contact points 60, and arms 55 with fixed first contact pads or 
points 58 matable with second contact points 60 of spring apparatus 54. 
Arms 55 extend through contact arm ports 62 defined by upper portion 36 of 
housing 34, which housing 34 also defines a receptacle 63 matable with a 
contacting element, as known in the art, but not shown herein. Housing 34 
is relatively thin and may be of an insulating or electrically 
non-conductive material. 
Lower portion 38 has a fluid sensing port 66 with a protuberance 64 
extending therefrom for a coupling or conduit 24. Protuberance 64 provides 
a communication between conduit means 24 through port 66 and second 
chamber 46, which is sealed by diaphragm operator 42 from first chamber 
44. 
A check valve 70, shown in FIGS. 4a and 4b, is positioned in chamber 46 to 
seal port 66. Check valve 70 includes a retainer 72, a calibrated valve 
plate 74, a valve seat 75 positioned about port 66 for seating valve plate 
74, and a mounting bracket 76 illustrated as an annulus to secure 
retaining means 72. A bleed port or restriction 78 is provided in valve 
seat 75 for continuous communication between port 66 and chamber 46. 
Restriction 78 allows dissipation of excess pressure in chamber 46 after 
seating of valve plate 74. Valve plate 74, shown in a plan view in FIG. 6 
as a generally square or rectangular shaped valve, is calibrated and 
gravity-biased to respond to a specific fluid pressure communicated from 
crankcase 18 when the oil level is below tube mouth 33 of tube 32. FIG. 7 
illustrates check valve retainer 72 in plan view as an annular segment 
defining port or passage 73, thus, for communication between chamber 46 
and passage or port 66. 
Switch actuator 52 includes a pin 80 with a lower portion 84 and an upper 
portion 85. Pin or plunger 80 is secured in a slidable upper collar 82 by 
a lower collar 86 at its lower portion 84. Spring actuator or flexural arm 
arrangement 54 with tabs 65 and an outer annulus 67 is mounted at its 
outer radius between upper and lower housing segments 36 and 38. Tabs 65 
extend radially outward from annulus 67 and rest on posts 47 in recesses 
39. The tabs 65 locate spring actuator 54 in chamber 44 and maintain it 
against rotational motion. Pin 80 and collar 82 extend from first chamber 
44 through a bore or actuator port 37 defined by top segment 36, and are 
slidable in bore 37 between a first and second position. Positioned on pin 
80 and collar 82 at upper portion 85 is a protective cap 87. At switch 50 
second position (shown in FIG. 4b), cap 87 may be utilzied to move pin 80 
and collars 82, 86 to their first position thus resetting switch means 50. 
Pin 80 and collars 82 and 86 are calibrated with spring actuator 54 to be 
operable by diaphragm operator 42 at a predetermined pressure indicative 
of a low-oil level in crankcase 18. 
Spring 54, shown in FIGS. 5 and 8 in plan view with a generally annular 
outer annulus 67, cooperates with extending arms 55 illustrated as spade 
plugs for coupling with an external indicating circuit 100. Spring 54 is a 
flexible, electrically conductive material and has a set of three V-shaped 
arms 102, 104 and 106 extending centrally from wall 67 with a first leg of 
each V in proximity at end points 103, 105, 107, respectively, to pin 80 
which end points are operatively secured between upper collar 82 and lower 
collar 86, but not fixed to pin 80. These first legs are noted as elements 
108, 110 and 112, respectively. The second or contact carrying leg of each 
V arm are noted as elements 114, 116 and 118, respectively. The first and 
second legs of the V-shaped arms or fingers move in unison from the first 
to the second switch position. Arms 108, 110 and 112 touch pin 80 at end 
points 103, 105 and 107, to form a generally triangular shaped arrangement 
about pin 80. Spade plugs or extending arms 55 are noted as elements 120, 
122 and 124 extending from arms 102, 106, and 104, respectively, however, 
the relative order of the plugs 55 is not a limitation. Similarly the 
V-shape of arms 102, 104 and 106 of actuator means 54 is utilized for 
illustration not as a limitation. 
Spring actuator 54 is an over-center arrangement, which is positioned about 
and contacts pin 80. As shown in FIGS. 4a and 4b the spring always 
contacts or is in proximity to pin 80 even though the first and second 
positions are displaced from the horizontal centerline. As spring 54 
transfers or moves between the first and second positions of switch means 
50, a known or predetermined force is required to overcome the 
interference fit and flexural strength of spring means 54. As the plunger 
80 and spring arms 108, 110 and 112 move from the first to the second 
switch position the annulus 67 and tabs 65 expand to accommodate the 
interference fit as the spring arms pass through the horizontal plane 
defined by the tops of posts 47 or annular wall 67. The force required to 
move diaphragm 42, pin 80 with collars 82 and 86 and to flex elements 108, 
110 and 112 is calculated to correspond to the pressure differential 
across diaphragm 42 between the reference pressure and the crankcase pulse 
pressure above the oil bath, which is generally a very low, positive 
pressure. However, the calibrated force is adequate to prevent false 
triggering of the swtich by mechanical shocks. 
Spring 54 of switch means 50, as shown in FIG. 8 is coupled to an 
illustrative indicating circuit 100. In the switch reference or first 
position, as shown in FIG. 4a, second contact points 60 of V-shaped arms 
102, 104 and 106 are disengaged from first contact points 58 which opens 
circuit 100. However, in the second or actuated position of FIG. 4b, all 
contact points 58 and 60 are engaged and circuit 100 is closed or 
energized. Connecting means or output spade plug 120 is coupled by lines 
130 and 132 to an ignition ground 134 of a magneto 136 of an engine, such 
as engine 10. Magneto or engine ignition circuit 136 has a second input 
138 with a line 140 coupled to an energy source 142, which energy source 
142 has a second line 144 coupled to an indicator means 146. Indicator 
means 146 is coupled to connecting means 124 by conductor 148. Indicator 
146 may be a lamp, an acoustic signal or a mechanical device. Lug or 
connector 122 is coupled to ground by conductor 15. Circuit 100 is 
operable to disable or kill the engine without indicator 146 and energy 
source 142. 
A passage 88 in FIG. 4b is defined by upper portion 36 and provides 
communication between a source of reference pressure, atmosphere in this 
case, a first chamber 44. This passage may be a small, narrow restriction, 
or it may be a larger restriction containing a pressed powder restrictor 
90 as known in the art and shown in FIG. 4c as element 90. A filter 92 
mounted on housing 34 at port 88 minimizes communication of entrained 
material to chamber 44. Passage 88, filter 92 and restrictor 90 are shown 
for illustration purposes only, which passages may be provided in the 
housing at other locations. Alternatively, chamber 44 may be sealed at a 
known pressure. 
Engine 10, which is illustrated and operable in a vertical orientation, has 
a sump 18 at its lower level 14. Sump 18 includes an oil bath, which at a 
normal or reference level 20 submerges tube mouth 33 of tube 32 to seal 
pneumatic communication between crankcase 18 and oil sensor 22. During 
normal operation, oil sensor switch actuator 50, as illustrated in FIG. 
4a, is in its reference or first position with first contacts 58 and 
second contacts 60 disengaged, thereby maintaining indicating circuit 100 
in an open condition and allowing magneto 136 in a parallel circuit to 
remain in an open or operating position. If the oil level in sump 18 falls 
below tube mouth 33, as shown by the dashed line in FIG. 3, the crankcase 
pneumatic pulse pressure is communicated to oil level sensor 22 through 
conduit 24 and port 66. Check valve 74 is biased by gravity to a position 
on seat 75 to seal port 66 when the oil level in sump 18 covers tube mouth 
33. Check valve 74 is calibrated and operable to open at a crankcase 
pressure correlative to the engine pneumatic pulse produced in crankcase 
18, which pulses are correlative to engine revolutions. Check valve 74 
operable with a periodicity related to the engine pulse opens 
communication between crankcase 18 and chamber 46. As the crankcase pulse 
pressure varies with the engine pulse the pressure change communicated to 
chamber 46 likewise varies with the pulsing action as valve 74 oscillates 
between an open and closed position until the pressure in chamber 46 is 
equal to the crankcase pulse pressure. 
A predetermined pressure differential between first chamber 44 and second 
chamber 46, which is a function of the crankcase pulse pressure, provides 
a force adequate to move diaphragm 42 and plate 48 to engage and move pin 
80 against their own weight. In addition, this force is adequate to move 
or deform over-center spring means 54 through its interference fit with 
pin 80, and thereby snapping or moving pin 80 to its second position to 
close switch means 50. As the interference fit of spring 54 as well as the 
biasing weight of pin 80 and valve 74, must be overcome before switch 50 
is closed, an indiscriminate or inadvertent single pulse of crankcase 
pneumatic pressure would not move diaphragm 42. Further, a single 
inadvertent pneumatic pulse would bleed back to crankcase 18 through bleed 
port 78. Plate 48 contacts and moves pin 80 and spring actuation means 54 
from the first or reference position to its second, closed or actuating 
position as shown in FIGS. 1, 4a and 4b. In this actuating position, 
contacts 60 and 58 are engaged and circuit 100 is closed. At switch means 
50 second position circuit 100 is closed and the circuit of indicating 
means 146 is closed and energized. Also, at the second position the 
magneto 136 circuit is grounded through lugs 120 and 122; lines 132, 130 
and 150; and, mating contacts 58 and 60 to kill the engine. 
While only particular embodiments of the present invention have been shown 
and described, it is manifested that these are in no way limiting on the 
scope of the invention described and claimed herein.