Measuring and testing – Probe or probe mounting
Reexamination Certificate
1999-10-06
2001-07-24
Williams, Hezron (Department: 2856)
Measuring and testing
Probe or probe mounting
C204S408000
Reexamination Certificate
active
06263748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to oxygen sensors used in association with the internal combustion engine of motor vehicles, and more particularly for a method of adjustment thereto.
2. Description of the Prior Art
Oxygen sensors are used in virtually all currently produced gasoline internal combustion engines used for vehicles operated in the United States. As shown diagrammatically at
FIG. 1
, the oxygen sensor
10
is located at exhaust pipe
12
so as to be in communication with the exhaust gas
14
therein in order to sense the oxygen content thereof, and provide an output signal voltage
16
responsive thereto. This oxygen sensor output signal voltage
16
is input, along with other sensor signals, to the engine computer control module (CCM)
18
, which then, based upon predetermined criteria, provides one or more signals
20
to various components associated with the engine
22
, particularly the fuel injector
25
, to optimally adjust operational parameters thereof to provide performance yet minimize both fuel consumption and exhaust gas pollution.
As shown at
FIGS. 2 and 3
, the oxygen sensor
10
is a device consisting of a zirconia ceramic sensor element tip
24
for producing the output signal voltage
16
(see FIG.
1
), wherein the sensor element tip is mounted to a housing
26
and protected by a perforated cover
28
, whereby the perforations
40
allow the exhaust gas to contact the sensing element tip. The oxygen sensor
10
has threads
30
for being threadably mounted to a mounting member
32
via a hex
36
. The mounting member
32
is, in turn, welded
34
to the exhaust pipe
12
. The housing
26
and the weld
34
prevent exhaust gas from escaping the exhaust pipe
12
thereat. An output signal wire
38
is connected to the sensor element tip
24
, emanates from the housing
26
, and is connected (typically) to the engine CCM
18
(see FIG.
1
).
The technique to locate an oxygen sensor in the exhaust pipe has not changed over the last twenty-five years: the housing
26
of the oxygen sensor
10
is screwed into the mounting member
32
, whereby the sensing element tip
24
extends outwardly a distance d equal to about fifteen millimeters. This technique while simple and repeatable, is not always optimal. The reason for this is that for the oxygen sensor is most accurate when operated within a specific temperature range set by the manufacturer. While heaters may provide a temperature at or above a low-side value of the specific temperature range for the oxygen sensor when the exhaust gas is cold (i.e., a cold engine start), there is no reasonable way to cool the temperature of the oxygen sensor to an allowable high-side value of the specific temperature range when the temperature of the exhaust gas exceeds this value. This later case may occur, for example, when the vehicle is towing and traveling upgrade, and heavy trucks are most prone to experiencing this behavior.
One way to limit higher end operational temperature of the oxygen sensor may be to place the oxygen sensor further downstream along the exhaust pipe. But, in practice, the excessive distance needed for a noticeable drop in exhaust gas temperature (as for example fifteen feet) renders this an impractical solution. Another solution may be to make the outward extension distance d smaller, on the theory that the reduced contact surface area will result in less heat exchanged with the exhaust gasses and the heat transfer rate away from the sensor element tip will be the same or improved (because of the shortened heat conduction path to the exhaust pipe sidewall). However, in practice the extension distance d may become too small, such that insufficient exposure to the exhaust gas can occur, resulting in less accurate output signal voltages.
Accordingly, what remains needed in the art is some method whereby the oxygen sensor may be installed and operate within the temperature limits set by the manufacturer under all operating conditions of the vehicle.
SUMMARY OF THE INVENTION
The present invention is a method for determining the mechanical installation parameters of an oxygen sensor so that it will operate within the temperature limits set by the manufacturer under all operating conditions of the vehicle.
The method according to the present invention sets the appropriate exposed area of the sensor element tip (S), calculated from a formula which takes into account: the recommended maximum operational temperature of the oxygen sensor (T
s
); the hottest expected temperature of the exhaust gas (T
g
); the effective heat transfer rate from the exhaust gas to the sensor element tip, and ultimately to the surroundings, i.e., the exhaust pipe sidewall which is in contact with the surrounding ambient air (Q/t); the thermal conductivity of the oxygen sensor (k); and an effective thickness, which depends on where the temperature is measured in the oxygen sensor (x).
This calculation is represented by equation (1):
T
s
=T
g
−(Q*x)/(t*c*k*S), wherein c has the value of unity when all units are in the same system (ie., all CGS units).
From equation (1) it is a simple matter to derive the signal voltage output V
s
of the oxygen sensor using equation (2):
V
s
=(T
s
*R)/(4*F)*(Log((PO2
a
)/(PO2
g
))), wherein R is the gas constant, F is the Faraday constant, PO2
a
is the partial pressure of oxygen in the atmosphere, and PO2
g
is the partial pressure of oxygen in the exhaust gas.
According to the method of the present invention, equation (1) is rearranged to derive S using equation (3):
S=(Q*x)/((T
g
−T
s
)(t*c*k)).
Now, to determine the appropriate exposed surface area S of the sensor element tip, the known values for Q, x, T
s
, T
g
, t, c and k are input into equation 3, and the calculation is then performed.
One way to mechanically effect the appropriate exposed area S of the sensor element tip may be adjusted using the configuration of
FIG. 3
, wherein d is obtained by threading the threads more or less into the mounting member, and wherein d=2*S/(pi*x).
Another way to mechanically effect the appropriate exposed area S of the sensor element tip is to utilize a collar having a cavity for receiving therein the perforated cover. The collar is in good contact with the exhaust pipe, as for example by welding thereto. The exposed area of the sensor element tip may be adjusted any of three ways, in any combination: by adjusting d as measured with respect to the collar; by adjusting A
1
, wherein A
1
is the concentric area of a gap between the sensor element tip and a cavity defining wall of the collar; and by adjusting A
2
, wherein A
2
is the area of a hole formed laterally in the collar which communicates with the perforated cover (and A
1
, if present) and has its axis parallel to the exhaust gas stream and faces directly thereinto; that is: S=pi*x*d/2+A
1
+A
2
, wherein the first term represents the surface area obstructive to the exhaust gas flow, and wherein the second term (A
1
) may have less weight in defining S, even neglected, as ascertained empirically for a specific structural configuration.
Accordingly, it is an object of the present invention to provide a mechanical method for adjusting oxygen sensor placement in an exhaust pipe so that the oxygen sensor will operate below a predetermined maximum temperature.
This, and additional objects, advantages, features and benefits of the present invention will become apparent from the following specification.
REFERENCES:
patent: 3844920 (1974-10-01), Burgett et al.
patent: 3847778 (1974-11-01), Riddel
patent: 4038034 (1977-07-01), Nakajima et al.
patent: 4040930 (1977-08-01), Dillon
patent: 4049524 (1977-09-01), Togawa et al.
patent: 4096050 (1978-06-01), Kobayashi et al.
patent: 4184934 (1980-01-01), Bobe et al.
patent: 4198279 (1980-04-01), Brown et al.
patent: 4407704 (1983-10-01), Mase et al.
patent: 5137616 (1992-08-01), Poor et al.
Garber C D
General Motors Corporation
Simon Anthony Luke
Williams Hezron
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