Method and apparatus for diagnosing automotive engine problems using oxygen

A novel method in accordance with the invention for generating diagnostic signals for power plants (e.g., automobile engines and other internal combustion engines, gas turbines, and the like) utilizes a digital storage lab oscilloscope (DSO), configured to display at least about five seconds of data at about at least 50 data points per second, to capture specific power plant information primarily from an oxygen sensor. A preliminary waveform analysis verifies that the oxygen sensor is functioning correctly. Then, the oxygen-sensor waveform is classified as to certain primary characteristics to produce gross-level diagnostic information. If necessary, the injector waveform can be used to further classify system or mechanical malfunctions. More specific diagnostic information is obtained by classifying certain secondary characteristics of the waveform. In some embodiments a portable DSO may be used to provide a low-cost way for a technician to connect the scope to a vehicle and actually drive the vehicle under varying conditions, thus increasing the chances of detecting and diagnosing intermittent problems. Such an approach is both more convenient and more economical than the use of expensive treadmill-type chassis dynamometers. The portability of the DSO also permits technicians to take the test equipment to various cars located in different repair bays instead of moving cars around to a fixed test instrument. In another aspect of the invention, a portable computer is used to digitize store, and display reference information, notably model waveforms for various types of oxygen sensors and other components. In still another aspect, a computer (portable or otherwise) can be used for automatic analysis and classification of individual engine waveforms.

1. BACKGROUND OF THE INVENTION 
The invention relates generally to a convenient system and method for 
efficient diagnosis and servicing of engine system problems in automobiles 
and other vehicles that employ feedback-loop engine management computer 
systems. The system and method take advantage in a novel way of the 
characteristic electronic "signatures" that are generated by various 
components of an engine management computer system, particularly the 
oxygen sensor that monitors oxygen levels in engine exhaust gases. In 
addition, the system and method advantageously permit efficient and 
reliable detection of malfunctioning oxygen sensors, reducing the 
likelihood that a properly functioning sensor or other component will be 
wastefully replaced for lack of correct diagnostic information. The method 
has a wide application in that it may be utilized with many if not all 
vehicles which possess oxygen sensor feedback-loop engine control systems. 
1.1. Vehicle Oxygen Sensors 
As is well known to those of ordinary skill in the field of servicing 
computer controlled vehicle engines oxygen sensors (depicted in FIG. 1 by 
reference numeral 101) are commonly built into modem vehicle exhaust 
systems to monitor engine exhaust gases. As the air-fuel mixture ratio 
introduced into the engine cylinders changes, the quantity of oxygen 
(O.sub.2) in the exhaust changes. The oxygen sensor 101 emits a voltage 
which is related to the amount of oxygen in the exhaust and the specific 
design of the sensor. 
Several types of oxygen sensors are currently used in computerized vehicle 
emission control systems. Zirconium dioxide sensors, commonly known as 
zirconia sensors, are perhaps the most common and are found with and 
without heating elements. Zirconia sensors generate a voltage when heated 
by exhaust gases in an oxygen-deficient atmosphere. These sensors have a 
nominal electrical output that typically ranges from zero to one volt, 
dependent on the oxygen content of the exhaust. More recently, titanium 
based sensors have been employed for essentially the same purpose as their 
zirconia predecessors, except that as the O.sub.2 level changes, the 
resistance across the sensor changes. When a reference voltage is applied 
to the sensor, the sensor returns a voltage to the computer which is 
directly related to the O.sub.2 level in the exhaust. 
1.2. Variation of Fuel-Air Mixture for Efficient Catalytic Conversion 
The primary use of O.sub.2 sensor information by vehicle engine management 
computer systems is in stoichiometric control of the fuel-air mixture 
introduced into the engine cylinders to aid catalytic conversion of the 
engine exhaust gases. The catalyst in most catalytic converters works most 
efficiently and lasts longer when subjected to a slight excess of air, 
followed by a slight excess of fuel, and so forth, as opposed to being 
subjected to a predominantly, non-oscillating mixture. Accordingly, the 
engine management computer system (sometimes referred to as an engine 
control module or ECM, identified in FIG. 1 by reference numeral 103) 
generates control signals to devices which alter the fuel-air mixture, 
e.g., fuel injectors. More specifically, the ECM 103 receives a voltage 
signal from the oxygen sensor 101 via an oxygen sensor lead 110. As noted 
above, that voltage is a function of the oxygen content of engine exhaust 
gases. The ECM 103 utilizes the voltage signal to vary the fuel-air 
mixture injected into the cylinders. 
1.3. The Problem of Servicing Malfunctioning O.sub.2 Sensors 
Plainly, an ECM cannot optimally control its engine's fuel-air mixture if 
the O.sub.2 sensor is malfunctioning. The Environmental Protection Agency 
(EPA) has stated that a large portion of engine emissions-test failures 
(i.e., engines that produce excessive pollutants) are due to 
malfunctioning oxygen sensors, by some estimates up to 50% of such 
failures. 
In the field of vehicle servicing, however, no feasible or economical test 
is known to exist for determining whether an oxygen sensor is in fact 
malfunctioning. As a result, unnecessary replacement of oxygen sensors and 
other parts frequently occurs because of erroneous diagnosis and/or 
guesswork on the part of mechanics. It has been reported by oxygen sensor 
manufacturers that a large portion of all supposedly defective oxygen 
sensors that are returned to the manufacturer under warranty are in fact 
not defective. 
Part of this diagnosis problem arises from the ECM's variation of the 
fuel-air mixture. Typically, an efficient predominantly fuel-air mixture 
results in an oxygen sensor output voltage averaging about 0.45 volts. By 
the same token, an efficient oscillating mixture likewise has an average 
voltage of about 0.45 volts. Use of a conventional volt-ohm meter (VOM), 
whether analog or digital (DVOM), cannot easily detect oxygen sensor 
problems because their sampling rates and averaging circuits do not give 
accurate representations of voltage vs. time. Due to the same limitations, 
scan tools are also inadequate. 
2. SUMMARY OF THE INVENTION 
A novel method in accordance with the invention for generating diagnostic 
signals for power plants (e.g., automobile engines and other internal 
combustion engines, gas turbines, and the like) utilizes a digital storage 
lab oscilloscope (DSO), configured to display at least about five seconds 
of data at about at least 50 data points per second, to capture specific 
power plant information primarily from an oxygen sensor. A preliminary 
"waveform" (graph of the deviation of an electrical signal including 
amplitude samples taken at about evenly spaced time intervals, stored in a 
digital memory and optionally displayed on a cathode ray tube) analysis 
verifies that the oxygen sensor is functioning correctly. Then, the 
oxygen-sensor waveform is classified as to certain primary characteristics 
to produce gross-level diagnostic information. If necessary, the injector 
waveform can be used to further classify system or mechanical 
malfunctions. More specific diagnostic information is obtained by 
classifying certain secondary characteristics of the waveform. 
In some embodiments a portable DSO may be used to provide a low-cost way 
for a technician to connect the scope to a vehicle and actually drive the 
vehicle under varying conditions, thus increasing the chances of detecting 
and diagnosing intermittent problems. Such an approach is both more 
convenient and more economical than the use of expensive treadmill-type 
chassis dynamometers. The portability of the DSO also permits technicians 
to take the test equipment to various cars located in different repair 
bays instead of moving cars around to a fixed test instrument. 
In another aspect of the invention, a portable computer is used to 
digitize, store, and display reference information, notably model 
waveforms for various types of oxygen sensors and other components. In 
still another aspect, a computer (portable or otherwise) can be used for 
automatic analysis and classification of individual engine waveforms.

4. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
4.1. Digital Storage Lab Oscilloscope 
Referring to FIG. 1, the illustrative system utilizes a hand-held digital 
storage lab oscilloscope (DSO) 111. In roll mode (in which the DSO 
continuously paints a signal trace on its display screen, with the oldest 
portion of the signal rolling off, e.g., the left side while the new data 
appears on the right side), the DSO is capable of displaying at least 
about ten seconds of data continuously scrolling across its display 
screen. One example of such an oscilloscope is the Tektronix 222X Series 
distributed by the Tektronix company of Beaverton, Oreg. Such an 
oscilloscope possesses sufficient accuracy and screen update rate to 
record the oxygen sensor's rapid voltage output changes as the ECM 103 
varies the fuel-air mixture. Several nonportable brands and models are 
also capable of displaying this measured data; while they can be used in a 
system in accordance with the invention, their lack of portability will 
eliminate some of the advantages of a portable system. 
4.2. Connector Cable 
Two problems commonly arising in vehicle repair facilities and especially 
in working in a vehicle's engine compartment, are engine heat and 
interference with access by engine components. One aspect of the 
illustrative system involves the use of a connector cable, connecting the 
oxygen sensor to the oscilloscope, the design of which advantageously 
addresses those problems. 
Referring to FIGS. 1 and 2, a connector cable 113 has two variations. Both 
versions comprise a teflon-coated coaxial cable 203 whose insulation is 
capable of withstanding the heat encountered in an engine compartment. The 
cable 203 also has a limited amount of bend to reduce both fatigue and the 
normal sagging of test cables which allows them to contact moving or hot 
engine parts. It has been found that a suitable high temperature coaxial 
cable is the RG-400/U teflon-coated cable available from Pasternack 
Industries in Irvine, Calif. under part number RG-400/U. 
For test lead 1, shown in FIG. 2, a probe-tip-to-BNC adapter 201 is slipped 
on the probe tip of the oscilloscope 111 to provide a quick disconnect 
capability, thus helping protect both the equipment and the technician 
should the cable get caught in the fan blades of the engine, for example. 
The BNC adapter 201 is connected to a modified BNC cable end 202, which is 
then attached to the cable 203. At the other end of the cable 203 is a 
silicone insulated test lead with a sheathed male banana-plug connector 
204. The test leads 204 are connected to cable 203 with a solder sleeve 
shield. All connections between the cables and test leads are covered with 
an adhesive lined heat shrink tubing 206. The tubing 206 helps in strain 
relief and preventing breakage at the connections. 
Test lead 2 allows the technician to measure a signal's frequency with a 
multi-meter while the waveform is displayed on the oscilloscope 111. At 
one end of the test lead is a BNC tee connector 207 which is connected to 
a breakout BNC male connector 208. The other end of test lead 2 is 
comprised of silicone insulated test leads with sheathed male banana plugs 
204. The connections between the test leads are covered with an adhesive 
lined heat shrink tubing 206. 
4.3. Overview of Diagnostic Use of the Oscilloscope 
In roll mode, the oscilloscope 111 permits the user, e.g., a service 
technician, to observe a signal's history to discern trends generated by 
the oxygen sensor 101. FIG. 3 is a high-level flow chart of diagnostic 
steps that can be taken using the scope as explained in more detail below. 
4.4. Test of Oxygen Sensor Function 
As shown in block 301 of FIG. 3, an optional first step in testing an 
engine with the system is that of testing whether the oxygen sensor 101 
itself is functioning properly. Once such initial testing has verified 
that the sensor 101 is operating within its proper parameters, the 
sensor's output can be used to verify the functionality of the engine's 
feedback loop system. Ideally, to confirm that an oxygen sensor is 
functioning properly the following parameters should be measured: 
1) Activation time: the time the oxygen sensor takes to reach its full 
level of activity from the initial start up of the engine; 
2) Amplitude, i.e., the maximum and minimum voltages the sensor can obtain; 
3) Response time: the minimum amount of time it takes the sensor to transit 
from its lowest to its highest voltage and vice versa. This time interval 
is not necessarily the same for both transitions; and 
4) Cool down time: the time it takes the signal to degrade after the engine 
load is reduced from part throttle to idle. 
Activation time is an area of diagnosis that is not specifically tested 
here. In most cases, activation time is primarily a function of the 
vehicle manufacturer's system design. Failure of the heating element 
contained within heated sensors has a significant negative effect on some 
systems. While artificially failing the heating element can allow a 
perceptive technician to note an unusually long time period from initial 
engine operating to the point of full oxygen sensor activity, actual 
diagnosis of the cause of failure is usually adequately addressed by the 
manufacturer. 
Amplitude can be tested with the following steps for the typical zirconia 
oxygen sensor 101. The engine is fully warmed up, e.g., by holding it at 
about 2500 rpm for about two minutes, then reduced to idle speed for at 
least 30 seconds. The negative probe of the oscilloscope 111 is connected 
to ground, e.g., the engine block or the negative battery post. The 
positive probe of the oscilloscope 111 is connected to the oxygen sensor 
lead 110 using the connector 113. 
The fuel-air mixture is forced to a lean level. This may be accomplished by 
creating a significant vacuum leak in the intake manifold, e.g., by 
disconnecting the positive crankcase ventilation (PCV) hose from the PCV 
valve or by removing a vacuum line from, e.g., a power brake booster 
reservoir. The forced lean mixture causes the oxygen sensor voltage to 
drop to its minimum level (or in some systems to rise to its maximum 
level). 
When the lean condition has been established, the fuel-air mixture is 
forced to a rich level. This may be accomplished by feeding additional 
fuel (e.g., propane) to the mixture via the previous vacuum leak, e.g., 
(PCV) hose or via a vacuum port such as the power brake booster hose. The 
amount of fuel delivered is gradually increased until engine RPM is 
noticeably reduced. The voltage should then rise to its maximum level (or 
drop to its minimum level). 
Response time can be measured by rapidly alternating between the injection 
of fuel into the induced vacuum leak and the removal of the same. The 
scope trace shows the transit time from full rich to full lean and vice 
versa. A typical zirconia sensor, when properly functioning, will exhibit 
a volts-to-time pattern approximating a step wave form as illustrated in 
FIG. 4A. The trace begins at a time indicated in the Figure as 
approximately 0.0 seconds with the engine operating in the full rich 
condition, producing a voltage exceeding 0.8 volts. At about 6.2 seconds 
the fuel enrichment is terminated simultaneously with the introduction of 
excess air; the sensor should accurately track this event with a vertical 
drop of sufficient speed that the middle third of the drop is 
approximately vertical, i.e., the middle third of the transition from 
maximum to minimum voltage occurs in no more than about 0.1 second. At 6.6 
seconds the trace displays a low voltage representative of the engine's 
lean condition. 
As shown in FIG. 4B, a malfunctioning oxygen sensor normally does not 
demonstrate the step wave shown in FIG. 4A. If a typical zirconia sensor's 
maximum measured voltage is less than about 0.8 volts, the sensor is not 
functioning properly. When the fuel-air mixture is suddenly forced lean at 
5.2 seconds, approximately 5 seconds elapse before the sensor's output 
reaches zero volts. 
Cool down time is tested by keeping the sensor active for a reasonable 
period of time when the engine is operating at idle speed. Because federal 
emissions tests have a maximum idle period of 40 seconds, a thirty-second 
wait between the time when the engine rpms are reduced to idle and the 
time that the sensor is exercised should be sufficient to establish that 
the oxygen sensor can meet the required parameters under the most severe 
conditions at which it is expected to be tested. 
4.5. Classification of Primary Oxygen Sensor Waveforms 
After the proper functioning of the oxygen sensor 101 has been verified, 
the waveform created by the oxygen sensor voltage signal can be used to 
distinguish between ECM-related problems (e.g., a malfunctioning computer 
system) and mechanical problems. Oscilloscope displays of specific 
irregular waveforms generated by the O.sub.2 sensor can indicate a 
specific problem with the vehicle's fuel combustion. Each irregular 
waveform indicates a particular approach to the identity of the root 
problem. 
Although the operating parameters of the various sensors vary with their 
design, it has been empirically determined that all the sensors tested 
exhibit typical waveform patterns for the same engine problem conditions. 
This allows the output of any sensor to be compared with a cataloged 
pattern for that sensor. Further, most sensors can be tested in a similar 
manner. Referring again to FIG. 3, four examples of specific waveforms are 
shown. 
Waveform 305, shown in expanded form in FIG. 5, indicates a "normal" 
waveform i.e., normal operation of the combustion system and thus a 
properly varying fuel-air mixture. Because the average fuel-air mixture 
over time should produce an average voltage of 0.45 volts, the signal as 
displayed on the oscilloscope 111 should spend about as much time above 
0.45 volts (indicating a rich mixture) as it does below that level 
(indicating a lean mixture). The number of transitions from rich to lean 
(cross-counts) will vary with, e.g., the system type (feedback carburetor, 
throttle-body injection (TBI), multi-port fuel injection (MPFI)) and 
engine RPM. Generally speaking, MPFI systems have the most cross-counts 
per second, followed by TBI and feedback carburetor systems. Hash or extra 
spikes on the signal display can indicate a vacuum leak, fuel pump 
cavitation, cylinder or injector imbalance, or misfire. 
Waveform 306, shown in expanded form in FIG. 6, indicates a fuel-air 
mixture that apparently is not being controlled by the ECM, indicating a 
systems failure of some kind, e.g., a loss of oxygen-sensor signal to the 
ECM (perhaps via a bad lead 110) or a computer problem with the ECM 
itself. Waveform 307, shown in expanded form in FIG. 7, indicates a 
predominantly rich fuel-air mixture, while waveform 308, shown in expanded 
form in FIG. 8, indicates a predominantly lean fuel-air mixture. 
4.6. Analysis of Fuel Metering Device Signals 
A predominantly rich or -lean condition can be analyzed further by 
obtaining a waveform of the fuel injector (or other mixture control 
devices) with the oscilloscope 111 to analyze the signal of the fuel 
injection pulse. As shown in waveforms 309 and 310 (and in expanded form 
in FIGS. 9 and 10 respectively), a too-wide injector pulsewidth indicates 
that the ECM 103 is generating a command for a rich mixture, while a 
too-narrow injector pulsewidth indicates that the ECM is generating a 
command for a lean mixture. The table in FIG. 11 shows some possible 
causes for various combinations of rich- and lean-mixture commands from 
the ECM juxtaposed with oxygen-sensor signals indicating predominantly 
rich or predominantly lean actual conditions: 
a. If the oxygen-sensor signal indicates a predominantly rich mixture but 
the injector pulsewidth indicates that the ECM is generating a lean 
command, a mechanical problem is indicated. Potential problems include a 
bad fuel pressure regulator causing too high a fuel pressure; a leaking or 
sticking injector; a bad purge system; and/or a clogged fuel return line. 
b. If the oxygen-sensor signal indicates a predominantly rich mixture and 
the injector pulsewidth indicates that the ECM is generating a rich 
command, the ECM likely is receiving a spurious sensor input or the ECM is 
not functioning properly. For example, various temperature sensors might 
improperly indicate that the engine is not warmed up, causing the ECM to 
generate a rich mixture. 
c. If the oxygen-sensor signal indicates a predominantly lean mixture but 
the injector pulsewidth indicates that the ECM is generating a lean 
command, a spurious sensor input or ECM failure is again indicated. 
d. If the oxygen-sensor signal indicates a predominantly lean mixture and 
the injector pulsewidth indicates that the ECM is generating a rich 
command, mechanical problems are again indicated. Potential problems may 
include, e.g., a bad fuel pressure regulator delivering too little fuel 
pressure; a clogged fuel injector; or a bad fuel pump. 
4.6. Analysis of Secondary Waveform Characteristics 
Waveforms whose primary characteristics resemble those of waveforms 306, 
308, or 309 will often exhibit secondary characteristics. Such secondary 
characteristics can show clearly recognizable forms attributable to 
specific combustion problems in one or more cylinders. Generally speaking, 
such combustion problems arise from either (1) failure to maintain 
consistent fuel mixtures in all the cylinders serving the oxygen sensor 
under test, or (2) ignition misfire or failure of ignition in one or more 
cylinders. 
A number of specific examples of such problems are described below along 
with the signal patterns resulting therefrom. The respective problems and 
associated patterns are referred to in the claims below as being in a "DSO 
Problem/Pattern Relationship." 
FIG. 12: Fuel charge imbalance, i.e., inconsistent fuel-air mixtures from 
cylinder to cylinder. One or more cylinders have a different fuel-air 
mixture than the other cylinders in the engine. As a result, oxygen sensor 
voltage transitions occur at a higher frequency than in a normal ECM 
control pattern; in effect, a higher-frequency signal is superimposed on 
the normal, lower-frequency ECM control pattern. Average voltage remains 
approximately normal. 
FIG. 13: Dripping injector. A predominantly normal pattern includes a 
sudden vertical rise followed by a comparatively long period (e.g., 
approximately one second or more) in which the oxygen sensor voltage 
indicates a predominantly rich mixture, then followed in turn by a slowly 
decreasing level in response to ECM control attempts. Example: in FIG. 13, 
the sudden rise and high signal trace between 3.0 seconds and 4.0 seconds 
indicates a dripping injector, followed by a gradual decrease resulting 
from ECM corrective attempts. Average voltage is thus biased high. 
FIG. 14: Dripping injector at high engine RPM. Average voltage is 
predominantly high--the trace seldom reaches its minimum voltage and has 
large areas of space under the curve. 
FIG. 15: Excess oxygen in exhaust stream. Even though the oxygen sensor's 
voltage continues to trace an approximately sinusoidal pattern throughout 
its full amplitude range, periods of predominantly low voltage (e.g., 
between about 2.0 seconds to about 4.5 seconds in the Figure) indicate 
excessive oxygen. Average voltage is thus biased low. 
FIGS. 16 and 17: FIG. 16 shows a trace resulting from one nonfunctioning 
(in fact disconnected) injector at high speed, creating a predominantly 
lean trace with an average voltage biased low and with excessive dwell 
time at minimum voltage. FIG. 17 represents the same engine after 
repairing (or reconnecting) the injector. 
FIG. 18: Fuel pump cavitation. The waveform initially indicates a lean 
mixture as discussed in connection with FIG. 15 during the first two 
seconds. From about 2.0 seconds to about 4.0 seconds, while the engine is 
running some of the injectors receive more fuel than others because of 
fuel pump cavitation, creating a pattern similar to FIG. 12 (fuel charge 
imbalance) inasmuch as all injectors have not received a full charge of 
fuel. At about 4.5 seconds the pump and injectors are fully charged with 
fuel; the voltage pattern indicates a full-rich condition, whereupon the 
ECM takes corrective action and restores normal operation at about 6.0 to 
6.5 seconds. 
FIG. 19: Vacuum leak in intake system indicated by very high-frequency 
signal at idle RPM. Fuel (carburetor cleaner) is sprayed on the intake 
system in the area around the leak, causing a rich indication at about 3.5 
seconds and lasting until spraying is discontinued at about 7.0 seconds. 
4.7. Computerized Storage and Display of Reference Information 
In another aspect of the invention, a portable computer may be 
advantageously used for convenient storage and display of reference 
information such as model waveforms for specific types of oxygen sensor 
and other components. A Compaq LTE 25C notebook computer with a 
120-megabyte hard disk drive and 8 megabytes of read-write memory (RAM) 
has been used satisfactorily, but any computer with adequate memory and 
disk storage space may be used. The computer may further be utilized for 
generating signals encoding digitized diagnostic information for 
particular vehicles such as the diagnostic information illustrated in the 
Figures, with vehicle being input to the computer via an RS-232 port on 
the DSO 111 and/or via manual means such as a keyboard; for generating 
customer reports; and so forth. The source code of an exemplar program for 
performing such functions is included in the enclosed appendix, which is 
incorporated herein by reference. 
The computer may be programmed to store, retrieve, and display specific 
information in any convenient manner. It will be appreciated that the 
programming itself will be highly implementation-specific, but 
nevertheless a routine undertaking for a software developer of ordinary 
skill having the benefit of this disclosure. It has been found, however, 
that it is particularly advantageous for the programming to permit a 
technician to search for model waveforms via a tree-like index. An 
exemplar embodiment is illustrated in FIGS. 20A, 20B, 20C, 21A, 21B, 21C, 
22A, 22B, & 22C. If the technician knows the vehicle identification number 
(VIN), the specific sensor for which information is desired (test point), 
or the make and model of the car, then the technician can begin traversing 
the search tree utilizing that information. 
In another embodiment, the computer may be programmed to perform automatic 
waveform analysis and classification as described above, e.g., using known 
pattern-recognition techniques. 
It will be appreciated by those of ordinary skill having the benefit of 
this disclosure that numerous variations from the foregoing illustration 
will be possible without departing from the inventive concept described 
herein. For example, the DSO and computer can be a single integrated test 
instrument, e.g., a computer with a data acquisition device and an 
appropriate display, e.g., a CRT, good/bad test result status lights, etc. 
As another example, functions described above as implemented in the 
software can equivalently be implemented in hardware and vice-versa. 
Accordingly, it is the claims set forth below, and not merely the 
foregoing illustration, which are intended to define the exclusive rights 
claimed in this application.