Method and apparatus for diagnosing faulty cylinders in internal combustion engines

This invention relates to a method and apparatus for diagnosing faulty cylinders in internal combustion engines. A probe produces a signal that is representative of the instantaneous angular velocity of the engine shaft. A Percent Power Contribution (PPC) parameter is computed by substracting the square of the instantaneous angular velocity at the beginning of a cylinder period from the square of the instantaneous angular velocity at the end of that cylinder's period and dividing this difference by the square of the average engine angular velocity. A value of this PPC parameter significantly less than zero indicates a faulty cylinder. An Average Energy Percent parameter (AEP) is computed by dividing the squares of the sampled instantaneous angular velocity values summed over a cylinder period by the number of samples in that period and by the square of the average engine angular velocity. A value of this AEP parameter significantly less than one indicates the previous cylinder in the firing order is faulty. The two parameters can be used independently or in combination to diagnose continuous and transient engine faults. Faulty cylinder indications are displayed on a monitor or communicated to an operator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of automotive engine 
diagnostics and more particularly concerns the problem of finding one or 
more intermittently faulty engine cylinders. 
2. Description of the Prior Art 
It is well known that the instantaneous angular velocity of an internal 
combustion engine at a fixed throttle setting varies over a small range as 
each cylinder fires. For an engine with all good cylinders, this variation 
resembles a sinusoidal function superimposed on a constant angular 
velocity. The pattern changes when one or more cylinders is not supplying 
its full power contribution. Even though this waveform is indicative of 
the overall performance of an engine; alone, it is not very useful for 
determining which cylinder is faulty, especially when there is more than 
one bad cylinder. 
Prior art methods have made various attempts to use this angular velocity 
waveform to determine the condition of a cylinder. Some methods arrive at 
an average or statistical evaluation of a given cylinder based on low 
resolution angular velocity estimates averaged over many engine cycles to 
lead to conclusions about each cylinder's performance. There are two major 
disadvantages with these methods. 1) Transient phenomena such as single 
mis-fires or conditions that occur only rarely cannot be detected. 2) The 
engine cannot be tested during acceleration or changing load where the 
most demands are placed on it for performance. 
Other methods have computed acceleration or have compared relative power 
against stored parameters for good engines. Acceleration methods suffer 
from numerical noise problems because estimating a mathematical derivative 
is a numerically noisy process. Comparing parameters against those of good 
engines requires large data bases of data for different engine types. 
Schroeder and Ryan in U.S. Pat. No. 4,539,841 discuss determining peak 
cylinder pressure and relative power output of each cylinder from a good 
engine at idle speed, as well as a total load torque and total engine 
inertia. These good engine parameters are stored and used for comparison 
during diagnostics. A diagnostic uses the difference in the squares of the 
instantaneous RPM values at three points during the cylinder firing cycle 
to derive a cylinder net power output. These numbers are displayed on a 
CRT screen for human interpretation. The method is cumbersome in that it 
requires pre-stored values of several parameters such as engine inertia 
and engine frictional torque from a good engine in order to derive net 
power output. The method presented by Schroeder and Ryan is primarily 
suited to a four cylinder engine. The technique becomes more and more 
difficult to use as the number of cylinders increases because of the 
overlap between compression and power phases of various cylinders. 
Schroeder et at. in U.S. Pat. No. 5,132,909 address a method where a very 
low resolution velocity estimate of six samples per engine revolution are 
used. Only gross statistical averages of engine speeds can be obtained. 
These estimates are used to derive average quantities called relative 
compression and relative power output for a given cylinder. While this 
method can possibly locate continuous or long term problems, it will 
entirely miss any cylinder condition that occurs intermittently or 
infrequently. 
Obayashi et at. in U.S. Pat. No. 4,691,286 teach a closed-loop method that 
also uses a very low resolution angular velocity estimate (twelve samples 
per revolution). While different from that of Schroeder, it is still 
highly statistical in nature and yields only average quantities. It 
consists of taking the difference in the squares of velocity at two 
different points in the engine cycle. This quantity is then statistically 
analyzed over one hundred engine cycles to determine the standard 
deviation of performance. The standard deviation is then used in a closed 
loop to control the fuel-air ratio or other operating parameters. The 
method cannot detect transient problems or problems that only occur under 
certain engine loading or acceleration conditions. It is aimed at the 
performance and control of the entire engine, not the diagnosis of one 
cylinder. 
Rizzoni and Connolly in their paper, "Real Time Estimation of Engine Torque 
for the Detection of Engine Misfires," presented at the 1991 ASME Winter 
Meeting, Atlanta Ga., teach a method of deconvolution from high resolution 
angular velocity data to torque values for each cylinder in the spatial 
frequency domain. Computations are performed on the 1/N th harmonic of the 
major rotational spatial frequency. An estimate is made of the 
root-mean-square of the indicated torque output of an individual cylinder. 
Identification of mis-fire is accomplished using statistical detection 
methods such as a likelihood ratio test on a non-uniformity metric. Again, 
the method is statistical and would miss a single event. Also, it is very 
compute intensive since it uses the Fast Fourier Transform and several 
statistical detection integrals. This makes it difficult to incorporate 
into a piece of standard diagnostic test equipment. 
The present invention overcomes the problems caused by statistical 
computation and low resolution sampling, as well as computation of 
acceleration and comparison against stored parameters of good engines. Two 
numerical quality parameters are computed for each cylinder during every 
firing cycle. The sequence of these numbers can be stored and used to 
detect single mis-fires and other transient events. The method is equally 
applicable to engines with from two to twelve cylinders. 
SUMMARY OF THE INVENTION 
The present invention comprises a probe that produces an electrical signal 
that is representative of the instantaneous angular velocity of the 
engine, a means of taking the square of the instantaneous angular velocity 
at the beginning and end of a cylinder period, a means of finding the 
average engine angular velocity, and a means of dividing the difference 
between the square of the instantaneous angular velocity at the end of a 
cylinder period and the square of the instantaneous angular velocity at 
the beginning of the cylinder period by the square of the average engine 
angular velocity. This computation produces a Percent Power Contribution 
(PPC) parameter that can be compared against a threshold to determine if 
the cylinder in question is good or faulty. A good cylinder produces a 
positive PPC parameter. A negative PPC parameter indicates a faulty 
cylinder firing event. The PPC parameter is an instantaneous value that 
can detect transient problems. The invention displays for an operator, 
records or communicates any determination of a bad cylinder. 
The present invention also comprises a means for computing an Average 
Energy Percent (AEP) parameter by dividing the average energy contribution 
during a cylinder period by the square of the average engine angular 
velocity for a cylinder cycle. This AEP parameter can be compared against 
a threshold to determine if the previous cylinder in the firing order is 
faulty or good. A good cylinder produces an AEP parameter near one, while 
a bad cylinder produces an AEP parameter significantly less than one for 
the next cylinder in the firing sequence. 
The two parameters can be used independently or in combination to diagnose 
an engine. When used in combination, the AEP parameter acts as a check on 
the PPC parameter. Generally, a faulty cylinder will have a PPC parameter 
considerably less than zero, and the AEP parameter for the next cylinder 
in the firing order will be considerably less than one (but still 
positive). A transient fault may affect one or both parameters. The use of 
two independent parameters leads to increased sensitivity to isolated 
events such as mis-fires.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning first to FIG. 1, it can be seen that the present invention contains 
a probe 2 that is capable of measuring the instantaneous velocity of an 
engine's crankshaft via a belt I or pulley that rotates at some angular 
velocity proportional to that of the crankshaft. This probe can be a 
magnetic pickup, a light beam, a rotating wheel directly in contact with 
the belt, pulley, or shaft, or any other means capable of measuring the 
engine shaft's angular velocity. The probe 2 produces a signal that is 
directly related to the instantaneous angular velocity of the crankshaft. 
This signal is conditioned by a signal conditioning means 3 for further 
processing. This signal conditioning means may simply measure or shift the 
level and/or frequency of the signal, or it may comprise an analog to 
digital (A/D) converter that changes the format of the signal to that of a 
sequence of numerical values suitable for processing by a microprocessor 
or digital signal processor (DSP). It may be a device that converts 
optical signals to electrical signals. 
The conditioned signal is routed from the signal conditioning means 3 to a 
means 4 for computing the instantaneous angular velocity or RPM of the 
crankshaft and to a means 5 for computing the average angular velocity or 
RPM of the engine. In an analog version of the invention, the means for 
computing the instantaneous angular velocity V.sub.i at the ith point of 
time in the cylinder firing cycle may be a voltage measuring device. In a 
digital or microprocessor version of the invention, the means for 
computing the instantaneous angular velocity may be a digital storage 
operation of the ith output word from a pulse width measurement or analog 
to digital (A/D) converter. In any construction of the invention, the 
output from the means 4 for computing the instantaneous angular velocity 
is a continuous or discrete sequence of angular velocity values 
representing a continuous variable V(t). In particular, there is a value 
of this variable at the beginning of a cylinder firing period 
V(t.sub.start) which will henceforth be named V.sub.start and a value of 
this variable at the end of that cylinder firing period V(t.sub.end) which 
will henceforth be named V.sub.end. 
The computation means 5 for computing the average angular velocity also 
receives the conditioned signal representing the instantaneous angular 
velocity of the crankshaft from the signal conditioning means 3. The 
average angular velocity V.sub.avg is computed in the continuous case by 
integrating V(t) over a period of time and dividing the resulting integral 
by the that length of time. In the discrete or digital case, the average 
angular velocity is computed by summing a certain number of instantaneous 
angular velocities samples V.sub.i and dividing the resulting sum by the 
number of samples summed. The computation means 5 can be, but does not 
have to be, a series of steps in a microprocessor of digital signal 
processor (DSP) program. It could also be an analog or digital integrator 
circuit or summing circuit. 
The two values of instantaneous angular velocity V.sub.start and V.sub.end 
as well as the average angular velocity of the engine V.sub.avg are routed 
to a computation means 6 for computing a Percent Power Contribution (PPC) 
parameter. This computation means 6 comprises a squarer for taking the 
square of a number, or the square of a voltage level in an analog 
embodiment, a subtracter to algebraically subtract two numbers or voltages 
values, and a divider to divide two numbers or voltage values. The output 
of the PPC computation means 6 can be represented by the formula: 
##EQU1## 
The computation means 6 can be, but does not have to be, a series of steps 
in a microprocessor or DSP program. It can also be accomplished by 
discrete squarers, subtracters, and a divider. 
The PPC parameter 7 thus computed is an excellent indicator of the 
performance of the cylinder in question. It will become less than zero for 
any cylinder that does not supply positive energy during its firing cycle. 
The PPC parameter 7 is responsive to a fault that occurs either 
intermittently or continuously. The PPC parameter 7 may be represented as 
a voltage in an analog system or a number in a digital or microprocessor 
system. 
The PPC parameter 7 is routed to a comparing means 15 that compares it 
against a threshold 12 that is near zero. If the PPC parameter is less 
than the threshold, the cylinder can be flagged with a fault condition and 
the fault condition can be displayed by a display device 35 such as a 
digital monitor. The comparing means 15 can be an analog comparator, a 
digital comparator or a comparison operation in a microprocessor or DSP. 
The instantaneous angular velocity values V(t) or V.sub.i and the average 
angular velocity V.sub.avg are also routed to a computing means 10 for 
computing an Average Energy Percent (AEP) parameter 11. For an analog 
version of the invention, an energy value for the cylinder is computed by 
integrating the square of the instantaneous angular velocity V(t) over the 
cylinder's firing cycle and dividing by the time period of the cycle. In 
the discrete or digital version of the invention, the squares of the 
sampled instantaneous angular velocity values V.sub.i.sup.2.sub.i are 
summed over the cylinder cycle and divided by the number of samples in the 
cylinder cycle. This energy value is then divided by the square of the 
average angular velocity over the cylinder cycle V.sub.avg.sup.2 to arrive 
at the AEP parameter. The means 10 for computing the AEP paramenter 
comprises a squarer for squaring voltage values or numbers, an integrator 
for integrating voltages, or an adder for summing numbers, and a divider 
for dividing voltage values or numbers. The output of the AEP computation 
is represented by the formula: 
##EQU2## 
The computation means 10 can be, but does not have to be, accomplished by 
a series of steps in a microprocessor or DSP program. It can also be 
accomplished by a discrete squarer, summer and divider. 
The AEP parameter 11 is near one for the next cylinder in the firing order 
for a cylinder that is contributing significantly to the power output of 
the engine. A faulty cylinder or mis-fire causes the AEP value of the next 
cylinder in the firing order to drop to considerably less than one. The 
AEP parameter 11 may be represented as a voltage in an analog system or a 
number in a digital system. The reason the AEP parameter appears to fall 
below one for the next cylinder in the firing sequence after the bad 
cylinder rather than for the bad cylinder itself is became of engine 
inertia. 
The AEP parameter 11 is routed to a second comparing means 14 where it is 
compared against a threshold 13 near one. If the AEP 11 parameter is less 
than the threshold 13, the cylinder is faulty or has suffered a transient 
condition that caused its energy output to temporarily drop. The comparing 
means 14 can be an analog comparator, digital comparator or a comparison 
operation in a microprocessor. 
The fault indications of the PPC comparing means 15 and the AEP comparing 
means 14 can be combined logically as shown with an OR gate 16 in FIG. 1 
to produce a composite fault indicator 17, or they can be analyzed and re, 
cored separately. Any faulty cylinder indications can be displayed on a 
display device 35 such as a digital monitor, CRT or television screen, 
primer, or they can be recorded or communicated to an engine analyzer 
system or operator. 
FIG. 2 depicts the behavior of the instantaneous angular velocity for a 
good cylinder and one possible case of a bad cylinder. It can be seen that 
the instantaneous angular velocity of a good engine follows roughly a 
sinusoidal waveform. Increases in angular velocity occur when a cylinder 
fires, and decreases occur when friction or engine loading slows the 
engine back down. It can also be seen that when a cylinder mis-fires or is 
faulty, the pattern deviates considerably from that of a sinusoid. 
Previous attempts to analyze the instantaneous angular velocity functions 
shown in FIG. 2 have proved unreliable. This is because, while the pattern 
indicates a fault, it does not indicate directly which cylinder is faulty. 
This effect is particularly acute when more than one cylinder is bad. This 
has led to the need for the PPC and AEP parameters of the present 
invention as described above. 
Referring now to FIG. 3, it is seen that for a good cylinder the PPC 
parameter is very near zero. There are occasional small negative 
deviations, but they are random, and do not attach to a particular 
cylinder. For a faulty cylinder with a 60% pressure leak, the PPC 
parameter can be seen to be quite negative. However, the PPC parameter can 
detect faults much less severe than a 60% pressure leak. 
Referring to FIG. 4, it is seen that for a good cylinder, the AEP parameter 
is near one, but for a bad cylinder, the AEP parameter is considerably 
less than one for the next cylinder in the firing order. The AEP parameter 
shown in FIG. 4 can be used as a check on the PPC parameter shown in FIG. 
3. If there is a single bad cylinder, or transient event in a cylinder, 
its PPC parameter will take a significant negative deviation followed by a 
substantial drop in the AEP parameter for the next cylinder in the firing 
sequence. The drop in the AEP parameter appears to occur for the next 
sequential cylinder firing sequence because of a time delay in the angular 
velocity caused by the engine's inertia. This can be seen by comparing 
FIG. 3 with FIG. 4. The cylinder with a 60% pressure leak is cylinder 
number 1 in both cases. The PPC parameter in FIG. 3 takes a negative 
deviation for cylinder number 1. However, in FIG. 4, it can be seen that 
the AEP parameter drops substantially below one for cylinder number 2. 
These two results point to cylinder number 1 as bad. 
Referring now to FIG. 5, it can be seen that the invention can be 
constructed using a microprocessor or digital signal processor (DSP) as a 
computing means. The signal representing the instantaneous angular 
velocity is sampled and converted to a sequence of numerical values by the 
signal converter 18. The signal converter 18 can be an analog to digital 
converter (A/D) or a digital timer. The resulting sequence of numbers 
enters the microprocessor 21 via its input port 20. This port can be a 
latch or standard port device well known in the art. The microprocessor 21 
executes a program stored in a program memory 23 and uses a work memory 22 
for local storage and computation in a well known manner. The program 
memory can be a read-only memory (ROM) or a random-access memory (RAM). 
The work memory is a RAM. Fault indications 17 are outputted via the 
microprocessor's output port 24 to be displayed on a display device 35, 
stored, communicated, or brought to the attention of an operator by some 
other method. The thresholds for the comparison of the PPC and AEP 
parameters can be stored either in ROM or in RAM. A microprocessor or DSP 
can perform the various mathematical operations that form part of the 
invention. For most applications, this is the best mode of practicing the 
present invention. However, a microprocessor based system is only one of 
many means of constructing the invention. 
FIG. 6 is a representative flow chart of a microprocessor or DSP stored 
program that could be used to construct the computational part of the 
invention. In block 25 and 26 respectively the endpoint angular velocities 
V.sub.start and V.sub.end are determined or sampled. In block 27, the 
average angular velocity V.sub.avg is computed and updated each engine 
cycle. Block 28 subtracts the squares of V.sub.end and V.sub.start, while 
block 29 divides by the square of V.sub.avg to obtain the PPC parameter. 
Block 30 computes the sum of the squares of the instantaneous angular 
velocity samples over the cylinder cycle while block 31 divides this sum 
by the square of V.sub.avg to obtain the AEP parameter. Blocks 32 and 33 
compare the PPC parameter and the AEP parameter against the thresholds to 
determine if the cylinder suffered a fault during the previous cycle, 
while block 34 displays, records, or reports any such faults directly to 
an operator or to an engine analyzer. 
It should be clear to one skilled in the electronic am that the invention 
can be constructed from analog circuits with devices such as operational 
amplifiers, integrators, comparators or similar devices using voltage 
values as variables. Alternatively, the invention can be constructed from 
discrete digital circuits such as TrL and CMOS integrated circuits as well 
as MSI and LSI integrated circuits, programmable logic devices such as 
PLD's, gate arrays, or custom integrated circuits such as ASIC's. The 
invention can also be constructed using a microprocessor or digital signal 
processor DSP with associated memories and stored program. 
It is to be understood that the above-described arrangements are merely 
illustrative of the application of the principles of the invention, and 
that other arrangements may be devised by those skilled in the art without 
departing from the spirit and scope of the invention.