Diagnosis of engine power and compression balance

This disclosure relates to a system for analyzing the performance of a reciprocating piston, internal combustion engine such as a diesel engine. Sensors are connected to the engine, which respond to various operating parameters, and signals representing the parameters are transmitted to computer processing equipment. A speed sensor responds to the movement of an engine part such as the teeth of the engine fly-wheel ring gear, and the processor calculates a function representing the change in the instantaneous engine kinetic energy. The kinetic energy change data are accumulated during an engine acceleration run and again during a deceleration run. The signals from an engine cycle event sensor are processed to indicate the firing intervals of the cylinders and thereby to correlate the engine acceleration and deceleration data. The changes in kinetic energy during the cylinder firing intervals are measured during engine acceleration, and the kinetic energy changes are again measured during deceleration, the two sets of measurements being at substantially the same engine speed. For each cylinder, the measurement on deceleration is subtracted from the related measurement on acceleration, to obtain the net work produced by each cylinder. A similar procedure may be followed to obtain the work done over segments of each firing interval, to thereby check the compression balance of the cylinders.

Frequently when a multicylinder internal combustion engine fails to deliver 
its rated power, the problem arises from the weakness or malfunctioning of 
one or two cylinders out of a total of, for example, six cylinders. In the 
case of a diesel engine, malfunctioning of a cylinder may be due, for 
example, to underfueling caused by failure of the fuel injector, or to 
worn piston rings and valves which cause loss of compression. Evidence of 
a malfunction may be roughness of engine operation or poor cylinder 
compression. 
It is of course desirable to be able to identify a weak cylinder, and there 
are highly skilled mechanics who are able to do so using time consuming 
techniques. This solution is not, however, satisfactory to an engine 
manufacturer where many engines must be tested, or to service facilities 
where skilled mechanics often are not available. Devices such as 
thermocouples, vibration detectors and cylinder pressure sensors may be 
used in a laboratory but they generally are not desirable or available 
elsewhere. 
U.S. Pat. No. 4,064,747 describes a method of identifying or isolating a 
weak or defective cylinder by measuring the instantaneous speed of the 
engine crankshaft as the engine goes through a complete operating cycle. 
When the instantaneous engine speed is plotted against time, a cyclically 
varying curve is obtained, and the maximum speed attained during the 
expansion stroke of a defective cylinder will normally not be as high as 
the speed attained for a normal cylinder. This method is not always 
accurate however, because it does not take into account variations in 
other engine parameters, such as the masses of the operating parts, 
inaccurate speed measurements, variations in cylinder compression work, 
etc. 
It is a general object of this invention to provide an improved system for 
testing the power and compression balance among the cylinders, which 
eliminates the foregoing disadvantages. 
In accordance with the present invention, means is provided for sensing the 
instantaneous engine speed and, during an acceleration run of the engine, 
measuring and storing the instantaneous engine speeds during at least one 
full cycle of the engine. At substantially the same engine speed during a 
deceleration run of the engine, the instantaneous speeds are again 
measured over at least one operating cycle. A function representing the 
change in kinetic energy during each firing interval is computed during 
acceleration and also during each firing interval during deceleration. For 
each firing interval, the two functions are subtracted to obtain the work 
from each cylinder.

DETAILED DESCRIPTION 
With reference to FIG. 1, an engine 10 is illustrated which may be a 
standard internal combustion engine such as the NH series, six cylinder, 
in-line reciprocating piston, diesel engine manufactured by Cummins Engine 
Company, Inc. Such an engine includes a head 11, a block 12, an oil pan 13 
and a rocker housing 14 fastened to the upper side of the head 11. The 
pistons (not shown) of the engine reciprocate within cylinders (also not 
shown) and are connected to rotate a crankshaft 66. A flywheel on the 
crankshaft has a ring gear 62 attached to it, teeth 63 on the gear 62 
being selectively engaged by a starter motor (not shown) for starting the 
engine. 
A plurality of fuel injectors 16 inject metered quantities of fuel into the 
cylinders after inlet air within the cylinders has been compressed 
sufficiently to cause compression ignition of the resultant combustable 
mixture. The injectors 16 may be a unit type embodying the features of the 
injectors shown in U.S. Pat. No. 3,351,288. A common fuel supply rail 17 
connects the injectors 16 with a fuel supply system including a fuel pump 
18 of the character shown in the U.S. Pat. No. 3,139,875. The fuel pump 18 
draws fuel 19 from a reservoir or fuel tank 21 and forms a regulated fuel 
source for the fuel supplied to the rail 17. A throttle is incorporated in 
the fuel pump 18 and permits the operator of the engine to regulate the 
fuel pressure delivered to the injectors. Also connected to each of the 
injectors 16 is a fuel return rail 24 which carries fuel from the 
injectors 16 to the tank 21. 
The engine 10 further includes a turbocharger unit 31 which may have a 
conventional design. The unit 31 includes a turbine that receives engine 
exhaust from an exhaust manifold 32 and it further includes a compressor 
that is connected by a duct 33 to an air intake manifold of the engine. 
The engine 10 further includes a lubricant system for circulating a 
lubricant such as oil through the various operating parts of the engine. 
The lubricant system includes a pump 41 that draws the lubricant from a 
reservoir in the crankcase and pan 13 and pumps the lubricant under 
pressure to a lubricant rifle passage 42 in the block. The pressure in the 
rifle 42 is regulated by a pressure regulator valve 43 connected in a 
bypass line 44 that is connected across the pump 41. 
A number of mechanical couplings, illustrated by dashed lines in FIG. 1 and 
indicated by the reference numerals 67 and 69, connect the crankshaft 66 
with the fuel pump 18 and the lubricant pump 41, respectively. 
A diagnostic system in accordance with the present invention is provided, 
and includes a cycle event marker (CEM) sensor 51 which is preferably 
mounted in the rocker housing 14 and responds to the movement of an 
operating part of the engine. For example, the CEM sensor 51 may be a 
magnetic coil proximity type sensor that is mounted adjacent the rocker 
arm that actuates the injector 16 of the number one cylinder. This rocker 
arm pivots during injection which occurs toward the end of the compression 
stroke of the piston of the number one cylinder and this movement causes 
the sensor 51 to generate a CEM signal toward the end of the compression 
stroke of the piston of the number one cylinder. The CEM signal is 
utilized in testing engine parameters as will be subsequently described. 
The diagnostic system still further includes an engine speed sensor 61 that 
is mounted adjacent to the outer periphery of the flywheel ring gear 62 of 
the engine 10. FIG. 3 illustrates an example of the sensor 61 and the 
circuits connected to it. The sensor 61 has two spaced elements 91 and 92 
which in the present specific example, are variable reluctance magnetic 
sensors. The teeth 63, moving clockwise, generate signals first in the 
element 91 and then in the element 92. An oscillator 93 is connected to a 
counter 94 which is controlled by the tooth pulses from the elements. A 
pulse from the element 91 operates through circuits 96 and 97 to enable or 
start the counter 94 and a pulse from the element 92 operates through 
circuits 98 and 97 to disable or stop the counter. The count associated 
with each tooth is read by the processor 29. Each count is directly 
proportional to the time interval (.DELTA.t) for a tooth to move from one 
element 91 to the other element 92, and inversely proportional to the 
instantaneous speed of the ring gear. A factor for converting counts read 
to engine RPM may be provided as an input to the processor 29 based on 
physical measurements, such as the spacing X between the elements 91 and 
92 and the radius R of the elements 91 and 92 or may be computed within 
the processor based on signals from the cycle event marker sensor 51. The 
CEM sensor 51 is connected through circuits 95, similar to the circuits 
96-98, to a CEM counter-register 100. The signal from the oscillator 93 is 
connected through the divider 99 to the register 100, and the register 100 
output is connected to data lines of the processor 29. 
The diagnostic system further includes a number of other engine sensors 
including a fuel pressure sensor 27 connected in the rail 17, a lubricant 
pressure sensor 46 connected in the rifle passage 42, and an intake 
manifold air pressure sensor 34 connected in the intake manifold. The 
sensors 51 and 61 are connected to a counter-timer module 22 and the 
sensors 27, 34 and 46 are connected to an A/D convertor 23, the components 
22 and 23 being connected to control and data lines of the processor 29. 
The processor 29 provides outputs of a readout device 70 which may 
provide, for example, visual indications and permanent records. 
FIG. 2 illustrates the diagnostic system in greater detail. The processor 
29 includes a processing unit 71 and a memory unit 72. An operator 
interface 73 is connected to the unit 71 and forms means whereby the 
operator may insert information and instructions and includes the readout 
70. The processor utilizes the signal from the CEM sensor 51 which is 
shown mounted in a position to sense the movement of a rocker arm 74 for 
an injector plunger 76. A cam 77 moves the plunger 76 in an injection 
stroke toward the end of the compression stroke. 
The components 22, 23, 29 and 73 may comprise, for example, standard 
products of Texas Instruments Company. 
FIG. 4 illustrates the variation in the engine torque output, at the 
crankshaft and the ring gear, for a six-cylinder four-stroke engine. The 
torque varies or fluctuates as shown about the mean absorbed torque with 
the engine operating at a steady speed. The crankshaft makes two complete 
revolutions for each engine cycle, and the firing interval of each 
cylinder is 120.degree. long. Torque peaks 101 to 106 appear during the 
combustion strokes of the cylinders, and the relatively low peak 104 
illustrates the characteristic of an underfueled cylinder. Top dead center 
(TDC) at the start of the combustion stroke of the number one cylinder is 
indicated by the number 108. If a cylinder firing interval is divided into 
three equal 40.degree. segments, for a normal cylinder, about 52% of the 
total work is produced in the first segment and 87% of the total is 
produced by the end of the second segment. For a cylinder that is weak due 
to improper burning, for example, about 40% of the total work is produced 
during the first segment and 80% is produced by the end of the second 
segment. 
FIG. 5 shows the torque output vs crank angle of a single cylinder over one 
firing interval, and illustrates the difference in torque between engine 
acceleration and deceleration. The acceleration curve 111 represents the 
condition when power is being generated as during a full open throttle and 
free engine acceleration, and shows the high peak torque caused by burning 
of fuel. The peak on the deceleration curve 112 is produced by the 
expansion of air in the cylinder without fuel combustion, as during 
deceleration when no power is being generated. The curve 112, particularly 
the shoulder 113 shows the effect of the compression in the next cylinder 
in the firing order and the inertia torques. Of course, if the compression 
and torque influences shown in curve 112 vary from cylinder to cylinder, 
the curve 111 would also vary and be an unreliable indication of power 
balance. Prior art systems based only on acceleration rate are not able to 
remove those factors from consideration. 
When the deceleration curve 112 is subtracted from the acceleration curve 
111, the torque or work output due to the combustion of fuel is derived 
for a single cylinder, and the torque for a normal cylinder over one cycle 
is represented by the curve 114 in FIG. 6. By subtracting the acceleration 
and deceleration curves to produce the curve 114, in accordance with this 
invention, a number of factors are eliminated, such as apparent speed 
variations due to ring gear faults, variations in the inertia of the 
rotating engine parts, variations in the compression work of the 
cylinders, and engine friction. The integral of the curve 114 thus 
represents the work produced by a single cylinder over one firing 
interval. The curve 116 is a similar curve but shows a deficient cylinder, 
and shows the lower work output or torque of a weak cylinder. 
To determine the work produced by each cylinder in accordance with this 
invention, and thereby to determine the power balance of the cylinders, 
the speed signals from the sensor 61 and the CEM signals from the sensor 
51 are transmitted to the processor 22 which operates in accordance with 
the Metacode shown in FIGS. 7-A to 7-S. 
The Metacode is an abstracted flow chart of the steps to be executed by the 
processor; a complete program will be obvious to those skilled in the art 
from the Metacode and the present description. While the Metacode is a 
sufficient basis for the preparation of a program to carry out the 
invention, the following discussion is provided to aid in understanding 
the system and the Metacode. 
Broadly, the present invention comprises measuring the instantaneous engine 
speed utilizing the sensor 61, the speed measurements being in terms of 
.DELTA.t or the time interval for a tooth 63 to move from one element 91 
to the other element 92. The angular distance, measured in ring gear teeth 
63, from a CEM signal to the next subsequent TDC, and the angular 
distances between adjacent TDCs, are determined to identify the TDC 
locations. During an acceleration run from a selected speed, the time 
interval data are obtained for at least a full cycle of engine operation. 
A function representing the change in kinetic energy from each TDC 
location to the next subsequent TDC is determined by squaring the angular 
speeds at both TDC points and finding their difference. Similar time 
interval measurements at the same engine speed are made during 
deceleration, and the functions representing the energy over the firing 
intervals are determined. Then, for each firing interval, the change in 
kinetic energy on deceleration is subtracted from the change in kinetic 
energy on acceleration. This change in kinetic energy is related to the 
work done by the engine by the relationship 
##EQU1## 
The work done during a firing interval is predominantly influenced by the 
combustion of fuel in the cylinder whose power stroke occurs during the 
firing interval being examined. The works for the various cylinders may 
then be compared or ranked to obtain the power balance. 
The kinetic energy at any instant is a function of the angular speed of the 
crankshaft 66 and the ring gear 62. In a test of a free engine, that is 
where no torque (T) is externally absorbed by a dynamometer or other load, 
the torque fluctuation is represented by a variation in engine 
acceleration as related by the function T=I.alpha. where I is the inertia 
and .alpha. is the angular acceleration. In the method disclosed herein of 
measuring the engine speed, the sensor 61 and the related circuitry 
determine the time interval .DELTA.t required for a tooth to move from one 
sensor element to the other. The time interval is an inverse function of 
ring gear angular velocity .omega. as follows: 
EQU .omega.=K.sub.1 /.DELTA.t 
The kinetic energy is 
EQU KE=1/2I.omega..sup.2 
or where I is the engine inertia and K.sub.1 is a constant. 
Thus, the instantaneous engine speed is related to the instantaneous torque 
output and to the kinetic energy of the engine. 
While the Metacode includes a number of definitions and comments, the 
following additional commentaries may be useful. CEMRK is produced by the 
sensor 51 which, in this example, produces a pulse just ahead of 
top-dead-center of the number one cylinder. The dual-pole sensor is the 
speed sensor 61. The variable DLTWSQ represents .DELTA..omega..sup.2 which 
is a function of the change in KE over an angular interval. TIMINT is a 
time interval .DELTA.t reading from the dual-pole sensor. PZX stands for 
positive zero crossing. When the kinetic energy over an engine cycle is 
plotted vs. time, the point at which torque stops being absorbed (negative 
KE) and starts to be produced (positive KE) is marked by a zero crossing 
from negative to positive (PZX). This crossing substantially coincides 
with the top dead center (TDC) when compression ends and expansion begins. 
Thus the TDC location of each firing interval may be located by plotting 
the change in KE and identifying the positive zero crossings. The crossing 
data are stored in (ARRAY) PZX. The OFFSET means the distance, measured in 
ring gear teeth, from CEMRK to the next PZX (or TDC). As to PROBCAL, the 
RPM equals PROBCAL divided by .DELTA.t and is a function of X/2.pi.R (see 
FIG. 3 for X and R). 
FIG. 7-C shows the power balance routine. The operator is prompted to 
insert various information describing the engine under test. Certain data 
may be stored on a conventional media such as cassette tape or floppy 
disc. These data may be accessed according to an engine model number on 
code supplied by the operator. Thus the operator need not supply the 
detailed information needed to perform the test. NUMCYL means the number 
of engine cylinders and FIRORD means the firing order. The test to be 
performed is the power balance and one or more runs or samples may be 
made. As to the threshold values, in an acceleration run of the engine 
from, for example, 600 RPM to 2400 RPM, the time interval data for the 
power balance test may be collected at approximately 1000 RPM. An engine 
of the type previously described accelerates at the rate of about 200 RPM 
per engine cycle; consequently the threshold instantaneous speed to 
initiate data collection may be approximately 800 RPM. During the 
subsequent deceleration run, the threshold instantaneous speed may be set 
at about 1050 RPM in order to accumulate data at about 1000 RPM, because 
an engine decelerates much more slowly than it accelerates. The system 
then calls the SETUP subroutine (shown in FIG. 7-D) and then calls the 
EXPANSION subroutine (shown in FIG. 7-I). SUMINT means twice the number of 
teeth on the ring gear, that is, the number of teeth passing the sensor 61 
in one full engine cycle. 
In the SETUP subroutine (FIG. 7-D), LOSPEED indicates a threshold value 
such as 600 RPM. TIMINT readings are taken over at least an engine cycle 
and stored in a buffer. The gear tooth corresponding to the occurrence of 
the CEM mark is determined. The positive zero crossings, which correspond 
to the top-dead-centers, are determined and the corresponding gear teeth 
are identified. This information and the firing order enables the TDC 
locations to be identified in terms of ring gear teeth. These data are 
utilized in the power balance test as previously noted. The SETUP 
subroutine calls up a number of other subroutines which accomplish these 
steps and are shown in the drawings. 
The ACQUIRE subroutine, (FIG. 7-F) acquires the .DELTA.t values and the CEM 
values over at least one engine cycle, from the counters that receive the 
oscillator 86 cycles during the .DELTA.t and the CEM time intervals. The 
CALIBRATE subroutine (FIG. 7-G) determines a factor used in determining 
RPM from the .DELTA.t measurements. The SMOOTH subroutine (FIG. 7-H) 
smooths and edits the data to eliminate wild points, in accordance with 
standard techniques. The EXPANSION subroutine computes the work done 
during a firing interval, and in EXPWORK, the work during acceleration is 
subtracted from the work during acceleration for each interval. In the 
IDENTIFY subroutine (FIGS. 7-I and 7-K) the cylinders and their TDC 
locations are identified relative to the CEMRK. The commentaries for the 
subroutines of FIGS. 7-L to 7-Q describe the functions. 
The operation of the system may again be briefly summarized as follows: The 
instantaneous speed data are accumulated in the form of time intervals 
.DELTA.t, the data being received from the sensor 61 and the counter timer 
94. Other data are initially loaded into the system such as the number of 
points or .DELTA.t measurements to be taken and the engine threshold 
speeds at which the measurements are to be taken. The values are stored in 
a buffer TIMINT which also receives the CEM signal. The buffer stores the 
.DELTA.t values, and it provides an index identifying the location of the 
.DELTA.t value that occurs at the same time as the CEM signal. The SETUP 
routine accumulates and processes the data in preparation for the 
acceleration and deceleration runs. The data in the TIMINT buffer are 
edited and smoothed using standard techniques. The instantaneous kinetic 
energy is computed from the inverse of .DELTA.t.sup.2 values. The 
processor utilizes the CEM signal and the zero crossing data, and provides 
an identification of the cylinder top dead center next following the CEM 
signal. The offset is computed in ring gear teeth from a CEM signal to the 
next positive crossing (PZX or TCD). From the engine firing order and the 
total number of teeth on the ring gear the processor computes the number 
of ring gear teeth between TDCs, which is the interval between power 
strokes (INTBPS). Thus, the number of gear teeth from the CEM to the TDC 
of each cylinder is computed. The processor also computes the 
instantaneous engine RPM using the PROBCAL conversion factor. 
After the data are accumulated during the acceleration run, the time 
interval measurements at the top dead centers of the cylinders are 
utilized to compute the change in kinetic energy from each TDC to the next 
TDC. Similarly, after the subsequent deceleration run, the changes in 
kinetic energy between TDCs are computed during deceleration. The kinetic 
energy changes are correlated with the associated cylinders or firing 
intervals, utilizing the OFFSET and the INTPBS data. Using the measurement 
for each cylinder or firing interval, the kinetic energy change during 
deceleration is subtracted from the kinetic energy change during 
acceleration, to produce the work of each cylinder. To compare the 
cylinders, the work values may be averaged and the average values of the 
cylinders may be ranked. 
Instead of accumulating data in only one acceleration run and only one 
deceleration run, there may be a number of acceleration runs and the data 
averaged, and a number of deceleration runs and the data averaged. 
The system may also provide for determination of the compression balance of 
the cylinders as shown by the routines of FIGS. 7-R and 7-S. The processor 
receives the time interval and the CEM signals, and the OFFSET and the 
INTBPS factors. A low threshold speed is set in it, which is below normal 
low idle speed. When the speed is sufficiently low and the fuel is cut off 
to produce deceleration at low speed, the variation in inertia forces 
between cylinders is small as compared to the gas forces, and consequently 
the kinetic energy change over a segment of a firing interval, such as 
40.degree., preceeding TDC represents the compression work done on the 
gas. The processor receives the data, divides each firing interval into a 
number of segments, and computes the change in kinetic energy in each 
segment. There may be, for example, three equal 40.degree. segments in 
each interval. The data for the segments just preceeding the top dead 
centers are compared and ranked in order to determine the relative work of 
compression for the cylinders. 
It will be apparent from the foregoing that a novel and useful system has 
been provided for diagnosing the health of the cylinders of an engine. The 
system determines the power balances and is able to identify a weak 
cylinder, by sensing the kinetic energy of the engine. By this method, a 
number of factors that could lead to error are eliminated by the 
subtraction of the energy during deceleration from the energy during 
acceleration. The work from each cylinder is thus determined. It is 
preferred that the work be determined by first measuring the changes in 
kinetic energy over the firing intervals both on acceleration and 
deceleration and then taking the difference, because the method may be 
carried out in a straight forward manner as described. However, the 
invention is also broad enough to encompass other systems for measuring 
the work, such as by taking a number of readings at a number of angular 
positions over each interval both during acceleration and deceleration, 
correlating the readings at each angle, taking the difference at each 
angle, and computing the work from the differences. The system also 
provides for determining the compression balance by measuring the changes 
in kinetic energy over the part of each interval that just preceeds 
top-dead-center during the compression stroke. 
When diagnosing power balance or compression balance, the values for the 
cylinders may be produced for evaluation by the system operator, or the 
processor may automatically rank the cylinders, or the values may be 
compared with acceptable reference values. Of course other calculations or 
tests may be performed based on the kinetic energy calculations.