Misfire detection in automobile engine

A simple, reliable, compact and inexpensive automobile engine misfiring detection system includes a basic capacitive pressure transducer, attached to a bleeder pipeline equidistantly from each point where the exhaust manifold is attached to the engine, and made of two closely spaced insulating plates whose opposed faces contain conductive layers, and one of which plates is a flexible diaphragm of low mechanical hysteresis. The transducer is thus coupled to a variable exhaust gas pressure source which under normal operating conditions remains at a substantially constant pressure level. The output signal from the transducer has its high-frequency AC component attenuated by a low-pass filter, whose output in turn has its DC component substantially removed by capacitive blocking, after which the remaining AC output is then amplified by a circuit including an operational amplifier. This AC-amplified signal is then compared with a "reduced-magnitude average" reference signal (produced by an AC-to-DC conversion side-circuit followed by magnitude-level adjustment). The comparator output triggers a one-shot monostable multivibrator used to produce an on-off switching signal which operates an LED alarm signal notifying the driver of engine misfiring and also sends an electronic signal to the car's central processor.

FIELD OF THE INVENTION 
This invention relates to practical detection of large abrupt changes in 
magnitude, called discontinuities, of exhaust gas pressure in conventional 
reciprocating internal combustion automobile engines, such as are 
occasioned by combustion misfirings. 
BACKGROUND OF THE INVENTION 
Under the leadership of a government agency, the EPA, and with the aid of 
modern vehicle electronics, most of the pollution from automobile exhausts 
has been eliminated. In a well-running car, engine fuel is burnt 
efficiently, producing in the process the normal exhaust gases. 
In an engine that misfires, however, unburnt fuel is expelled as well, 
contributing measurably to atmospheric pollution. In addition, of course, 
misfiring causes a significant loss of power of the engine, and may result 
in severe damage to the engine. 
A means of detecting misfiring in an engine is therefore very desirable, so 
that corrective action can be taken. 
Back pressure in the exhaust system of a car increases with engine speed 
(measured in revolutions per minute or "rpm") and is relatively constant 
for a given rpm. For a V-8 engine, as an example, this pressure, measured 
just before the catalytic converter, goes up to 10 psig at the highest 
rpm, as shown in FIG. 3 below. 
The continuous succession of firings in the car engine produces relatively 
small pressure pulsations around the back pressure, as shown in FIG. 3. In 
the event of a misfire the back pressure decreases from lack of enough 
exhaust from the misfiring cylinder and recovers its normal level when 
gasses from the cylinder that fires next are forced into the exhaust 
system, causing much larger pressure pulsations, as shown in FIG. 4. This 
difference between the amplitudes of normal and misfire pressure 
pulsations, hereinafter called a pressure discontinuity, forms the basis 
of the present invention for detection of misfires. 
Accordingly there has been a need for a simple, compact, reliable and 
inexpensive real-time pressure discontinuity analysis system capable of 
detecting such exhaust-gas pressure abnormalities and alerting either a 
human operator or/and another automatic correction system to the 
occurrence of a misfire event. 
While relatively expensive alarm systems of this type have been proposed, 
these tend to be complicated and to operate by indirect sensing methods 
which require sophisticated data-processing procedures and techniques in 
order to be of practical utility. For example, the "Method of and System 
for Detecting Misfire in Internal Combustion Engine", U.S. Pat. No. 
4,083,234, issued Apr. 11, 1978 (and assigned to Nissan Motor Co., Ltd. of 
Japan), involves two separate indirect transducers, namely an acoustic 
transducer, such as an earphone receiver placed near the output of the 
engine exhaust gas, together with a rotary electromechanical engine-speed 
responsive frequency generator which produces a frequency variable 
proportional to the output speed of the engine. Furthermore, the signals 
from these two separate transducers must be processed by a fairly 
complicated electromechanical frequency analyzer (involving four separate 
band-pass filters, and two rotary contact mechanical elements operated at 
variable frequencies dependent upon engine speed). 
It might be possible to apply the output of pressure transducers to 
suitably adjustable low-pass filters, having a high-frequency cut-off 
equal to the lowest firing frequency to be attenuated and a high-frequency 
band-pass maximized to the highest misfire frequency to be detected. 
Unfortunately the resultant filter design can be optimized only for a 
given engine rpm, and the development of an adaptive filter, whose 
frequency-domain shape varies with the engine rpm, though possibly 
realizable, is not believed to be the most practical or cost-effective 
approach. 
Accordingly there has been a need for a more direct pressure-discontinuity 
analysis system, which utilizes only one transducer, namely a pressure 
transducer, and which operates successfully regardless of the values of 
other related variables (such as engine speed, in the present case of 
automobile engine misfiring detection systems), such as can be provided by 
a fixed-parameter passive RLC (Resistance, Inductance, Capacitance) analog 
circuitry, or a modified circuit of this type containing active elements 
whose characteristics are fixed independently of such external variables 
as engine speed. 
Also there has been a need for an analysis system which does not include 
any rotating mechanical parts, and whose mechanical aspects are limited to 
the simple flexing of a diaphragm of low mechanical hysteresis, which 
simplification greatly improves reliability and extends lifetime 
durability, without need of scheduled service, and essentially eliminates 
the possibility of breakdown-mandated repairs. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the present invention, there is 
provided a simple, reliable, compact and inexpensive automobile engine 
misfire detection system which includes a single basic capacitive pressure 
transducer, made of two closely spaced insulating plates whose opposed 
faces contain conductive layers, and one of which plates is a flexible 
diaphragm of low mechanical hysteresis. 
The pressure pickup point in the exhaust system should be at a point in the 
exhaust system prior to the catalytic converter, as shown in FIG. 10 in 
the case of an automobile (such as the Cadillac on which the invention has 
been tested) having a single exhaust system exit. The pressure pickup 
point should be at a single common point on a system of bleeder pipelines 
for vehicles with multiple manifold exhaust systems, and the lengths of 
the several bleeder pipelines used to connect the various exhaust systems 
to a common point should be substantially equal. In the case of engines 
having dual exhaust manifold systems, as shown in FIG. 11 (which depicts a 
test on a Lincoln Continental) and FIG. 12 (which depicts a test on a Ford 
TownCar), the transducer is coupled to each engine exhaust manifold by two 
bleeder pipelines of substantially equal length, thus being coupled to a 
pressure source which under normal operating conditions remains at a 
substantially constant pressure level. (By a "substantially" constant 
pressure level is meant one with only minor fluctuations, such as are 
depicted in FIGS. 3 and 7, where "minor" means whatever is normal in the 
absence of actual misfirings; likewise, by "substantially" equal pipeline 
lengths is meant sufficient approximation to exact equality of length that 
the pressure sensor experiences "substantially" constant pressure in the 
absence of actual misfirings.) 
The output of the transducer is operated upon by a fixed-gain, 
constant-parameter, time-independent active circuit composed of passive RC 
circuits combined with operational amplifiers ("op amps") and transistors. 
The output signal from the transducer has its high-frequency AC component 
attenuated by a low-pass filter, whose output in turn has its DC component 
substantially removed by capacitive blocking, after which the remaining AC 
output is then amplified by a circuit which may include an operational 
amplifier. This AC-amplified signal is then compared with a 
"reduced-magnitude average" reference signal (produced by an AC-to-DC 
conversion side-circuit followed by magnitude-level adjustment). The 
comparator output triggers a one-shot monostable multivibrator used to 
produce an on-off switching signal which operates an LED alarm signal. 
In accordance with a further aspect of the invention, a misfire detection 
and analysis system includes a pressure transducer for providing 
electrical signals corresponding to input pressure, a comparator, and a 
detection use circuit. Applied to the inputs of the comparator are an AC 
signal corresponding to the AC component of the output of the pressure 
transducer, and a DC reference signal which is a function of the average 
magnitude of the AC component of the output of the pressure transducer. 
Circuitry is also provided for energizing the detection use circuit only 
when a pressure discontinuity occurs, causing the AC component to exceed a 
predetermined level. 
Other objects, features, and advantages will become apparent from a 
consideration of the following detailed description and from the 
accompanying drawings.

DETAILED DESCRIPTION 
Referring more particularly to the drawings, FIG. 1 shows a cylindrical 
pressure transducer 11 coupled to a fluid-carrying pipe 15 by a Tee-joint 
17 and providing electrical output signals to the discontinuity analysis 
circuit 19 shown in greater detail in FIG. 2 (wherein the pressure 
transducer 11 is specialized to a capacitive pressure sensor 20). The 
presently preferred placement of these four components (numbered 11, 15, 
17 and 19) in three different types of automobile engines is illustrated 
in FIG. 10 (for a Cadillac), in FIG. 11 (for a Lincoln Continental), and 
in FIG. 12 (for a Ford TownCar). The requirements governing the choices of 
these arrangements have already been discussed above in the summary of the 
invention. 
In the preferred embodiment of the present invention, the transducer 11 is 
of the type of a capacitive pressure sensor 20, such as are manufactured 
by Kavlico Corp., 14501 Los Angeles Ave., Moorpark, Calif. 93021 under 
such patents as U.S. Pat. No. 4,329,732, issued May 11, 1982 to Fred Kavli 
et al for "Precision Capacitance Transducer," and U.S. Pat. No. 4,388,668, 
issued Jun. 14, 1983 to Fred Kavli et al for "Capacitive Pressure 
Transducer." In these transducers there is an insulating plate closely 
spaced from a flexible insulating diaphragm of low mechanical hysteresis. 
The plate and diaphragm are coated on facing surfaces with a conductive 
layer; thus the capacitance between the plate and the diaphragm varies 
with the diaphragm's flexing, which is proportional to changes in the 
fluid pressure on the non-coated side of the diaphragm. This type of 
sensor is preferred because of its superior signal to noise ratio and 
because of its tailored time response characteristics. The sensor response 
time is roughly 5 to 10 milliseconds, which indicates that it would take 
about 5 to 10 milliseconds to shift 63% of the way from indicating one 
pressure level to indicating a new pressure level. 
The output waveform of such a sensor in a normally running automobile 
exhaust gas output stream is presented in FIG. 3. 
The same sensor, with abnormal pressure conditions produced by repeated 
engine misfirings, gives the output waveform presented in FIG. 4. 
Before discussing the particular details of the pressure discontinuity 
analysis circuit provided in FIG. 2, it may be helpful to the reader to 
consider the information-flow architecture of this circuit as presented in 
block-diagram form in FIG. 9. Here a variable pressure source 10 presents 
pressure changes 12 to a pressure transducer 20 whose output consists of 
time-varying signals 30. These signals are operated on by low-pass filter 
40, which suppresses any extraneous high-frequency oscillations which may 
be present. The resultant smoothed signal is passed to a 
signal-conditioning sub-circuit 50, which consists of DC blocking circuit 
52, producing a DC-attenuated signal 56, which is the input to 
AC-amplifying circuit 54. The result is the AC-amplified signal 56, which 
is both the input to comparator circuit 80 and 
averaging-and-level-adjusting circuit 60. The adjusted average provides 
reference signal 70, which is compared with AC-amplified signal 56 by 
comparator circuit 80. The difference signal 80 triggers switch circuit 
90, which provides on/off signal 92 to detection use circuit 98. The 
preceding information-processing algorithm could be implemented by a 
digital filter or a hybrid digital-analog filter, but for simplicity and 
cost-effectiveness the preferred embodiment comprises the analog-circuit 
of FIG. 2, which will now be discussed in greater detail. (Operational 
amplifiers will be referred to as "op amps".) 
The sensor output 30, denoted by "pressure voltage" V.sub.p as shown in 
FIG. 2, is filtered by a signal-conditioning circuit 45. In a presently 
preferred embodiment, this signal-conditioning circuit 45 comprises three 
sub-circuits: a low-pass filter 40, a DC-blocking filter 52, and an 
AC-amplifying circuit 54. 
The low-pass filter 40 is comprised of resistor R1 (174 kilo-ohms) and 
capacitor C1 (0.1 micro-farads). The output of the low-pass filter is 
buffered by op amp U6B, and then subjected to the DC-blocking circuit 52, 
which is formed by capacitor C2 (0.47 micro-farads) and resistor R2 (200 
kilo-ohms). 
The resultant signal has its AC component amplified by the AC-coupled 
inverting amplifier 54 implemented by op amp U6A and its associated 
circuits, including resistors R3 (1 megohm) and R4 (10 kilo-ohms). 
The output 56 of the conditioning circuit, denoted by "signal voltage" 
V.sub.s, is shown in FIG. 2 as providing an input to both a comparator 
subsystem 85 (which includes a comparator 80 and a monostable 
multivibrator 90) and an averaging side-circuit 60. 
The side-circuit 60 uses an AC-to-DC conversion circuit as a means of 
averaging the AC signal V.sub.s to produce a reference signal 70, denoted 
by "reference voltage" V.sub.r, which is the other input to the comparator 
80. The side-circuit 60 includes capacitors C4 (4.7 micro-farads), C6 (10 
pico-farads), C7 (150 pico-farads), C8 (30 pico-farads), C9 (10 
micro-farads), C10 (10 micro-farads), and C11 (4.7 micro-farads), together 
with resistors R6 (20 kilo-ohms), R7 (20 kilo-ohms), R8 (10 kilo-ohms), R9 
(22.6 kilo-ohms), R10 (20 kilo-ohms), R11 (15 kilo-ohms), R12 (6.2 
kilo-ohms), R13 (97 kilo-ohms), and R14 (23 kilo-ohms), as well as op amps 
U2, U3, U4, and U5B, and transistors D1 and D2. 
The comparator 80 is based upon op amp U5A, and its output is the input to 
the switch circuit 90 implemented by monostable multivibrator (one-shot) 
U1A, which produces an output voltage pulse at the collectors of 
transistors Q1 and Q2 for every occurrence of a pressure discontinuity 
detection. The pulse duration is determined by the timing components of 
circuit 90, namely resistor R5 (50 kilo-ohms) and capacitor C5 (10 
nano-farads). The output resistors R15 and R16 are both of 10 kilo-ohms 
resistivity. The resistor R17 (200 kilo-ohms) precedes transistor D3, 
which is a Light Emitting Diode (LED) and which is illuminated for the 
duration of the pulse at transistor Q1. The resistor R18 (1 kilo-ohm) is 
connected to the collector of transistor Q2, whose output may be monitored 
by a digital filter or microprocessor as indicated by the output connector 
BNC. 
As shown in FIG. 2, the sensor output V.sub.p is filtered by a low-pass 
filter 40, formed by R1 and C1, to attenuate very high-frequency 
oscillations of the type which may occur during normal operation and are 
of no consequence for discontinuity detection or reference level 
determination, The filtered signal is buffered by op amp U6B and coupled 
through capacitor C2 to an inverting amplifier U6A. AC coupling is used to 
block the DC level of the sensor. The gain of the amplifier 54 is set by 
resistor R3 to give a suitable peak-to-peak voltage at the output (U6A pin 
1), the DC level at the output of the amplifier being set by resistor R4. 
The output V.sub.5, of the amplifier U6A ,shown in the plots of FIGS. 7 and 
8, is applied to the non-inverting input of the comparator 80 (U5A pin 3). 
The same signal V.sub.s is coupled via capacitor C4 to an AC-to-DC 
converter circuit. The DC output of this circuit, appearing on pin 6 of 
U4, is level-shifted by op amp U5B and then applied on lead 70 to the 
inverting input of the comparator 80 (U5A pin 2) to serve as a reference 
voltage V.sub.r. The comparator output (U5A pin 1) acts as a trigger for 
the monostable multivibrator U1A. The one-shot circuit U1A produces a 
pulse output on output pins 4 and 13 on every positive-going voltage 
transition on its input on pin 2. 
For the case of normal operating conditions, not shown in the plots of the 
drawings, the signal V.sub.r is lower than the bottom peak of V.sub.s, and 
the output of the comparator stays high and there is no output from the 
one-shot. This would correspond to a plot of the type of FIG. 5 wherein 
the inverted signal V.sub.s would be represented by a fluctuating but 
almost constant-level straight line beneath the inverted reference signal 
V.sub.r ; in this case, the comparator would never send an output pulse. 
For abnormal conditions, involving pressure discontinuities due to engine 
misfiring, the amplitude of V.sub.s is larger, causing V.sub.r to increase 
and the bottom peak of V.sub.s to move lower as shown in FIG. 5 and the 
corresponding FIG. 6, wherein the pulse-width is about 0.5 milliseconds 
and the time between rising pulse edges is about 5 milliseconds in one 
example tested of a misfiring automobile engine at 6,000 rpm. At the 
crossover points of signal voltage V.sub.s and reference voltage V.sub.r 
the comparator switches state, producing a square wave at its output. The 
positive-going pulse on output pin 13 of the circuit U1A turns transistor 
Q1 to its ON state, lighting up the LED for the duration of the pulse. 
This blink of the LED serves as a visual indicator of the abnormal 
pressure discontinuity event. The negative-going pulse on output pin 4 of 
circuit U1A is inverted by transistor Q2 to produce a positive-going pulse 
at its collector. This pulse can be monitored by a central processor 
coupled to the output collector labeled BNC. 
In conclusion, it is to be understood that the foregoing detailed 
description, and the accompanying drawings relate to the presently 
preferred illustrative embodiment of the invention. However, various 
changes may be made without departing from the spirit and the scope of the 
invention. Thus, by way of example and not of limitation, the transducer 
per se may be made of other materials than those mentioned hereinabove. 
Furthermore, it is possible to use a variable-resistivity sensor instead 
of a variable-capacitance sensor; for example, the facing surfaces of the 
plate and diaphragm can be coated with film resistive layers whose 
resistivity changes as the diaphragm is flexed. In addition, the parts 
need not have the precise configuration described hereinabove, but may 
have alternative arrangements. Further, instead of the structural parts 
being made of metal, they may in many cases be formed of high strength 
composite materials. The analog circuit of FIG. 2 may be replaced by a 
functionally equivalent hybrid analog-digital filter or purely digital 
filter having the same information-theoretic architecture, as depicted in 
FIG. 9. Also a threshold device can be inserted between the comparator and 
the monostable multivibrator, in order to reduce the detection sensitivity 
to minor pressure discontinuities; and this threshold device can be 
operated either upon an absolute threshold level-setting, or upon a 
relative level-setting which depends upon the level of the reference 
signal and varies as that signal varies; and such a circuit could be used 
to supplement or in place of the circuit 60 of FIG. 9. Accordingly, it is 
to be understood that the detailed description and the accompanying 
drawings as set forth hereinabove are not intended to limit the breadth of 
the present invention, which should be inferred only from the following 
claims and their appropriately construed legal equivalents, rather than 
from the example given.