Device for measuring exhaust flowrate using laminar flow element

An apparatus for determining the exhaust flowrate from an internal combustion engine (112) includes a laminar flowmeter (116). The flowmeter includes a capillary section (18) of ceramic material including an array of capillary tubes. Signals from a differential pressure sensor (128) which measures pressure loss across the capillary section as well as an absolute pressure sensor (126) and a temperature sensor (130) are input to a computer (134). The computer is programmed to calculate the flowrate of exhaust gas from the engine through its operating range.

TECHNICAL FIELD 
This invention relates to gas flow measuring devices. Specifically, this 
invention relates to a system and device for measuring the exhaust 
flowrate from an internal combustion engine which utilizes a laminar flow 
element. 
BACKGROUND ART 
Internal combustion engine exhaust is characterized by many inherent 
qualities which render it difficult to accurately measure quantitatively. 
Typical flowmeters cannot be used to measure the exhaust flowrate because 
they create a backpressure which affects engine performance. 
Other gas flow measuring devices are not suitable because they are 
incapable of operating at the high temperatures of the exhaust. The 
temperature of the exhaust varies from ambient to as high as 800.degree. 
F. The temperature varies as the engine speed and fuel consumption rate 
varies. The temperature changes as the exhaust gas composition changes. 
The composition and temperature both affect the viscosity and density of 
the gas, creating inherent inaccuracy in most flowmeters. 
The exhaust gas contains corrosive components as well as water. This 
chemical combination creates a inhospitable atmosphere for materials 
typically used in flowmeters. The high temperatures multiply the corrosive 
nature of the exhaust. The water vapor in the exhaust can also produce 
condensation, which may plug flowmeters and sampling lines. 
The exhaust gas flowrate can vary widely. The flowrate can range from a 
very low level which renders it difficult to accurately measure, to a very 
high level. Most flow measurement devices are incapable of maintaining 
their sensitivity over such a wide range. Internal combustion engines can 
produce step changes in exhaust flowrate in time periods as short as 15 
milliseconds. The exhaust flowrate often pulsates and can even reverse its 
flow direction. Most flowmeters are incapable of reacting to such quick 
flow changes, while maintaining their sensitivity. 
Many devices which measure the engine intake air flowrate are known in the 
prior art. These devices operate satisfactorily in the ambient 
temperatures and innocuous chemical atmospheres of the intake air, but are 
not suitable for measurement of exhaust gas. 
Thus there exists a need for a device and system to quantitatively measure 
exhaust flows from internal combustion engines and which can operate in 
such corrosive atmospheres while maintaining high sensitivity over a wide 
turndown flow range. 
DISCLOSURE OF INVENTION 
It is an object of the present invention to provide an apparatus for 
measuring exhaust flows from an internal combustion engine. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which can maintain high accuracy over a wide range 
of exhaust flowrates. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which maintains flow measurement accuracy over a 
wide temperature range characteristic of internal combustion engine 
exhaust. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which minimizes the backpressure in the exhaust 
conduit to avoid interference with internal combustion engine performance. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which is capable of accurately measuring 
volumetric flowrates characteristic of internal combustion engine exhaust 
flowrates. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which is capable of accurately reacting to and 
accurately measuring step changes in volumetric flowrate which are short 
in duration. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which can withstand the corrosive compositions and 
high temperatures characteristic of internal combustion engine exhaust. 
It is a further object of the present invention to provide an apparatus for 
measuring exhaust flows which can prevent water saturated exhaust gas from 
condensing and interfering with precise measurement of the exhaust. 
Further objects of the present invention will be made apparent in the 
following Best Modes For Carrying Out Invention and the appended claims. 
The foregoing objects of the present invention are accomplished by an 
apparatus and system which includes a laminar flow element device or LFE. 
The LFE includes a cylindrical body which is connected to an exhaust pipe 
which carries the exhaust from an internal combustion engine. The LFE 
further includes a capillary section which is positioned within the body. 
A pressure drop in the exhaust is induced by the capillary section as the 
exhaust gas flows through the LFE. The pressure drop is measured by a 
pressure differential sensor which in the preferred embodiment of the 
invention comprises a pressure transducer positioned in fluid 
communication with the fluid at each end of the capillary section. 
The capillary section is comprised of an array of capillary tubes, which 
are aligned in parallel with the longitudinal axis of the body. The 
aggregate area of the capillary section through which the exhaust flows is 
sized so that the exhaust flow therethrough is in the laminar range. This 
size and design of the capillary section acts to produce a very low 
backpressure even at high exhaust flow. 
In the preferred embodiment the body is made of a material that can 
withstand the high temperatures and corrosive properties of the exhaust 
gas. The capillary section is made from a ceramic material. The low 
pressures produced in the gas are virtually unaffected by the thermal 
expansion of the body and capillary section. 
The preferred embodiment of the apparatus further includes a heating 
element adapted to transmit heat to the body and capillary sections of the 
LFE. The LFE is heated by the heating element to prevent the temperature 
of the LFE from falling below the dewpoint of the. exhaust gas. 
The apparatus further includes a pressure sensing element positioned to 
measure the pressure of the exhaust entering the capillary section, and a 
temperature sensing device positioned to measure the temperature of the 
exhaust entering the capillary section. The pressure sensing element, the 
temperature sensing element and the pressure differential sensor produce 
signals which are input to a computer. The computer samples the signals 
from the sensors and uses the readings to calculate the volumetric 
flowrate of the exhaust.

BEST MODES FOR CARRYING OUT INVENTION 
Referring now to the drawings, particularly to FIG. 1, there is shown 
therein, a laminar flow element device or flowmeter 10 used in the 
preferred form of the present invention. Laminar flowmeter 10 includes a 
cylindrical body 12. The body 10 is made of a material that can withstand 
the high temperature and exhaust gas corrosive properties. In the 
preferred embodiment of the invention the body is manufactured from 
stainless steel. Alternatively nickel alloys may be used. The body 10 
accepts the exhaust from an engine at an exhaust inlet 14. Clamps or high 
temperature hose connections known to those skilled in the art can be used 
to connect the exhaust inlet to the exhaust pipe or other conduit through 
which exhaust flows from the engine. The body 12 also has an exhaust 
outlet 16 which provides an outlet connection to the rest of the exhaust 
system. 
Laminar flowmeter 10 also includes a capillary section 18. The capillary 
section 18 consists of an array of ceramic capillary tubes. These tubes 
are shown in cross section in FIG. 2. Each parallel tube 20 is square in 
cross section with an open internal area of preferably about 0.05 by 0.05 
inches and extends traverse to the direction of exhaust flow. The 
aggregate open internal area of the array is preferably about 72 percent 
relative to the interior area of the body. The length between the inlet 
and outlet of the capillary section is preferably about three inches. Each 
tube has a hydraulic diameter of about 1.27 millimeters and is 
individually sufficiently large to minimize potential blockage due to 
exhaust gas particles. The tube size is selected to produce a differential 
pressure, which is a permanent pressure loss, which minimizes the exhaust 
backpressure. This small differential pressure minimizes the effect of the 
flowmeter on engine performance. 
The capillary section 18 is rigidly secured inside body 12. Capillary 
section 18 nests precisely in a pocket 17 formed in the interior of body 
12. The capillary section is held in place by a sleeve 19. This rigidity 
insures geometric integrity and accuracy and repeatability of the 
relationship between volumetric flowrate and differential pressure. This 
construction also prevents flow bypass around the capillary section. 
Because the laminar flowmeter is a mechanically rigid unit, it does not 
change calibration with cleaning or proper handling. As explained later in 
detail, by maintaining close machining tolerances for the body, high 
accuracy, including repeatability of plus or minus two percent of readings 
for a 10:1 turndown flowrate are attainable. 
The body 12 also includes sensing ports to detect the pressure adjacent to 
the inlet and outlet of the capillary section 18. The sensing ports are 
also utilized to measure line static pressure. A first sensing port 22 is 
preferably about 1.5 millimeters in diameter and extends perpendicular to 
a longitudinal axis of body 10. Port 22 is also located adjacent to an 
inlet plane of the openings in capillary section 18. A second sensing port 
24 is similarly sized and also positioned perpendicular to the body's 
longitudinal axis. Port 24 is positioned adjacent to an outlet plane of 
the openings of the capillary section. Both sensing ports are fluidly 
connected to secondary readout devices with coupling and reducer 
assemblies 26. 
The capillary section is preferably comprised of ceramic material. Ceramic 
material provides desirable resistance to the corrosive properties of the 
exhaust gas and resistance to high temperatures. The high temperatures of 
the exhaust gas do not affect the dimensions of the ceramic capillaries 
because of the inherent properties of the refractory material. 
The laminar flowmeter 10 further includes a heating element 28. The heating 
element 28 extends in coiled fashion about the exterior of the body 12. 
The heating element preferably is of the electrical-resistance type and is 
positioned centrally on the body to transmit heat into the interior of the 
body and into the capillary section. The heating element 28 is preferably 
operated under the control of appropriate electrical control circuitry. 
The heating element is operated in response to a thermocouple or other 
temperature sensing device which is mounted through port 23 of body 12. 
The heater operates to maintain the interior of the laminar flowmeter at a 
predesignated minimum temperature. Although the electrical-resistive 
heating element is preferred because of its low costs and operating 
capabilities, other suitable heating elements could alternatively be used. 
In the preferred form of the invention the heating element is covered 
externally by an insulating cover (not shown) to minimize heat loss and 
avoid burns to individuals who may come in contact with the device. 
The laminar flowmeter uses the principle of flow through a capillary tube 
to determine the volumetric flowrate of gas. The principles of capillary 
flow were developed by Poiseuille and Hagan. The Hagan-Poiseuille Law 
relates the fluid actual volumetric flow through a capillary tube to the 
differential pressure across the capillary tube. This relationship is: 
##EQU1## 
where: .DELTA.P=differential pressure across the capillary tube, 
A=constant, 
Q=actual volumetric flowrate, 
.mu.=absolute viscosity, 
L=capillary length, 
D=capillary diameter, 
B=constant, 
.rho.=density of the flowing fluid. 
The Hagan-Poiseuille Law was derived assuming a constant property, fully 
developed laminar flow. Most laminar flowmeters have a non-linearity of 
five percent or less over their normal 10:1 turndown flow range. To 
account for this non-linearity an expanded quadratic form of the 
Hagan-Poiseuille equation is: 
##EQU2## 
where: C=a constant. 
This equation relates the volumetric flow to the differential pressure in a 
more representative fashion for laminar flowmeter performance. This 
equation can be used to determine what pressure differential would be 
generated for a given flow rate Q. 
The design of the ceramic laminar flowmeter enables the total pressure that 
is lost as the exhaust gas passes through the laminar flowmeter to be less 
than two inches of water. Because of the low pressure loss, the gas stream 
can be treated as a compressible. It is therefore possible to rearrange 
the equation in a quadratic or cubic least square fit relationship: 
##EQU3## 
where: A, B, C, D=constants determined by calibration data of each 
individual laminar flowmeter. This equation allows for the calculation of 
the flowrate for a known .DELTA.P. 
The flow in the exhaust pipe connected to the inlet of the body 12 may be 
turbulent, transitional, or laminar. However the proper sizing of the 
laminar flowmeter assures that the flow is laminar through the capillary 
section of the flowmeter. A laminar flowmeter is sized properly when the 
Reynolds number of the gas of the exhaust flow is less than or equal to 
37.5 times the .DELTA.P through the capillary section of the laminar 
flowmeter, where the Reynolds number equals: 
##EQU4## 
Re=Reynolds number, Sg=specific gravity of the flowing exhaust, 
P=line pressure of the exhaust in inches of mercury, 
.DELTA.P=differential pressure generated by the capillary section at 
flowing conditions in inches of water, and 
.mu.=absolute viscosity of the flowing gas in micropoise. 
When the Reynolds number is greater than 37.5 times the .DELTA.P, the 
flowrate to differential pressure relationship will not be linear. 
However, if the laminar flowmeter is calibrated at the operating 
conditions, the calibration is useable and repeatable. 
The calibration of a laminar flowmeter is possible. FIG. 3 shows a typical 
manual calibration setup. A precalibrated laminar flow element device 30 
which has a laminar flow element that has been calibrated with instruments 
traceable to the National Institute of Science and Technology, is placed 
in a closed conduit 33 in series with a laminar flow element 32 to be 
tested. Air or other gas is supplied by a blower 34. Alternatively a 
compressor or pressurized gas source could be used as the gas supply. The 
volumetric concentration of the gas must be known for purposes of 
calculating both the viscosity and the density of the gas. A flow control 
valve 36 is used to regulate the flow of the blower 34. The to-be-tested 
laminar flow element is placed downstream of the control valve 36. 
Instrumentation, including a manometer 38, a thermometer 40 and a 
barometer 42 is attached to the precalibrated laminar flow element device 
30. Likewise a manometer 46, a thermometer 48 and a barometer 50 are 
attached to the laminar flow element device to be tested. Finally a filter 
and/or separator 52 is placed upstream of both laminar flow elements to 
prevent contaminants from entering and plugging the capillary sections of 
the laminar flow elements. 
The parameters required to determine gas flowrate through the laminar flow 
element devices are the differential pressure across the laminar flow 
element, the inlet absolute static pressure and the exit fluid 
temperature. A micromanometer is preferably used to measure the 
differential pressure. An absolute barometer is preferably used to measure 
the static pressure, and a half-degree graduated thermometer is preferably 
used to measure the fluid temperature. 
Although a laminar flow element device can accept a large turndown, its 
accuracy and turndown are limited by the accuracy of the measuring device 
available for low differential pressures. Laminar flow elements will 
produce a maximum differential pressure of 8 inches of water. At a 10:1 
turndown, a readout device must be capable of measuring 0.8 inches of 
water to within plus or minus 0.002 inches of water to achieve an accuracy 
of plus or minus 0.25 percent. Until very recently, the only device 
capable of these accuracies was a micromanometer. 
With the manual calibration system assembled, the laminar element device 
can be calibrated. A constant flowrate is first established through the 
conduit, and the readings of manometers 38 and 46, thermometers 40 and 48 
and barometers 42 and 50 are taken and recorded. These readings are 
similarly taken after the flowrate has been adjusted over increments 
throughout the desired range. The recorded data is then used to regress 
the calibration constants A, B, C, D. These calibration constants can then 
be used in the calculation of volumetric flowrate when using the now 
calibrated laminar flow element. This manual method of calibration is a 
tedious and time-consuming task. Calibration becomes especially cumbersome 
when the temperature of the gas flowing through the conduit changes over 
the course of the test. The changing temperature requires the adjustment 
in readings of the barometer and micromanometer. 
Because of the problems associated with the manual method of calibration, a 
computerized calibration system is the preferred method. The computer 
controlled calibration system is coupled with a data acquisition control 
system. This type of system can provide the benefits of proven system 
accuracy, reduced calibration time, and condensed permanent data storage. 
Such a system is shown in FIG. 4. The calibration train consists of a 
prefilter 54, the to-be-tested laminar flow element device 56, the 
precalibrated laminar flow element device 58, a programmable flow control 
valve 60 and a blower 62. The system alternatively can include a source of 
pressurized gas of a known volumetric concentration. The absolute pressure 
and differential pressures cross the capillary sections of each LFE are 
sensed through the LFE sensing ports 64. The system includes for each LFE 
a differential pressure transducer 66 and an absolute pressure transducer 
68. Preferably the absolute pressure transducer is a digital gauge such as 
Meriam Instrument Model AN0030PA and the differential pressure transducer 
is a capacitance sensor made by MKS Model 220CD. The system further 
includes a temperature sensor 70 which is preferably a one hundred ohm 
resistance temperature device (RTD) with transmitter. The analog signals 
are passed through an analog to digital (A/D) converter 72 which outputs 
digital signals to a computer 74. 
The data acquisition control system comprises the computer 74 and the 
analog to digital converter 76. The computer includes a real time clock 
which enables it to calculate intervals of time. The start and stop times 
are operatively input into the computer and the real time clock allows the 
computer to assign a time to all data input, relative to these two times. 
The computer 74, is preferably programmed so the calibration system may 
perform an entire calibration at an operator's command. With a programmed 
input of the maximum flow to be calibrated, the system can perform a 
10-point calibration by incrementing flow every one-tenth of full scale. 
The flow is adjusted by the control valve 60 under the control of the 
computer via a digital to analog (D/A) converter 76 which outputs a 
variable pressure 3-15 PSIG signal on a line 78 to a pneumatic actuator 80 
of the flow control valve 60. At each increment the computer will first 
allow the system to balance for several seconds as required. The computer 
will then scan the inputs to store the data for later use. The data can be 
stored in the memory 82 associated with the computer, which can be a 
floppy disk, hard drive or EPROM. At the completion of the test sequence, 
the computer will perform the necessary calculations and save the results 
to disk storage and/or on a data printer 84. 
FIG. 5 shows a computer controlled calibration system flow diagram. At the 
start of the calibration, the operator enters the model number, the serial 
number and calibration information for both the LFE to be tested and the 
precalibrated LFE 88 via a keyboard (see FIG. 4) 89. The computer then 
zeros the sensors 90, and waits for the operator to set the flow 
calibration point 92. After the point is set, the computer selects a 
proper sensor 94 and samples the differential pressure, line pressure, and 
temperature at step 96. This data is stored until the test is complete as 
determined through decision point 98. 
A preferred calibration approach uses eight flow points. At the completion 
of the test, the Reynolds number, viscosity and density are calculated at 
step 100. The results of these calculations are used to calculate the 
actual flowrate in cubic feet per minute at step 102. The pressure and 
temperature data are then used to calculate the standard flowrate at step 
104. 
The computer then regresses the calculated data and calculates the 
calibration coefficients using the appropriate quadratic or cubic 
equations at step 106. The computer then compares the actual results to an 
assemblage of calculated results at step 108 and prints out the results as 
a calibration sheet at step 110. The flowrate through the tested laminar 
flowmeter device can then be calculated by using the coefficients supplied 
in the calibration sheet. 
FIG. 6 schematically depicts a system for measuring the exhaust flowrate 
from an internal combustion engine using a laminar flow element device. 
The engine 112 is supported on a test stand and is supplied with fuel and 
air in the conventional manner. The products of combustion from the engine 
flow via an exhaust pipe 114 to a laminar flowmeter 116. Alternatively, 
the laminar flowmeter can be in fluid communication with the exhaust using 
a heat-resistant conduit extending from the exhaust pipe and the laminar 
flowmeter 116. The exhaust flows to the atmosphere through the closed 
conduit consisting of the exhaust pipe and the laminar flowmeter. 
The temperature of the laminar flow element is maintained via a heating 
element 118 at a predetermined elevated temperature. This elevated minimum 
temperature is preferably maintained at about 150.degree. F. prior to 
engine start to avoid condensation of water vapor in the device. As 
discussed hereafter, the power input to the heating element is controlled 
by the computer to heat the device only to the extent necessary to avoid 
condensation therein. 
Exhaust pressure information is sensed through an upstream sensing port 120 
positioned before the capillary section of the laminar flowmeter and a 
downstream sensing port 122 positioned after the capillary section. The 
temperature information of the exhaust is obtained from a thermocouple 
sensing port 124 positioned upstream of the laminar flowmeter. Absolute 
pressure is measured at the upstream sensing port using an absolute 
pressure transducer 126. A differential pressure transducer 128 measures 
the pressure differential across the capillary section via sensing ports 
120 and 122. Temperature of the exhaust gas entering the laminar flowmeter 
is sensed using a temperature sensor 130 positioned in the thermocouple 
sensing port 124. 
The three sensors 126, 128 and 130 produce analog signals which are input 
to an analog to digital converter 132 and then to a computer 134. The 
computer also receives information from a memory 136 and from an operator 
input via a keyboard 138. The computer can transmit results via a printer 
140 and a monitor 142. The computer in some embodiments also operates the 
control circuitry to control the heat transmission rate of the heating 
element 118 and thereby controls the temperature of the laminar flowmeter 
116. 
The apparatus is specifically designed to measure the engine exhaust 
flowrate. Several characteristics of engine exhaust gas and its flow make 
it difficult to measure the flowrate directly. The exhaust gas changes 
composition over time. The temperature of the gas varies from ambient, 
which can be as low as -20.degree. F. to approximately 800.degree. F. at 
full engine throttle. The exhaust flow pulsates and may actually reverse 
its direction. All of these characteristics must be accounted for in 
measuring the exhaust gas flowrate. 
The absolute viscosity has a direct effect on the measurement of flowrate. 
The absolute viscosity of exhaust gas changes with its composition and 
temperature. Therefore, in order to measure real time flowrate, an exhaust 
gas composition model must be generated. This model depends on engine 
type, engine size and time. The viscosity of the engine exhaust gas can be 
calculated at various temperatures using the formula: 
##EQU5## 
where .mu..sub.mix =absolute viscosity of exhaust gas, 
.mu..sub.i =absolute viscosity of the in component of exhaust gas, 
Y.sub.i =percent volume of the i.sup.th component of exhaust gas, and 
M.sub.i =the molecular weight of the i.sup.th component of the exhaust gas. 
This absolute viscosity information is stored in the memory of the computer 
and used to calculate flowrate. 
The density of the exhaust gas also directly affects the calculation of 
flowrate and is dependant upon exhaust gas composition. The density can be 
calculated using the formula: 
##EQU6## 
where: .rho..sub.mix =density of the exhaust gas at operating conditions, 
Y.sub.i =volume percentage of the i.sup.th component of the exhaust gas, 
M.sub.i =molecular weight of the i.sup.th component of exhaust gas, 
28.962=molecular weight of air, 
P=absolute line pressure, and 
T=absolute line temperature. 
This density information is stored in the memory of the computer to 
calculate flowrate. 
As the laminar flowmeter converts the velocity profile of the exhaust into 
a differential pressure, it responds to step changes in flowrates in about 
10-15 milliseconds. Therefore, the differential pressure sensor must 
respond as quickly as the laminar flowmeter for accurate sampling. Other 
system parameters that must be measured need not react as rapidly as the 
pressure sensors. 
In accordance with a preferred embodiment of the invention, for an engine 
produced by a manufacturer an exhaust gas composition model is developed 
which details the exhaust gas constituency. This model details the 
CO.sub.2, CO, O.sub.2, H.sub.2 O, NO.sub.x and hydrocarbon percent volumes 
as a function of real time from start to idle and at the full range of 
higher engine speeds. The remaining volume of exhaust gas is nitrogen. 
Prior to testing, a model is developed for each engine to be tested that 
profiles the exhaust constituency from engine start to 5 seconds, 5 
seconds to 10 seconds, and so on. In many situations the volume 
percentages of NO.sub.x and hydrocarbons in the exhaust are so small that 
they do not effect the viscosity and density calculations and need not be 
included in the model. The model exhaust constituency data is stored in 
the memory of the test computer prior to testing for later data retrieval. 
A flowchart for the computer program executed by the apparatus in making 
engine exhaust flowrate measurements using a laminar flowmeter system is 
shown in FIG. 7. The operator is first prompted to input the LFE 
information into the memory of the computer and the LFE's calibration 
coefficients in step 144. The operator is then prompted to input the test 
identification data and the information regarding the engine size, fuel 
mix used in the engine and the duration of the test 146. This information 
will allow the computer to correlate the volumetric concentration of the 
exhaust gas in real time by retrieving the appropriate exhaust 
constituency model data from memory. After all the pertinent test 
information is input into the computer, the test can be started. 
The first step in the test is the computer preheating the laminar flowmeter 
by sending a signal to an associated rheostatic controller 148. The 
controller enables electrical flow through the heating element. The 
computer preferably calculates the dewpoint temperature of the exhaust gas 
concentration for the start of the test and controls electrical flow to 
preheat the laminar flowmeter to a temperature which exceeds the dewpoint. 
This preheating is necessary to prevent the thermal inertia of the ceramic 
laminar flowmeter from condensing exhaust gas water which will plug and 
occlude the capillary tubes of the capillary section and result in 
inaccurate readings. Alternatively, the computer may preheat the flowmeter 
to a temperature that is sufficiently high to avoid condensation for all 
engine types and conditions. Once the computer receives a signal from 
circuitry associated with the thermocouple or other sensor adjacent to the 
heating element that the laminar flowmeter has achieved the desired 
temperature, the engine is started at step 150. 
Once the engine is started, the sampling begins. Data for differential 
pressure, absolute line pressure and temperature are acquired by the 
computer every 20 milliseconds at step 152. With this data, the computer 
then calculates the viscosity and density of the exhaust gas at step 154. 
Every 100 milliseconds a five-point average of differential pressure, 
absolute pressure and temperature are used to calculate the actual 
volumetric flowrate at step 156. The computer calculates the standard 
flowrate using the actual temperature and pressure data at a step 158. The 
flowrate and averaged parameters are then downloaded into a file in memory 
at step 160. 
Once the temperature signal indicates that the exhaust temperature exceeds 
the calculated dewpoint temperature of the exhaust gas, the computer stops 
electrical flow to the heating element. Alternatively the computer can 
compare the temperature signal to a set point for purposes of stopping 
external heating of the capillary section. 
Steps 152 through 160 are repeated until the engine is turned off to 
designate the end of the test 162. The operator can then instruct the 
computer to print out the report or may choose to keep the report in its 
respective file in memory. The form of the printed report can be conformed 
to meet the needs of the entity conducting the test. 
The preferred embodiment of the present invention enables the accurate 
measurement of engine exhaust flowrates under varying conditions. The low 
pressure drop of the LFE enables testing under conditions which closely 
simulate those found when the LFE is not present. The system is 
particularly useful in the refinement and testing of electronic engine 
control systems to attain desired operating parameters and optimal engine 
performance. The apparatus, though highly sensitive, is also sufficiently 
durable and tolerant of the harsh service environment to provide a long 
useful life. 
Thus, the apparatus for measuring exhaust flowrate using a laminar flow 
element of the present invention achieves the above-stated objectives, 
eliminates difficulties encountered in the use of prior devices and 
systems, solves problems and attains the desired results described herein. 
In the foregoing description, certain terms have been used for brevity, 
clarity and understanding, however, no unnecessary limitations are to be 
implied therefrom because such terms are for descriptive purposes and are 
intended to be broadly construed. Moreover, the descriptions and 
illustrations herein are by way of examples and the invention is not 
limited to the details shown and described. Further in the following 
claims any feature that is described as a means for performing a function 
shall be construed as encompassing any means capable of performing that 
function and shall not be limited to the particular means shown in the 
foregoing description or mere equivalents. 
Having described the features, discoveries and principles of the invention, 
the manner in which it is constructed and operated and the advantages and 
useful results attained; the new and useful structures, devices, elements, 
arrangements, parts, combinations, systems, equipment, operations and 
relationships are set forth in the appended claims.