BACKGROUND OF THE INVENTION In January of 1972, a"Carbon Dioxide (C02) Tracer Technique for Modal Mass Exhaust Emission Measurement"paper was presented to the Society of Automotive Engineers by authors Ward W. Wiers and Charles E. Scheffler. This paper described the CO2 tracer technique as a method for measuring automotive exhaust emissions during arbitrary modes of operation of a car on the 1972 Federal Emission Test schedule. This technique allows modal mass measurements of low-emission cars based on undiluted exhaust gas concentrations. The CO2 concentration at the tailpipe is compared with the CO2 in the diluted stream to obtain exhaust flow. This flow multiplied by tailpipe concentration of hydrocarbon, carbon monoxide, and nitric oxide, and integrated over the driving mode, gives modal mass emissions. Problems associated with the lag between the time at which a transient maneuver takes place in the engine and the time at which measurements are recorded are also discussed.

In March of 1993, the paper titled"A Sampling System for the Measurement of PreCatalyst Emissions from Vehicles Operating Under Transient Conditions"was presented to the Society of Automotive Engineers by authors Jon McLeod, Don Nagy, Pat Schroeder, Sue Thiel, Mark A. Dearth, Alex D. Colvin, Tim Webb, Keith R.

Carduner, Dennis Schuetzle, Rick Middleton and Ann M.

Schienker. The sampling system disclosed in this paper provides a proportional sampler for vehicle feedgas and tailpipe emissions that extracts a small, constant fraction of the total exhaust flow during rapid transient changes in engine speed. Heated sampling lines are used

to extract samples either before or after the catalytic converter. Instantaneous exhaust mass flow is measured by subtracting the constant volume sampling (CVS) dilution air volume from the total CVS volume. This parameter is used to maintain a constant dilution ratio and proportional sample. The exhaust sample is diluted with high-purity air or nitrogen and is delivered into modular sample bags. These transient test cycle weighted gas samples can be collected for subsequent analysis of hydrocarbons and oxygenated hydrocarbon species. U. S.

Patent No. 5,650,565 was issued based on a patent application filed incorporating the subject matter of the above-identified paper.

In February of 1997, a paper on"Measurement of Exhaust Flow Rate: Helium Trace Method with a Mass Spectrometer"paper was presented to the Society of Automotive Engineers by Masayuki Adachi, Takashi Hirano, Kozo Ishida, Yasushi Nagata, Akira Kubo, and Shizuo Nakamura. This paper presented a detailed description of a flow rate measurement technique for automotive exhaust.

The system consists of a sector field mass spectrometer for continuous analysis of helium concentration in the exhaust gas and a mass flow controller which injects pure helium at a constant rate into the intake manifold of an engine. The exhaust flow rate can be calculated by helium injection flow rate divided by the concentration since the concentration value is a measure of the ratio of helium dilution taking place in the engine. The advantages of the technique consist of (1) no disturbance from strong pulsed flow present when an engine is idling, (2) easy time alignment with gas analyzers, and (3) measurement of dry-based flow rate that can be directly multiplied by dry-based gas concentration to obtain mass emission rate. Errors in the measured flow rate could arise from several factors such as (1) time of diluted helium to travel from a helium injection point to an exhaust sampling point, (2) existence of helium in the

ambient air, (3) mixing, and (4) analyzing capability of the mass spectrometer. Experiments were conducted to investigate degrees of contribution to the error from the above factors. As a result, calibration of the technique requires ambient helium compensation and vacuum pressure compensation. Comparison with CVS-bag method for C02 mass emission from FTP hot transient mode showed correlation within 1%.

An exhaust sampler for use in evaluating exhaust emissions of an exhaust source such as an internal combustion engine is disclosed in U. S. Patent No. 5,184,501. This patent uses a calibrated subsonic venturi for measuring the exhaust or exhaust/dilution air flow rate. Controls are provided for selecting and controlling a variety of flow rates between or during test periods. The system also provides a variety of control methods for extracting a sample from the bulkstream flow of exhaust and dilution air or from raw exhaust flow.

SUMMARY OF THE INVENTION The previously known exhaust mass flow measurement techniques are technically difficult to put into practice. In addition, these techniques can incorporate large errors in the measurements due to pulsating exhaust flow. Furthermore, in order to directly measure the exhaust flow at high speeds these techniques require the use of flow measurement equipment having a high flow rate capability, while the exhaust flow rate measurement taken when the engine is idling requires the measurement of a relatively low flow rate in comparison with the high speed exhaust flow measurement.

Typically, a single exhaust flow measurement instrument is used for these measurements, and consequently the instrument introduces a magnitude of error in the low exhaust mass flow measurement that is unacceptable.

It would be desirable in the present invention to provide an exhaust mass flow measurement system that

is technically easy to apply and put into practice. In addition, it is desirable in the present invention to provide an exhaust mass flow measurement system that does not incorporate a magnitude of error that is unacceptable when measuring low exhaust mass flow. Furthermore, it is desirable in the present invention to provide an indirect exhaust mass flow measurement system.

The present invention provides a method and apparatus for measuring engine exhaust mass flow. To avoid condensation, engine exhaust gas is mixed with clean, ambient air (balance air), with enough volume as to virtually eliminate water condensation in the mixed air/exhaust gas stream. The inlet is heated sufficiently to vaporize any water before entering a mixing chamber.

The temperature is high enough, such as at least 350°, to prevent water from entering the fan or blower. A pump or blower is used to draw the mixed engine exhaust flow and balanced air flow through a first flow measurement device (Ql). A second pump or blower is used to deliver balance air to the exhaust flow. The flow of the balance air is measured with a second flow measurement device (Q2). The outlet of the second pump or blower is routed to a plenum which allows the balance air to be routed to the mixed air/exhaust flow stream, or routed through a third flow measurement device which measures the bypass flow (Q3).

The balance air blower will satisfy the flow requirement of the mixed air blower, any flow in excess of the flow required by the mixed air blower will be routed through the bypass flow measurement device. The balance air is mixed with the exhaust sample in a chamber of sufficient volume to absorb and dampen exhaust flow pulsations. The balance air flow (Q2) must be maintained higher than the mixed air/exhaust flow (Q1) at all times. The ratio of balance flow air to mixed flow (Q2/Q1) should be maintained above 1.0, at a level sufficient to achieve good resolution with the flow measurement device for the

bypass flow (Q3). The exhaust flow (Qexhaust) can be calculated by using the following algorithm: Qexhaust= (Q3-Qbias)- (<32-Ql) Prior to flow measurement, the bias flow (Qbias) is determined with the exhaust flow (Qexhaust) initialized to zero according to the following algorithm: Qbias=Q3- (Q2-Ql); {when Qexhaust °} Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: Fig. 1 is a simplified flow schematic illustrating an indirect exhaust mass measurement system according to the present invention; Fig. 2 is a simplified flow diagram illustrating the method of operation of the indirect engine exhaust mass flow measurement system according to the present invention; Fig. 3 is a simplified right side schematic view of an apparatus for indirect engine exhaust mass flow measurement according to the present invention; Fig. 4 is a simplified front schematic view of the apparatus illustrated in Fig. 3; Fig. 5 is a chart illustrating the performance of the present invention in comparison to a real time vehicle intake mass air flow plus fuel flow derived from the air to fuel ratio where the y axis is in standard cubic feet per minute (scfm) and the x axis is in time in seconds.

DESCRIPTION OF THE PREFERRED EMBODIMENT The indirect exhaust mass measurement system according to the present invention is shown schematically in Fig. 1. Engine exhaust is introduced into a mixing chamber 10 through an inlet port 12. The engine exhaust exits from the mixing chamber 10 through outlet port 14 in communication with a mixed air/exhaust flow measurement device 16 for measuring a first fluid flow (Q1). The engine exhaust is drawn through the first flow measurement device 16 by a first blower or pump 18 for drawing mixed air/exhaust flow through the first flow measurement device 16 from the mixing chamber 10.

A second blower or pump 20 provides balance air to the mixing chamber 10 through a second flow measuring device 22 for measuring the balance air flow (Q2). The inlet port or pipe 12 is heated sufficiently to vaporize any water before entering the mixing chamber 10. The temperature is raised sufficiently high, such as to at least 350°, to prevent water from entering the system.

The balance air is preferably clean, ambient room air supplied in sufficient volume through the second blower or pump 20 to virtually eliminate water condensation in the mixed air/exhaust gas stream formed in the mixing chamber 10. The second blower or pump 20 is of sufficient size to supply balance air flow (Q2) greater than the mixed air/exhaust flow (Q1). The excess flow passes through a third flow measurement device 24 for measuring bypass flow (Q3) that is not mixed with the engine exhaust in the mixing chamber 10. The mixing chamber 10 is of sufficient volume to absorb and dampen exhaust flow pulsations while the balance air is mixed with the engine exhaust. The outlet of the second blower or pump 20 is in fluid communication with the mixed air/exhaust flow stream (Q1) and the bypass flow stream (Q3). The second blower or pump 20 will satisfy the flow requirement of the first blower or pump 18 and excess flow will be routed through the bypass flow measurement

device 24. Optionally, a recirculation flow device 26 may be provided for diverting a portion of the flow from the outlet of the second blower or pump 20 to the inlet of the second blower or pump 20 in order to properly adjust the effective output of the second blower or pump 20 for the engine exhaust flow rate to be tested.

By way of example and not limitation, the engine exhaust stream can typically exhibit temperatures of up to 1100°F, with a low flow rate of 4 cubic feet per minute (CFM) and a high flow rate of 80-90 CFM for a small four cylinder 1.9 liter engine under current mandated testing criteria. Under proposed testing criteria, an eight cylinder 7.5 liter engine may require a maximum flow rate of 400 CFM. With the optional adjustable recirculation flow device 26, the present invention is capable of adapting to various size engines and various test requirement criteria with proper adjustment of the recirculating flow device 26 with respect to the output of the second blower or pump 20.

In each case, it is adjusted to provide the balance air flow (Q2) at a higher flow than the mixed air/exhaust flow (Q1) and to provide a sufficient excess flow to achieve good resolution with the flow measurement device for the bypass flow (Q3), even under 0 exhaust flow conditions. The increased volume of air through the system allows flow measurement at idle engine speeds, since the excess air flow increases the total air flow for each of the flow measurement devices into an acceptable resolution range without incorporating errors of a magnitude that would be unacceptable, as was previously experienced with attempts to measure the low flow at idle speed of engines with previously known direct engine exhaust mass flow measurement systems.

Although shown schematically in Fig. 1 as single flow measurement devices for the mixed air/exhaust flow measurement, the balance air flow measurement, and bypass flow measurement, it should be recognized that

multiple flow meters can be provided at one or more of these locations to calculate flow based on an average of the measured flows in order to eliminate or reduce any error due to faulty flow measurements by one of the devices. In addition, this provides multiple signals for comparison to detect improperly calibrated or malfunctioning flow measurement devices at each location.

Referring now to Fig. 2, the method of indirect engine exhaust mass flow measurement according to the present invention is illustrated in a simplified flow diagram. In step 100 a control device, such as a microprocessor, control circuit, or computer reads flow measurement inputs in analog or digital form after performing any required signal conditioning and filtering of the raw signals. The inputs that are read by the control device include Q1 (corresponding to the mixed exhaust/balance air flow measurement), Q2 (corresponding to the balance air flow measurement), and Q3 (corresponding to the bypass air flow measurement). The query of step 102 determines if the operator desires to initialize the system with engine exhaust flow set to zero prior to test. This step is required prior to testing an engine in order to calibrate the system by calculating Qbias-If the answer to the query in step 102 is yes, the method according to the present invention continues on to step 104 where Qbias is calculated as equal to the bypass air (Q3) minus the difference between the balance air (Q2) minus the mixed exhaust/balance air (Ql) t with Qexhaust held to zero flow. Once Qbias has been determined for the system, or if the answer to the query in step 102 is no, the method according to the present invention continues to step 106 where the engine exhaust flow is calculated according to the following algorithm: Qexhaust= (Q3-Qbs)-(Q2-Qi) After calculating the engine exhaust flow in step 106 based on the input signals and the calculated bias, the method continues to step 108 where the output signal,

either analog or digital is generated by the control device, corresponding to Qexhaust is equal to the engine exhaust mass flow.

Referring now to Fig. 3 and 4, an apparatus according to the present invention is illustrated in simplified schematic views. Engine exhaust is introduced into the mixing chamber 10 through inlet pipe or port 12 and is withdrawn from the mixing chamber 10 through outlet pipe or port 14. Preferably, the mixing chamber 10 is substantially enclosed in high temperature insulation 28. As best seen in Fig. 3, balance air is introduced into the housing 30 defining the plenum 32. A filter 34 is disposed within the plenum 32 to filter the balance air entering the housing 30. After passing through the filter 34, the balance air is diverted by a baffle 36 defining a narrowed passageway carrying the balance air flow passed the second flow measurement device 22. The second flow measurement device 22 can include one or more hot wire anemometers (HWA), or suitable substitutes. Acceptable substitutes for any of the flow measurement sensors used to measure the mixed exhaust/balance air flow (Q1) or the balance air flow (Q2) can include by way of example, and not limitation, hot wire anemometers, vortex shedding, differential pressure, turbine, swirlmeter, ultrasonic doppler, laser doppler, variable area, electronic mass flow, critical flow venturis, smooth approach orifices, variable flow venturis, positive displacement pumps, electronic mass flow controllers or the like. The suitable substitutes for the bypass air flow (Q3) can include by way of example, and not limitation, hot wire anemometers, vortex shedding, differential pressure, turbine, swirlmeter, ultrasonic doppler, laser doppler, variable area, electronic mass flow or the like. After passing by the second flow measurement devices 22, the balance air enters a second chamber 38. As best seen in Fig. 4, the balance air within chamber 38 communicates with one or

more inlet ports 40 of one or more second pumps or blowers 20. The discharge of each second blower or pump 20 is expelled through outlet port 42 into a third chamber 44 within the housing 30. Optional recirculation flow device 26, such as an actuator and valve assembly, is disposed between the second chamber 38 and the third chamber 44 allowing a predetermined amount of recirculating flow to occur between the outlet port 42 and the inlet port 40 of the second blower or pump 20.

Referring again to Fig. 3, the third chamber 44 is in fluid communication with the bypass flow meter 24 through a series of baffles 46a, 46b, 46c, and also in fluid communication with the mixing chamber 10 through a series of air foils or baffles 48a, 48b. A pulsation dampening device, such as a diaphragm 50, is positioned adjacent the mixing chamber 10 external to the high temperature insulation 28. The air foil or baffle 48a directs the balance air flow over the diaphragm 50 to maintain a cooler temperature for the diaphragm 50 prior to entering the mixing chamber 10. The diaphragm 50 is exposed on the opposite side from the mixing chamber 10 to atmospheric pressure allowing the diaphragm 50 to dampen the pressure fluctuations attributable to the engine exhaust flow sufficiently to insure good resolution of the flow measurement devices. Preferably, the diaphragm 50 is constructed of teflon material.

Suitable substitutes for the diaphragm 50 can include by way of example, and not limitation, a low mass, flexible film, predominantly non-permeable material, such as teflon, viton, rubber, tedlar, buna, latex, conductive polyethylene or the like. If analysis is to be conducted after the flow measurement, it is desirable to provide a diaphragm material that is inert to the gases passing through the system. In addition, if static charge is considered a potential problem in the system, the material for the diaphragm is preferable selected to be conductive. Membranes for the diaphragm 50 have been

used in the range 5 mils to 15 mils thickness, inclusive.

Preferably, a teflon membrane of 5 mil thickness is used for the diaphragm 50. The mixed balance air/engine exhaust exits the mixing chamber 10 through outlet pipe 14. As previously described, the mixed air/exhaust is drawn through first flow measurement device 16 by first blower or pump 18, while the second blower or pump 20 is supplying balance air in a volume exceeding the demands of the first blower or pump 18. The excess balance air flow exits through bypass flow measurement device 24 which experiences sufficient flow for good resolution with the flow measurement device 24, even under engine idle operating conditions.

Referring now to Fig. 5, a graph is provided illustrating engine exhaust mass flow measurement in standard cubic feet per minute (scfm) versus time (seconds) compared with real time vehicle measurements of intake mass air flow plus fuel flow derived from the air to fuel ratio, and vehicle speed. As can be seen from the graph of Fig. 5, the indirect engine exhaust mass flow measurement system according to the present invention illustrated by line 200 closely follows the real time vehicle measurements of intake mass air flow plus the fuel flow derived from the air to fuel ratio illustrated in graph line 202. Graph line 204 is provided to illustrate the vehicle speed for reference purposes. The data provided in graphic form by line 200 illustrates the accuracy and responsiveness of the indirect engine exhaust mass flow measurement system according to the present invention.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the present invention, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.