Mini-dilution apparatus and method for exhaust emission testing

A mini-diluter apparatus and method are provided for preparing dilute samples of exhaust gas for emission testing. The apparatus includes first and second mass flow controllers which respectively control the mixing of exhaust gas and diluent gas samples at a constant ratio. The mixture is fed at a pressure, controlled by a back pressure regulator, to a third mass flow controller. This third controller provides a controlled sample rate (normally proportional to exhaust flow from an engine under test) to a bag sample system, or other sampling system, with excess mixture being exhausted through a pressure regulator to atmosphere. Improved accuracy and reduced calibration complexity are results of the improved apparatus and method.

FIELD OF THE INVENTION 
This invention relates to measurement of automobile engine emissions and in 
particular to a mini-diluter apparatus and method for preparing dilute 
samples of exhaust gas for emission testing. 
BACKGROUND OF THE INVENTION 
SAE Technical Paper 930141 published Mar. 1, 1993, entitled A Sampling 
System for the Measurement of PreCatalyst Emissions from Vehicles 
Operating Under Transient Conditions provides a full description of such a 
system. That system incorporates a so called "mini-diluter" utilizing a 
pair of mass flow controllers externally controlled to provide a mixture 
of exhaust gas and diluent gas in a prescribed ratio and at a flow rate 
related to the flow of exhaust through the vehicle. The mixture of diluted 
exhaust and diluent is provided to a plurality of bag samplers for 
collection of samples. 
FIG. 1 illustrates the mini-diluter portion of this prior art sampling 
system as described in the SAE paper. It shows the appropriately labeled 
exhaust gas pipe (exhaust), an exhaust mass flow controller (MFM) actuated 
by a control circuit and exhaust flow signal, a diluent mass flow 
controller (MFM) connected with a source of zero air, a sample pump (metal 
bellows pump) and three sample bags connected with the pump. 
While the mini-diluter sampling system discussed has been successfully 
utilized, it is found necessary to periodically check the calibrations of 
the two mass flow controllers at a number of points across their complete 
range of operation and to establish, at least initially, the relative 
accuracy of the flow rates of the two mass flow controllers over the range 
of flow conditions. Because the flow rates of the two mass flow 
controllers at various flow settings are not linear and may have differing 
response times, the repeated calibration of these devices requires 
considerable time and the resulting accuracy of the measurements is 
limited, although adequate for the purposes intended. 
SUMMARY OF THE INVENTION 
The present invention provides an improved mini-diluter apparatus for 
preparing dilute samples of exhaust gas for emission testing. The 
apparatus may be used in a bag testing system as described in the above 
mentioned SAE Paper 930141, as well as in continuous emission testing 
systems, and has been found to improve both the ease of calibration of the 
apparatus and the accuracy of the results obtained therefrom. 
In essence, the improved mini-diluter apparatus allows the first pair of 
mass flow controllers to be operated at a selected constant flow rate to 
provide a constant flow controlled mixture of exhaust gas and diluent gas 
for subsequent exhaust sampling operations. The apparatus adds a third 
mass flow controller in a pressurized transfer line downstream of the 
mixture ratio controllers. The third mass flow controller is operated to 
vary the flow of mixture delivered to the emission collection bags or 
other testing equipment. In accordance with the currently preferred 
sampling method, the sample collection flow is varied proportionally to 
the exhaust flow from the engine being sampled. Excess mixture provided is 
exhausted by a pressure regulator. The apparatus thus simplifies 
calibration since the first pair of mass flow controllers need to be 
calibrated only at the constant flow point or points for each controller 
and the variable flow controller is checked against its single curve for 
repeatability. Accuracy of the sample repeatability is improved from about 
20% error to less than 5% error. 
These and other features and advantages of the invention will be more fully 
understood from the following description of certain exemplary embodiments 
of the invention taken together with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 2 and 3 of the drawings in detail, numeral 10 
generally indicates a bag type sampling system including sample receiving 
and control apparatus. The sample apparatus 12 (FIG. 3) is a known 
arrangement and it should be understood that other types of sampling 
apparatus, such as apparatus for continuous sampling, could equally well 
be connected with the mini-diluter apparatus 14 (FIG. 2) of the present 
invention. 
Sampling apparatus 12, shown in FIG. 3, includes a suitable support 16 for 
carrying, for example, three sample bags 18, each of which is removably 
connected through a vacuum switch 20 and solenoid valves 22 and 24 with 
supply and purge apparatus 26, as well as with exhaust lines 28 and a 
separate bag analysis port 30. The supply apparatus 26 includes solenoid 
valves 32, 34, 36, 38 connecting with an evacuator pump 40 connected with 
a vacuum gage 42 and vacuum switch 44 and a pressure gage 46 adjacent to a 
needle valve 48 controlling the feed from a purge source of zero air or 
nitrogen gas 50. Apparatus 26 connects the sample bags 18 with the 
mini-diluter apparatus 14 through the connection of valve 32 with a 
transfer line 52. 
The mini-diluter apparatus 14, shown in FIG. 2, includes a heated exhaust 
gas line 54 extending into a temperature controlled oven 56 where it 
connects, through a thermocouple 58 and filter 60, with a first mass flow 
controller 62. A temperature sensor 64 and pressure transducer 66 are also 
connected with the exhaust gas line 54 for measuring temperature and 
pressure of the gas fed to the mass flow controller 62. At its other end, 
exhaust gas line 54 is adapted for connection with a tailpipe or other 
portion of the exhaust system of a vehicle or engine for obtaining raw 
exhaust gas samples therefrom. 
Also provided is a source 68 of diluent gas, such as nitrogen or zero air 
(that is free from water and other measurable emission constituents). 
Source 68 connects via diluent gas line 69 through a toggle valve 70, 
forward pressure regulator 72, pressure gage 74, and an axial heater 76 
with a second mass flow controller 78. Located within the oven 56, a 
temperature sensor 80 is also provided to indicate the gas temperature 
being supplied to the second mass flow controller 78. 
Mass flow controllers 62, 78 have outlets which are connected together 
through the transfer line 52 to mix their outgoing gas constituents. The 
mixed flow samples in the transfer line are directed from the two 
controllers 62, 78 past a pressure transducer 82 and a temperature sensor 
84, through a sample pump 86 and past a pressure gage 88, a bypass line 
90, a pressure transducer 92 and a temperature sensor 94 to a third mass 
flow controller 96 which connects with valve 32 of the supply apparatus 
26. The bypass line 90 connects with an accumulator 98 and a back pressure 
regulator 100 which exhausts excess flow through an exhaust port 102. 
FIG. 4 shows an associated calibration apparatus 104 that includes a 
calibration gas inlet port 106, connected through a solenoid valve 108 and 
a controlling needle valve 110, with an outlet 112 which is connectable 
with the exhaust gas heated line 54 for calibration purposes. A rotometer 
114 is also provided to indicate calibration gas bypassed during the 
calibration process. 
In operation, the heated line 54 is connected with an engine or vehicle 
exhaust system and receives exhaust gas samples at a rate controlled by 
mass flow controller 62. The sample pump 86 is operated to provide at 
least the minimum pressure drop across this first mass flow controller 62 
which is required for accurate flow control. Concurrently, diluent gas 
line 69 receives a diluting gas such as zero air or nitrogen from source 
68. The diluent gas is fed at a pressure controlled by forward pressure 
regulator 72 through the axial heater 76 where it is heated to the 
temperature of the oven 56. From heater 76, the gas is passed into the 
oven and through the second mass flow controller 78 which controls the 
rate of diluent gas flow through line 69. 
The controlled flow rates of exhaust gas and diluent gas through flow 
controllers 62, 78, determine the ratio of exhaust gas to diluent in the 
mixture that is formed by joining of the outlets of the two mass flow 
controllers at their connection to the transfer line 52. This diluted 
mixture is then drawn into the transfer line 52 by the sample pump 86 
which, in effect, sucks the exhaust gas samples from the engine or vehicle 
exhaust pipe for mixing with the diluent gas from line 69. 
The pump 86 delivers the dilute mixture under pressure past the bypass line 
90 to the third mass flow controller 96. This controller 96 controls the 
flow rate of the diluted samples from the mini-diluter into the supply 
system and the sample bags 18 fed thereby. This flow, as in the sampling 
system discussed in the previously mentioned SAE paper 930141, is 
controlled as a function of gas flow through the engine exhaust system 
being sampled and therefore varies with the engine exhaust flow 
conditions. 
To provide accuracy of the mixture ratio, the flow rates through the first 
and second mass flow controllers 62 and 78 are maintained constant for 
each test condition although the rate of constant flow may be varied for 
differing conditions. Accordingly, excess mixture, beyond that allowed to 
flow through the third mass flow controller 96, is delivered to the bypass 
line 90. This excess mixture is exhausted through the back pressure 
regulator 100 and exhaust port 102. The back pressure regulator, thus 
controls the pressure at a preselected value between the sample pump 86 
and mass flow controller 96 in the pressurized portion of the transfer 
line 52. This creates a minimum pressure differential across the third 
mass flow controller 96 to assure its accurate control of the mass flow 
according to a pre-measured and known flow curve. 
The accumulator 98 is provided to damp out pressure pulses in the transfer 
line 52 which would otherwise be caused by the pumping action of the 
sample pump 86. Location of the accumulator in the bypass line 90 prevents 
mixture exhausted from the accumulator from affecting the composition of 
dilute mixture samples flowing through the transfer line 52. 
Referring to FIG. 4, when calibration of the system is required, 
calibration gas is received through port 106 into apparatus 104. The 
calibration gas is admitted through solenoid valve 108 at a rate 
established by needle valve 110 and delivered through outlet 112 to the 
gas line 54. The calibration gas is fed through the sampling system 10 as 
if it were engine exhaust gas and the known constituents are measured by 
the sampling system to determine and calibrate the system for accuracy. 
The rotometer 114 in the calibration system indicates the amount of 
calibration gas being bypassed from the system and shows that only 
calibration gas is being fed through the outlet 112. 
The improved mini-diluter apparatus 14 provides accurate and easily 
controllable mixture samples to the sample bags 18. This results from the 
fact that the first and second mass flow controllers 62, 78 are operated 
at selected constant flow rates to provide a predetermined mixture ratio 
of exhaust gas sample to diluent gas, and this mixture is then fed through 
the third mass flow controller which is able to accurately deliver varying 
flow rates of mixture to the sample bags. The sample flow rates are 
preferably controlled by the third mass flow controller 96 to be 
proportional to exhaust flow of the vehicle under test. The use of the 
three mass flow controllers in this manner, accompanied by temperature and 
pressure controlling devices for maintaining accuracy of flow and 
delivery, provides an improved mini-diluter apparatus requiring a minimum 
of calibration complexity and providing significantly improved accuracy 
and repeatability of gas samples over the prior mini-diluter system. 
While the invention has been described by reference to various specific 
embodiments, it should be understood that numerous changes may be made 
within the spirit and scope of the inventive concepts described. 
Accordingly, it is intended that the invention not be limited to the 
described embodiments, but that it have the full scope defined by the 
language of the following claims.