North American Edition

DIESEL PROGRESS NORTH AMERICAN EDITION June 2003

For on-highway engine manufac-turers, meeting model year 2007emissions levels represents a ma-

jor measurement challenge. However,future off-road engine emissions test-ing requirements are anticipated toinclude not only lower steady-stateemissions levels, but also the possibilityof using transient cycle data.

As a result, engine manufacturers areseeking more cost-effective methodsof developing engines to meet regula-tions regardless of their size and re-quired test cycles. The measurementchallenge for 2007 development, com-bined with development cost concernsfor both on- and off-highway engines,have provided the impetus for improv-ing the efficiency, accuracy and flexi-bility of engine test cells.

At the Caterpillar Technical Center,Mossville, Ill., new partial flow samplingsystems (PFSS) are being evaluated foruse in accelerating on-road engine devel-opment using transient test cycles in testcells not equipped with full-flow dilutionsystems. The overall goal is reduction ofanticipated future expenditures by substi-tuting raw exhaust sampling systems thatprovide equal or better data for the morecostly full dilution tests.

The challenge is that during tran-sient test cycles, engine speed, load,airflow and fuel flow values exhibitlarge variations of change over veryshort time frames. For example, themagnitude of engine inlet air mass flowexcursions can approach 10:1 in lessthan two seconds. The challenge for aPFSS is to maintain constant propor-tional flow from an exhaust streamwith a highly variable mass flow rate.

In 1990, Caterpillar and Sierra In-struments entered into a technologypartnership to develop emissions testequipment. Rob Graze, senior engi-neering specialist at Caterpillar, said thatCaterpillar granted Sierra an exclusive

worldwide license to its patented micro-dilution tunnel design incorporated inthe Transient Dilution Airflow Controlsystem (TDAC).

Sierra Instruments, Inc./EmissionsDivision, Monterey, Calif., is a suppli-er of transient and steady state partialflow sampling systems.

“This is an example of the ongoingCaterpillar effort to make its extensiveinventory of technology and know-how available through a proactive li-censing program,” Graze said.

Sierra’s model BG-3 with TDAC is aPFSS that provides accurate, repeatableparticulate matter measurements fortransient testing of all engine types andsizes. The model BG-3 utilizes a Cater-pillar advanced patented dilution tunneldesign and Sierra’s flow apportionment,measurement and control system to exe-cute proportional sampling effectively.

The model BG-3 utilizes overall sys-tem sampling speed and a real-time

measurement of intake air or exhaustflow to ensure correlation with full-flowconstant volume sampling.

In the model BG-3, conditioned dilu-tion air is measured and controlled by adilution air mass flow controller locatedin the TDAC system. A flow controlvalve system in a feedback loop with afast-response intake or exhaust mass flowdevices provides feedback loop control ofthe proportional flow control valve.

Graze said Sierra’s model BG-3 withTDAC and its ancillary exhaust flowmeasurement instrumentation (Exhaust-Trak) will become an important devel-opment tool for Caterpillar. “Repeatabledata is the key to being able to accuratelyquantify the small “building blocks” thatrepresent progress toward meeting emis-sions goals.

“This is crucial information, as en-gine design and hardware changes aremade during the development cyclebased on those measurements. The

test equipment

Partial Flow Sampling Systems For Transient Engine Testing

Meeting model year 2007 emissions levels for on-highway engine manufacturers is amajor measurement challenge, as well as a development challenge. Especially as futureoff-highway engine emissions testing requirements are anticipated to include not onlylower steady-state emissions levels, but also the possibility of using transient cycle data.

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BG-3 has been engineered to achievethe accuracy and repeatability demandsof organizations developing engines tomeet 2007 and beyond on-highwayparticulate limits without sacrificing the flexibility required to develop off-highway engines at today’s limits,” hesaid. “The fundamental goal for BG-3is to provide particulate measurementswith a high degree of correlation to fulldilution from engines against currentknown transient cycles.”

The model BG-3 acquires raw ex-haust or intake airflow data, in the formof a zero to 5 Vd.c. or zero to 10 Vd.c.

signal from an air or exhaust mass flowmeter, at a rate of 160 Hz. The TDACsystem utilizes an operator-providedalgorithm to convert the voltage signalto engine flow in pounds per hour. Thisinformation is used to specify the setpoints of the dilution system. This tech-nique ensures that dilution ratios arelocked in proportion with the exhauststream for the duration of the test cycle.

The Exhaust-Trak mass flow meter is amultivariable venturi system that incor-porates a differential pressure transducer, atemperature sensor, and a pressure trans-ducer in a single device. As the raw

North American Edition

exhaust flow accelerates through thecontraction in the flow meter, the stagna-tion pressure of the flow is reduced. Thisreduction in pressure is sensed to create aflow signal, and the temperature and pres-sure measurements are used to calculateand output a mass flow rate signal.

Downstream of the meter’s contrac-tion is a gradual expansion, or pressurerecovery section, which permits therecovery of approximately 90 percent ofthe pressure loss, Sierra said. The perma-nent pressure drop at the full-scale flowrate is less than 1 in. H2O, and does notaffect the engine’s exhaust system.

Absolute pressure and temperaturemeasurements are also available as out-put signals and may be used for diagnos-tic, calibration, or quality assurance pro-cedures. Exhaust-Trak’s speed ofresponse is on par with the raw pressuredifferential signal from an intake air lam-inar flow element (LFE) so it is anappropriate alternative to a cell LFE fortransient control of BG-3.

Transient testing standards requirethat engine manufacturers determineexhaust mass flow either through inte-gration of intake air and fuel mass flowor by direct measurement of exhaustmass flow. Direct measurement of rawexhaust flow provides a means to im-prove the accuracy of partial flow sam-pling systems and enhance their corre-lation with full-flow systems.

The over-arching goal of the BG-3with TDAC and the Exhaust-Trak sys-tem is to reduce the capital and opera-tional costs of full-flow dilution testingand accelerate development and certifi-cation cycles by the use of raw exhaustsampling and measurement systems. H

test equipment

With the 2007 numbers looming, engine manufacturers are seeking more cost-effec-tive methods of developing engines to meet regulations regardless of their size andrequired test cycles. The measurement challenge for 2007 development, combinedwith development cost concerns for both on- and off-highway engines, have providedthe impetus for improving the efficiency, accuracy, and flexibility of engine testcells.

REPRINTED FROM JUNE 2003 DIESEL PROGRESS North American Edition Copyright Diesel & Gas Turbine PublicationsPrinted in U.S.A.

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