FEATURE ARTICLE The MDLT-1302T Partial-Flow …

1 34 FEATURE ARTICLEThe MDLT-1302T Partial-Flow dilution tunnel for transient Test Cycle PM SamplingAbstractThe current steady-state test cycles used to measure particulate matter (PM) in engine exhaust will be replaced bytransient test cycles in near future. Horiba has recently developed a PM sampling system using a partial flow dilutionmethod, the MDLT-1302T , to perform sampling during transient test cycles. The MDLT-1302T has been evaluatedusing a heavy-duty diesel engine operating under the Euro transient Cycle (ETC) and Feaeral Test Procedure (FTP) transient cycle, and the results prove that a fast flow control has been achieved with a response time of less than This paper describes the performance of the MDLT-1302T and the technical challenges encountered duringits MDLT-1302T Partial-Flow dilution Tunnelfor transient Test Cycle PM SamplingYutaka YamagishiMini TunnelsFull TunnelsFull / Mini / Micro TunnelsMicro TunnelsMini TunnelsFull TunnelsMicro Tunnelsflow rate of exhausted gas I/mFlow rate sample gas I/mDriving modePM measurementin transit testcar speedtimeFlow rate change of exhausted gas andsample gascorrelation between flow rate of exhaust gas ofsample gasflow rate of sample gas I/mtime(s)

2 Flow rate of exhausted gasflow rate of sample gasflow rate of exhausted gas I/mFEATUREARTICLE35 Technical Reports3 IntroductionThis paper describes the performance of the MDLT-1302T Partial-Flow dilution tunnel , a system that willallow highly reliable particulate matter emissionsmeasurements of diesel engine exhaust when dieselengines are tested under the transient test cycles that willsoon be required by a diesel engine is driven under simulated roadconditions, condensed carbon particles and high-boiling-point hydrocarbons emitted from the tailpipe generateparticulate matter (PM). To measure PM emissionsduring simulated driving conditions in the laboratory, adilution tunnel is used. Until recently, only full dilutiontunnels can be used to operate effectively when theengine was operated under transient test cycles andpartial flow tunnels could not apply to transient new transient test regulations place additionaldemands on dilution sampling system which shall be metby fast response flow control systems, if partial flowsamplling method was used for required transient of the dilution tunnel andTransient Test CyclePM measurement has previously used a full flow dilutiontunnel where the entire amount of engine exhaust gas iscollected and made available for sampling.

3 However, underthis sampling method the size of the dilution tunneldepended on the exhaust gas volume: the greater thevolume of exhaust gas from the test engine, the larger thedilution tunnel system must be. The Partial-Flow dilutiontunnel eliminates this problem by permitting sampling of apart of the total exhaust flow. The Partial-Flow dilutiontunnel system first separates the flow of engine exhaust bya constant split ratio before the exhaust stream is fed intothe dilution tunnel . The dilution tunnel dilutes and coolsthe exhaust gas using dilution air. Then the diluted exhaustpasses through filters after the tunnel to prepare for PMsampling. The following equation shows the relationship ofthe engine exhaust gas flow-rate and the diluted exhaustgas flow-rate:r === const. ------- (1)Gsam = Gtot GdilWhere Gexh : Total flow-rate of engine exhaust gasGtot: Flow-rate of diluted exhaust gasGdil: Flow-rate of dilution airr: Split ratioGsam: Flow-rate of engine exhaust gas to be sampled1As shown in equation (1), to maintain a constant dilutedexhaust flow-rate (Gtot), it is necessary to control thedilution air flow-rate (Gdil) at an early stage before theengine exhaust gas flow (Gexh) changes rapidly.

4 Thus,high-speed measurement and control of the dilution airflow is required for the Partial-Flow dilution tunnel if it isto operate under transient test DevelopmentOur approach to developing the MDLT-1302T Partial-Flow dilution tunnel started with improvements made todilution tunnels used for steady-state emissions began by making improvements in the speed ofmeasurement and control for steady-state test provides a schematic of the a dilution tunnel usedin steady-state cycle PM emissions Flow Schematic of the dilution tunnel for aSteady-State Test Cycleexhausted gasenginemicro-tunnelPM picking filterventuri flow Venturi Flow MeterHoriba s dilution tunnel uses highly accurate venturiflow meters that are free from errors caused bydifferences in the densities of the many different gaseouscomponents of the sample, especially CO2 and H2O. Thedensity of engine exhaust gas is nearly constant and thusdoes not present measurement problems when venturiflowmeters are shown in , the venturi flow meter has aconfiguration where the diameter sharply decreases fromthe inlet to the throat and gradually expands towardsoutlet.

5 Here the flow-rate measurement is based on theproportional relationship of the velocity of the gas andthe square root of the differential pressure caused by twodifferent velocities of the gas-the pressure at the inlet andthe pressure at the throat. Because the equation relies onthe square root of the pressure differential, measurementsusing a venturi flow meter are more accurate at highflow-rates. Chart 3 shows a measurement accuracy curvethat illustrates this GdilGsam36 FEATURE ARTICLEThe MDLT-1302T Partial-Flow dilution tunnel for transient Test Cycle PM Partial-Flow Sampling by the Blower Pump andPredictive ControlIn the Partial-Flow dilution tunnel system, the dilutedexhaust flow rate is maintained by controlling therotation speed of the pump installed after the tunnel andPM sampling filter. Because moving a large flow of airgenerates a large inertia, if a large-volume pump is usedto control the flow of dilution air a response delay isunavoidable.

6 This is why different types of blowers areused. Control of the dilution air requires fast response tocontrol inputs so the dilution air pump uses a low-inertiavortex blower. The diluted exhaust flow does not requiresuch a fast response, so a positive displacement rootblower is shows a typical step response of dilution air flow-control when a venturi flow meter is used with a vortexblower. Here the delay time (Td) is approximately and the response time at 90% of indication (T90)is only the exhaust gas flow rate from the testedengine varies rapidly but gradually than the step change,a response time of second for T90 is an acceptableresult in , the delay time (Td) needs a furtherimprovement. The predictive control method is used inthis purpose. Under this method the test engine performsa certain simulated driving cycle before PM samplingbegins. This allows collection of time sequence data onthe exhaust gas flow.

7 Based on this data, the actual PMtest is performed using the delay-time correction thusderived. shows that this method reduces samplingerror to 5% or Configuration of the Venturi Flow Measurement Accuracy of the Venturi Flow Meterflow rate accuracy ( %)flow rate (l/m)goal flow rateresponse curve ofthe dilution airmeasured response curvesTd= 4 The Step Response of Flow Control Using theAir Flow Control Error by the PredictiveControl Method37 Technical The Step Response of Flow-control Using thePiezo-Valve4 Further Speed-Up Development of MDLT-1302T To satisfy the need for fast flow-rate control, Horiba hasdeveloped the MDLT-1302T Partial-Flow dilution tunnelfor use with transient test cycles. shows a flowschematic of the MDLT-1302T . The followingparagraphs discuss some of the new technologies appliedin developing this new dilution Flow Schematic for the Fast Response dilution Air Flow Control byPiezo ValveThe two major factors that affect delay in performingaccurate measurement during a transient cycle are: 1) thetime delay between the sampling of exhaust gas and itsmeasurement and; 2) the delay in the flow control of thedilution tunnel .

8 The first factor can be corrected by usinga predictive control method as discussed in section the problems posed by the second factor, , speeding up the response time of the dilution tunnelflow control, is discussed in the following first approach replaced the vortex blower rotationcontrol method with a new flow control method that usesa supply of compressed air controlled by a piezo-electricvalve. This idea came from a design for a hot-wire typemass flow controller that used a control valve. Suchdevices are in common use, although their accuracy andresponse speed were not satisfactory for use in innovative method developed by Horiba combinesa piezo-valve with a venturi flow meter. In addition tothis, by using a critical flow orifice (CFO) with thepiezo-valve, the response time can be reducedsignificantly. As shown in , these approachescontribute to dramatic improvement of the step responseto second for both Td and rate of exhausted gas (I/m)time(s) Flow-Control ElectronicsTo achieve fast-response flow control, all elements mustperform at high-speed-the signal processing and controlsystem, as well as the flow meter and flow controldevice.

9 The electronics system is vitally important. Itmust perform virtually instantaneous flow-ratecomputations using the analog signals from differentialpressure sensors of the venturi flow meters as well astarget flow-rate values and controller feedback example, the piezo-valve has a characteristichysteresis. The valve-opening signal voltage supplied tothe piezo element is variable. To permit adjustments tothe timing of signals controlling the piezo-valve, aclosed-loop control system with variable target voltage control theory requires that the control signalbe faster than the target response time by one order ofmagnitude. To meet this requirement we made our flowcontrol response time target within 200 ms, andconcentrated our efforts toward minimizing delays fromprocesses such as A/D conversion, flow operation, andD/A conversion. This produced a control loop with anotably high speed of 20 ARTICLEThe MDLT-1302T Partial-Flow dilution tunnel for transient Test Cycle PM Flow Rate Change of Engine Exhaust Gasand Sample Gas in the ETC Mode Transit-TimeEvaluation of transient Response inthe shows a flow rate change of diesel engine exhaustgas and sample gas during a Euro transient Cycle (ETC)test.

10 Shows the correlation between two flows whenmeasured second-by-second throughout the 1800-secondduration of the ETC test. The sampling flow-rate shownhere is obtained from the diluted exhaust gas flow-rateand dilution air flow-rate in equation 1 in section 2. As isobvious from , the exhaust gas sample flow into thetunnel takes only second to catch up to the exhaustgas flow rate (input). The correlation factor betweenthese two flows is (refer to ), which showsthat a reliable proportional control is successfullyachieved. The result of this evaluation clearly indicatesthat the MDLT-1302T provides steady performance withminimum delay time and fast response. The MDLT-1320T is especially useful in measuring exhaust gas incold-start tests . This performance is also supported ( ) by the evaluation of operation in mode testcycle used in of the shows an overview of the MDLT-1302T . Thesystem is made up of the model DLT-1302 dilutiontunnel (set-up inside the engine emission test laboratory),the model DLS-2300 dilution flow control unit, and a setof operator and indicator panels to be installed in theoperation room.