SIMULTANEOUS Cf}NT‘R()L OF NOX AND PM
`
`FROM DIESEL ENGINES
`
`Mr M K Khair.
`
`Southwest Research Institute,
`
`-. USA
`
`ABSTRACT
`
`96EN008
`
`in North America, particulate matter omission limits were imposed on l‘lcavy—Duty Diesel
`‘Engines in l988. Since then. the NO! and particulate tnzttt-or tradc~ofl cltamcteristic became the
`Achilles heel of heavy-duty diesel engines. Most NO,‘ reduction nicasuros invariably lead to higher
`particulate matter emissions.
`In 1998. NO,‘ emissions from heavy-duty diesel engines will not be
`allowed to exceed 4.0 gm/bhp-'nr while particulate matter will still be regulated at 0.1 gIbl1p~hr.
`Post» I998 N03‘./PM emissions standards are expected to be drastically tighter. Ultra-Low Emission
`Vehicle (ULEV) standards are proposing a combined NOX and HC standard not to exceed 2.5
`g/bl1p~hr with particulate matter regulated at 0.10 g/blip» hr.
`
`This paper also considers the impact of exhaust gas rocirculzstion on particulate matter
`composition.
`in addition, the potential of combining both exhaust gas recirculation and passive trap
`aftcertrcatmcnt is demonstrated as a potential solution for simultaneous control of these two emissions
`species.
`
`lN'l'ROD-UCTION
`
`The diesel angina inns long been the most cncrgy efficient power plant for transportation.
`Moreover, dicsclfi emit extremely low levels of hydrocarbon and carbon rnonoxidc that do not
`require aftcnretntrncnt to comply with current and projected standardis. However. it is admittedly
`difficult for this power plant to simultaneously meet NOK und particulate. matter projected standards.
`Traditionally, tlcsign changes aimed at reducing one of those two exhaust gas species. have led to
`an iucrcase in the other. This physiizal characteristic. which is l-tr-own as t\‘Ox/Pitt trad-coff remains
`the subject of an intense research effort.
`
`As mentioned in the. abstract. future emissions standards are proposing extremely low NO:
`and p2tl'llCul£.t1t.' mztttcr limits. A nzasonablc objective for NO,‘ control is 2.0 glblip-hr (2.7glkWli\.
`in order to !l'lC»t'.‘.l the N0‘ +« HC limits of 2.5g/bhp-hr (3.49,/‘l:\‘v‘ht proposed in the Statement of
`Principles (SOP) agreed to by the North An-icrican Engine Manufacturers.
`
`TECHNOLOGIES FOR LOW N03 IN l)l’F.SEL ENGINES - A REVIEW
`
`Many teclinologie.-5 for NO“. reduction have been considered over the last two decadcs. Sonic.
`of these technologies are:
`
`Pilot injection and irriection rate shaping
`lntako charge air cooling
`Injection timing retard
`Water omulsionlinjection
`E1’-;l’I'.‘1|lSl aftertreatrnont
`
`lixliuust gas rccinsulation
`
`BASF-2002.001
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`

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`Some other technologies were adopted for their impact on PM reduction rather than NO: reduction.
`However. it should be noted that reducing PM provided engineers more freedom in controlling NOX
`than would otherwise have been possible. For instance, increasing injection pressure led to reducing
`PM through better mixing and higher air utilization.
`in the meantime, higher injection pressure,
`reduced ignition delay and allowed engineers additional injection timing retard leading to lower NOx
`emissions as well.
`
`EFFECT OF PILOT INJECTION AND i.NJECTION RATE SHAPING
`
`Researchers at Nagasaki University have shown that pilot injection reduced ignition delay
`by 50 percent from that of the conventional injection systcms.m This ignitiondelay reduction
`allowed more injection timing retard with good combustion stability. Significant reductions in flame
`temperature. NO‘, and fuel consumption were also reported by the same researchers. These
`irnpmvcntcnts were attributed to a slower rate of combustion pressure rise resulting in better
`mechanical efficiency and lower cooling loss. Their work, however, documented an increase in
`smoke emissions and pointed to the need for flexible control of pilot injection timing and quantity
`as a function of engine speed and load. Similar results were reported earlier by researchers from
`the ACE institute C0,, Ltd. and JARI, Inc. in iapanm These researchers achieved 35 percent NO‘
`reduction and 60-80 percent smoke mduction sirnttltaneously without fuel consumption penalty.
`They have achieved these results by combining pilot injection with high injection pressures.
`
`Pilot or staged injection research and development results indicated that an optimum injection
`strategy would be a slow injection rat: (small pilot) for N01 control. The latter portion of the
`injection would be characterized with it fast injection rate for particulate control where most of the
`fuel would be injected at the highest combustion temperature near TDC. Work by researchers at
`the University of Wisconsin, Madisottm, using multiple injections (more than two). generally
`supported the above conclusions. Figures la and lb were adapted from rcfcrcncem to illustrate the
`effects of low injection rate on NO,‘ reduction and the adverse impact of higher rates after the bulk
`of the fuel had released its heat. respectively.
`
`EFFl£C'l' OF INTAKE CHARGE AIR COOLING
`
`Most if not all published data agree that for high specificpout ut diesel engines (high BMEP).
`there is a beneficial effect for lower temperature charge air.“*5'
`’ Many engine niattufacturers
`replaced their water-jacket with air-to-air intercooled systems.(4’7’ For a typical North American
`7.5 liter turbocharged diesel engine a fuel consumption improvement of 2.49 percent was seen when
`intake charge air tetnpcruture was reduced from 2()0”F to 120°? (93"C to 49'C).m Meanwhile, a
`N0,‘ reduction of 17.5 percent was reported at the same injection titning.m Figure 2 was adapted
`from reference 7 and gives the effect of rated speed intake manifold temperature on EPA he;-.vy—duty
`transient test NOX emissions.
`
`EFFECT OF INJECTION TIMING RETARD
`
`The effect of retarding injection timing on NO‘ emission in diesel engines has been well
`docu:1tented.i5'5'7) Retarding injection tinting by 8-10’ crank angle was found to help most heavy-
`duty DI diesel engines to meet the 1991 NOX standards of 5.0g/bhp-hr (6.8glkWh) from their pre-
`l99l calibrations of 940.5 g/bhp—hr (l4.3g/kWh). Adverse NOX emission behavior was noted
`however where injection timing was extended beyond this margin.“‘) This plicnonienon was
`attributed to an increase in the premixed fuel fraction due to a significant increase in ignition delay.
`Figure 3 which was adapted from reference 7. ties the influence of both injection timing retard and
`
`BASF-2002.002
`
`

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`BASF-2002.003
`
`

`
`EFFECT OF WATER EMULSION AND WATER INJECTION
`
`This subject was researched by SOl!'le(S'9) including the authors of SAE Paper 920464. While
`this work was conducted, understandably. at one engine speed it showed good promise for
`substantial N0‘, and smoke reduction with a minimum impact on fuel consumption. This work also
`advocated low compression ratios which. together with water emulsion. extend the ignition delay
`but avoid the excessive increases in heat release by increasing the fuel fraction burned during the
`constant volume portion of the cycle.
`in general. lower combustion pressures were observed as
`well. Results obtained with emulsified fuel at various injection timings and compression ratios gave
`the NO,‘-peak combustion temperature relationship shown in Figure 4.
`1,200
`
`
`
`MostEmlastona.ppm
`
`Mun Putt In-Cylinder Gas Temperature. Dog. 0
`
`Figure 4. Effect of Water Emulsion on Not: Emissions
`at Various Timing and Compression Flallos
`
`EFFECT OF EXHAUST GAS AF'I'ERTREATMEN'I‘
`
`In spite of these design changes the basic NO,‘/PM tradeoff remained. Therefore. further
`NO; reductions. without
`increasing PM emissions,
`required new ttftertrezttmt-.nt
`techttologies.
`Normally. using a NO‘ decomposition catalyst in diesel exhaust environment would be preferable.
`However, diesel exhaust
`is typically oxygen-rich and characteristically low on HC and CO.
`Researchers‘-10'} "12’ quickly realized that NO‘ decomposition in the oztidanbricli and retluctant-lean
`environment was a formidable task. Even though the search for a NOX decomposition catalyst for
`diesel exhaust continues, current emphasis is on systems that rely on supplementing entltaust
`hydroc:trbons.(”) This selective catalytic. reduction SCR) method is commercially applied in
`stationary installation using ammonia as it ruductantfl“) Some work was carried out to adopt the
`use of ammonia in the mobile fleetum, but the impracticality of an on—bourd ammonia tank and the
`potential of reductant leakage were found to be detrimental. Therefore, experimentation using diesel
`fuel appeared to be favored over other reductants such as ammonia or urea”3'”). even though it
`may not produce the highest possible N0‘ conversion.
`
`ically given peak NO‘ conversion
`type catalysts such as Pt have t
`Precious metal
`cfficicncics at exhaust temperature levels of 200-250‘ C.“ ’ Base metal zeolitc catalysts on the
`
`BASF-2002.004
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`and base metal catalysts. supplemental hydrocarbon is added in varying amounts of supplemental
`fuel-to~N(') ratio (F/I‘-JO). While good NOX conversion ratios were reported using bench tests and
`simulated stcady—statc. conditions, performance of these catalysts with real diesel exhaust gas yielded
`disappointingly low conversions (less than 15 percent) in EPA transient testing.(”" Dcceleration
`modes enjoy the highest NO‘ conversions‘ while acceleration modes suffer the lowest conversion
`r:rtios.“6’ This disclosure may point
`to the need for sophisticated and flexible control of the
`supplemental hydrocarbon. Published work related to this subject indicates that precious metal NO,‘
`reduction catalysis have a conversion
`of 30 to 50 percent at approximately 225’C, while metal
`Zttoliu: catalysts give the sauna NGX cuiwersion at 350~400'C. The most appealing results were
`obtained when supplemental HC (diesel fuel) was injected about 6 inches {_ 15.25 cm) upstream of
`the catalytic converter. Although high conversion rates were possible at steady-state engine
`conditions associated with the narrow exhaust temperatures mentioned earlier, the overall EPA
`heavy-duty transient emission test resulted in about l5 percent NO reduction, 25 percent particulate
`matter iniprnveiiierit, and 3 percent fuel consumption increasedléfi
`
`The rernalnder of this paper will be devoted to EGR and particulate trap technology for the
`simultaneous redttrtion of NO): and particulate matter. Exhaust gas recirculation is certainly not it
`new technology for gasoline or light-duty diesel cnginesm) However, its application in heavy-duty
`diesels ta-as not required. With the prospect of lighter NO; limits (2.0 glbhp-hr or 2.7gfkWh,l,
`engine malltlfélClllTCl’S_ are currently testing and developing sophistictued EGR systems for NOK
`control in dirccbinjected heavy-duty diesel eiigines.’-ml
`
`EFFECT OF ISXHAUST GAS RECIRCULATION ON NOX AND PM EMISSIONS
`
`it has been shown that EGR is it very etfective method for NC)‘ redu_ction.“7’ Two
`principles are believed to control the rate of NO! reduction when using EGR. The firs! premise is
`that of reducing peak combustion temperatures where EGR acts as it heat siril:_‘w’ In this case, the
`heat absorbed by EGR is thought to be directly proportional to the product of EGR flowrate, the
`specific heat at constant pressure, and the temperature differential. Therefore, the absorbed heat (Q)
`can be expressed as follows:
`
`Q-,«1n"XpL_p)<(_A!)
`
`(I)
`
`The second and more important principle by which EGR reduces N0‘ emissions; is displacing some
`of the oxygen induced with the fresh air charge.
`in It simplified way, thermal N01 is formed an a
`function of t“_~_. O3, combustion temperature. and residence time in an environment of high NOX
`fortnatioc, modilirsd by the dissociation of NO and N03, i.e.
`‘iljf’
`= K, (N1, 0,) v x1(N0,No,)
`
`(2)
`
`where K, and K1 are reaction rate cotistzutts that are strong functions of cornbustiott temperature.
`Controlling any of the basic variables (N2, 02, Twmh. and t‘) would control
`the rate of NO‘
`formation. Therefore, reducing the fresh charge air oxygen constant by means of EGR reduces NO;
`formation through reducing one of the four factors contributing to NO‘ formation.
`In addition, it
`can be concluded that EGR cooling would increase the temperature differential term in Equation- I.
`thus increasing the EGR heat absorbing capacity and further reducing NO‘.
`
`One of the negative consequences of using EGR is its adverse influence on particulate
`matter. A test conducted on a Series 60 1 1 L turbocharged and intercooled Detroit Diesel engine
`where EGR was systematiczilly increased from Level A through D (given in Figure 5) shows the
`corresponding total particulate matter emission increase during a series of EPA heavy-duty transient
`
`BASF-2002.005
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`constant and concluded that the insoluble fraction (mostly carbonaceous) of the particulate increased
`as EGR rates increased from A to D. Oxidation type catalytic converters were designed to reduce
`particulate SOF and, unfortunately, do not curb the insoluble fraction (mostly carbonaceous matter).
`The only available aftcnreatment alternative for the insoluble fraction became the diesel particulate
`filter (DPF) commonly known as diesel particulate trap.
`
`
`
`EMISSIONS.qfhhp-hr
`
`A
`
`B
`
`C
`
`v DIRECTION or mcnensen can D
`
`FIGURE 5. EFFECT OF EGR ON PAFITICULATE COMPOSITION
`
`Particulate trap systems were extensively investigated in the 1970's. They are characterized
`with wzillflow trap clement designs which force the exhaust to flow through their porous walls and
`deposit
`its solid matter on the walls of the filter elemcntfzm Soot and other solid matter
`accumulated on the filter surface are periodically incinerated to maintain good engine performance
`and fuel economy by holding down backprcssure. An important feature of such actively regenerated
`traps was their complex regeneration system and their sophisticated controls. These features were
`also associated with high cost and the entire trap system had low reliability. which led a major
`North American manufacturer to abandon it in 1993.
`
`Extensive development work was carried out in the last five years on catalyzed trap systems
`with good and encouraging resu1is.m‘22’23'24) Some preferred adding the catalyst on the surface of
`the trap while others introduced the catalyst with the fuel.(25) The introduction of catalyzed fuel
`and the intimate mingling of the catalytic material with the accumulated soot
`in the tra
`led to
`lowering the ignition tetnpcrature of carbon from 600°C to 500.400 or even below 300°C.‘ 33 This
`phenomenon made tmp self-regeneration (passive regeneration) systems possible.
`in summary.
`passively regenerated traps are making EGR a more viable NO‘ control method by passively
`reducing excessive insoluble matter resulting from aggressive EGR control strategies.
`
`DEMONSTRATION OF LOW NOXAND PM USING EGR AND PASSIVE TRAP SYSTEM
`
`A demonstration using two hea.vy—duty North American diesel engines was carried out at
`Southwest Research Institute as part of an internal research project. This dcmonstrzition was
`performed using a 5.9 1.. B—series Curnmins and an ll L series 60 Detroit Diesel heavy-duty diesel
`engines. A Coming wallflow EX—80, l0.5-inch diameter x l'2-inch long, cordiciitc trap was used.
`The catalyst was a cerium-based fuel additive supplied by Rhone-Poulcnc.
`inc.
`Extensive
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`BASF-2002.006
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`NO; emissions to between 2.0 and 2.5 gfbhp-hr (2.7 and 3.4 g/kWh) on both engines. Analysis of
`the PM satnple filters assisted in estimating the carbonaceous particulate matter content typical of
`an EPA heavyduty emission transient cycle. A corresponding additive concentration was then
`recommended by the supplier to maintain quit9i—continuotis trap regeneration.
`
`Exhattst gas recirculation was implemented in a high pressure loop (HPL) configuration. A
`scliematic diagram of the HPL EGR system is given in Figure 6, where EGR is taken from a point
`upstream of the turbocharger turbine and introduced into the intake manifold. Results of hot-start
`EPA heavy-duty transient emissions tests and cycle average specific fuel consumption for the Series
`60 and the B-series engines are tabulated in Tables I and 2. respectively. Starting from NO, levels
`of 3.89 and 4.67 g/blip-hr (5.3 and 6.35 glkwh) for the Series 60 and B-series engines, respectively,
`both engines were successful in achieving 2.02 and 2.2‘) g/bhp—hr (2.73 and 3.10 gfkwhl NO‘.
`emissions.
`in both cases, particulate matter engine-out emissions had more than doubled (from
`0.085 to 0.204 g/bhp-hr for the Series 60 and from 0.14 to 0.35 g/bhp-hr for the B Series).
`However. using a trap and cerium—hase.d fuel additive these PM levels were reduced to 0.014 and
`0.06 glbhp-hr (0.02 and D.08g/kWh). respectively. Fuel consumption increased by 4 percent for the
`Series 60 engine and by 5.5 percent for the 13 series engine. Both engines experienced significant
`increases in HC and C0 emissions. Although EGR contributed to this increase, the presence of the
`trap appeared to have added to the problem. This development was especially noticeable in the case
`of the B series cnginc with HC emissions where an older trap already laden with carbonaceous
`mztterial was used without regenerating it prior to the test. For the same reason it appeared that
`NO‘ emissions were exceptionally low as it result. of the increased bnclcpressure which must have
`led to atiditioniil residual {internal} EGR.
`
`'\
`
`reign {I
`
`“ ‘(:9/1 “:2;
`
`{I
`
`.ii
`
`"Figure 5.NoxfPM FtEDUcTtON SYSTEM
`(men PRESSURE LOOP Eon)
`
`TABl.l€.
`
`1. SERIES 60 TRANSIENT EMISSIONS RESULTS
`
`usrc,
`_ wb_ lb/bl: hr M;
`Test Descri ton
`‘O35
`0.09
`1.30
`3.82
`0.085
`
`0.398
`
`0.204
`
`0.403-
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`BASF-2002.007
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`TABLE 2. B SERIES TRANSIENT EMISSIONS RESULTS
`
`Test Date u ti
`
`Baseline
`
`Active HPL EGR
`Active HPL EGR + OX. CAT.
`
`_
`
`0.55
`
`0.331
`0.50
`
`W __
`2.27
`
`4.20
`2.96
`
`-
`
`__h
`4.67
`
`2.53
`2.30
`
`b
`
`M T
`lblb ._ i
`
`0.424
`
`0.421
`
`A
`1.36“
`ctie I-{PL EGR + Tra
`3 ‘ High HC and CO attributed I part to elevated nne backpre ti
`ricted a .
`
`SUMMARY, CONCLUSIONS, AND RECOMMENDATTONS
`
`Engine design changes including impressive advanws in fuel injection systems have already
`made a major positive impact on engine performance and emissions profile. However,
`further improvements required to achieve NO‘ emissions as low as 2.0 g/bhp—hr (2.7 glkwh)
`will require other technologies.
`Using EGR coupled with passively regenerated traps demonstrated that diesel engines have
`the potential of meeting future proposed low NO,‘/PM emissions standards.
`The capability of achieving 2.0./<0.l glbhp-hr (2.7/<0. l4gll«:Wh) NOX/PM was demonstrated
`on two heavy-duty diesel engines.
`With high pressure loop EGR and passively regenerated trap, EPA transient cycle average
`fuel consumption increased by about 5 percent.
`Increases in HC and C0 emissions were experienced especially with the B series engine
`when the trap was loaded with PM.
`It is believed that the penalty in fuel consumption and increases in HC and CO emissions
`could be curbed by properly optimizing the system. System optimization would involve
`further adjustment
`to the EGR schedule and control algorithm, selection of it more
`appropriate fuel additive concentration. and the investigation of a low pressure loop EGR
`system configuration.
`In the low pressure loop EGR design, exhaust from downstream of
`the trap is returned to the turbocharger (compressor) inlet.
`Variable EGR cooling should also be investigated.
`insoluble particulate matter increased with EGR. This increase was mostly carbonaceous
`matter and could be detrimental
`to engine durability especially with HPL EGR
`configurations. Therefore. long term durability of the diesel engine equipped with I-IPL EGR
`should be investigated and potential advantages of the LPL EGR should be assessed.
`
`REFERENCES
`
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`
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`BASF-2002.008
`
`

`
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`‘
`
`Kharas. K, Thais. 1.. "Pert‘onnancc Demonstration of a Precious Metal Lean NOX Catalysts
`in Native 'Dit:scl F.xhntist," SAE Paper ‘J50’/'5l, February 27-Mztrcli 2, 1995.
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`lS!<_‘I0.
`
`Nccdltam. 1., Doyle, D., Nicol. A., "The Low NO,‘ Truck I:'.nginu_.“ SAI3 Paper 9lO73t\
`Felirttary ?.5AMar-:l'.t l, I991.
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`Ciratz, L.. Lcddy. D., "A Review of Diesel Particulate Control
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`a Heavy-Duty Diesel Engine,” SAE Paper 942067, October l7-20. 1994.
`Demcnthon, 1., Martin. B.. Richards, P4’ Rush, M., Williams, 1)., Bcrgonzini, L., Motclli, P.,
`“Novel Atitlitivt: for Particulate Trap Regeneration," SAE Paper 952355. 1995.
`llammcrlc, R.. Kctchar. D.. Horrocks. R., Lcppt-.rhol'l} CL, Huthwohl, G., Lucrs, B..
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`Particulate: Traps," SAE Paper 952391. 1995.
`Run. V.. Ciltztnclt. H._. Htirrnclnz, R.. “Dit:scl Particulate. Control System for Ford l.lil. Sicrrrt
`Ttll‘l.10-DlCSCl to Matt! 1997/3003 Pttrticulatc Stztndztrtls."
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`I-1., B3I1l1B. P.. Lcmttire. 1.. "Quztsi-Continuous Particle Trap
`Lcpperltoff, G.. Lutlcrtt,
`RCgCDCl"dll€Il'| by Cerium Adtlitivc:t_.” SAP. Paper 950369, February 27-March 2, 1995.
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`BASF-2002.009