Methods for Fast Catalytic System Warm-Up During Vehicle Cold Starts
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`Methods for Fast Catalytic System Warm-Up During Vehicle Cold Starts
`
`Technical Paper
`
`Paper #: 720481
`DOI:
`
`10.4271/720481
`
`Published: 1972-02-01
`
`Citation:
`
`Bernhardt, W. and Hoffmann, E., "Methods for Fast Catalytic System Warm-
`Up During Vehicle Cold Starts," SAE Technical Paper 720481, 1972,
`doi:10.4271/720481.
`
`Author(s): W. E. Bernhardt
`
`E. Hoffmann
`
`Affiliated: Volkswagenwerk AG
`
`Abstract: During vehicle cold start, emissions, mass flow rates, and catalytic
`converter space velocities vary by orders of magnitude. Therefore,
`catalytic exhaust control systems must be designed to operate at high
`efficiency almost from the moment of engine start-up. Catalysts must
`reach their operating temperature as quickly as possible. Therefore,
`the utility of different methods
`for
`improving
`the warmup
`characteristics of catalytic systems is illustrated.
`A very elegant method to speed the warmup is the use of the engine
`itself as a "preheater" for the catalytic converters. High exhaust gas
`enthalpy to raise exhaust system mass up to its operating temperature
`is obtained by the use of extreme spark retard, stochiometric mixtures,
`and fully opened throttle. Intensive studies to investigate the effects of
`concurrent changes of spark timing and air/fuel mixtures on exhaust
`gas temperature, enthalpy, NOx and HC emissions are discussed.
`
`Finally, NOx catalyst characteristics are dealt with, because the NOx
`catalyst is the first in a dual-bed catalytic system. The NOx catalyst
`should have high activity, low ignition temperautre, and good warmup
`performance. If the NOx has a fast warmup rate, this would result even
`in a significant improvement in the warmup characteristic of the HC/CO
`bed.
`
`Sector:
`
`Automotive
`
`Topic:
`
`Emissions control
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`©2015 SAE International. All rights reserved.
`
`http://papers.sae.org/720481/
`
`1/20/2015
`
`BOSCH-DAIMLER EXHIBIT 1007
`
`Page 1 of 25
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`
`Methods for Fast
`Catalytic System
`Warm-Up During
`Vehicle Cold Starts
`7~o48/
`
`W. E. Bernhardt
`and E. Hoffmann
`VOLKSWAGENWERK AG
`
`INTRODUCTION
`To achieve the emission targets prescribed by law for
`1975/76 a number of emission concepts with conven(cid:173)
`tional internal combustion engines and emission control
`systems have been examined by the automotive indus(cid:173)
`try. Catalytic converters, thermal reactors and a com(cid:173)
`bination of these two have been considered as emission
`control systems (1) *. Low emission values have been
`attained with these concepts when the engine is under
`warm working condition. However, the difficulties
`
`lie mainly in the warm-up phase during cold vehicle
`start-up.
`
`To improve the over-all effectiveness of catalytic sys(cid:173)
`tems at vehicle start-up, extensive experimental tests
`were carried out during the warm-up phase on various
`after burning systems by the Research Department of
`the Volkswagenwerk AG. The intent of this paper is to
`illustrate the utility of improving the warm-up charac(cid:173)
`teristic of catalytic emission control systems for achiev(cid:173)
`ing very low emission levels.
`
`ABSTRACT _________________________ _
`
`During vehicle cold start, emissions, mass flow rates, and catalytic converter space velocities vary by orders of
`magnitude. Therefore, catalytic exhaust control systems must be designed to operate at high efficiency almost from
`the moment of engine start-up. Catalysts must reach their operating temperature as quickly as possible. Therefore,
`the utility of different methods for improving the warm-up characteristics of catalytic systems is illustrated.
`
`A very elegant method to speed the warm-up is the use of the engine itself as a "preheater" for the catalytic con(cid:173)
`verters. High exhaust gas enthalpy to raise exhaust system mass up to its operating temperature is obtained by the
`use of extreme spark retard, stochiometric mixtures, and fully opened throttle. Intensive studies to investigate the
`effects of concurrent changes of spark timing and air/fuel mixtures on exhaust gas temperature, enthalpy, NOx and
`HC emissions are discussed.
`
`Finally, NOx catalyst characteristics are dealt with, because the NOx catalyst is the first in a dual-bed catalytic
`system. The NOx catalyst should have high activity, low ignition temperature, and good warm-up performance. If
`the NOx catalyst has a fast warm-up rate, this would result even in a significant improvement in the warm-up charac(cid:173)
`teristic of the HC/CO bed.
`
`*Number in ( ) indicates reference at end of paper.
`
`1
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`
`WARM-UP METHODS FOR CATALYTIC SYSTEMS
`Catalytic emission control systems described in this
`paper operate mainly with
`the dual-bed catalytic
`process. The first bed contains the reduction catalyst
`which reduces the oxides of nitrogen (NOx) by carbon
`monoxide
`(CO), hydrogen (H2), and hydrocarbons
`(HC) which are present in the exhaust gases. The
`reaction between NOx and CO will only take place
`providing that the amount' of oxygen (02) present in
`the exhaust gas is strictly limited to low concentrations.
`This oxygen limitation is met by adjusting rich fuel/ air
`mixtures.
`
`The second catalyst bed contains the oxidation cata(cid:173)
`lyst which burns the carbon monoxide and hydrocarbons
`after introducing secondary air between the first and
`second beds. The quantity of secondary air is set high
`enough to ensure that there is excess oxygen for all
`driving conditions.
`
`Figure 1 illustrates a dual-bed axial-flow converter.
`Such a concept using fresh catalysts when tested accord(cid:173)
`ing to the CVS cold-hot test procedure gave emission
`values which were still twice as high as the exhaust
`emission standards specified for model year 1976. The
`results would be considerably better if the catalytic
`emission system could be warmed up very quickly from
`
`the moment of cold engine start-up. Methods to speed
`the warm-up are listed below:
`
`• Reduce the heat capacity of the exhaust system be(cid:173)
`tween the engine and the dual bed converter.
`
`• Reduce the heat capacity of the catalyst, use smaller
`catalyst quantities, and smaller catalyst particles.
`
`• Mount the converter very near to the engine exhaust
`valves.
`
`• Introduce secondary air in front of the first bed dur(cid:173)
`ing the initial 120 seconds after cold engine start-up;
`then switch the secondary air to the connecting pipe
`between NOx and HC/CO beds (staged secondary
`air).
`
`Even when these features were used, the dual-bed
`system illustrated in Figure 1 could not reach the targets
`of 0.41 gm. HC/mi., 3.4 gm. CO/mi., and 0.4 gm.
`NOx mi. as proposed in the Federal Register for 1976.
`
`It is particularly difficult to fulfill the emission stan(cid:173)
`dard for oxides of nitrogen as the temperature in the
`NOx bed increases very slowly in systems which are
`designed to allow an adequate residence time for the
`exhaust gases. In Figure 2 is illustrated the mid-bed
`temperature of a VW radial-flow converter with a 1.3
`liter NOx bed during the CVS cold start. It is plain to
`
`DUAL BED CATALYTIC CONVERTER
`(AXIAL FLOW)
`
`--INSPECTION HOLES--
`
`LOUVER PLATE---"'
`
`EXHAUST
`GAS OUT
`~
`
`EXHAUST
`GAS IN
`
`HC/CO CATAL VST BED
`
`NOxCATALVST BED
`
`SECONDARY A IR /
`
`Figure 1
`
`2
`
`Page 3 of 25
`
`

`
`N01 MID· BED TEMPERATURE AND CONCENTRATION DURING
`COLD START PORTION OF CVS TEST
`VW 1.7 LITER (TYPE 4}. RADIAL FLOW DUAL-BED CONVERTER, PELLETED CATALYSTS
`TEMP.- oc
`)PEED-KM/HR
`
`SECONDARY AIR SWITCHED
`TO BETWEEN BEDS
`
`SPEED -
`
`1
`
`r-.......... "
`I
`~
`',
`I
`I
`
`1000
`800
`500
`400
`
`100
`80
`60
`
`40
`
`~ r-"-
`.
`I\ f\ I
`'\j IJ
`I
`20
`200
`I
`I
`0~~--------._~ __ ._._ __ ~----~----~--~-L----~~~~----~ 0
`NOx-PPM
`
`POOR REDUCTION
`(LEAN MIXTURE)------.
`
`GOOD REDUCTION
`(RICH MIXTURE)---..
`
`STAGED SECONDARY AIR
`
`o~~~~--~~~_.~~~=-~-L--~~----~--~~~~~~~
`400
`100
`150
`350
`450
`500
`START-UP
`50
`
`Figure 2
`
`see that the catalyst ignition temperature of approx.
`250°C was reached in the pelleted NOx catalyst bed
`after 195 seconds in spite of the use of the staged secon(cid:173)
`dary air feature. The ammonia problem can also be seen
`in Figure 2 although this is not being dealt with in this
`connection. For more detailed information refer to
`Meguerian (2).
`
`To illustrate the problems of catalytic exhaust emis(cid:173)
`sion control systems Figure 3 shows the tail pipe emis(cid:173)
`sions as well as the exhaust gas flow rate during the
`first 240 sec. of a CVS cold start test. Both the concen(cid:173)
`trations and the mass flow rate vary by orders of mag(cid:173)
`nitude during the CVS cold start test procedure. Further(cid:173)
`more, the concentrations of CO and HC are particularly
`high during the first 80 sec. For this reason, catalytic
`emission control systems must be designed to operate
`
`with high efficiency, that means high reduction and
`conversion rates, as quickly as possible after the engine
`start-up. The catalysts should reach their operating
`temperatures within 20 sec, so that the emissions which
`are produced during the warm-up period of the engine
`can be controlled as quickly as possible.
`
`To do this, further methods for improving the warm(cid:173)
`up characteristics of dual-bed systems should be in(cid:173)
`vestigated.
`
`One method which promises success is a thermal
`reactor acting as a "preheater" for improving catalytic
`converter performance. The thermal reactor is located
`at the cylinder heads. When starting with a rich fuel/ air
`mixture, oxidation of carbon monoxide and hydro(cid:173)
`carbons after adding air, ensures rapid warm-up of the
`
`Page 4 of 25
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`

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`
`CVS COLD START RAW EMISSIONS IN COMPARISON WITH VEHICLE
`SPEED ·AND EXHAUST GAS FLOW RATE
`VEHICLE VW TYPE 4, 1.7 LITER ENGINE
`
`SPEED(cid:173)
`MPH
`
`co-%
`
`C02-%
`
`8
`6
`4
`2
`0
`
`12
`8
`4
`0
`0
`
`NOx(cid:173)
`PPM
`
`HC(cid:173)
`PPM
`
`GAS
`FLOW*
`
`3000
`2000
`1000
`0
`
`1500
`1000
`500
`0
`
`0.75
`0.50
`0.25
`0
`
`40
`
`80 120 160 200 240
`TIME- SEC
`
`0 40
`
`80 120 160 200 240
`TIME- SEC
`
`Figure 3
`
`*STANDARD CUBIC METER/MIN
`
`catalyst. In this system, secondary air is introduced in
`front of the thermal reactor at the cylinder head, and
`the reduction catalyst works as an oxidation catalyst in
`the starting phase. Due to the burning of high HC and
`CO emission levels directly after start-up a rapid warm(cid:173)
`up of the after burning system and therefore a rapid
`attainment of operating temperature is ensured.
`
`A catalytic reduction of NOx is not necessary during
`the cold start phase because the engine operating tem(cid:173)
`perature during this period is not high enough to pro(cid:173)
`duce very high NOx emissions. After the catalysts in
`both beds have reached their operating temperatures
`(100-120 sec. after CVS cold start-up), the thermal
`reactor must be switched off. This is brought about by
`the transfer of secondary air introduction to the conne~t­
`ing manifold between the first and second catalyst beds.
`
`Figure 4 illustrates a catalytic emission control system
`together with major hardware components which has
`been developed for research purposes. It consists of a
`monolithic dual-bed converter, series connected thermal
`reactor, by-pass system, exhaust gas
`recirculation
`(EGR), EGR cooler, regulating valve, and EGR filter.
`The efficiency of such an emission control concept can
`be improved by the introduction of an additional igni(cid:173)
`tion system in the thermal reactor. By enrichment of the
`air/fuel mixture, an improvement can be obtained as
`
`can be seen in Figure 5. With 10% rich fuel/air mix(cid:173)
`tures (A/F = 13.0), the exhaust gas temperature at the
`thermal reactor outlet reaches 300°C five
`to six
`seconds earlier than with normal mixture strength
`(A/F = 16.0).
`
`This high exhaust gas temperature increases the
`reactor warm-up rate, too. This means that the catalysts
`also reach their operating temperatures of about 300°C
`at least five seconds earlier.
`
`Another method of improving the warm-up rate of
`the catalytic system with series connected thermal
`reactors is to cause an ignition failure of a single cylinder
`charge (which contains approx. 20,000 ppm HC) and
`at the same time to increase the idling speed of the
`engine together with wide open throttle. The technique
`produces an increased flow rate of exhaust gases with
`high unburned components during the cold start phase;
`the chemical energy of which can be converted into high
`exhaust enthalpy. This comparatively rich fuel/air mix(cid:173)
`ture can be ignited in the reactor by an additional spark(cid:173)
`plug. With an automatic control device, the ignition
`failure of a particular cylinder can be controlled in
`accordance with the firing order. After the exhaust gas
`has attained the operating temperature required by the
`catalysts the ignition system will revert to normal.
`
`4
`
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`
`

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`
`COMBINED REACTOR-EGR-CATALYST SYSTEM
`MAJOR HADWARE COMPONENTS
`SECONDARY AIR INJECTION------..
`
`EGA ON-OFF VALVE-----.
`
`ELECTRONIC
`CONTROL UNIT
`
`REACTOR - - - - - - l
`
`EG R SYSTEM __ _,
`
`EGA COOLER---'
`NOBLE METAL HC/CO CONVERTER----
`
`Figure 4
`
`INFLUENCE OF RICH AIR-FUEL MIXTURE ON REACTOR OUTLET TEMPERATURE
`FOR BETTER SYSTEM WARM-UP
`
`100 CU IN THERMAL REACTOR
`WITH ADDITIONAL IGNITION
`SYSTEM IN THE REACTOR
`WITHOUT WARM- UP SPARK RETARD
`
`sou~--------------------~
`
`REACTOR
`OUTLET
`TEMP. -°C
`
`600
`
`400
`
`200
`
`0
`
`0
`
`....... ___ _
`
`' - - - - - - A/F = 13.0
`' - - - - - - - A/F = 16.0
`
`40
`
`80
`
`120
`
`160
`
`200
`
`240
`
`TIME- SEC
`
`Figure 5
`5
`
`Page 6 of 25
`
`

`
`Downloaded from SAE International by Bianca Hamilton, Wednesday, January 21, 2015
`
`Figure 6 shows the results of an engine cold start test
`employing this warm-up system. The illustration shows
`the time dependence of the exhaust temperature in the
`reactor core and at the reactor outlet as a function of
`throttle opening. The test was carried out at an engine
`speed of 2800 rpm. The throttle valve angle was in(cid:173)
`creased from 5° to 35°. A maximum speed governor
`controlled the ignition failure. It can be seen in Figure
`6, for example, that with a throttle valve opening of
`35° a reactor outlet temperature of 360°C is achieved
`in 10 sec. Even when considering the heat loss of the
`exhaust system between the thermal reactor and the
`catalytic system, it can be ensured that the operating
`temperature of the first bed can be reached in a very
`short interval of time.
`
`Especially when operating under rich fuel/ air con(cid:173)
`ditions with thermal reactors mounted at the cylinder
`heads temperatures could be produced which are above
`the melting temperature of the monolithic materials,
`such as Cordierite (Mg2Al4Sis01a), Mullite (3AbOJ·
`2Si02), and Alumina
`(a-Ab01). In Figure 7, the
`monolithic catalyst reached temperatures of more than
`1350°C. It can be seen by the figure that the center of
`the monolithic catalyst has been melted when operating
`
`of the thermal destruction of the material could lay in
`the unequal distribution of the active components on
`the support material. This unequal distribution of
`material, as for example CuO, could have led to a drastic
`reduction of the melting temperature from 1350 down
`to 975°C. Similar symptoms in the outer coating were
`observed during the aging process of catalysts by J. F.
`Roth (3).
`
`In place of the thermal reactors, monolithic noble
`metal catalysts could be employed as warm-up elements
`because the majority of the.HC and CO emissions pro(cid:173)
`duced by an engine are emitted in the first two minutes
`of the 42-min. CVS cold-hot test, while the NOx emis(cid:173)
`sion in general is produced over the whole test period.
`For this reason, a monolithic HC/CO converter at each
`side of the engine which reduces the high carbon mon(cid:173)
`oxide and hydrocarbon raw emissions with high effi(cid:173)
`ciency could be used to improve the warm-up character(cid:173)
`istic. Due to the good cold start performance and the
`low ignition temperature, the platinum monolith is
`particularly suitable. The ignition temperature for CO
`is approximately between 200 and 280°C and for HC
`(hexane) between 240 and 340°C.
`
`together with a front mounted reactor. Tests have
`proved that a too high inlet concentration of HC/CO is
`not the cause of this high bed temperature. The cause
`
`To give a complete presentation of warm-up possi(cid:173)
`bilities, other more sophisticated methods for rapid
`warm-up of catalytic systems should be mentioned.
`
`EFFECT OF CONTROLLED IGNITION FAILURE ON REACTOR WARM-UP
`AS A FUNCTION OF THROTTLE OPENING
`80 CU IN THERMAL REACTOR WITH SPARKPLUGS IN THE
`INLET TUBES, IDLE SPEED 2800 RPM, IGNITION FAILURE
`CONTROLLED BY A MAX SPEED GOVERNOR
`
`1000
`
`GAS
`TEMP.-
`oc
`
`800
`
`600
`
`400
`
`200
`
`0
`
`0
`
`20
`
`80
`60
`40
`TIME- SEC
`
`100
`
`120
`
`Figure 6
`
`6
`
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`
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`
`MONOLITHIC CATALYST FAILURE
`During Rapid Warm-up Performance
`
`Figure 7
`
`7
`
`Page 8 of 25
`
`

`
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`
`One such approach involves the use of a gasoline
`heater, the other approach involves an electric heater.
`An electrically heated HC/CO radial-flow converter in
`which high temperature resistance heating rods were
`installed was tested within the IIEC Program (4).
`These heating rods were capable of heating the first
`layer of the catalytic bed (approx. 0.5 lb.) from the
`ambient temperature to 300°C within 30-40 sec. at
`start-up. Power requirements were available from
`vehicle electrical system. Similar results were obtained
`by the VW Research Department with pelleted catalysts
`in axial-flow converters which were however supplied
`by an external electrical system. The power available
`from the battery alone was not sufficient.
`
`In most cases the gasoline heater has the disadvantage
`of not being able to operate against the relatively high
`exhaust gas back pressures. A further disadvantage is
`that an auxiliary heater could produce considerably
`high emissions. Without taking the engine emissions
`into consideration, the following table shows the values
`produced during a CVS test by a particular heating
`system:
`
`HC 0.58 gm./mi.
`
`CO 0.42 gm./mi.
`
`NOx 0.03 gm./mi.
`· It may be surprising that for HC this is more than
`40% above the 1975 target, while the CO emission is
`approx. 12% and the NOx emission is approx. 8% of
`the emission standards proposed for 1976. Fortunately,
`a properly designed gasoline heater operating only 100
`to 120 sec. after engine start-up has considerably lower
`emissions.
`
`Another method of improving the initial reaction
`temperature of the catalytic system is to increase the
`exhaust gas temperature by altering the ignition timing
`into the region after T.D.C. By the use of extreme re(cid:173)
`tarded timing at vehicle start-up for example, the cata(cid:173)
`lyst operating temperature of 250°C was reached 25
`sec. earlier in the CVS test than by normal ignition
`timing. The influences which the ignition timing has
`upon the combustion process (exhaust gas temperature
`and exhaust gas emissions) are discussed in more detail
`in the next chapter. Based on extensive single cylinder
`measurements the utility of this warm-up technique is
`illustrated because of its particular importance for
`speeding the warm-up performance of catalytic systems.
`
`WARM-UP TECHNIQUE BY SPECIAL
`ENGINE OPERATION
`Only by employing after-burning systems, can the ex(cid:173)
`tremely low emission targets for model year 1975/76
`be reached. Therefore, one of the most important tasks
`
`of the internal combustion engine is to ensure high effi(cid:173)
`ciency almost immediately after engine start-up by
`changing the engine conditions especially for this re(cid:173)
`quirement. It has been found that under appropriate
`operating conditions the engine itself is able to act as
`a preheater for the catalytic system. Warm-up spark
`retard and an increased idling speed of the engine with
`full open throttle lead to higher exhaust temperatures
`and thereby to a greater enthalpy of the exhaust gases,
`so that the after burning system could be brought rapidly
`up to its operating temperature.
`
`Figure 8 shows schematically an internal combustion
`engine as an open thermodynamic system. If steady flow
`is assumed the application of the First Law of Thermo(cid:173)
`dynamics gives important information about possibili(cid:173)
`ties of increasing the exhaust gas enthalpy (see Figures
`8 and 9).
`
`As shown in the equations in Figure 9 the total chem(cid:173)
`ical energy of the exhaust gases can be used to increase
`the exhaust gas enthalpy if the shaft work is zero
`(W1z = 0). In practice this operation condition cannot
`be achieved because the engine could not overcome its
`own mechanical friction. The maximum heat of the
`exhaust gases (Oi'z) m•x is therefore not at the no work
`condition, but at an indicated output which is appro(cid:173)
`priate to the mechanical friction of the engine. A second
`factor is the quantity of the exhaust gas flow rate which
`can be increased by opening the throttle. In an engine
`the condition W1z = 0 can be attained by altering the
`ignition timing to "retard". In this case the energy re(cid:173)
`lease rises very late so that the work done on the piston
`becomes less.
`
`To illustrate the influence of the ignition timing on
`the combustion process more clearly, Figure 10 should
`be considered. This figure illustrates the results of a
`thermodynamic analysis of two combustion processes
`at low load. The combustion cycles differ only in the
`ignition timing (9° B.T.C. as opposed to normal adjust(cid:173)
`ment of 27° B.T.C.) whereas other engine parameters
`such as air/ fuel ratio and volumetric efficiency remained
`equal. Essential differences can be seen already in the
`pressure-time history (upper diagram) . In the case of
`extreme retarded timing the maximum pressure is re(cid:173)
`duced from 16.5 • 105 to 7.8 • 105Pa (pascal, newton/
`sq. meter) at 20° and 50° A.T.C., respectively. The
`expansion process of the working fluid can be noticed
`very late, runs at comparatively high pressure level and
`due to the opening of the exhaust valve it is cut off at
`a high pressure. Due to this energy loss to the ambient
`(that is loss of the work done on the piston) the indi(cid:173)
`cated mean effective pressure was reduced from
`3.90 • 105 to 3.05 • 105Pa. This result can be taken
`directly from the energy release diagram (lower dia(cid:173)
`gram).
`
`8
`
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`
`APPLICATION OF FIRST LAW OF THERMODYNAMICS
`TO AN INTERNAL COMBUSTION ENGINE
`Open Steady-Flow System
`
`W12
`
`CONTROL SECTION --"""" 1 - - - - CONTROL SECTION
`A I
`I
`V .....,0
`I
`- -
`a"' """/'
`',....., 1
`..,.,...
`--Tt(-- INFLOW : - V1
`y
`_,"' I
`' ,
`___ ....
`I
`I
`CD
`
`t 2 I
`- - -
`.,"
`I
`'-,
`\
`.........:...-+"'
`~ OUTFLOW
`•
`;
`t II
`'
`I
`I
`®
`
`/
`' - _,.
`
`/
`.....,
`Va ...... o
`
`V2
`
`.....,
`
`012-HEAT, W12-WORK, m-MASS FLOW RATE, h-ENTHALPY
`PER UNIT MASS, V-VELOCITY, SUBSCRIPT a-"AMBIENT".
`
`Figure 8
`
`APPLICATION OF FIRST LAW OF THERMODYNAMICS TO INCREASE THE
`SENSIBLE HEAT OF THE EXHAUST GASES
`
`If No Work W12 Is Done By The Engine The Sensible Heat 012
`Reaches Its Maximum
`
`W12 = 0 - - - · 012 = m [ h2- h1 + Yz ( V22-v12) 1
`012 = m [ h2 + Yz V22 - ha 1
`P- DENSITY
`
`A-OUTLET AREA,
`
`Cp ::::: Cp (t);
`
`t-TEMP; Cp-SPECIFIC HEAT AT CONSTANT PRESSURE
`
`(SAME SYMBOLS AS IN FIGURE 8)
`
`Figure 9
`
`9
`
`Page 10 of 25
`
`

`
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`
`INFLUENCE OF IGNITION TIMING ON THE VARIATION OF PRESSURE AND
`ENERGY DURING COMBUSTION
`SINGLE CYLINDER ENGINE, VW 1.6 LITER WITH MECHANICAL FUEL INJECTION,
`ENGINE CHOKING CONDITIONS, ENGINE SPEED 1500 PRM
`EXHAUST GAS WARM· UP RATE 66°HIGHER ( 646 VS. 580° C)
`25 x10-5 - - - - - - - - - - - - - - - - - - - - .
`20
`15
`10
`5
`0 ~-~~~-~--~-~-~-~--~-~
`0.15 x10-3 - - - - - - - - - - - - - - - - - - - - .
`
`PRESSURE-Pa
`
`RELEASED ENERGY(cid:173)
`KW-HR
`
`0.10
`
`0.05
`
`0
`-30
`
`CUMULATIVE~·---~--::-:-:::=-:.---
`HEAT
`RELEASE
`-QB
`
`_._, __
`
`10
`-10
`TDC
`CRANKANGLE-DEGREE
`Figure 10
`
`Due to the late energy release rise, the period in
`which the high temperatures are produced in the exhaust
`gases is considerably shortened. For this reason the
`heat loss to the cylinder walls during combustion is less.
`The same quantity of fuel is converted into energy in
`both cases, but in the case of extreme spark retard
`the piston work L; and the heat loss to the walls
`Ow = Os - OH is reduced, and the internal energy of
`the working gases U; = QH - L; is increased by the
`appropriate amount. The exhaust gas temperature is
`thereby increased by 66°C from 580 to 646°C. The
`exhaust gas emissions are thereby also strongly influ(cid:173)
`enced; the HC emission is reduced from 132 ppm to
`60 ppm by the high exhaust temperature, and the NOx
`emission is suppressed from 1888 ppm to 720 ppm due
`to the retarded ignition timing.
`
`For the experimental investigation a VW 1.6 liter
`single cylinder engine with a production type combus(cid:173)
`tion chamber was used. A mechanical fuel injection
`system was chosen with which optimum fuel/ air ratios,
`good mixture preparation, and an independance from
`the distributor setting was available.
`
`The measurement of the exhaust gas temperature was
`carried out in the exhaust with an insulated thermo(cid:173)
`couple of 1.5 mm. 0 .D. The holding device for the
`thermo-element was fitted with a radiation shield.
`
`The tests began with an increased idling speed of
`2500 rpm and were later continued at 1500 rpm. The
`engine speeds were selected with respect to the highest
`possible exhaust flow rate. After adjusting the ignition
`timing the individual measuring points were chosen to a
`particular value in relation to the air flow rate, while
`the required air/fuel ratio was regulated by the fuel
`quantity. Deviations from the mean effective pressure
`Pme = 0 had to be balanced out by adjusting the supplied
`air and fuel flow rates in stages.
`
`In addition to the measured exhaust gas temperatures
`Figure 11 shows the HC and NOx emissions as well as
`the exhaust gas enthalpy as a function of air I fuel ratio
`and ignition timing. These results are in good agreement
`with the theory. As expected the exhaust gas tempera(cid:173)
`ture increased very rapidly when the ignition timing
`was retarded to a region after T.D.C. However, due to
`alteration of the ignition timing the indicated output
`was decreased, as already mentioned. In order to keep
`the engine running the cylinder charge was repeatedly
`increased as the timing became more retarded until
`finally the throttle was fully open. Therefore, the lean
`limit in the ignition region after T .D.C. is given as the
`left hand limit on the performance diagrams of Figure
`11. The highest temperatures measured at the lean limit
`were 794°C at the engine speed of 1500 rpm and 912°C
`at 2500 rpm. The lean limit with spark advance is
`
`10
`
`Page 11 of 25
`
`

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`
`ENGINE WARM-UP PERFORMANCE TEST
`
`Engine Speed 1500 RPM, Steady State, No Load.
`
`THROTTLE ADJUSTMENT
`
`EXHAUST GAS
`TEMP.- °C
`
`EXHAUST GAS
`ENTHALPY - KCALIHR
`
`(
`
`700
`
`600
`
`500
`
`(
`
`500
`
`NO-PPM
`
`HC-PPM
`
`.,.._ __ VOLUMETRIC
`EFFICIENCY
`
`0.8
`I
`
`/
`
`,.,
`
`/
`
`..,0.4
`"'
`/ 50[ VOLMETRIC
`EFFICIENCY
`
`0.3
`
`/
`
`I
`I
`\
`
`'
`
`I
`1 100
`-~,--_,;.;.~~--
`
`20~\
`~\
`
`0.25
`
`/
`
`I
`{
`
`0.3
`
`400
`
`1.2
`
`1.1
`
`1.0
`
`0.9
`
`0
`1-
`<C 0.8
`a:
`w
`u
`2 0.7
`w
`.....
`<C >
`::;)
`d
`w
`.....
`<C
`u
`0 1.2
`a:
`
`a.. -~ 1.1
`
`a:
`
`1.0
`
`0.9
`
`0.8
`
`0.7
`
`30
`
`20
`
`10
`
`0
`
`TDC
`
`ATC
`
`10
`
`20
`
`30
`
`30
`BTC
`ATC
`CRANKANGLE- DEGREE
`
`20
`
`10
`
`0
`
`TDC
`
`10
`
`20
`
`30
`
`BTC
`
`Figure 11
`
`11
`
`Page 12 of 25
`
`

`
`Downloaded from SAE International by Bianca Hamilton, Wednesday, January 21, 2015
`
`dependent on the air/fuel ratio in the same manner as
`it is with spark retard. However, the criteria for this
`lean limit is not the throttle opening but the breakdown
`of the combustion cycles which results in a rapid in(cid:173)
`crease of the HC emission.
`
`The exhaust gas temperature diagram in Figure 11
`illustrates the influence of the air/fuel ratio and the
`ignition timing on the exhaust gas temperature. The
`exhaust gas temperature reaches its maximum at a
`reciprocal equivalence ratio of l/</> = 1.0 because the
`combustion temperature is highest at stoichiometric mix(cid:173)
`tures.
`
`The increase of air/fuel ratio at a constant spark ad(cid:173)
`vance brings the exhaust gas temperature up to a max(cid:173)
`imum at greatest possible air/fuel ratio.
`
`This is explained by the postponement of the com(cid:173)
`bustion process into the expansion stroke which is in
`turn caused by the lower flame speed at lean mixtures.
`Furthermore, the volumetric efficiency increases as the
`A/F ratio becomes larger. The low exhaust tempera(cid:173)
`tures at rich mixtures are attributed to the cooling effect
`of the fuel in very rich mixtures. In combination with
`the volumetric efficiency, however, the largest influence
`on the exhaust gas temperature is exerted by the spark
`timing which brings about a large increase in the ex(cid:173)
`haust gas temperature.
`
`As mentioned above, the volumetric efficiency for the
`desired indicated output is dependent on the spark
`timing and increases from about 0.25 with spark ad(cid:173)
`vance to a full load value of over 0.8 with spark retard,
`see Figure 11 (bottom). The curves of constant volu(cid:173)
`metric efficiency have the same tendency as the curves
`of constant exhaust gas temperature so that the desired
`objective of providing the hottest possible exhaust gas
`in largest possible quantities is achieved by the spark
`retard. This result is shown clearly by the enthalpy dia(cid:173)
`gram. The increase in fuel flow corresponding to the
`increase in volumetric efficiency with spark retard raises
`the exhaust gas enthalpy
`(which is related to the
`enthalpy at ambient temperature)
`to a maximum of
`about 2750 kcal./hr. at 1500 rpm (Figure 11) whereas
`it is approximately 5600 kcal./hr. at 2500 rpm.
`
`In the enthalpy diagram only the sensible heat portion
`of the exhaust energy is illustrated. The chemical energy
`still contained in the exhaust gas, particularly when
`there is a shortage of air, is not taken into account. By
`the use of appropriate devices (i.e. thermal reactor
`with secondary air injection) this energy can be used
`to warm-up the converters in the start-up phase.
`
`In Figure 11 one can see that the spark timing has a
`very intensive influence on the composition of the ex(cid:173)
`haust gases. The HC emissions are
`lowered
`from
`400 ppm to less than 50 ppm by spark retard despite
`
`the increasing amount of charge. Down to the spark
`retard running limit only a minimum rise can be deter(cid:173)
`mined. The cause of the low HC values are the high
`temperatures in the exhaust gas due to the delayed com(cid:173)
`bustion which lead to a reaction of the still unburned
`hydrocarbons particularly in the presence of excess
`oxygen.
`
`The NO emissions do not show the expected tendency
`due to the overlapping influence of the volumetric
`efficiency. The spark retard causes a drastic increase
`in NO emissions because the volumetric efficiency in(cid:173)
`creases. This effect increases the temperature level in
`the combustion chamber due to the higher compression
`pressure. The NO maximum occurs in the region of
`l/(f> = 1. Furthermore the region in which the NO
`emissions are almost independent of the spark advance
`(l/<£ ~ 0.8) is shown in the NO emission diagram
`(Figure 11). Only excess oxygen in adequate quantities
`at lean mixtures (l/</> > 1) increases the NO emis(cid:173)
`sions when the spark advance is varied.
`
`In order to ensure that hot exhaust gases in the largest
`possible quantities are available not only in the initial
`idling phase of the CVS test but