La Ópera y el salón de baile.

Electrically-heated catalysts (EHC) have been the subject of in-depth investigation to

reduce the period of low catalytic conversion, Hurley et al, 1991 and Heimrich, Albu

and Osborn, 1991. Heimrich, Albu and Osborn, investigated two different

configurations of electrically-heated catalysts, each being fitted to then current production vehicles and subjected to the FTP emissions tests. Electrically-heated catalysts are pre-heated prior to engine cranking to achieve temperatures sufficient for catalytic activity from cranking, consequently reducing cold-start and warm-up emissions. The catalyst may also be subjected to additional heating once the engine has started in order to maintain a pre-set operating temperature. The duration of post heating depends upon the location and size of the EHC.

Due to the cold-start enrichment, emissions of HC and CO are high with relatively

low concentrations of O2 present. O2 is required for satisfactory oxidation levels of

is to operate at its full potential though a detrimental effect on NOx emissions will be incurred if either the volume flow rate o f air is too great, or it is supplied for an excessive period of time. Table 1.4 shows the results of altering the air flow rates supplied to the EHC performed by Heimrich, Albu and Osborn, 1991, for the cold- start portion of the FTP emissions tests. The researchers stated that the unexpectedly low value of NOx at the highest air flow rates was presumed to be an anomaly, and reported high scatter for the data collected. Catalyst cooling due to excess air was thought to be the reason for the increase in CO levels at the highest air flow rates. Emission levels of the vehicle measured during the FTP test cycle for both standard and the EHC systems averaged reductions o f non-methane hydrocarbons (NMHC) of up to 75% and 85% for CO are extremely impressive and approach the ULEV emission levels, though the increase in NOx appears to be unavoidable. Concerns include electrical power requirements as high as 3-8 kW, durability and the delay in

reaching operating temperature. Hurley et al, 1991, Anon, 1994a and Anon, 1994b.

Table 1.4

Cold-start emissions with preheated catalyst and air injection, 3.8 litre V-6, from Heimrich, Albu and Osborn, 1991

; Bag 1A A[r Injection Flowrate, L/min 'Em issions," ""1... ""'T. ? g/fpj IN oA iraj 170 ' 300 ’ 370 ; HC 1.50 I 0.36 * 0.40 0.42 CO = 9.68 : 1.96 : 1.16 2.92 L NOx ' 2.24 2.80 2.61 ; 1.80 i &Heat only.

j Air Injection for 75 seconds. ! Average of repeat tests at 300 L/min,

Hurley et al, 1991, investigated the problem of durability but only looked as far as

4000 miles. The reduction in emission levels of both new and aged EHCs were substantial when compared with the ‘baseline’ catalyst system, however, whilst the aged catalyst still produced lower emission levels after aging than the baseline system it suffered an increase of 115% of NMHC emissions and CO output rose by 250%. The drop in conversion efficiency is clearly worrying considering the proposed

160,000 km durability emissions tests. Hurley et al suggests that repositioning of the EHC further downstream of the exhaust manifold and downstream of the three-way catalyst may provide a level of protection from both exhaust contamination and high

exhaust temperatures, thereby increasing durability. This repositioning incurs

electrical power penalties due to the loss of exhaust gas heating which requires the electrical power be maintained for a period of time after the engine has fired. The current requirement averaged approximately 650W for a duration of 20 seconds when pre-heating the catalyst and throughout the post-heating period.

Ma, Collings and Hands, 1992, describe an alternative method for the rapid heating of the exhaust catalyst known as Exhaust Gas Ignition (EGI). Operational catalyst temperatures were reached by utilising an afterburner situated just upstream of the catalyst, see figure 1.6. The engine was calibrated to run extremely rich to provide a sufficiently flammable hydrogen concentration in the exhaust gas when mixed with an auxiliary air supply pumped into the exhaust.

Figure 1.6

Schematic of EGI system, from Ma, Collings & Hands, 1992

addirlonal air e x c e ss f u e l exhaust + air I g n it e r

Table 1.5

Comparative results for the emissions during Bag 1, Cycle 1 o f the FTP Drive Cycle, from M a, Collings & Hands, 1992

grams/mile HC CO NO,

No EGI 4.1 31.3 2.4

With EGI 1.0 4.8 1.3

Reduction 76% 85% 54%

The rich fuelling excursion was required to achieve a flammable exhaust air/fuel ratio such that ignition within the afterburner was achieved. Details of fuel mapping for the cold-start and warm-up conditions were not disclosed, although light off time for the catalyst was reduced from 100 seconds to 20 seconds indicating a rich excursion time of approximately the same duration. Results recorded for Bag 1, Cycle 1 of the FTP Drive Cycle show substantial reductions, see table 1.5, however, these gains are not achieved without penalty. Heavy carbon deposits, increased roughness, poor fuel economy and performance were cited by the authors as some of the adverse effects. The arrangement also required a substantial volume of air pumped into the exhaust to achieve reliable afterburner ignition, with the required flow rate equalling that o f the engine air flow rate. Figure 1.7 shows the difficulties in achieving a flammable exhaust mixture. Whilst this study was prompted by the expected durability failings of EHCs no durability tests of the EGI system were discussed, in addition, there may be problems of oil dilution and coke deposits due to the rich mixture required throughout the initial period of operation which will adversely affect the engine’s longevity. The afterburner required an idling period of 20 seconds to achieve catalyst light-off temperatures. Anon, 1994a.

Both BMW and Mercedes Benz took the concept o f an exhaust burner further by the use of a separate fiiel supply introduced to the exhaust system just upstream o f the catalyst, see figure 1.8. Unlike the afterburner system of Ma, Collings and Hands, the

Figure 1.7

Flammability of cold exhaust/air mixture, from Ma, Collings & Hands, 1992

%;% flammable CO ool\ CO C O u H C 112 % Oxygcjj % vo! a: afterburner - 1.25 12,00 02 H 2.25 ! I - ;.5o F.xri - A i r E.v.

intallation required no modification of the engine’s operating strategy. Figure 1.9 shows the reduction of emissions, comparing a variety of catalytic converter heating

methods. The principal benefit of this system was the reduction in the energy

conversion efficiency when compared to an EHC. Whilst it may appear wasteful to put neat fuel into the exhaust, burning it directly in the exhaust to provide heat allows a higher heating efficiency than burning it in the combustion chamber, SI engines operate at approximately 25% thermal efficiency which is then used drive the alternator to provide electrical power, with an associated efficiency of some 75%,

before being used to power the heating elements within the EHC, One of the

operation in service, with failure to achieve ignition resulting in higher emissions than with a conventional closed-loop catalyst. BMW also cited the problem of coke build­ up in the injector nozzle with an associated deterioration in the air/fuel mixture presented to the ignition unit due to the short duration of operation of the burner (< 1% of the vehicle’s service life). Anon, 1994a. For the remainder of the time the injector was subjected to the hot flow of exhaust gases which burn any residual fuel creating the coke deposits.

Figure 1.8

Catalyst heating options, from Anon, 1994a

j C 111 E)9etrUMttf iMeatelyl «wtvwW Fuaf tnartumir Figure 1.9

Reduction of emissions due to catalyst heating, from Anon, 1994b

starting point

s l a n a a f t l oow«rc>utr.’ w rtn cto & W «000 ca ta -'A C convo<',»i SMCXi

Maihunmi etmtÉm 9unm (1SkW>

A further alternative to the EHC was proposed by Hiemrich, Smith & Kitowshi, 1992. A method of hydrocarbon collection was suggested and tested using a zeolite molecular sieve situated in the exhaust system. Since the catalyst is below operating temperature under cold-start conditions, the unburned hydrocarbons pass through the catalyst and into the sieve. Zeolites are highly porous aluminosilicate crystals. The collector, a standard type of catalyst substrate with a zeolite coating, readily adsorbed certain sizes of molecules and therefore collecting the UHC emissions until the catalyst reached light-off temperature. The sieve was then purged using exhaust gas which fed back into the inlet manifold as a form of EGR. Zeolite has a desorption temperature of 83 °C and must be situated in the exhaust at a point where this

temperature will not be reached prior to catalyst light-off. The exhaust system

Figure 1.10

Experimental cold-start hydrocarbon collection exhaust routing, from Hiemrich, Smith & Kitowshi, 1992

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8 2

TWC

tah«u«t Ga# Bypaaa

HC measurements taken both upstream and downstream of the adsorption unit. Bag 1 of the test was split into two parts, with Bag 1A representing the first 140 seconds of the 505-second duration cycle, see figure 1.11. It can be seen from this figure that desorption occurs at temperatures between 45-50°C, well below the predicted 83°C desorption temperature and thought to have been due to higher than expected HC concentrations, contamination from other exhaust gas pollutants or short containment times within the sieve caused by the exhaust gas velocity during engine transients. Regardless of the system’s inability to function as predicted, a 35% reduction in cold- start HC emissions during this 140-second period was recorded, table 1.6. Areas for further study were cited as the long term durability of such a system and development of the chemical adsorption kinetics to produce a successful adsorbent that can readily target specific species following a cold-start.

Figure 1.11

FTP cold-start hydrocarbon collection, 1.9 litre Honda Accord, from Hiemrich, Smith & Kitowshi, 1992

Raw Exhauat Hyârocartwn Concentrât ton. (X10.000 ppm G

Catalyst Temperature, 0-1000 fc Adaortïent Temperature. 0-500 C Vehicle Speed. 0-100 ml/hr

10.000 100

! Adsort>ent Exhaust Flow for

I 140 Seconds (Test No. 3) HC Concentration 8,000 - Upstream of ' Adsorption Unit - 80 HC Concentration Downstream of Adsorption Unit 6,000 60 Catalyst Temperature 4,000 40 \ Adaortxtnt \ Temperature \ 20 2.000 0 40 SO 160 Time. Seconds Table 1.6

FTP cold-start hydrocarbon collection, 1.9 litre Honda Accord, cumulative emission levels, from Hiemrich, Smith & Kitowshi, 1992

; Test ; Number Cold-Start Hydrocarbon Collection Time. Hydrocarbon Improvement i

Emissions, gram s over Collection , sec _Bag 1 A (0-140 sec) Baseline. % ,

i 1 No ^ 0 _1.72 - i

! 2 1 Yes 70 1.11 1 35 1