FEATURE With the catalytic converter, 1975 and later model autos have cleaner exhausts, better driveability and improved fuel economy; however, problems remain to be solved With the increasing intensity of attention to shortages of fuel and energy, every step taken for environmental reasons has become more subject to questioning by those concerned with the energy supply. Thus, President Ford urges a 40% improvement in fuel economy. But the automobile companies are increasingly concerned about the conflicts that accompany requirements for fuel economy and for cleaner exhausts. Before requisite technology was available and before the emission limits became severe enough to necessitate use of the catalytic converters, emission control measures generally were accompanied by deterioration of both fuel economy and driveability (see Figure 1). With the 1975 introduction of catalytic emission control systems this trend was reversed. And, good fortune it was that a measure directed toward reducing exhaust emissions also helped fuel economy. After this fortuitous instance, however, a return can be expected to a condition of giving something up in exchange for that which was gained. If exhaust emission standards became more stringent than the 1975-76 standards, and no new practical system is developed for controlling emissions, a downward trend in fuel economy can be anticipated. In the early days of emission control, modification of airfuel ratio and retarding the ignition timing were two basic measures utilized: intake air was heated and the compression ratio was decreased. These steps, however, carried the penalties of higher fuel consumption (Figure 2) and poorer driveability. When substantial NO, control became necessary, it was accomplished largely by spark retard and exhaust gas recirculation. By 1974, then, average or sales-weighted fuel economy for a General Motors car was about 16% less than for its 1970 counterpart. Sales-weighted fuel economy for 1974, however, improved over that of the preceding year, because the proportion of smaller cars sold had increased, even though the car-for-car decline continued. Catalytic system development Early in vehicular emission control development, it was generally believed in the industry that a catalytic or thermal aftertreatment system would eventually be needed, particularly when exhaust standards became too severe for other control methods. Consequently, development programs for such systems were instituted prior to the date for designing a specific system for a given set of standards. When William D. Ruckelshaus, then the EPA Administrator, imposed the 1975 standards, the catalytic system was the only technology available to meet compliance without making further major compromises in vehicle driveability and fuel economy. Catalytic systems, therefore, were almost universally applied to 1975 model year cars. Platinum-palladium was selected as the catalyst combination. Other major changes were also made to the powerplant, including modification in carburetion and application of an electronic ignition system. Spark timing and fuel air mixture for 1975 cars were restored substantially to pre-control settings. Both fuel economy and driveability were‘ improved as a consequence. 820
Environmental Science & Technology
Ernest S. Starkman General Motors Corp. Warren, Mich. 48090
The oxidizing catalytic system introduced on most 1975 cars built in this country (see Figures 3 and 4) represents a major departure in emission control steps. It is the most discussed, the most praised, and the most criticized. It has also been the most effective. The most important achievement of the catalytic system is that it has enabled automobile manufacturers to meet the 1975 interim exhaust hydrocarbon (HC) and carbon monoxide (CO) standards-1.5 g/mi for HC and 15 g/mi for CO. It reduced the 1974 exhaust emissions by 50 % (Table 1). Field trials Both before and continuing on after the interim standards were established, field trials were conducted to evaluate the progress being made, and to learn what changes were needed to produce an effective, commercially acceptable catalyst emission control system. An account of two field trials may help in understanding some of the painstaking and extensive programs that lead from design to tooling. The product cannot go from drawing board, or even from laboratory, directly to production. Accumulation of major field experience on vehicles with systems much like that for 1975 began in November 1971, when GM’s AC Spark Plug Division started a fleet of 18 Buicks equipped with underfloor catalytic converters in a field durability test intended to cover 50,000 mi per vehicle. They FIGURE 1
Fuel economy trends changes (gain/loss) in GM city-suburban schedule economy -
+lo
--
I
Potential “best” economy (no emission control)
0-
-
% change from 1970 levels -10
-
-20
-1970
Vehicle weight increases 6%
1971
1972
1973
1974
1975
Model year Fuel economy of GM cars has deteriorated in recent years as a result of weight increases and emission control. Use of the catalytic converter for exhaust emission control reversed that trend for 1975. Analysis of fuel economy changes:
1975 GAIN results from engine optimization (EGR, spark and A/F ratio with catalytic converter) 1975 “Adjusted” to 1970 weight and compression ratio approaches 1970 best economy
FIGURE 2
Factors affecting emissions and fuel consumption
10 12 14 "S" 16 18 -Rich Airlfuel ratio LeanAir-fuel ratio affects both exhaust emissions and fuel consumption
20
Table 1. Exhaust standards (g/mi using 1975 Federal Test Procedure) HC
1974 1975-6
CO
NO,
Federal California Interim Federal California
3.0 2.9
28 28
3.1
1.5 0.9
15 9.0
3.1 2.0
All states
0.41
2.1
Statutory
L
I
I
I
I
. Spark timing . Advance
-Retard
Ignition timing retard adversely affects fuel consumption
3.4 0.4 _____ ~-
were started with base metal or non-noble metal catalysts. In six of the cars, a decision was made to replace the original base metal catalyst with platinum and palladium after only 12,000 mi of operation. The HC and CO emission levels had already far exceeded the statutory standards, which at that time were scheduled for 1975. This whole 18-car program was terminated after 602,000 mi of mixed results. It was one of the main information sources that subsequently lead to the adoption of the noble metal catalyst. A larger and more comprehensive field durability program included almost 250 cars equipped with catalytic systems built under actual assembly line conditions. It was initiated with 13 Oldsmobiles built in October 1972, and put into full operation with 222 Chevrolets and 12 more Oldsmobiles as-
Table 2. Fleet mileage results System
Standavds
Group la Chev CCS Underfloor Group 2 Chev C C S Manifold Group 3b Chev AIR U nderfloor Group 4 Chev CCS Manifold Group 5 Chev T-MECSC Group 6 Olds CCS U nderf I oor Group 7 Olds AIR Underfloor
'75'76 Federal 1.511 5.013.1
75
3,772,000
'75'76 Federal 1.511 5.013.1
71
3,189,000
'75'76 Calif. .9/9.0/2.0
36
1,265,000
'75-'76 Calif. .9/9.0/2.0
22
788,000
'78 Federal .4113.41.4
18
171,000
'77 Federal .41/3.4/2.0
13
408,000
'77 Federal .41/3.4/2.0
12
TOTAL a Contro!led C o m b u s t i o n System. single m a n i f o l d - m o u n t e d u n i t .
A i r Injection Reactor.
No. of cars
247
Mileage total
9,852,000
T r i p l e M o d e Emission C o n t r o l System-Oxidizing
a n d r e d u c i n g catalysts i n
Volume 9, Number 9, September 1975
821
FIGURE 3
260 cu. in. catalytic converter Weight = 26.2 tbs. 18.70 in.
Table 3. Fuel economy Oldsmobile converter fleets vs. certification data vehicles (5,000 Ib inertia weight)
1975 G M vehicles (Federal) 1975 G M vehicles (California) 1977 Experimental Oldsmobile
sembled in May 1973. This program was begun when the statutory 1975 HC and CO standards, 0.41 g/mi HC and 3.4 g/mi CO, were still on the books, and the decision to go from base metal to platinum group catalysts had been made; noble metal catalysts were used in all vehicles. While this field trial was designed to provide some of the evidence needed to demonstrate that the 1975 catalytic emission control system could be made ready for release to the public, engineering for production, even before such trials were conducted, had to be in process in order to be ready for the 1975 model year. The fleet completed 10 million miles of operation early in October 1974, at about the time the 1975 production cars began to appear on the streets with the catalytic system (see Table 2). In the course of the field test, the original 1975 standards were suspended. As a consequence, some of the vehicles, which were divided into seven groups, were recalibrated for different sets of standards. Two groups of cars were calibrated for the new 1975 interim standards for California-0.9 g/mi HC, 9.0 g/mi CO, 2.0 g/mi NO,, and two for the 49-state interim standards of 1.5, 15.0, and 3.1 g/mi HC, CO, and NO,, respectively. Two other groups were set up for the more seFIGURE 4.
1975 emission control system
h a d rn-~nilnl~l
Carburetors contain / quick pull choke and / mechanicalventing
Em issio n test fuel economy
Fuel economy penalty
12.1
-
14
10.9
1 0%
12
9.6
2 1%
25
No. of cars
vere standards then scheduled for 1977, and the seventh was continued at the statutory standards-0.41, 3.4, and 0.4 g/mi. In the groups that were recalibrated, settings were adjusted for the improved fuel economy, driveability, and performance that could then be realized with the catalytic system because the emission constraints were less stringent. Important as this step turned out to be, it was still secondary to the trial’s primary purpose: evaluation of durability and performance of catalysts and systems in a diverse range of service conditions while meeting the applicable exhaust emission standards. The fleet test also provided a means to evaluate maintenance requirements and any other problems resulting from operation in areas with special geographic characteristics. To assist in acquiring road experience quickly, the cars were placed in taxi, police, and governmental fleets throughout the country. In general, the converter systems were successful in controlling emissions at their prescribed levels. The four groups calibrated for the 1975-76 emission requirements, both Federal and California, finished their test with average emissions meeting the limits for all pollutants (Figure 5). The two groups of Oldsmobiles set for exhaust emission standards of 0.41, 3.4, and 2.0 g/mi were running satisfactorily in mid-June 1975, with mileage accumulation of 26,000 to 37,000, with some individual cars exceeding the emission limits but the average meeting the standards. Keep in mind that to receive
Table 4. President’s fuel economy objectives (miigal) Model year
1974 1975 1980
All Weighted other average
GM
Ford
Chrysler
12.2 15.7 18.7
14.4 13.3 18.7
13.8 15.8 18.7
20.1 21.2 24.7
14.0 15.9 19.6
53%
30%
36%
23%
40%
(goal)
percent improvement over 1974 Carbon canisterThis schematic illustrates the major components of a typical 1975 GM emission control system
certification, of course, every certification car must pass. A margin of safety below the actual standard is necessary. The groups meeting the statutory standards for HC and CO showed fuel economy that demonstrates the impact on fuel consumption of increasingly stringent emission standards. The average fuel economy of these 25 cars was 9.6 milgal, 21 YO below the average of similar vehicles calibrated for the 1975 interim Federal standards. Those vehicles set for the California 1975 standards, at 10.9 mi/gal, showed a 10% penalty (Table 3). Economy effects With technology at its present state, to meet standards much more severe than the present Federal standards would again require compromise of spark timing and air-fuel ratio. The condition would be particularly aggravated by a more
2.4 2.0 1.6 .--g E 1.2 &m
.8 .4
stringent NO, standard, which would require increased exhaust gas recirculation (EGR), since EGR shifts the maximum economy point toward a richer mixture and more advanced spark timing, and concomitantly increases HC emission (Figure 6). Both fuel economy and driveability would again be impaired by measures taken to improve emission control. Last year, President Ford requested, by 1980, a voluntary 40 YO improvement in automobile fuel economy over the 1974 level. General Motors reacted affirmatively, projecting that, with decreases in car weights, increased power train efficiency, and improved aerodynamic qualities, as well as a refinement of the catalytic emission control system, GM could reach the President's goal of 18.7 mi/gal in 1980 models. This forecast was based on the premise that the exhaust emission regulations would not be made more severe than the 1975 standards for at least five years. Although President Ford's request was expressed as a 40% fuel economy increase over 1974, GM's improvement would have to be 53%, since the company's 1974 sales mix contained a greater proportion of larger cars than the average of the other companies in the industry (Table 4). Sales-weighted fuel economy of 1975 GM cars is shown by the EPA city test schedule to be 28% greater than in 1974. The composite average fuel economy for 1975 GM cars is 15.5 milgal, computed on the basis of 55% city and 45% highway driving. This figure is expected to improve to about 16 mi/gal in 1976. With retention of 1975-76 emission standards, approximately 17 milgal is projected for 1977, and 18
1
1.4
1.2
p.+.-.+:-#:+%
-
.-E .8 _ --.
__---L.-.&.-.+.**.......f
.4
,..... ....... .... *
""""
e..
0
4.8
3.2
-
4.0
.-62
..-.-.++..+-+J-3 T-'
3.2
2 2.4
1.6 .8
.8
-
Volume 9, Number 9, September 1975
823
FIGURE 6
Emission and economy tradeoffs at 55 mph road load condition (Numbers on curves give percent EGR)
2.5
2.0
. .-E
w
1.5
I 0
1.o
0.5
n
"20
30
40
50
60
70
80
Total Spark - 'BTCa *Before tcp center
The bold solid line shows the increasing spark advance required to n a i n t a i n best fuel economy w i t h increasing EGR rates, and Ihe r e s u l t i n i detrimental effect on HC emissions. The dashed l i v s u p e r t r b s e d shows the eflect of increasing the EGR rnte wLlile m a i n t a i m g a cons:ant spark s e t t i n g
milgal may be exceeded in 1978. These projections, on the basis of experience in California and with experimental systems, will have to be discounted substantially if Congress does not see fit to order an extension of the 1975 standards. Further problems The catalytic converter's function is to oxidize unburned material in the exhaust. Unfortunately it also tends to further oxidize the small amount of sulfur present in gasoline (about 0.03% by weight as a national average). Some of the sulfur dioxide resulting from the burning of the sulfur is further oxidized to sulfur trioxide (which hydrolyzes to H2S04)in the exhaust system. The magnitude of the resulting sulfate emissions is very small, and there should be no immediate health hazard. At the very least, several years are available for analysis of that possibility. During this period the benefits of decreased HC and CO will surely outweigh any adverse effects of sulfuric acid from automobile sources. The most pessimistic outlook is that, after a substantial portion of the Averican automobile population consists of cataiyst-equipped cars, a concentration of sulfuric acid potentially adverse to health could develop at a heavily traveled roadside under the worst possible conditions. The probability of such occurrence appears slight. Present projections are based on highly questionable atmospheric models. General Motors is, therefore, conducting a large-scale experiment to collect data intended to test and improve these models. In this program, which is being conducted in the fall of 1975 at the GM Proving Ground near Milford, Mich., more than 350 catalyst-equipped cars will be driven on a 4-iane track under schedules duplicating heavy traffic on a freeway. Samples of air will be taken from a matrix of locations in the area. Another possible obstacle to the continued use of the catalyst, the cos: of platinum, is not expected to be a controlling factor in future dccisions regarding catalyst use, but the fact that it is imported from South Africa could present political awkwardness in the future, Alternate powerplants While alternative engines, under continuing evaluation by the auto industry, may ultimately include a candidate that can decrease air pollution while providing adequate transportation, none has yet been shown to have this capability. More in the realm of near-term probability would be the spark ignition
engines designed to burn extremely lean air-fuel mixtures efficiently. Both stratified charge engines (SCE) and the "leanburn" engines that have been under investigation by the auto companies are in this group. The stratified charge became a production reality in the 1975 model year. As with ail lean-running engines, carbon monoxide emission is inherently low in the stratified charge engine (SCE), and NO, is low relative to a conventional engine. While NO, can be brought below 2.0 g/mi, apparently NO, from an SCE car will have difficulty in meeting the 0.4 g/mi. GM has achieved the statutory 1977 HC and CO standards in the laboratory with a stock engine modified for stratified charge, with fuel economy levels comparable to that of 1974 cars. Improved economy could be expected with this design if the engine were recalibrated for less stringent levels, but there presently appears to be little practical near-term prospect of meeting the standards for 1978, when all three statutory levels become mandatory. The diesel engine appears to have the ability to meet the statutory HC and CO standards, along with an NOx standard of about 2.0 g/mi. While it does have a fuel economy advantage over the gasoline engine, part of this gain is at the cost of performance, which is definitely poorer than that of the comparable gasoline engine. The NO, reduction measures used up to the present, exhaust gas recirculation and spark retard, have been effective at moderate control ievels, but spark retard impairs fuel economy. Even to approach the 0.4 g/mi. level, inowever, catalytic reduction appears to be required. So far none of the experimental reducing catalysts or combinations of methods tried have met the statutory NO, level for any sustained mileage. One of the systems under investigation, that offers some promise, entails simultaneous oxidation of HC and CO and reduction of NO, by a single catalyst. However successful the U.S. Congress and the automobile manufacturers are in establishing and meeting exhaust emission standards appropriate to the atmospheric needs of the nation, mass transit can be expected to play a more prominent role in the future. The individually owned car, nevertheiess, will remain the dominant form of transportation for the near-term, but that car is expected to become smaller and lighter. Additional reading Stempel, R . C., and Martens, S. W., "Fuel Economy Trends and Catalytic Devices," Paper 740594, presented at SAE West Coast Meeting, August 1974 Gurnbieton, J. J., Boiton, R. A., and Lang, H. W.. "Optimizing Engine Parameters with Exhaust Gas Recirculation," Paper 740 104, presented at SAE Automotive Engineering Congress, February 1974. Miles, D. L., Faix, L. J., Lyon, H. H.,and Niepoth, G. W., "Catalytic Emission Control System Field Test Program," Paper No. 750179, presented at SAE Automotive Engineering Congress, February 1975. Report by the Committee on Motor Vehicle Emissions, National Academy of Sclences, Commission on Socio-Technical Systems, National Research Council. Washington, D.C., November 1974,
Ernest S. Starkman is vice president of General Motors in charge of the Environmental Activifies Staff. He recently served on the White House Task Force on Air Pollution and on the Office of Science and Technology Ad Hoc Panel on Unconventional Automotive Vehicle Propulsion. Coordinated by LRE
