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6. ALTERNATIVE SYSTEMS FOR LOW-EMISSION AUTOMOBILES The Committee also considered power systems other than Otto- cycle gasoline engines . It became apparent quite early in t his study that no alternative power system could be produced in sufficient num- bers by 1975 or 1976 to displace an appreciable part of present engine- production quantities. Several power systems (e.g., Rankine, Stirling, batteries, fuel cells) show promise for eventually meeting 1976 stan- dards, but development time and cost reduction are necessary before these can become competitive. Two engines (diesel and gas turbine) show promise of meeting 1975 emission standards. However, even though such engines have already been adapted to passenger cars, little devel- ment is being done on them for 1975 and 1976 because they are costly and have other detractive characteristics. The present diesel is heavy, tends to smoke, and its exhaust is odorous. The gas turbine has poor fuel economy at part load, and the NOx emissions are not pres- ently controllable to low enough levels. Although it is unlikely that any alternative engine will be in appreciable mass production by 1975 or 1976, some of them will be phased in within the next decade. Thus, summaries of the findings con- cerning the various systems are given below. 6.1 Diesel Engines Recent data show that several current four-stroke, and one two- stroke, diesel engines can meet 1975 standards for carbon monoxide and unburned hydrocarbons. A typical NO value for a current Mercedes Benz 220D under the CVS-CH test is 1.65 Simile. There have been no results obtained on diesel engines showing ability to meet the 1976 NO stan- dard of 0.4 g/mile. Daimler-Benz estimates that the lowest NO levels achievable for diesels at the present state of the art would be about 0.8 Simile. - 104 -
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New developments in diesel engines, such as a two-stroke engine with a new, low-emission combustion method, and the use of positive- displacement rotary prime movers, such as the Wankel-engine configura- tion, offer the future possibility of meeting, or nearly meeting, 1976 standards with an engine that is smaller and cheaper than the present (1970) gasoline engine. Much work must still be done to prepare even suitable prototypes of these concepts. There is a good possibility that a diesel engine of sufficient power density, light enough weight, and emissions nearly satisfactory for 1976 automobile can be built. But much engineering work must still be done before there can be a proven concept. Potential problems of smoke, white smoke, odor, and noise still remain. It appears that good single prototypes of the advanced engine will not be available before 1975. Limited production might be possible by 1980. A passenger-car diesel engine designed according to existing technology may have a possible disadvantage in slightly greater weight and larger size over a spark-ignition engine of comparable output. It may cost more basically, but the difference shrinks when the emission controls for gasoline engines are added in, since the add-one for diesels to meet 1975 standards are minimal. It will give better fuel economy and require less maintenance, which should quickly make up any first-cost difference. The efficient diesel will tolerate a wide range of fuels and becomes of greater interest as our concerns with energy conservation increase. Because fuel of lower volatility is used, diesel engines have an additional safety factor, and also there would be less fuel-vapor emissions at the filling station. 6 2 Gas Turbines . . Gas turbines are a feasible method of propulsion for standard- size U. S. passenger cars. In prototype form, they have demonstrated acceptable or superior weight, size, fuel consumption, driveability, - 105 -
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maintainability, resistance to abuse and neglect, and safety. Carbon monoxide and hydrocarbon emissions are below the 1976 standards; NO emissions are presently above the 1976 limits, but several approaches have shown that it is technically feasible to lower NO to 1976 require- ments especially for low-pressure-ratio engines. The concepts can probably be incorporated in a prototype by 1976. The added controls or costs of reaching 1976 NO standards are not yet known. Gas turbines to date have all shown poor fuel consumption at low design power and while operating at low fractions of the design power. Highly regenerated units tend to limit the effect, but the possibility of economic gas turbines having design power below 150 horsepower and operating under lightly loaded conditions is still a controversial matter. The retail costs of future gas turbines installed in automobiles are highly uncertain. Estimates made by vary ous highly qualified indi- viduals or organizations run from a price below that of the cleaned-up spark-ign~ Lion engine to one three or four times higher. These esti- mates are based on the use of materials similar to those in today's engines. Future possibilities for gas turbines improve as the use of ceramics for many parts is proven. If ceramics become widely avail- able for the hot parts of gas turbines, it is generally agreed that the engines would eventually cost less than the spark-ignition alternative. In addition, the employment of critical resources would be greatly reduced. A realistic schedule for advanced gas turbines to be produced in quantity would be for advanced limited-production engines by 1982, followed by mass production by 1984. - 106
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6.3 Stirling Engines At the present state of development, Stirling engines are very ef ficient engines that could allow high-performance full-size automobiles to meet the 1976 emission standards. Any form of heat energy or fuel source can be used to operate it. The engineering problems that remain to be solved before it would be possible to adopt them as practical engines for limited application relate to the reliability of sealing the working fluid inside the engine, to the cost and reliability of the heater assembly, and to the development of a simple, versatile power- output control system. Considerably more engineering is necessary to allow the engine to be considered as an entirely suitable automobile power plant. Additional developments necessary to make this possible red ate to cost, operation in the hands of the customer, and integration into the automobile. The two sets of problems are best attacked simul- taneously and may involve changes in the present form of the engine. The potential of the engine goes well beyond its present state. Size, weight, producibility, safety, response to abuse and neglect, starting ease, driveability and versatility, control ease, fuel eco- nomy, noise, emissions, and cost potential all show indications of being competitive with or better than diesels in the present generation of development, and equal to or better than gasoline engines in the next generation of development. Thus, the engine could fit into the auto industry, truck industry, and other segments of the transportation industry, independent of the eventual outcome of the energy crisis or the fuel controversy. Approximately 4 to 10 years of additional devel- opment will be required to solve the outstanding engineering problems and produce a prototype advanced Stirling engine suitable for present- type automobiles. · 107 -
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6.4 Electrically Driven Vehicles Electrically driven vehicles in principle provide freedom from pollution and are characterized by high energy efficiency, flexibility of performance, good durability, and low maintenance requirements. At present, the limiting factor relating to the technical and economic feasibility of electric vehicles is the vehicular power source. E lec- tric drive systems (motor and controls) having excellent characteristics have been demonstrated; development of optional drive systems is not con- sidered to be limiting in the ultimate realization of electric automo- biles . Fuel-cell-powered electric vehicles in which the free energy of fossil fuels is directly converted into electrical energy for motive power do not emit CO or NOx; unused hydrocarbons can be easily removed from the exhaust. Fuel cells are not heat engines and are not subject to the Carnot limitation. For this reason they may operate at very high energy-convers ion efficiency, resulting in superior fuel economy. Although some fuel-cell systems have been successfully deployed in space missions, these are not adaptable for applications where low cost is important. Current advanced developments directed toward sta- tionary applications in commercial and consumer markets are in the field-test stage. These represent important cost reduction and perfor- mance Improvements relative to the aerospace units. With further sig- nif~cant cost and performance improvements, vehicular applications in small quantities may become fees ible within 10 to 15 years . Vehicles that employ rechargeable batteries as a power source do not have emissions resulting from the combustion of fuels; the site of emissions is transferred to central power stations where such emis- sions are understood to be more effectively controlled, and at a lower cost. Because of the high efficiency of batteries and of electric drives, the net fuel economy of such vehicles promises to be better - 108 -
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than that of present automobiles. Furthermore, if we move toward an electric economy, batteries may assume a unique role in the transporta- tion system. In contrast to fuel cells, extensive experience exists with re- spect to the performance characteristics of at least one battery system- lead/acid. This battery is rugged, efficient, reliable, and can respond instantaneously to large changes in load. Presently available special- purpose vehicles can provide ranges of up to 50 miles and modest accel- eration marginally acceptable under urban driving conditions, at a high cost. Other currently available rechargeable batteries, such as zinc/ silver-oxide and cadmium/nickel-oxide, while superior in some respects to the lead/acid system, are inherently unsuitable for vehicular applications because of cost and/or limited availability of materials. Still other battery systems concurrently in various stages of develop- ment offer significant performance improvements, and may meet the cost and materials requirements for vehicular applications. The zinc/nickel-oxide battery is expected to allow a vehicle design with acceptable acceleration and a range of about 80 miles be- tween recharges. The most promising of the advanced battery systems are sodium/ sulfur and lithium/sulfur batteries, which operate at temperatures in the range 300-400°C, and are maintained at operating temperature by their reject heat and appropriate thermal insultation. These batteries are expected to have specific energies of 100 watt-hours/pound and specific powers of 100-200 watts/pound, permitting the design and construction of electric vehicles with excellent acceleration capabil- ities and a range of about 200 miles between recharges. About 7 or 8 years of optimum effort will probably be required for the development of pilot quantities of these batteries for vehicle test purposes. Still other promising nonaqueous systems are in early stages of exploration. - 109 -
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Hybrid Electra c/heat-engine powerplants are claimed to enable reduction of the emission of air pollutants. The expected improvement in driveabil ity by using the electric motor for power surges should allow the heat engine to operate cleanly and economically at one setting or with a slowly varying setting over a range, There are significant penalties in the areas of cost and complexity that must be overcome before the hybrid can be considered a viable contender. Even if the technical and economic criteria can be met, it is doubtful whether in- troduction of this new and complex power-plant scheme w'11 represent any more than an interim solution with respect to pollution abatement and ef fee Live use of natural resources . Rankine Engines Tests made on Rankine-engine components have shown that the 1976 standards could probably be met with Rankine-engine-powered, stan- dard-s ize automobiles . Various approaches to the design indicate that Rankle engines can be made to fit into full-size automobiles. These findings are to be demonstrated with working units in real automobiles by 1975. Engine noise promises to be low except for the condenser fans, which could be troublesome due to large air-flow requirements. Start- ing should be easy, although time-consuming (one minute being a practi- cal es timate). The driveability of Rank~ne-powered automobiles should be satisfactory if a sufficiently high power-to-weight ratio can be achieved. One full-size automobile has been fitted with a 150-horsepower steam engine. Emissions did not meet 1976 standards and there were other detracting features, which can be traced partly to the under- developed nature of the engine. Lower-power steam engines have been fitted into compact-size automobiles and demonstrated, Low power density is a general characteristic of these engines, traceable to poor e f ficiency. - 110 -
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Newer forms of Rankine engines that use organic f luids f lowing through either reciprocating or turbine machinery offer the possibility of trouble-free operation (no freezing, easy starting) at the expense of poorer fuel economy as compared with steam. These units will be larger and more difficult to integrate than will steam engines. The Rankine cycle in any version will tend to have relatively uniform specific fuel consumption over the operating range. This leads to reasonable fuel economy (but less than that of gasoline-powered auto- mobiles of similar size) over typical driving schedules when steam, or the best organic-fluid, engines are considered. To achieve an engine with reasonable fuel economy, the controls have to be complex and the engine has to be as large as possible within the allowable envelope. Thus, any Rankine engine will be pushed to the allowable limits on s ize, weight, and cos t for a given application, and the automobile will be considerably underpowered and overpriced as compared with a gasoline engine in the same application. Despite its potentially good emissions, driveability, and low noise, most of the other realistic evaluation features for automobile engines (such as size, weight, cost, fuel economy, and starting time) are missed by the Rankine engine, independent of type. It is problematic whether even limited production of full-power engines could be feasible before 1980. Limited production of existing designs for low-power applications could begin by 1976-77. Major questions remain to be answered affirmatively with respect to safety, operability, reliability, and overall driving versatility in the hands of the public. Unit cost and the ability to be phased into production present even larger questions for which affirmative answers are lacking. - 111 -
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A suitable full-size, prototype Rankine engine will not be available until 1975 (EPA schedule). Development of a manufacturable prototype must follow this by several years, which must in turn be followed by normal development. 6.6 Other Engines A wide variety of other engines with some potential advantage over the gasoline engine or diesel engine have been considered over the years. Most of these have not been developed even as far as the automobile gas turbine, Rankine engine, or Stirling engine. None of them seem to offer a clear-cut advantage in emissions over the other types, and they all offer some increase in complexity, weight , volume , and probably cost. Systems using positive displacement machinery but with combustion taking place outside the cylinder (out-of-cylinder combustion systems) have been studied for engines operating on the diesel cycle, the Otto cycle, the Brayton cycle, and many variations. They all suffer from lowered efficiency, larger size, and probable high NOx values. None of these systems appear to offer any basic advantage that cannot be achieved ultimately by diesels, gas turbines, and Stirling engines, all of which show promise of lower cost. - 112
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