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OCR for page 104
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.
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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,
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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.
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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.
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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
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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.
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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.
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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.
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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
Representative terms from entire chapter:
fuel economy
