Alternative vehicle-propulsion systems.

Recent and projected emissions standards are forcing the automotive industry to develop and adopt new technologies for piston engine design and operation. Development costs, the first cost of the vehicle, the cost of operating the vehicle due to expensive new technologies, and the cost of alternative clean fuels are all rising.

Of the alternative propulsion systems that have been explored, three promising ones are electric or electric-hybrid systems, gas turbines, and fuel cells. In general, low-polluting alternative fuels present less of a problem to alternative propulsion systems than to the currently used piston engine system.

Background

Passage of the Clean Air Act Amendment of 1990 and the outlook for even more-stringent standards in California are driving development efforts toward low emissions in all segments of the engine-vehicle industry. Many different systems and component have been invetigated and tested, including steam engines, organic Rankine systems, various bottoming systems, combined piston/turbine compound engines, Stirling engines, piston engine accessories, and gas turbines.

Light-duty vehicles are usually tested and certified on chassis dynamometers, which are run over the specified Federal Driving Schedule. Tail-pipe emissions (HC, CO, and [NO.sub.x] are measured and presented in grams/mile and compared to established standards. Emissions from heavy-duty vehicle engine are measured over the U.S. Federal Transient Test Procedure on an engine dynamometer. Emissions of [NO.sub.x], HC, CO, and particulates are measured continously; results are provided in grams per horsepower-hour for comparison against established standards.

Current Requirements and

Systems

Light-duty vehicles. Legal requirements and standards adopted by the EPA and regulatory agencies in California drive the development of conventional spark-ignition engines to achieve lower emissions and higher fuel economy. Emission limits are now specified for up to 100,000 miles.

So far, the industry has been able to devise the technology needed to achieve the mandated goals--at a price. The marginal cost to the customer for every increment of improvement is increasing. Eventually, performance may be compromised.

The challenges presented by the California requirements for low-emission and ultralow-emission vehicles will not be satisfied easily--or cheaply. Some of the emission control approaches being explored or developed are improved cold-start control; improved air-fuel ratio control; low crevice volumes in piston and gasket areas; fast burn combustion; variable valve timing; low thermal inertia exhaust manifold; exhaust port liners; detailed design changes in piston, piston ring, and liner to minimize oil consumption; catalyst improvements such as metallic substrates or electrically heated lower light-off temperature catalysts; exhaust treatment improvements including an electric air pump, portmounted catalysts, a hydrocarbon absorber, and hydrocarbon traps; and fuel system improvements including proportional air-fuel balancing, improved fuel preparation (air-assisted injection and heated spray targets), dual feedback loop control, reduced deterioration of ignition components, catalyst, and oxygen sensors.

If required emission levels are achieved with new cars, it must be proven that they will remain within the limit for the life of the vehicle. Since emission levels must be maintained for 100,000 miles with minimal maintenance, emission system deterioration is an important consideration.

Ultimate solutions and final designs are far from fixed. U.S. automakers have estimated that emission controls could add $500 to $700 to the price of 1994 cars (first-tier emission-control requirements). The same companies say the technology does not yet exist to meet the second-tier standards (model year 2003). On the other hand, EPA officials estimate the first- and second-tier standards can be met for $100 and $500 per car, respectively. More-expensive alternative fuels would also add to the consumer's costs to own and operate the vehicle.

Recent advances in the use of fuel-injection systems, electronic control systems for ignition and injection timing, and fuel-air ratio controls have stimulated automakers in the United States, Japan, and Australia to reconsider the use of two-stroke engines. Inherent advantages of such engines include light weight, compactness (a power stroke every revolution), good fuel economy with injection, simplicity, and potentially lower cost. The challenges include achievement of the low-emission standards, demonstration of acceptable durability, and combustion stability at idle and light loads.

Alternative fuels. Because piston engines are sensitive to variations in fuel properties, mere substitution of a potentially clean-burning alternative fuel does not ensure low emissions from an engine. Proper integration of the combustion system, controls, and catalyst system are imperative. A poorly adapted engine will negate the clean-burning potential of any fuel.

Various alternative fuels under consideration include M100 (pure methanol), M85 (85 percent methanol, 15 percent gasoline), natural gas, enthanol, liquefied petroleum gas (LPG), reformulated gasoline, and hydrogen.

Methanol's proponents claim it is the best alternative fuel because it appeards to produce lower emissions, except aldehydes, from a modified engine and has a higher octane number, permitting higher engine compression ratios, which implies higher efficiency. Methanol can be manufactured from natural gas, which is available from abundant foreign sources or, at more expense, from domestic coal or wood sources.

However, concerns and problems associated with methanol include flame invisibility, tank flammability, poor low-temperature starting characteristics, and toxicity. Aldehyde emission will come under regulation in California by 1994 with a standard of 15 mg/mile. Vehicle range (or storage space) will be reduced due to the lower volumetric energy content of methanol.

The small amount of gasoline in M85 circumvents the flame invisibility problem of pure methanol and virtually eliminates the cold-starting problem in vehicles built specifically for methanol. Although still highly toxic and corrosive, M85 appears to be a reasonable transition fuel for market introduction. Indeed, most major automobile companies are building limited numbers of flex-fuel vehicles, which can operate on any combination of methanol and gasoline. Although the price of these fuels will be slightly higher than that of pure gasoline, such fuels may be integrated into current supply and handling systems. However, since methanol is not permitted in existing pipelines because of its affinity for water, it must be trucked. Ultimate consumer acceptance is uncertain.

No infrastructure change would be required for reformulated gasoline (clean fuel), except in the refineries. There is probably some small to moderate reduction in emissions to be expected without engine modifications. A key advantage is that reformulated gasoline, unlike other alternate fuels, may be usable by the entire vehicle population, not just new cars.

Emission benefits and cost of reformulated gasoline are still highly uncertain. Ford Motor Co. (Dearborn, Mich.), General Motors Corp. (Detroit), and Chrysler Corp. (Highland Park, Mich.) are working jointly with 14 oil companies to study the potential of reformulated gasoline relative to other alternative fuels. They are looking at the effect on emissions of aromatics, olefins, sulfur, volatility (90 percent point), and oxygenates. Atmospheric modeling and cost-benefit assessment are also being investigated.

Ethanol, manufactured from domestic sources (corn), is a familiar liquid fuel with benefits similar to methanol. Organic emissions are lower than gasoline but higher than methanol. Lower toxic emissions result, and engine efficiency should be higher than for gasoline.

Unfortunately, ethanol costs much more than gasoline; vehicle range may be one-third less unless larger fuel tanks are used (low energy density). Cold starting is a problem below 50[degrees]F for pure ethanol. At high production levels ther will be a food/fuel competition.

Gasahol is a mixture of 90 percent gasoline and 10 percent ethanol.

Compressed natural gas (CNG) has low emission characteristics except for a potential of somewhat higher [NO.sub.x] emissions. Gas is abundant worldwide but for moderate production rates, the equivalent of a million barrels per day, suitable North American sources are available. CNG can be derived from coal. It has advantages over methanol relative to aldehyde and evaporative emissions.

Challenges related to CNG include lower vehicle performance due to lower power and energy density, low cruising range, and safety. Larger fuel tanks would be needed. Liquefied natural gas has better range, about the same as methanol. To date, the limited number of vehicles outfitted for natural gas still have significant development problems. Among them is the need for a retail fuel distribution system. Refueling is also slower than for more-conventional liquid fuels.

Hydrogen-powered vehicles would have low emission characteristics with minimal hydrocarbons. Production would be domestic; hydrogen has potential for fuel cell use. However, the range is limited by heavy bulky fuel storage. Projected vehicle and total operating costs are high; extensive research and development and an entirely new infrastructure are needed. Although hydrogen is usually considered a very long-term alternative, some development work is being done.

Heavy-duty trucks and buses. This class of equipment also contributes to pollution problems in U.S. urban areas. In addition to hydrocarbons, [NO.sub.x], and CO, particulate emissions are also controlled for trucks and buses under federal and California regulations.

Fuels contribute to the emission problems of diesel engines. (Many of the previous comments on fuels for light-duty vehicles also apply to diesel engines.) Starting in October 1993, diesel fuel specifications will limit sulfur content, an important factor in particulate emissions, to a maximum of 0.05 percent by weight and the Cetane index to a minimum of 40.

The industry appears to be meeting the 1991 standards with cleaner fuels and engine improvements. The outlook for 1994 however, is uncertain. No one knows how--or if--those standards can be met. The first approach is to use in-cylinder modifications combined with higher injection pressures; precise electronic control of the fuel system (timing and injection rate); high-stress cams for better rate shape of injection; and careful trade-off among [NO.sub.x], particulates, and thermal efficiency. Exhaust gas recirculation is effective in reducing [NO.sub.x] emissions. Meeting the 1994 standards will be complex and may cost 15 to 30 percent more than current technology. If particulate traps or oxidation catalysts are required, it could add another 15 to 20 percent to the cost.

With the continued reduction of particulate and [NO.sub.x] standards after 1994, other factors, such as lubricating oil from the piston/ring belt area and from turbocharger seals, are becoming more important. These factors can add to the particulate emissions problem.

Other unknowns include the effect of exhaust gas recirculation on durability due to a faster wear rate. Recycling of soot particles increases the wear rate by degrading the lubricant. Higher soot concentration in the lubricant aggravates wear by reducing the effectiveness of antiwear additives, which leads to degradation of the oil surface film. Valve train components and the top ring are also susceptible to increased wear.

The experts seem to agree that in addition to further refinement of fuel specifications (such as no sulfur and use of oxygenation) and extensive exhaust gas after-treatment, the 1998 standards will require additional breakthroughs or new technology. Industry has been resisting the use of particulate traps because of their high cost, complexity, and unproven durability. It is expected that catalytic after-treatment will be required to treat the aldehyde elements in the exhaust gas. Clean fuels will be a key issue.

One new chemical approach to [NO.sub.x] reduction is the Raprenox system. Invented by Robert Perry while at Sandia National Laboratories (Albuquerque, N.M.), this process is being developed and commercialized under a license to Cummins Engine Co. (Columbus, Ind.) and has been demonstrated on 50-kilowatt naturally aspirated, 150-watt turbocharged, and 1000-kilowatt turbocharged/after-cooled diesel engines. All achieved greater than a 95 percent reduction in [NO.sub.x] emissions with no loss of performance. The process is based on the use of cyanuric acid (a low-cost solid compound made from urea). With heat, the solid sublimes into a gas and then dissociates, producing isocyanate, which reacts with [NO.sub.x] in the exhaust stream to give [H.sub.2.O], [N.sub.2] and [CO.sub.2]. Present operating temperatures are 900[degrees] to 950[degrees]F. The first commercial prototype application of the process is in a stationary 6-megawatt power system comprised of four 1500-horsepower Cummins KTA5D engines in Hawaii. They are expected to produce no more than 0.5 grams of [NO.sub.x] per horsepower-hour. These engines have been delivered. Installation was originally planned for the first quarter of 1991.

The system is not yet practical for vehicle (or mobile source) applications. It is too large, heavy, and complex. However, a program aimed at the evolutionary development of a practical system for such applications is in progress.

Other types of advanced after-treatment systems are also expected to emerge; the major truck engine companies continue their efforts to meet anticipated standards. Details of technical approach, cost, and schedules are proprietary.

Alternative fuels are expected to be an integral part of the attack on heavy-duty-engine emissions. Methanol appears to be the leading contender due to cost and availability. Detroit Diesel Corp. (Detroit) has some 75 M100-fueled engines in the field in trucks and buses. The general implementation strategy of the industry is to introduce the fuels through private and government fleet operations in many of the nonattainment areas around the country (10 or more vehicles capable of central fueling seems to be the criterion for a fleet).

Low-heat-rejection diesel engines. The techniques for reducing emissions and achieving higher fuel economy via the low-heat-rejection approach are being tested and developed in the heavy-duty-engine arena. Many of these tehcniques may also be applied to light-duty diesel engines.

The present thrust of advanced heavy-duty engine development, aside from emissions reduction, is toward low-fuel-consumption low-heat-rejection engines. The DOE has set targets at 0.25 pounds per BHP-hour (about 53 percent thermal efficiency) in their LHR25 advanced heavy-duty-engine program. Caterpillar Industrial Inc. (Peoria, Ill.), Cummins, and Detroit Diesel are working toward this goal. The intent is energy conservation without exceeding emission standards. Current truck engines have brake-specific fuel consumptions in the 0.30-to-0.35 pound-per-BHP-hour range.

The general approach for high-efficiency low-heat-rejection engine development is to reduce the thermal losses from the engine by eliminating or reducing the cooling system and recovering more of the resulting increased energy from the combustion gases in the exhaust system. This is generally accomplished using advanced high-temperature materials such as monolithic ceramics or by insulating pistons, liners, fire deck (cylinder head), valves, and ports with thermal barrier coatings or air-gap insulation systems. This, of course, results in much higher operating temperatures in the engine. Top-ring reversal temperatures in the 1000[degrees] to 1200[degrees]F range can be expected. This constitutes a major lubrication problem since no liquid lubricants can survive such temperatures. This appears to be the pacing problem in low-heat-rejection engine development. No manufacturer has defined or openly discussed a definite development path or schedule to the LHR25. Many say that with time and money it is achievable. However, when and whether it will be a practical cost-effective system that can meet [NO.sub.x] emission standards are unknown.

The low-heat-rejection engine is considered an extention of current engine development rather than an alternative system. Admittedly this is a borderline case; without government support and continuity, achieving LHR25 objectives would be even longer-range.

Gas Turbines

The potential advantages of gas turbines in vehicle applications captured the fancy of the technical community from the mid 1950s until the early 1970s. Enthusiasm waned when it became apparent that automotive turbine development was not keeping up with piston engine improvements especially in the area of fuel economy and manufacturing cost. However, the basic long-term advantages preducted for the turbine--very low emissions, light weight (high power density), multifuel capability, and customer appeal (smooth vibration-free power delivery)--provided sufficient incentive for further development to overcome the perceived deficiencies.

In the late 1970s, it was apparent that mush higher fuel economy had to be achieved if gas turbine engines were to beome serious contenders for road vehicle propulsion. The target get set by the DOE was that fuel economy should be 30 percent better than that of a comparable spark-ignition piston engine. It was further apparent that the most likely avenue to this high fuel economy was markedly increased turbine inlet temperatures. Previous work showed that temperatures in the 2300[degrees] to 2500[degrees]F range were necessary to achieve the fuel economy goals. Because of the small size and cost constraints on the engine, the use of high-temperature alloys and complex cooling schemes did not appear practical. Thus, the application of new high-temperature ceramic materials to critical turbine components appeared to be the most promising approach. Primary efforts were redirected toward the solution of this materials problem through the DOE/NASA. The Advanced Turbine Technology Application Program (ATTAP) is being pursued in two parallel contracts, one with the Allison Gas Turbine division of GM (Indianapolis) and one with the Garrett Auxiliary Power division of Allied-Signal Aerospace Co. (Phoenix, Ariz).

As of October 1991, Allison reported the following on its ATTAP contract: "...over 3340 rotating test rig and engine test-hours have been accumulated on over 2170 ceramic components. Ceramic rotor designs have shown survivability in cases of extreme foreign object ingestion, high-speed rubs, severe start-up transients, and cyclic durability testing. One loader has successfully accumulated more than 1000 test-hours, including 507 cyclic durability hours and 5170 starts." The AGT-5 hot gasifier rig with a ceramic rotor has operated successfully at 1395[degrees]C (2543[degrees]F) rotor inlet temperature and 100 percent speed.

So it appears that the original DOE/AGT objectives will be achieved: at least 30 percent improvement in fuel economy over vehicles powered by conventional spark-ignition piston engines of the same weight and performance based on equal energy content of the fuel used: gaseous emissions and particulate levels less than existing and planned federal and state stadards; ability to use alternative fuels; and competitive initial and life-cycle costs.

The powertrain design for the ATTAP turbine (based on ceramic compoent performance and known vehicle performance) had 57.3 percent better fuel economy over the Federal Composite Driving Cycle than the 1988 Pontiac Grand Am reference vehicle. (This is just over 35 percent on a Btu basis when correcting for the difference in heating values between diesel fuel and gasoline.) Such improvement in fuel economy has important implications for automobile manufacturers should higher corporate average fuel economy standards be enacted. Acceleration of the turbine vehicle (0-60 mph) was 13.1 seconds versus 13.5 seconds for the baseline car.

Assuming the high rotor inlet temperature can now be achieved, the following technical problems remain before commercialization can be implemented.

1. Low-emission burners: To meet the low emission requirements mandated by the 1990 Clean Air Act amendments and the state of California, the combustion system must be operational in the vehicle and operating under the conditions set forth in the Federal Driving Cycle. Since the capability of operating an engine uncooled in the 2500[degrees]F range has only recently been demonstrated, there is very little documented or detailed information available. However:

* United Turbine AB (Malmo, Sweden) reported the first road test of a ceramic turbine engine in March 1982. Emissions and fuel economy data were not reported.

* Daimler-Benz (Stuttgart, Germany) demonstrated a ceramic gas turbine car at the ASME International Gas Turbine Conference in Brussels, Belgium, in June 1990. The car was driven 660 kilometers (390 miles) from Stuttgart to Brussels. Emissions were reported to be "below 1995 California requirements." Maximum turbine inlet temperature during acceleration can reach 1350[degrees]C (2462[degrees]F). These emissions were achieved even though the fuel economy was about 20 percent lower than for a comparable diesel engine. (This implies lower emission levels when fuel consumption is reduced. This engine has not yet been fully developed or optimized for fuel economy. Note: in a subsequent corporate reorganization of Daimler-Benz, further development of the automotive gas turbine was dropped from the research division agenda and referred to another division for possible further product development).

* Work on ATTAP has focused on the development of practical ceramic components that can operate at design speeds and temperatures with acceptable durability. Some parallel low-emission burner work has been done at Allison in Indianapolis and by the GM advanced engineering staff in Warren, Mich. In essence, steady-state combustor rig tests have been run with ceramic (silicon carbide) burners over the operating range of the engine with elevated burner inlet air temperatures (on the order of 1800[degrees]F) corresponding to the higher operating temperatures of the ceramic engine. Temperatures of the incoming premixed prevaporized fuel-air mixture are typically above the auto-ignition temperature of the mixture. This typically inhibits "lean blow-out" (fuel-air ratio below which the flame goes out). Thus, the high burner inlet temperatures really help in the design of low-emission premixed prevaporized burners. Local hot spots due to stoichiometric droplet burning of the fuel are avoided; uniform temperatures are achieved with maximum local temperatures only up to about 2800[degrees]F. Under these steady-state conditions, emissions, including [NO.sub.x], well below EPA standards have been demonstrated.

Areas that require further development and demonstration include reducing emissions during cold-start and transient operating conditions (acceleration and deceleration). It is also imperative minimize carbon formation in the burner to alleviate possible foreign object damage to downstream ceramic components. The necessity to adopts some form of variable-geometry burner design is still uncertain because the burner and its control system must be integrated with the engine and vehicle for demonstration on the chasses dynamometer and on the road.

* The Japanese Ceramic Gas Turbine Program is still in the design and component-development phases. Nissan has published some of its high-temperature burner work, which was initiated in the mid-1970s. It appears that they have adopted a premixed prevaporized approach to their ceramic burner.

2. Regenerator core and seals: Other components impacted by the higher cycle temperatures are the regenerator core ant its seal system. It has been found that the materials and design for both the core and the seals must be upgraded to withstand the 2000[degrees] to 2100[degrees]F regenerator inlet temperatures that accompany the 2500[degrees]F turbine inlet temperatures. Both performance and durability need to be demonstrated under long-term cyclic temperature conditions.

Equally important is the need to reduce the cost of the regenerator system. Conversion from a wrapped to an extruded manufacturing process for the core is being explored. Although the aluminum silicate material is suitable for the higher operating temperatures, it appears to be difficult to process, and hence expensive.

3. Remaining ceramic materials questions: Short-term capabilities of the critical ceramic components (nozzles, wheels, and burner) have been successfully demonstrated. However, ATTAP requires 3500 hours of durability testing. The data base on ceramic materials and components under long-term cyclic loads and temperatures is not yet established. The time-dependent properties are not really well known yet.

Evolutionary development questions seem to center on low-cost processing of components in quantities suitable for automotive production. High yield of quality parts is important. The larger scrool-type components have been difficult. Current slip-casting and plaster molds do not seem to be the answer. Further process development is needed. However, the apparent success of Japanese manufacturers with ceramic turbocharger rotors (in production since 19850 is encouraging.

4. Heat management: Another element necessary for the achievement of maximum fuel economy is control of thermal losses from the cycle. Insulation of the engine housing is to be accomplished on the Allison AGT-5 engine by a proprietary material (Cerachrome-[Al.sub.2.O.sub.3]-[SiO.sub.2]-[Cr.sub.2.O.sub.3] of the Manville Corp. (Denver). Development is directed toward injection-molding and hardening in place. The material process must demonstrate adhesion, thermal cyclic durability, and erosion resistance.

5. Aerodynamic components: Downsizing the power for smaller more-fuel-efficient vehicles to around 100 horsepower and significantly increasing operating temperatures and pressures greatly reduce the physical size of the aerodynamic components. It is difficult to maintain high component efficiencies and seal losses at acceptable levels for such small sizes. Based on the measured component performance from the AGT and ATTAP programs, the projected fuel economu of the reference powertrain design turbine-powered vehicle should be as mentioned earlier. Of course, achievement of this performance must be demonstrated by the engine in the vehicle on the road. Careful tuning and judicious optimization of the aerodynamic elements and integration of the engine, controls, and drive systems into the vehicle will be a significant part of such a demonstration.

6. Conclusions and timing: It appears that there are no technological barriers to preclude possible production of automotive turbines, but there is much engineering and development work still ahead. The configuration of a potentially competitive turbine-powered automobile can be defined. The technology is essentially in place, but the economic and business strategies are open questions that can only be answered by the manufaturers involved. With decisions to proceed and adequate funding, production could begin in five to eight years.

Electric and Hybrid Systems

California law mandates that 2 percent of the cars sold in that state in 1998 must be zero-emission vehicles (ZEV). By 2003, this rate will rise to 10 percent of the cars sold. Only electric vehicles qualify as ZEVs. Other states, including Massachusetts, New York, New Jersey, and Pennsylvania, are following California's lead. A number of European cities are also considering requirements for electric vehicles. Consequently, manufacturers in the United States, Europe, and Japan are developing electric and electric-hybrid vehicles. Initial penetration of these vehicles is expected in fleet operations and commuters segments of the market.

Electric vehicles have o tailpipe emissions. Further, these vehicles significantly reduce the overall dependence on foreign oil. And if the power station generating the electricity to recharge batteries uses nuclear, hydro, natural gas, or solar energy, air pollution is also reduced. If the power station burns coal, however, emissions may increase depending on the power station's emissions-control equipment. Regardless, the energy can be supplied from domestic sources and the power stations can be distributed outside urban areas. Vehicle battery recharge could be accomplished overnight in off-peak-demand periods. This eases the supply/demand problems for the increased electrical capacity, at least in the early transitional phases.

On the other hand, when compared to conventional gasoline-fueled vehicles, electric vehicles have some drawbacks.

For example, energy density in gasoline is about 12,000 watt-hours per kilogram compared to current lead-acid batteries, which provide about watt-hours per kilogram. This translates into a relatively short operating range for electric vehicles--some 50 to 120 miles per charge if no battery-charging system is used during operation.

Another drawback is that battery packs are heavy; even with a fiberglass body over a space-frame chassis, gross vehicle weight can easily exceed 3000 pounds. Even the GM two-passenger Impact electric vehicle weighs about 2200 pounds.

Moreover, battery packs are expensive. Present lead-acid packs can cost from about $1500 to $8000 depending on the system design. They need to be replaced every 20,000 to 30,000 miles. Operating cost estimates also seem to vary widely, reflecting not only the differing designs, status of the technology, and the assumptions, but also the attitude of the analyst.

These batteries can take six to eight hours to recharge, depending on the battery--and they lose capacity at low ambient temperatures--but an emergency charge can be completed in two hours. Here again, the technology is in a state of change. Recent estimates for a special Japanese battery/charge system go down to 15 minutes.

In response to these recognized shortcomings, different types of batteries are under development by various U.S., European, and Japanese companies. Some of the most prominent types under development include sodium-sulfur, nickel-cadmium, lithium aluminum-iron sulfide, sodium-metal chloride, zinc-bromine, iron-air, and zinc-air. All are trying to alleviate these problems, but usually incorporate disadvantages such as high cost, high operating temperatures (safety problems), availability of strategic materials, recyclability, and waste disposal problems. In view of the need for greatly improved battery systems, the U.S. Advanced Battery Consortium has been formed by Ford, GM, and Chrysler to share half the cost of the program ($100 million annual budget) with the government to develop longer-lasting higher-performing electric car batteries. For now, sodium-sulfur batteries appear likely to be the best alternative because they have four times higher energy density than lead-acid batteries; tests indicate they would last about 100,000 miles. However, these batteries use liquid sodium and sulfur electrodes and operate at elevated temperatures (about 300[degrees]C), so safety is a concern.

To alleviate the short range of electric vehicles, various types of hybrid systems have been demonstrated or are being developed. An auxiliary power unit (APU), such as a small spark-ignition engine or diesel engine-generator, is incorporated into the system. This generator set, fueled by gasoline, propane, compressed natural gas, diesel fuel, or methanol, runs at relatively steady-state optimum conditions (very low emissions and high efficiency) and recharges the batteries. Some designs allow the driver to select either the APU or the battery system (this is sometimes called a dual-power system). The strategy is to used low-emission APUs burning alternative or clean fuels and operating under optimum conditions, so emissions are minimized. In urban or high-smog areas, the APU would be turned off to achieve zero emissions. Prototype gas-turbine hybrid-electric systems were built and tested by Toyota Motor Corp. in the late 1960s.

The problem associated with hybrid systems is the cost of the two engines. In addition, the cost of added controls and system integration cannot be ignored. Of course, when the APU operates, the car is not a zero-emission vehicle. Hence, early attempts at hybrid systems were dropped.

Fuel Cells

Electrochemical generation of electrical power may be the most radical departure from conventional vehicle-propulsion systems. Long term, it could solve the dilemma of polluting emissions generated by the heat engines powering light- and heavy-duty vehicles. Since fuel cells are not heat engines, efficiencies are not constrained by Carnot principles. Fuel cells offer the potential for extremely low emissions and relatively high thermal efficiency (more than twice that of typical spark-ignition pistion engines).

Basically, a fuel cell uses hydrogen and oxygen to generate do electricity and water plus waste heat. Various types of fuel cells are named for their electrolytes, such as phosphoric acid, molten carbonate, solid oxide, or solid polymer. Hydrogen generated from methanol, natural gas, or coal-derived fuel in a reformer (part of the system) is supplied to the anode; air ([O.sub.2]) is supplied to the cathode; and the electrolyte is sandwiched in between. Ions migrate through the electrolyte from the cathode to the anode; electrons flow from the anode back to the cathode in the external electric circuit. A voltage is generated across the catalyst-treated electrodes. Currently, an operational fuel cell is rather complex, unlike the simpe lead-acid battery often used for comparison.

Phosphoric acid and molten carbonate fuel cells are relatively advanced in their development. A number of serious commercial efforts are ongoing with support of the utilities, the chemical and processing industry, the Electric Power Research Institute (Palo Alto, Calif.), the Gas Research Institute (Chicago), and the DOE involving U.S., Japanese, and European companies. Commercialization in electric utility applications is expected to be well along by the mid-19902, with power ratings ranging from 200 kilowatts to 11 megawatts. In a DOE demonstration program, a phosphoric acid cell was incorporated into a bus; that type of cell was selected because of its availability and experience base at the time rather than its compatibility with vehicle requirements.

For various reasons, including large size, weight, lack of operational flexibility, and transient response, such systems are not appropriate for transportation systems; however, recent advances in fuel cell technology have demonstrated characteristics that do appear suitable for vehicle applications. These are proton exchange membrane (PEM) fuel cells (originally called solid polymer electrolytics cells) and solid oxide fuel cells; development of both is being pursued by General Motors and Allied-Signal. It is of interest that proton exhchange membrane fuel cells provided electric power for the Gemini spacecraft (circa 1964).

1. Proton exchange membrane fuel cell: Principal incentives for development of such fuel cells are extremely low emissions and excellent part load fuel economy, exceeding projected capabilities of the traditional piston engine. Proponents claim the power density will be similar to that of spark-ignition engines. A 6 1/2-year cost-shared government/industry program is in progress to demonstrate an 80-kilowatt prototype system in a vehicle by 1997. Allison Gas Turbine is the lead company coordinating the efforst of the General Motors technical staff, Los Alamos National Laboratory (Los Alamos, N.M.), Dow Chemical Co. (Midland, Mich.), and Ballard Power Systems Co. (North Vancouver, Canada). It is expected that the 80-kilowatt system could be scaled up for heavy-duty bus and truck requirements in the 250- to 300-kilowatt range.

Numerous technical problems and engineering developments need to be resolved, however, including:

* The PEM system includes a reformer that receives methanol fuel and provides hydrogen to the anode. The fuel reforming or processing is not instantaneou; development is necessary to reduce the response time and cost of the system. Of particular interest are cold start-up, transient operation, and low-temperature survivability. The system must be small and light enough for vehicle application. System integration is an important part of the prototype development.

* Both electrodes use platinum catalysts, which are expensive and readily poisoned by small concentrations of carbon monoxide (a typical by-product of the reforming process). A preoxidizer or equivalent is necessary to reduce the small amount of CO coming from the reformer to only a few parts per million to avoid catalyst poisoning.

* Other by-products of the reforming process are water and [CO.sub.2]. It is important for the electricity generation to keep the two electrodes and membrane saturated with water vapor. Otherwise, internal electrical resistance will increase. Water is injected to keep the fuel cell gases saturated and to provide cooling. This all adds complexity to the system.

* Mass flow instrumentation is needed to control ratios of fuel and air.

* The PEM system operates at modest temperatures (about 200[degrees]F) and pressures (about 2.5 atmosphere); thus an air pump is required.

* Continual effort toward reducing the cost of the materials, system components, and drivetrain components will be a necessary part of a successful development program.

Availability of a prototype preproduction PEM system is at least 10 to 15 years away based on current levels of effort.

2. Monolithic solid oxide fuel cells: The monolithic solid oxide fuel cell (MSOFC) is in an even earlier stage of development, but it offers potential advantages over the proton exchange membrane system. They are high power density (order of 10 kW/kg); low emissions; high efficiency (order of 60 percent); the fuel reformer is internal due to a high operating temperature (1800[degrees]F); a separate reformer reactor is not necessary; the system is smaller, simpler (and costs less), and more responsive that the PEM fuel cell; and carbon monoxide poisoning of platinum catalysts is avoided.

Technology development and prototype demonstration efforst are still in early stages. These efforts include:

* AirResearch Los Angeles division of Allied Signal Corp., which is working on MSOFCs. So far, the company has been testing single cells and small stacks. Manifolding has not been tested yet; however, proof of the concept has been demonstrated on a small laboratory scale.

* Monolithic solid oxide fuel cells are primary compact corrugated ceramic structures. The keys to success for the development of this system lie in the materials and fabrication methodologies, material and process quality assurance, and cell performance tests under specified conditions, stack performance modeling, and stress analysis.

At current spending levels of $2 million to $3 million per year, the first prototype power system is expected for demonstration and test by the year 2000. Initial applications will probably be in the high-dollar segments of the aerospace and military markets; from there, application would be in civil aircraft auxiliary power units, industrial power, and cogeneration. The implementation would be for mass-produced automobiles, perhaps by the year 2010 or 2020.

In consideration of the technical status, required economic commitments, and risk involved, it appears that the most likely alternatives systems to be introduced will be (in descending order of probability): electric and electric-hybrids, gas turbines, and fuel cells. However, the conventional piston engine will not be replaced easily or quickly. This points up the need for continued government participation in the higher-risk and long-range technologies in which financial limitations and risks may preclude full development by industry alone.

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Title Annotation:	electric systems, gas turbines and fuel cells
Author:	Harmon, Robert
Publication:	Mechanical Engineering-CIME
Date:	Mar 1, 1992
Words:	6017
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