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Thus, compliance of production engines to meet the 1994 particulate levels means the design goal must be about 0.07 g/bhp-hr in order to meet the 0.10 g/bhp-hr limit. A cursory review of emissions work shows a relationship between NO and particulates such that engine conroustion parameter changes taken to reduce NO tend to increase particulates and vice versa. Emissions compliance with current production engines by timing modification would require timing retardation of fuel injection. This retardation would incur an 8 to 10 percent increase in fuel consumption.

The leading after-treatment technologies for particulate emissions reduction below 0.5 g/bhp-hr are the particulate trap and the flow through oxidation catalytic surfaces. Truck operators and the engine manufacturers seem to be in general agreement that they would prefer not to use particulate traps. The initial cost is high, about $3K, the units,' performance as a functi on of time is expected to detèriorate, they incur a 1-2 percent fuel efficiency penalty and require additional maintenance. There is considerable activity in developing emissions acceptable engines without using particulate traps as well as development of particulate trap/oxidizer systems. Engine companies are not revealing their strategies or development results in the emissions compliance area.

There are some engine development directions that can be perceived through open literature and public displays. Fuel Injector pressures are being investigated well beyond the current production capability of 20,000 psi. In fact we may well see injection pressures approaching 50,000 psi introduced in the 90's. Injection of fuel with thermally insulated combustion chambers requires the major portion of the fuel to be introduced over a small crank angle and after top dead center. The injection pattern should achieve proper air-fuel mixing. Injection cut-off should be without further fuel introduction or leakage. Electronic fuel control will be further advanced such that timing control is camless and is controlled by combustion diagnostics and operating requirements. Injectors will either be cam-spring loaded, electrohydraulic or of another variation. Electronic fuel control is currently available. Computerized control of air and fuel management should be available soon.

Combustion chamber designs appear to continue to be direct injection with quiescent combustion chamber designs as opposed to the high swirl, deep combustion bowl piston design currently popular in Germany. The top piston ring will be close to the piston head land in order to minimize crevice volume which contributes to particulate emissions. Piston rings are much more complex than suggested by their appearance and the low heat rejection engines will require new ring materials and designs for the higher top ring reversal temperatures.

DOE is encouraging development of diesel engines with improved fuel consumption. However better fuel efficiency for the most part involves increased operating pressures and temperatures which tends to reduce particulates but increase NO levels. Thus aftercooling of turbocharged air becomes of increasing importance. There is an after-treatment developed at Sandia-Livermore by Dr. Robert Perry on a DOE Energy Conservation and Utilization Technology (ECUT) program. Perry developed a cyanuric acid addition to diesel engine exhaust which can reduce the NO level by more than 90 percent with |xhaust temperatures in the 1012 to 1830 F (600 to 1000 °C) temperature range. The lower limit for the process is 750°F (400°C) and at 2000 F (1200 C) the efficiency of NO reduction is 60 percent. This additive costs $.20/lb. The economic justification involves alternative engine timing retardation (8-10 percent fuel penalty) which appears to be one approach for the 1991 and 1994 NO standards. Metering of the cyanufic acid introduction can probably be controlled by the same electronic control signal for fuel injection control. This NO reduction system Perry calls "Raprinox", allows concentration of efforts on reducing particulate emissions and use of the higher pressures and temperatures and later injection associated with improved engine fuel economy. There may be a , spin-off advantage of with one variation of Rapernox that includes as one of the products of the system, nitrous oxide (N20). Recent scientific work suggests that N^O has a reaction with chlorine in the stratosphere which reduces the chlorine degradation of the oz.one layer. There are serious concerns about the ozone layer depletion being attributed to the air conditioning equipment leakage of freon from vehicles. This N„0 may also increase the "greenhouse effect."

Particulant emissions in current engines attributable to petroleum base lubricating oils are around 0.07 g/bhp-hr. This level will have to be reduced by improved sealing or use of synthetic oils. Engine testing at Komatsu with an ester base synthetic, SDL-1, indicates roughly a 50 percent reduction in oil consumption compared to equivalent testing with one of the better Japanese petroleum base lubricating oils. About 0.35 g/bhp-hr of particulates in current engine use is traceable to fuel hydrocarbons. A major fraction of the hydrocarbons results from unburned fuel in the combustion chamber. However, recent engine test work indicates a significant contribution of hydrocarbons involves their solubility in the lubricating oil and migration out of the oil film into the exhaust gas. The solubility of hydrocarbons into petroleum base lubricating oils is much greater than in ester base synthetic lubricants. There appears.to be ever increasing data indicating that compliance with the 1994 particulate emissions standards may require use of a synthetic liquid, probably an ester base lubricant similar to SDL-1 or a commercial equivalent (7).

Lightweight Reciprocating Components

Lighter weight reciprocating components such as the piston assembly reduce inertia forces as well as the necessary balance weight. Reduced inertia forces reduce the bearing load which is becoming increasingly important with the trend to ever-increasing Brake Mean Effective Pressure (BMEP).

The piston skirt typically has about 1/5 of the total piston assembly weight. However, reduction of the skirt length could impact piston guidance and stiffness.

There are a wide range of materials that allow the designer to reduce mass. Success in the high temperature liquid lubrication program will be a major impetus to use of higher temperature materials for the reciprocator components. Some of the new material candidates for these applications are the metal matrix composites, ceramic matrix composites, the RST powder alloys and fiber or whisker reinforced alloys.

Aluminum is used as a piston material as a lower half section of either a composite piston or an articulated piston with a steel or nodular cast iron crown or as an entirely aluminum piston. The last case usually requires internal passages for oil spray cooling, referred to as gallery cooling. Conventional aluminum alloys have a fall-off in strength at relatively low temperatures. Reinforcement of aluminum pistons with ceramic fibers or whiskers in the combustion bowl, piston crown and the ring groove area has.provided roughly a 2-fold increase in mechanical and thennal strength properties around 570 F (300 C) which is a critical temperature for pistons. Current approaches involve about.20 percent fiber or whisker reinforcement. Aluminum oxide fibers and silicon carbide whiskers have been used to date.

Currently the cost of ceramic fibers and whiskers is high but they should soon begin to reflect the economies of large quantity production. It is projected that a fiber reinforced aluminum piston without the complexities of gallery cooling could be cost comparable with an aluminum piston incorporating gallery cooling (9).

Silicon carbide whisker aligned and distributed in a designed fashion have been used in forged A6061 aluminum alloy connecting rods. These rods are approximately 45 percent lighter than comparable conventional steel connecting rods. These whisker reinforced aluminum connecting rods improve engine' performance as well as reduce vibration and noise.

Rapid solidification technology (RST) provides the means of developing a new range of aluminum alloys such as the Al-Fe-Mo alloy which has useful operating strengths at 650°F (343°C) compared to about 350°F (180°C) for conventi onal preci pi tati on-hardened aluminum alloys (2000 and 7000 series) for the same strengths. The RST process improves aluminum alloys by extending the range of solid solubility of the transition elements. Strength is increased from additional solid-solution strengthening and from the formation of a very fine uniform dispersion of metastable phases. In addition a high volume fraction of fine, thermally stable dispersoids is obtained during subsequent thermomechanical processing. It is projected that advanced dispersion -strengthened aluminum alloy can be developed with RST which would have useful strength to 800 °F (425°C). (10) It would appear logical that further improvements of RST aluminum alloys were obtainable with fiber reinforcement. Other lightweight alloy systems should be capable of similar improvements with RST and fiber reinforcement.

Reduction in weight of exhaust valves is another area for possible efficiency gain.. Since the inertia of the moving mass of the valve is related to the square of the engine rpm, interest in lightweight valves is stronger in the automotive spark ignition engines where engine speeds are in the 3,000 rpm and higher range compared with heavy duty diesel engines with a maximum 2,100 rpm. Current heavy duty diesel exhaust valves operate with temperatures low enough to use austenitic steels. Higher temperatures could be handled by nickel base alloys or sodium filled valves. Sodium cooled valves have been used for aircraft reciprocating engines and stem cooled valves are being used in some light weight truck engines. Lighter weight valves could be provided by drilled out steel valves, that is valves without sodium but with the passage, titanium and silicon nitride. Current heavy duty engine designs don't produce much improvement with lightweight valves.

New thinking on valves will most probably emanate from engine designs going to higher peak cylinder pressures with attendant higher temperatures and higher valve wear rates. Control of valve event timing by electronics could provide significant advantages. Concerns will be with durability.

Coatings for Heat Engines

Gas turbine engines have used coatings on hot-section parts for over 25 years. Thermal barrier coatings, deposited by plasma spraying have been used in commercial and military engines on combustor liners for over 20 years. Pratt & Whitney pioneered the thermal spray technology which is now used in all major aircraft and marine gas turbine engines in the Free World. Successful application of thermal barrier coating to turbine airfoil surfaces has been a development goal for nearly 20 years. Columnar structure zirconia coating systems were first deposited by Bush and Patten at Battelle Northwest in the mid 70's. Tom Strangman improved this concept using electron-beam physical vapor deposition (EB PVD) while at Pratt & Whitney and later at Garrett Air Research. Ernest Demaray at Airco-Temescal independently advanced this technology. Although not yet qualified for man-rated aircraft engines, the EB-PVD columnar structure zirconia coatings are doing very well in advanced engine testing.

Metallic coatings of the M Cr A1 X where M = Fe, Ni, Co or combinations thereof and X = Y and/or Hf have proved very successful in marine gas turbines in Naval service, in VSTOL aircraft and industrial gas turbines. The bulk of the coatings have been deposited by EB PVD. However, recently the low pressure plasma spray (LPPS), also referred to as vacuum plasma spray (VPS), has become competitive with EB PVD. Plasma spray deposition under vacuum minimizes oxidation during deposition. M Cr A1 X bond layers are used with zirconia outerlayers to improve attachment to substrates and provide oxidation protection to the substrate as zjrconia coatings are pervious to oxygen.

The application of coatings on diesel engine components has been at a much lower level than with the gas turbine engine. The initial use of chromium plating on piston rings or liners resulted from severe wear encountered in British 8th Army tank engines in North Africa in World War II. Chromium plating had a very long term usage because it combined low friction, high hardness and good corrosion resistance. Plasma sprayed molybdenum composite coatings are used on top piston rings where high operating temperatures and minimum lubrication are encountered. Engines intended for dual fuel (natural gas and heavy fuels) often use plasma sprayed chromium carbide-molybdenum type coatings. This coating is compatible with soft and hard liners. High BMEP engines with high temperature seal rings often have a tungsten-molybdenum coating to improve scuff and wear resistance.

Successful development of a high temperature liquid lubricant will allow top ring reversal temperatures over 900 F (482° C) which will require new thinking on the ring design and materials including coatings.

The low heat rejection diesel engine represents a potential major use of plasma spray thermal barrier coatings. Commitment to an engine line could require coating several million parts per year.

Foreign Competition

The application of high performance ceramics to reciprocating engines is a critical area of intense international competition. Foreign governments are supporting considerable engine application development as well as ceramic materials development which in many cases is directed at engine requirements.

The most similar program to the US Department of Energy's Low Heat Rejection Diesel Engine Program is the British Ceramics Applied to Reciprocating Engines (CARE) Program. CARE is a consortium of British engine builders, component manufacturers and materials suppliers including ceramic developers supported on a 50-50 cost share basis by their Department of Trade and Industry. The CARE budget is about $10M over the initial 3 years.

The CARE Program is comprised of 4 major sections; Insulation, Substitutional, Turbocharger and Materials. The Substitutional Section's efforts involves developing ceramic components to replace some conventional material components in engines. Emphasis of this group is with valve train and injection train areas as well as valves, valve followers, guides and fuel plungers. The Insulation Section is developing insulated combustion chambers. The Turbocharger Section is developing a ceramic as well as an improved aerodynamically efficient, nickel-based superalloy turbocharger. The Materials Section focuses development on requirements developed by the other 3 sections.

The diesel engine development in the CARE project is primarily done with the Leyland Research Center and Perkins Engines. Leyland had several years experience with the insulated engine concept before participation in CARE. They use a number of component concepts. Current engine'work is with pistons with air gaps and thermal barrier coatings and thermal barrier coatings also on the combustion chamber head and on the valves. Leyland uses precision coating in the head to minimize thermal gradients and reduce hot-spots between valves and around the injector tip. They use a synthetic lubricating oil provided by the Shell Oil Company. The first insulated engine approach by Leyland was with a 30 percent smaller engine block water cooling system which provided a 6 percent fuel improvement due to reduced parasitic losses. This insulated engine was installed in a 16 ton truck for testing and it has been shown at various Motor Shows (11).

In response to a sense that Europe was falling behind the US and Japan, the French Prime Minister Mitterand proposed a European Research Coordination Agency, referred to as "Eureka", which requires participation from industrial concerns from two or more European countries. One program is a 5 year, $13.4M program involving development of fiber-reinforced ceramics for diesel engines for commercial vehicles with French and German partners. (12) Renault and Peugot are also involved in government supported low heat rejection diesel engine development, Renault is nationaliz.ed so all their R & D is government supported.

The German government supports diesel engine development or materials development oriented towards diesel engine development via several programs. One German program is called KEBOD and involves KHD-Deutz., Audi, KKK (turbochargers) and Hoechst CeramTec organizations working on various aspects of advanced diesel technology. The German nuclear agency KFA also supports several diesel related projects.

There are also a number of European Economic Community (EEC) supported programs with diesel and turbine related materials work. The EEC supports the COST 601 project which is a several million dollar a year engine materials program. Another EEC program is the Basic Research in Industrial Technologies (BRITE) program which was recently initiated with significant engine related projects. However, the greatest government support for engine related materials development is in Japan. This effort has spawned significant and systematic improvements in design with brittle materials as well as development of ceramics and ceramic composites.

Summary and Conclusions

This review was intended.to outline the breadth and scope of some of the potential performance and durability improvements for the heavy duty diesel engine skewed to the materials perspective. In the process the myth that the diesel engine was a mature engine with little future growth was dispelled. The diesel engine .is the most fuel efficient engine yet devised and this has been achieved primarily with unsophisticated steel, iron and aluminum alloys. If further diesel engine performance improvements are related to materials, then the diesel engine should further extend its fuel efficiency lead over all challengers.

The low heat rejection diesel engine concept is viable. Woschni has pointed out the inadequate understanding of insulated diesel engine combustion and heat transfer. There are major differences between US low heat rejection engine development and Woschni's test engine. Performance gains with insulated combustion chambers are achieved with thermal barrier coatings on all combustion chamber components as opposed to air gaps near the piston crown, quiescent combustion with shallow combustion bowls, very high pressure fuel injection with electronic control as opposed to a high swirl approach with deep combustion bowl pistons and moderate fuel injection pressures used by Woschni. High efficiency turbochargers well matched are important.

Tribology applied to diesel engines is going to be receiving increasing attention. Virtually all those responsible for diesel engine development agree that liquid lubrication will be used well into the 21st Century. This is because of the functions the lubricant must perform and it is strongly reinforced by the major advances made recently in synthetic liquid lubrication. The DOE/NASA High Temperature Liquid Lubrication program should produce a Virtually ash free low-cost liquid lubricant capable of top ring reversal (TRR) temperatures over 900°F (482°C). This lubrication capability will stimulate use of more advanced materials in the engine.

Probably the biggest fuel efficiency improvement achievable in the 90's is in advanced turbomachinery. The most advanced turbine and compressor aerodynamic design techniques developed for gas turbine engines could provide a 15 point improvement in combined efficiency which would be a 30 percent improvement compared to current production turbochargers. Since the reciprocator requires air as a linear function of power and the turbocharger output is exponential, better tank mileage could be obtained by operating at higher efficiencies over a wide power range by using variable geometry or sequential turbocharging. The use of pulse turbocharging instead of the constant pressure manifold could provide further advantage.

The greatest problem facing the heavy duty diesel engine community is compliance with the EPA emissions regulations going into effect in 1991 and 1994, and doing it without uiing the very expensive traps. Many of the approaches to reduce NO result in increased particulate emissions. Retardation of timing to help meet emissions can result in an 8 to 10 percent increase in SFC. A very promising method of reducing N0X

levels by as much as 90 percent through addition of cyanuric acid ($.20/lb) to the exhaust gas was developed at Sandia Livermore by Dr. Perry. Success could allow concentration on reduction of particulates. Oil consumption levels with current petroleum based lube oils, and the fact that solubility of current oils with hydrocarbons from the fuels contributes to particulates, suggest that particulate emission compliance could require a synthetic lube oil such as SDL-1.

There are major programs supported by European and the Japanese Governments directed at improving some facet of the low heat rejection diesel engine concept. This competition should accelerate diesel engine development.

References

1. French, C.C.J., "Internal Combustion Engines for the Nineties and Beyond-Challenges and Opportunities", The 1985 Calvin W. Rice Lecture for ASME, presented in New Orleans, Feb.

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