US Department of Energy

Introduction

Rudolph Diesel received academic training in late 19th Century classical thermodynamics. About 100 years ago he was working in the Linde Ice Machine Company where he noted the large amount of heat given off by reciprocating ammonia compressors. This observation lead to his ideas about compressing air in a similar manner to over 10 times its initial volume such that the air temperature rose high enough to ignite an injected fuel charge. He first published a thermodynamic analysis of his engine concept in 1893 and 5 years later in Munich displayed a practical engine built on the compression ignition concept which now bears his name.

While there have been a large number of improvements in the evolution of the diesel engine, there have only been 3 major innovations. First there were the fuel injection systems of the jerk pump type that were introduced by Bosch in the early 20's. Next was the turbocharging development attributed to Buchi in the 1911-14 period. However, turbocharging heavy duty truck diesel engines did not begin .until 1954. By the mid 70's almost all U.S. made heavy duty diesel engines were turbocharged, a trend which was well ahead of European and Japanese efforts. The third innovation is electronic controls with microprocessors for fuel introduction, engine operation, emissions management (1). The low heat rejection .(LHR) diesel., concept, sometimes referred to as the "Adiabatic Diesel", could well be a candidate for the next major diesel innovation. The LHR engine concepft is based on thermal insulation of the combustion chamber in order to minimize heat transfer to the engine block water cooling system thereby increasing the available energy in the exhaust gas where it can be more readily used. Thus, the LHR concept allows for the reduction in size, or elimination of the engine block cooling system with the attendant reduction in the parasitic losses of the water pump and fan. However, Professor Woschni of the University of Munich challenges the advantages for an insulated engine.

The diesel engine has emerged as the most cost-effective propulsion engine for surface transportation applications. Almost all new commercial ship construction is with diesel propulsion. There is also a significant retrofit market for marine diesel engine replacement of steam turbine plants as was recently done on the QE-II passenger ship as well as numerous tankers and freighters. The only 4 commercial ships fitted with aircraft derivative gas turbine engines were retrofitted with diesel engines less than 8 years after the gas turbines had been installed. Virtually all of the railroad locomotives operating on non-electrified lines are diesel. Essentially all of the heavy duty class 8 trucks are diesel powered and there is an increasing trend of diesel engine use in the medium and light duty truck field.

The diesel powered automobile was becoming popular in the U.S. in the early 80's when 3 events reversed this trend. These were the fuel economy improvement of the spark ignition engine, the many problems with engine life encountered with a good spark ignition engine modified to become a diesel and the increase of diesel fuel prices in urban areas considerably above the price of gasoline. The oil industry for the U.S. market has a product slate optimized for gasoline while in Europe it is optimized for diesel /home heating fuel. While the sale of diesel cars in the U.S. is less than 1 percent, it is about 20 percent of the German market.

The military selection of propulsion plants has an extended range of requirements which include cost-effectiveness. The British Ministry of Defense (MOD), which had committed to an all gas turbine surface fleet in the 70's, shifted to a combined diesel electric and gas turbine plant (CODLAG) for its first new class after Falklands, the Type 23 Frigate. This approach of using the diesel engine for cruise, which is 90 percent of underway time, and the gas turbine for sprint is a concept adapted by most navies with the notable exceptions of the U.S. and Russian navies. The U.S. Army currently has a competition between an advanced diesel engine and a gas turbine for its next main battle tank. One interesting point for this new Army engine is that both the diesel and gas turbine engines will provide the same power output but will fit in a 30 percent smaller volume than the AGT 1500 gas turbine propulsion plant requires in the Abrams M-l tank. The military is looking at the possible use of diesel engines for missile, airship and helicopter propulsion.

The diesel engine is the most fuel efficient production heat engine yet devised. Large marine diesel engines now have a thermal efficiency of 52% while the smaller, higher rpm heavy duty truck engines are about 42% efficient. These efficiencies have been achieved primarily with iron, steel and aluminum alloys with only a modest introduction of advanced materials recently. Thus, if further engine efficiency and durability gains are related to using advanced materials, then the diesel has a very great potential for further advances. Diesel parts are massive compared to gas turbine parts. Cost considerations suggest components be developed with low cost alloys used for low temperature, lightly loaded sections with higher loaded sections fiber reinforced, or, alternatively, with a second material braz.ed or joined. Surface modifications for improved wear, insulation or oxidation/corrosion resistance are achievable with coatings. Diesel engines require moving components which have very low wear rates. Dense silicon nitride against metal surfaces has produced significantly improved wear resistance. Candidate wear coatings should have wear rates comparable to these under engine operating conditions. Fuel economy improvements are anticipated to be achieved through use of materials and lubricants that allow higher pressures, and temperatures thereby providing greater design flexibility. Effective design integration of materials and lubricants can lead to a large number of small improvements. Candidate improvements must conform to emissions standards or they won't be used. Successful development will most logically flow from innovative engine designers working with capable materials developers. This review provides some insight into candidate technology for diesel engine fuel savings with emphasis on the materials aspects. The advanced diesel technology to be discussed includes the 3HR diesel concept, applied tribology, advanced turbochargers, thermoelectrics, lightweight moving parts and emissions.

Low Heat Rejection Diesel Concept

The major emphasis in the DOE/NASA Heavy Duty Transport Technology program is focused on, but not limited to, the low heat rejection (LHR) diesel engine concept, also referred to as the "Adiabatic Diesel'1. The LHR engine concept basically involves thermal insulation of the combustion chamber and exhaust passage to minimize heat loss to the engine block cooling water system, thereby increasing the available energy in the exhaust gas where it is more readily usable. This insulation on the fireside of the combustion chamber components causes many interacting complexities that must be properly managed to achieve a net reduction in fuel consumption. The high temperature surfaces of the insulated combustion chamber components decrease the density of the air during the intake stroke.

This lower density air results in an air compression penalty.

So success with the LHR engine concept requires exhaust energy utiliz.ation which provides a net gain; i.e., a reduction in specific fuel consumption (SFC). This exhaust gas utilization must include turbochargers well matched to the engine. The much higher surface temperatures of the insulated combustion chamber components affects combustion which according to Professor Woschni, University of Munich, results in increased fuel consumption. This challenge by Woschni to the LHR engine concept directly impacts the rate of advanced materials introduction into the diesel engine. Therefore, a short review of Woschni's concerns is presented, along with identification of some problems with Woschni's contentions.

Woschni's Challenge

The most controversial topic in the diesel community is the Woschni challenge to the LHR diesel engine concept which he presented at the SAE Congress in Detroit, February 1987. Professor Woschni of the University of Munich contends that when a diesel engine piston is insulated there will be an increase in the wall surface temperature from 380°C (716°F) to 700°C (1282°F) and that the heat transfer co-efficient will increase by a factor of 4. He attributes this to a reduction in the boundary layer thickness and the flame penetrating the boundary layer. He calls this effect "convection vive". Woschni contends that, except for low power settings, the insulated engine will have a higher SFC than the conventionally cooled engine for. either the naturally aspirated or turbocharged configuration.

One should look closely at Woschni's work to help place it in perspective. He conducted his testing with deep combustion bowl pistons from Daimler-Benz^and Maschinenfabrik Augsburg Nurnburg A.G.(MAN) modified with a Nimonic (high temperature metal alloy) crown and air gaps in a single cylinder test engine. There are several differences from a practical engine perspective. Most Investigators working on the LHR diesel engine concept use modified zirconia coatings, referred to as thermal barrier coatings, to provide insulation not only on the piston crown but also on the cylinder head or firedeck and on exhaust valve heads excluding the seating surfaces. These zirconia coatings have a very low specific heat, low thermal conductance and have one of the closest thermal expansion matches of a ceramic to a metal. Some investigators use these coatings to minimize heat transfer into the piston but once it is in, have highly conductive thermal paths to the piston skirt where the heat is transferred to the oil covered cylinder liner. The point is that the cyclic thermal characteristics of the z.irconia are very wide and with its very low specific heat suggests less heat transfer to the intake air than the air gap Nimonic crown design. The thermal excursions of these two insulated engines will be quite different.

There is general agreement that the hot surfaces of an insulated engine do transfer some heat to the intake air and thus there will be fewer air molecules admitted before the compression stroke begins. This penalty is described as volumetric efficiency loss. Most investigators agree that an insulated naturally aspirated engine will be slightly less efficient than a conventionally cooled engine. The increased available energy in the exhaust as a result of an insulated combustion chamber must compensate for the volumetric efficiency penalty and provide a net gain compared to a conventionally cooled engine. Woschni does not discuss measuring the increased energy in the insulated engine exhaust and since it is difficult to measure, it is quite possible he did not do it. His simulated turbocharger air should reflect this difference and it is not apparent that he uses a different simulation for the 2 cases.

The MAN "M" wall burning combustion system appears to be one of the high swirl systems Woschni uses. This system includes spiral intake ports to produce a high-speed rotary air motion in the cylinder during the air intake stroke.

Fuel is injected such that it strikes the wall of the spherical combustion chamber in the piston where it spreads to form a thin film which evaporates under controlled conditions. The hot air in the combustion chamber develops a cyclone type action that sweeps over the fuel film peeling it layer by layer from the wall for progessive and complete combustion (2). Clearly the hotter insulated wall surface will affect this combustion. Caterpillar and Cummins use a quiescent combustion chamber wherein they impart high level energy to the fuel and then atomize the fuel through spray nozzles to achieve the desired fuel-air mixing. Fuel droplets essentially do not impinge on any surfaces. Woschni is believed to be using a Bosch injector which operates to about 12,000 psi while the injectors used in production engines by both Cummins and Caterpillar operate to 20,000 psi. Injection pressure is important because it affects ignition delay. The injector has to introduce the fuel charge such that proper air-fuel mixing takes place so that a smooth pressure characteristic is achieved and pressure isn't built up before top dead center (TDC) which could be counterproductive.

When an engine is insulated there are a number of interacting effects that must be considered. The increased cylinder wall temperature wil1 increase the lube oil temperature which results in a thinner film thickness near TDC. Cylinder liners distort from a true circle in a conventionally cooled engine and this distortion is magnified in an insulated engine. The liquid lubricant seals the out-of-round area thereby minimizing blow-by. Increasing the valve and valve seat temperatures can lead to distortion and blow-by. The fuel temperature can have a significant effect on ignition delay. The British Ceramics Applied to Reciprocating Engines (CARE) program uses precision cooling in the cylinder head as weil as thermal barrier coatings. In fact, the British say that in many ways precision cooling is more effective in reducing heat transfer to the engine block coolant than is insulation.

There are a number of ways to insulate a diesel engine to achieve an LHR diesel engine. Woschni has chosen air gaps and a high temperature metal crown while most investigators use thermal barrier coatings and insulate more than the piston cap. Woschni reports very well what he tested with his specific engine. One would not expect an academic institution to have the facilities or the practical engine experience to effect the necessary engine modifications and adjustments to achieve a fuel savings. British investigators agree with Woschni that the heat transfer coefficient increases significantly in an insulated combustion chamber. However, they disagree with his assertion that this leads to an increased fuel consumption (11).

These major differences between Woschni's single cylinder test engine and US diesel engines are particularly significant as Woschni appears to be backing out his heat transfer coefficient and heat flux from his fuel consumption data based on his "convection vive" boundary layer heat transfer. DOE/NASA has supported a detailed analysis of heat transfer in the insulated diesel engine conducted by Tom Morel at ITI. Morel shows a reduction in heat flux for the same case that Woschni gets an increased heat flux. Morel's analysis is validated with engine testing at Purdue University (3). It should be noted that typically there is considerable uncertainty involved with fuel performance measurements derived from single cylinder engine tests.

Woschni implies his "convection vive" concept applies to all insulated engines. This appears to be a phenomenon of a reactive boundary layer which must have a substantial amount of premixed and unreacted fuel/air mixture present such that the reaction rate is influenced by the surface temperature (4). Typical US heavy duty engines have high fuel injection pressures and good atomizing noz.zles in a quiescent combustion design which tends to have a much smaller amount of unreacted fuel/air mixture at the insulated surfaces than do the high swirl, lower, injection pressure German eng.ines. Consequently the heat transfer coefficient should not be significantly influenced by the surface temperatures in the US insulated engine designs. Woschni's insulated engine effect appears to be engine specific. Woschni has made the point that the engine developers do not understand well enough what is going on in heat transfer, combustion and heat release in an insulated engine to use to best advantage. This area is the subject of significant investigation. U.S. efforts will also be burdened with strict emissions restrictions.

Thermoelectrics

A novel method for fuel saving using waste heat is the replacement of conventional alternators with thermoelectric units. The modern use of direct conversion of thermal energy to direct current electrical power by the Seebeck Effect emerged in the late 30's. Thermoelectric devices have demonstrated unattended life of over 20 years in space, undersea and terrestrial applications with very high reliability. Several diesel engine manufacturers took a look at thermoelectrics during the past 10 years before curtailing the study because of cost-effectiveness considerations. There may be some recent events that justify another look at the thermoelectric concept. Specifically, there is a company 1n California that is convinced they could produce a large quantity production module with a price to the operator of under $1 per watt. Since comparable prices today are roughly $25 per watt the obvious question is how realistic is the projection?

Current thermoelectric devices are essentially fabricated by labor intensive "cottage industry" methods. The cost of raw material for 500,000 lb per year quantities would be about $0,071 per watt compared with small batch costs of $0.41 per watt. There is considerable labor cost in the materials which explains this cost differential. Adoption of automated production techniques for forming, assembly, potting and quality control should bring costs below the $1 per watt level (5). There appears to be sufficient incentive for a more exacting evaluation.

Thermoelectric arrays are comprised of P-type and N-type thermoelectric elements electrically connected in series. The two thermoelectric couples best suited for diesel engines are bismuth telluride which is limited to 617 F (325 C) and lead telluride which can be used up to 1067°F (575-C). The units have to operate across a temperature differential. The hot side can be provided by attachment to the exhaust piping or manifold. Air cooling should be adequate for the cold junction. Technical problems include the installation geometry and the ability of the semiconductor devices to withstand the engine environment. Developments in the semiconductor industry have been very rapid. Microelectrics have played an important role in reducing the microprocessor cost such that they are now cost-effective in heavy duty diesel applications. Can we expect similar advances that will make thermoelectrics cost-effective in heavy duty truck operation? Can a thermoelectric generator replace the alternator in a heavy duty truck on a cost-effective basis?

Tribology-Diesel Engine

The emerging science of tribology covers the combined effects of friction, wear and lubrication. Roughly 10 percent of the fuel energy introduced into a diesel engine is used to overcome friction. A reasonable goal is to reduce this friction loss by 50 percent. With the trends to higher cylinder pressures and higher temperatures, coupled,with engine Project Managers requiring a minimum of 10,000 hours durability, while actually striving for 20,000 hours, the challenge in tribology for the heavy duty diesel engine developers is difficult.

Lubrication in the upper cylinder liner area is the key engine operational parameter. There are several approaches to achieve this necessary lubrication. Conventional mineral oil lubricants provide adequate lubrication with top ring reversal (TRR) temperatures under 400 F (204 C). However, oil consumption rates for current engines result in particulate emissions very close to the 1994 EPA limit of 0.1 gm/hp-hr even before considering the particulates generated in the combustion process. Thus, it appears that either the oil seals in the engine and turbocharger will have to be tighter or a synthetic oil with a lower rate of oil consumption will be necessary.

In order to improve the current liquid lubrication the functions performed by the mineral oils should be outlined and their problems or limitations be defined. Liquid lubrication in the diesel upper cylinder performs five major functions. The primary function is to provide an oil film that prevents contact between the ring and cylinder liner. This is a hydrodynamic oil film characterized by a thick oil film for most of the stroke. As the piston approaches top dead center, the oil film makes a transition to a much thinner oil film referred to as boundary lubrication. Fortunately the piston velocity is slower as it approaches top dead center and wear is not severe. Generally cylinder liner wear is most extensive in the top ring reversal (TRR) area. Current liquid lubricants provide a very low friction factor of around 0.01.

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