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The U.S. Army Tank-Automotive Command is developing a future high power, low heat rejection military diesel engine. Performance requirements for the engine result in a predicted cylinder wall temperature of 560°C at the top piston ring reversal location. Thermal stresses imposed on the lubricant will therefore be unusually severe. Midwest Research Institute is developing the tribological system for this engine. A new general concept for high temperature diesel engine lubrication has been formulated. Our concept includes advanced synthetic liquid lubricants, solid lubricant additives, and self-lubricating materials. The lubricants, additives, and materials that have been selected for initial laboratory and engine evaluations of the concept are reported here.

Contrary to the thick thermal barrier coating approach used in adiabatic diesel engines, the authors have investigated the merits of thin coatings. Transient heat transfer analysis indicates that the temperature swings experienced at combustion chamber surfaces depend primarily on material thermophysical properties, i.e., conductivity, density, and specific heat. Thus, cyclic temperature swings should be alike whether thick or thin (less than 0.25 mm) coatings are applied, Furthermore, thin coatings would lead to lower mean component temperatures and would be easier to apply than thick coatings. The thinly-coated engine concept offers several advantages including improved volumetric efficiency, lower cylinder liner wall temperatures, improved piston-liner tribological behavior, and improved erosion-corrosion resistance and thus greater component durability.

High thermal stresses in the cylinder heads of low heat rejection (LHR) engines can lead to low cycle fatigue failure in the head. In order to decrease these stresses to a more acceptable level, novel designs are introduced. One design utilizes scallops in the bridge area, and three others utilize a high-strength, low thermal conductivity titanium faceplate inserted into the firedeck (combustion face) of a low heat rejection engine cylinder head. The faceplates are 5mm thick disks that span the firedeck from the injector bore to approximately 10mm outside of the cylinder liner. Large-scale finite element models for these four different LHR cylinder head configurations were created, and used to evaluate their strength performance on a pass/fail basis. The complex geometry of this cylinder head required very detailed three-dimensional analysis techniques, especially in the valve bridge area. This area is finely meshed to allow for accurate determination of stress gradients.

A large scale, high resolution, finite element methodology for analysis of generic thermomechanical behavior of complex, low heat rejection engine components has been developed. This paper describes this process and presents an example evaluation of a low heat rejection cylinder head. Because of symmetry considerations, a one cylinder section of the head was modeled. However, the geometric nature of this cylinder head section required very precise three-dimensional analysis techniques. The completed three-dimensional model contains 40,696 elements and 48,536 nodes. The results of this example model show high stresses at the valve bridge and injector bore. These stresses result from a constrained thermal expansion of the head, and are generally compressive and radial in nature. A comparison of three different material types indicated that two of the three exceeded, and one was below the elastic limit.

The future of maintaining a superior mobile military ground vehicle fleet rests in high power density propulsion systems. As the U.S. Government desires to convert its powerplant base to heavy fuel operation, there arises the opportunity to incorporate new advanced materials into these heavy fuel engines. These newer materials serve the purpose of decreasing powerplant weight and develop new component designs to take advantage of improved strength and temperature capability of those materials. In addition, the military continues the effort for a non-watercooled Low Heat Rejection (LHR) diesel engine. This type of engine demands the use of ceramic and advanced ceramic composite material hardware. Furthermore, today's higher pressure fuel injection systems, coupled with reduced air/fuel ratio as a means of increasing horsepower to size and weight, will require thermal protection or change in material specification for many of the engine's components.

Thermal barrier coatings (TBC) are perceived as enabling technology to increase low heat rejection (LHR) diesel engine performance and improve its longevity. The state of the art of thermal barrier coating is the plasma spray zirconia. In addition, other material systems have been investigated for the next generation of thermal barrier coatings. The purpose of this TBC program is to focus on developing binder systems with low thermal conductivity materials to improve the coating durability under high load and temperature cyclical conditions encountered in the real engine. Research and development (R&D) and analysis were conducted on aluminum alloy piston for high output turbocharged diesel engine coated with TBC.

This paper discusses the goals, progress, and future plans of the TACOM/Cummins Adiabatic Engine Program. The Adiabatic Engine concept insulates the diesel combustion chamber with high temperature materials to allow hot operation near an adiabatic operation condition. Additional power and improved efficiency derived from this concept occur because thermal energy, normally lost to the cooling and exhaust systems, is converted to useful power through the use of turbomachinery and high-temperature materials. Engine testing has repeatedly demonstrated the Adiabatic Engine to be the most fuel efficient engine in the world with multi-cylinder engine performance levels of 0.285 LB/BHP-HR (48% thermal efficiency) at 450 HP representative. Installation of an early version of the Adiabatic Engine within a military 5 ton truck has been completed, with initial vehicle evaluation successfully accomplished.

Diesel engine cold-start problems include long cranking periods, hesitation and white smoke emissions. A better understanding of these problems is essential to improve diesel engine cold-start. In this study computer simulation model is developed for the steady state and transient cold starting processes in a single-cylinder naturally aspirated direct injection diesel engine. The model is verified experimentally and utilized to determine the key parameters that affect the cranking period and combustion instability after the engine starts. The behavior of the fuel spray before and after it impinges on the combustion chamber walls was analyzed in each cycle during the cold-start operation. The analysis indicated that the accumulated fuel in combustion chamber has a major impact on engine cold starting through increasing engine compression pressure and temperature and increasing fuel vapor concentration in the combustion chamber during the ignition delay period.

The study relates air/fuel ratio, fuel injection timing, and engine speed to exhaust smoke levels and performance of the diesel engine. Additional data were obtained under supercharged and turbocharged inlet air conditions to investigate the applicability of the derived relationships under these conditions. Limited data using a variance in fuel type were obtained. Insight into the basic mechanism of smoke formation in diesel engines was gained. The relative percentages of fuel injected before ignition (i.e., premixed fuel) and after initiation of combustion (i.e., unmixed diffusion burning fuel) were found to be extremely significant in determining smoke levels. A smoke factor (the ratio of equivalence ratio in the combustion chamber at initial ignition to overall equivalence ratio) was formulated and found to be useful in predicting smoke phenomena in diesel engines.

This paper documents the successful operation of a modified Cummins T46 turbocharger with a ceramic rotor. This turbocharger is modified to incorporate a 4.6 inch diameter ceramic turbine rotor (pressureless sintered silicon nitride) on the hot end. These results document the most complete ceramic turbine rotor performance map, for a large ceramic turbocharger rotor, available to date.

Integrated control strategies for the O.P.E.R.A.S. (Optimization of injection Pressure, EGR ratio, injection Retard or Advance and Swirl ratio) are demonstrated. The strategies are based on an investigation of combustion and emissions in a small bore, high speed, direct injection diesel engine. The engine is equipped with a common rail injection system and is tested under simulated turbocharged engine conditions at two loads and speeds that represent two key operating points in a medium size HEV vehicle. A new phenomenological model is developed for the fuel distribution in the combustion chamber and the fractions that are injected prior to the development of the flame, injected in the flame or deposited on the walls. The investigation covered the effect of the different operating parameters on the fuel distribution, combustion and engine-out emissions.

This paper presents tribological modeling, experimental work, and validation of tribology parameters of a single cylinder turbocharged diesel engine run at various loads, speeds, intake boost pressures, and cylinder liner temperatures. Analysis were made on piston rings and liner materials, rings mechanical and thermal loads, contact pressure between rings and liner, and lubricant conditions. The engine tribology parameters were measured, and used to validate the engine tribology models. These tribology parameters are: oil film thickness, coefficient of friction between rings and liner, friction force, friction power, friction torque, shear rate, shear stress and wear of the sliding surfaces. In order to measure the oil film thickness between rings and liner, a single cylinder AVL turbocharged diesel engine was instrumented to accept the difference in voltage drop method between rings, oil film, and liner.

A high output experimental single cylinder diesel engine that was fully coated and insulated with a ceramic slurry coated combustion chamber was tested at full load and full speed. The cylinder liner and cylinder head mere constructed of 410 Series stainless steel and the top half of the articulated piston and the cylinder head top deck plate were made of titanium. The cylinder liner, head plate and the piston crown were coated with ceramic slurry coating. An adiabaticity of 35 percent was predicted for the insulated engine. The top ring reversal area on the cylinder liner was oil cooled. In spite of the high boost pressure ratio of 4:1, the pressure charged air was not aftercooled. No deterioration in engine volumetric efficiency was noted. At full load (260 psi BMEP) and 2600 rpm, the coolant heat rejection rate of 12 btu/hp.min. was achieved. The original engine build had coolant heat rejection of 18.3 btu/hp-min and exhaust energy heat rejection of 42.3 btu/hp-min at full load.

The instantaneous frictional torque (IFT) of many components of a single cylinder diesel engine was determined by considering the forces acting on each component and the resulting change in the angular velocity. The IFT for the basic system, consisting of the crankshaft with the flywheel and oil pump, was first determined. The effect of adding each of the following to the basic system was determined: balancer shaft, cam shaft, piston with different ring combinations, inlet valve, exhaust valve and fuel injection pump. All the tests were conducted without gas pressure in the cylinder in a coast down mode. The results indicated the contribution of each component in the total frictional torque and its mode of lubrication. The energy absorbed by the valve springs and released back to the system was clearly Identified. The effect of speed on IFT and energy lost in friction was determined.

This paper deals with the analysis of heat release characteristics of an insulated turbocharged, six cylinder, DI contemporary diesel engine. The engine is fully insulated with thin thermal barrier coatings. Effect of insulation on the heat release was experimentally verified. Tests were carried over a range of engine speeds at 100%, 93%, 75% and 50% of rated torque. Fuel injection system was instrumented to obtain injection pressure characteristics. The study shows that rate of heat release, particularly in the major portion of the combustion, is higher for the insulated engine. Improvement in heat release and performance are primarily attributed to reduction in heat transfer loss due to the thin thermal barrier coating. Injection pressure at the rated speed and torque was found to be 138 MPa and there was no degradation of combustion process in the insulated engine. Improvements in BSFC at 93% load are 3.25% and 6% at 1600 and 2600 RPM, respectively.

Experimental results focused towards developing tribological surface coatings coupled with liquid lubricant boundary layer effects, for advanced high temperature military diesel engine applications are presented. The primary focus of this work is in the area of advanced, low heat rejection (LHR) high output diesel engines, where high temperature boundary lubrication between the piston ring and the cylinder liner wall surface is critical for successful engine operation. The target temperature focused upon in our research is an operating top ring reversal (TRR) temperature of approximately 538°C. The technology advancement used for this application involves treating porous iron oxide/titanium oxide (Fe2O3/TiO2) and molybdenum (Mo) based composite thermal sprayed coatings with chemical binders to improve coating strength, integrity, and tribological properties. This process dramatically decreases open porosity to form an almost monolithic appearing coating at the surface1.

Significant progress has been achieved in the development of advanced high-temperature, insulated, in-cylinder components for high-power-output miliraty diesel engines. Computer aided modeling and small-bore engine component testing have both been utilized extensively during the exploratory development process. Specific insulated optimal designs for the piston, cylinder headface, and cylinder liner have been identified. The designs all utilize thermal barrier coatings, titanium alloy, and interfacial air-gaps to provide thermal resistance. Finite element modeling including diesel cycle simulation has been utilized to screen and optimize material and design concepts relative to program objectives, while small-bore engine testing has been utilized to demonstrate component integrity. An improved slurry densified thermal barrier coating has been demonstrated by testing on a high temperature small-bore engine.

High-pressure fuel injection combustion is being applied as an approach to increase the power density of diesel engines. The high-pressure injection enables higher air utilization and thus improved smoke free low air-fuel ratio combustion is obtained. It also greatly increases the injection rate and reduces combustion duration that permits timing retard for lower peak cylinder pressure and improved emissions without a loss in fuel consumption. Optimization of these injection parameters offers increased power density opportunities. The lower air-fuel ratio is also conducive to simpler air-handling and lower pressure ratio turbocharger requirements. This paper includes laboratory data demonstrating a 26 percent increase in power density by optimizing these parameters with injection pressures to 200 mPa.

This paper presents a global friction model of a diesel engine. The model accounts for the individual contributions of the main components of the mechanical losses and the influence of specific design and operating parameters on the mechanical losses. The main components considered in the model are: the piston-ring assembly, the valve train, the bearings and auxiliaries (injection pump, oil pump and coolant pump). For each of these components, the model was developed based on geometric parameters, operating conditions and the physics governing the friction. The individual models were assembled in a global friction model of a multicylinder diesel engine, and a computer code was developed to simulate the total mechanical losses of the engine. The experimental validation of the model was obtained by comparing the simulated crankshaft's speed variation with the instantaneous speed measured by a shaft encoder.

An experimental method for determining the Instantaneous Frictional Torque (IFT) using pressure transducers on every cylinder and speed measurements at both ends of the crankshaft is presented. The speed variation measured at one end of the crankshaft is distorted by torsional vibrations making it difficult to establish a simple and direct correlation between the acting torque and measured speed. Using a lumped mass model of the crankshaft and modal analysis techniques, the contributions of the different natural modes to the motion along the crankshaft axis are determined. Based on this model a method was devised to combine speed measurements made at both ends of the crankshaft in such a way as to eliminate the influence of torsional vibrations and obtain the equivalent rigid body motion of the crankshaft. This motion, the loading torque and the gas pressure torque are utilized to determine the IFT.