The adiabatic engine has many advantages over conventional diesel engines, thus it is well worthwhile to perfect it. The immediate benefits of an installed power plant without a conventional cooling system include improvements in survivability, maintainability, fuel efficiency, packaging, and allowance for greater design flexibility.

Improvements in survivability will result from the elimination of failures due to damaged radiators, fans, belts, pumps, and common coolant leaks. Maintenance will be greatly reduced by the elimination of the coolant, belts, and fans. Fuel efficiency gains will result mainly from the reduction of parasitic power losses associated with driving air-handling fans which must force large volumes of cooling air through ballistic grilles, ducts, radiators, and tight crevices. Small improvements of in-cylinder efficiency can be expected by proper insulation and waste-heat utilization. Packaging size requirements will be greatly reduced with the elimination of the radiator, ducting, and fans. Ballistic grille design complexity requirements will be greatly reduced, thereby reducing greater projectile protection level requirements.

In addition to the above advantages, an adiabatic engine is expected to have a wider fuel usage tolerance as a result of increased combustion chamber surface temperatures. White smoke dissipation and quicker warm-up can also be attributed to the hot combustion system.

Overall Program Objectives

The objective of the work performed under the TACOM contract is to identify, develop, and demonstrate advanced tribological and insulating technologies with the following major targets:

• Brake Specific Heat Rejection (BSHR) of 12.0 Btu/bhp-min.

• Cylinder liner Top Ring Reversal (TRR) temperature to 1100 degree F

• Durability of 100 hours on a single-cylinder engine test with a minimum life expectancy of 1000 hours.

Other important operating specifications include:

• Non-water-cooled (adiabatic) diesel engine

• Brake Mean Effective Pressure (BMEP)

- 300 psi at peak torque

- 240 psi at rated power

• Cylinder heat loading, 4HP/piston in2

• Brake Specific Fuel Consumption (BSFC) .320 lbs/bhp-hr

The BSHR target of 12.0 Btu/bhp-min requires that the engine performance, cooling system, intake air handling system, tribological system, and insulation package all be closely reviewed. It is well known that all of these factors influence heat rejection. Heat rejection has been defined as the heat energy which is rejected at all radiators, including the engine surfaces. A typical high-bmep, conventional water-cooled and after-cooled (intake air) diesel engine (at a rated-power BSHR level) is approximately 27.0 Btu/bhp-min. With the removal of the water-cooling system, the elimination of intake air after-cooling, and the application of combustion chamber insulation, it has been estimated that the heat rejection target of 12.0 Btu/bhp-min can be achieved. A generalized energy balance comparison between a conventional engine and the program's engine is illustrated in Figure 1. A review of the energy balance indicates that in order to achieve the heat rejection goal, a minimum reduction of in-cyUnder heat transfer of 57% is required, even with the elimination of water-cooling and intake air after-cooling.

Cylinder liner temperatures approaching 1100 degrees F have been predicted for the adiabatic engine version addressed in this program. The program assessment is that advanced liquid lubricants can be formulated for diesel engines to provide reasonable life at Top Ring Reversal (TRR) temperatures up to 840 degrees F. At TRR temperatures above 840 degrees F, solid lubricant suspension and decomposition films take over. At TRR temperatures above 1040 degrees F (at which point a solid lubricant itself decomposes or cannot fully coat) self-lubricating materials can be used.

Work Schedule

This program, which extends over approximately five years, has been divided into four major phases:

(1) Selection of Concept,





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