Adiabatic Engine Tacom Activities

The history of adiabatic engine activities at TACOM dates back to the early seventies. A great deal of feasibility demonstration has been accomplished sirtce that time utilizing both single-cylinder and multicy1inder engine testing phases. Mai y of TACOM's early efforts (as well as the current activities) emphasized high output military dieSel engines. Payoffs from this technology for military type applications were previously discussed within the Adiabatic Engine Overview section. Early engine testing demonstrated a mu1ticy1inder turbocompounded adiabatic engine with performance levels of 0.285 LB/BHP-HR (48% thermal efficiency) at 450 hp. Installation of an early version of an adiabatic engine (Figure 4) was performed in a military 5-ton truck. This engine used ceramic coatings on all combustion surfaces and eliminated the water cooling. The engine is "waterless" with limited oil-cooling, and successfully eliminated 361 parts from the standard production engine (i.e., fans, radiators, pumps, coolant manifolds, etc.). The engine in addition significantly reduced engine system volume, and presented many interesting "design freedom" opportunities for future truck applications. Specific heat rejection of the adiabatic version engine was reduced 38% from the conventionally cooled baseline Cummins NHC-250 engine. Dynamometer data indicated a fuel economy gain in the range of 510% over the speed range (at full load) of the adiabaitic engine. Within a vehicle, an adiabatic engine impacts upon factors such as: (a) fan and water pump parasitics, at various loads and speeds, (b) insulation effects, (c) higher oil temperatures, (d) newly configured aerodynamic characteristics and weight reduction, and (e) other transient and steady-state performance factors, (particularly at low road load power requirement levels where parasitic losses have a much greater effect on fuel economy). Using the adiabatic engine configuration, vehicle data indicated that tne above factors produced a fuel economy benefit in excess of 20% when a domestic road cycle of transient load and speed points was followed. This adiabatic engine version truck has accumulated in excess of

12,000 miles to date without failure. Follow-on evaluation of this truck from both performance and reliability standpoints is currently being funded by the Department of the Army (under TACOM's technical direction) in order to better understand the effect of low heat rejection engines within a vehicular installation. As results of this further evaluation become available, further publications will follow.

Activities at TACOM also are deeply involved within higher horsepower combat vehicle applications where again the benefits of installed fuel economy appear to be significantly greater than predicted as a result of "bare" dynamometer data alone. Work has been on-going on a 600 hp low heat rejection, "adiabatic type" engine. The engine was developed as a spin-off of adiabatic technology for possible near-term combat military vehicle application. The program used a technical approach which addressed the following elements: (1) low heat rejection techniques, (2) strategic oil-cooling, (3) high-temperature lubricants (over 700F top ring reversal), (4) high-temperature materials (including ceramic coatings and mono-lithics), and (5) advanced component integration. The engine was designed to be a "drop-in" to an existing infantry fighting vehicle. Performance improvements for this engine versus the production engine are: (1) horsepower density (20% im provefl'ient), (2) brake specif.io heat re9ection (49% improvement), (3) peak torque (20% improvement), and (4) fuel economy "bare engine" <13% improvement). Aga in, an installed vehicle fuel economy for this engine 1« projected to be significantly greater thaft the bare engine; howivet, evaluation of this factor has not as yet been quantified.

Work continues within the low heat rejection "adiabatic" engine area with respect to future main battle tanks (i.e., approximately 1500 hp). Propulsion systems for future tanks utilize advanced adiabatic engine technology as one of their critical building blocks. As work with these efforts progresses, available results will be reported.

A significant TAC0H program within Lb a low heat rejection area includes near-term product improvement of the engine within our current M109 Howitzer. The program is representative of TACOM adiabatic engine strategy to spin-off technology when available, at the earliest opportunity. The program utilizes (1) cast-in-phase insulative exhaust shields, (2) better engine efficiency, and (3) reduced parasitic losses to produce significant benefits. These demonstrated benefits include: (1) 19% heat rejection reduction, (2) 7-9% (depending on load) fuel economy improvement, and (3) improved driveability (i.e., 18% torque rise). With successful durability demonstrati6n, these engine improvements will be incorporated within the M109 Howitzer fleet in the very near future.

In addition to the programs discussed, a number of TACOM advanced technology base efforts are directly related to the low heat rejection "adiabatic" engine area in topics such as: (1) advanced high-temperature tribology (lubrication, wear, and friction), (2) adiabatic heat transfer (both steady-state and transient), and (3) design with brittle materials (including brittle (ceramic) material finite element analysis). The details of these programs and others will be the subject of future publications.

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