If the key to keeping the internal combustion engine viable lies with increasing its efficiency, then engineers have a long way to go. Garden-variety internal combustion engines unlock maybe 20 to 30 percent of the energy available in their fuels. The most advanced gasoline engines these days — the ones with bleeding-edge technology and decades of research and development behind them — manage a thermal efficiency of about 40 percent.
More than 30 years ago, however, an experimental adiabatic engine reportedly set thermal efficiency records its first time out with some relatively simple modifications in a relatively ugly 5-ton Army truck.
Anybody who’s heard the term adiabatic probably immediately thinks of Smokey Yunick and his “hot-vapor” four-cylinder Pontiac Fiero. Smokey reportedly recorded some impressive numbers — 50-plus miles per gallon, 250 horsepower, 6-second 0-to-60 times — by, more or less, ensuring that as much combustion heat as possible remained in the engine. He wrapped the 2.5-liter four-cylinder with insulation, repurposed a turbocharger as a fuel mixture “homogenizer,” and installed a slightly revised camshaft to create his “expander cycle” engine. Car and Driver didn’t quite buy it, but Hot Rod went all in on Yunick’s claims.
Yunick, though, was hardly the first to tinker with adiabatic (“a process or condition in which heat does not enter or leave the system”) modifications to internal combustion engines. Plenty of engineers — dismayed at the internal combustion engine’s low thermal efficiency numbers and the gobs of waste heat that the engines produce — have offered up fuel delivery systems that rely on fuel vapors rather than a mist of fuel. The Pogue carburetor even claimed to do as much, but the first we see that anybody tried a complete adiabatic engine were a series of Shell high-mileage experimental cars from the late Sixties into the early Seventies.
Then in 1975, the U.S. Army’s Tank Automotive Command and Cummins began a joint development program focused on adiabatic diesel engines, which they termed Low-Heat Rejection engines. Fred Crismon, in his book U.S. Military Wheeled Vehicles, implied that the main goal of the program was to reduce vehicle failures by eliminating the cooling system.
The Army has found that historically, 60 percent of the failures which occur in their vehicles relate to problems with the cooling system. Theoretically, at least, if the cooling requirement could be eliminated, there would be 60 percent fewer vehicle failures.”
While that may have proven a side benefit (along with the 338 pounds the engine shed in cooling system components), the actual aim of the program was to increase fuel efficiency. Or, as Cummins and TACOM engineers wrote,
to take a giant step toward improving the energy and material conservation efforts of the future vehicular power plants. These efforts were not to compromise with engine emissions characteristics.
Thus, Cummins modified its NHC 250 855-cu.in. six-cylinder engine with ceramic-metallic components (head, cylinder liners, pistons, valves, and exhaust ports — anything that encountered the combustion chamber) to handle temperatures of up to 2,000-degrees Fahrenheit. Heavy insulation of the intake path helped direct that heat to the fuel/air mixture.
In 1981, TACOM then placed the engine in an M813 5-ton six-wheeled truck with a special hood touting the adiabatic engine underneath and put about 10,000 miles on the truck. Over that time period, TACOM and Cummins noted a 38 percent increase in fuel economy over a stock M813. According to a TACOM/Cummins SAE paper, that testing program “has repeatedly demonstrated the Adiabatic Engine to be the most fuel efficient engine in the world” with fuel consumption numbers that indicated 48-percent thermal efficiency. Crismon didn’t offer any hard numbers on thermal efficiency, but noted that it improved on the NHC 250’s efficiency by 100 percent.
The adiabatic M813’s success led to a number of other TACOM experimental engines, including a turbocharged adiabatic V-8 medium-duty truck engine reportedly good for 600 to 750 horsepower, larger adiabatic tank engines reportedly good for 1,200 to 1,500 horsepower, and a fully ceramicized Minimum Friction Engine designed to use no lubricants at all and still reduce by half the amount of friction in the engine.
Mentions of the TACOM adiabatic engines seem to peter out by the mid- to late 1980s, however, possibly because internal combustion engine efficiency efforts swung more toward direct injection and more advanced electronic fuel injection, neither of which seemed to work well with adiabatic systems.
As far as the adiabatic M813, it appeared to remain in the Army’s possession until a circa 2009 surplus sale, after which it went to an Illinois-based collector.