History | Air & Space Magazine
Why General Electric put an airplane engine on a truck and drove it to the top of Pikes Peak.
Moss was hardly deskbound, posing with the pilot who held the Army's altitude record, J.A. Macready (left).
(NASM)
Donald Sherman
Pikes Peak, the second highest mountain in Colorado, reaches 14,109 feet above sea level. In 1916, race cars began to compete over a road built to the top of the mountain. Each car ran against the clock, and the one that took the least time to reach the top was the winner. The Pikes Peak International Hill Climb is still run every year on the Fourth of July, its basic format unchanged, and the climb is still a severe test for man and machine: The lack of oxygen induces altitude sickness in humans, and engines begin to wheeze as they lose power at high altitude.
In 1918, Sanford Moss, a General Electric engineer on loan to the U.S. Army Air Service and a man with a keen interest in engines, believed he had solved the problem of engine power loss at altitude. In order to demonstrate that his solution would work, he too would find himself climbing Pikes Peak, not to win a race but to perform engine research in the thin air at the summit.
At the time, Moss' immediate problem was that his solution worked too well. He had built a turbo-supercharger, a device that draws energy from an engine's exhaust gases to drive a compressor that pumps an extra charge of air to the engine's intake - supercharging the cylinders. Moss' device could easily generate the requisite air pressure in the intake manifold of a Liberty test engine, but in U.S. Army tests it caused the fuel-air mixture to ignite prematurely, thereby triggering destructive detonation - a death rattle that could burn or break engine components in seconds. A report filed by two engineers at the Army's labs at McCook Field in Dayton, Ohio, neatly summed up the problem: "When using the supercharger, 470 horsepower [versus a standard Liberty's 420 horsepower] was developed at 1700 rpm. It was, however, difficult to make many tests with the supercharger operating. Even when only subjecting the engine to a small amount of supercharge at this low altitude, the spark plugs failed and numerous other difficulties developed."
Moss, a slight, owlish man with a gray beard and professorial aspect, knew that the only logical way to proceed was to test his turbocharger at altitude. Instead of sending unproven equipment heavenward in the hands of a test pilot, Moss suggested testing the turbocharger on a mountaintop. A crew would mount the test engine on a truck, and then it would simply be a matter of finding a mountain that had a road all the way to the summit.
From the moment when the first airplane rose from the surface of the earth and headed skyward, aircraft engine designers have faced a dilemma: Power fades with a gain in altitude, and eventually an airplane reaches its maximum ceiling - a point at which it can no longer climb. An engine capable of 500 horsepower at sea level puts out only 420 horsepower at 5,000 feet, then 355 horsepower at 10,000 feet. By 20,000 feet, loss of air density has sapped half of the sea-level output. Early aeronautical engineers expressed this dilemma mathematically; devising practical solutions in an era when airplanes were mainly used as attractions at county fairs was not a priority.
But war changes everything, and as soon as the shooting began in World War I, military strategists made for the high ground. In aviation terms, that meant airplanes that could fly higher and faster than one's adversary.
Pretty soon, engineers could read about theoretical solutions in the technical literature. Only nine years after Nicolaus Otto created the first four-stroke-cycle engine in 1876, Gottlieb Daimler, another German inventor, conceived the means to improve it. His patent for supercharging states: "With this engine greater amounts of combustible mixture are delivered into the cylinder and at the same time the exhaust gases are more effectively removed. This is done by means of a pump alongside the cylinder."
In the early 1900s, the supercharger tree sprouted several branches in Europe. Frenchman Louis Renault developed a centrifugal compressor, and in Switzerland, Alfred Buchi proposed using the engine's exhaust gases to spin a turbine wheel and drive a centrifugal compressor plumbed to deliver air to the engine's intake manifold. This bootstrap approach, called turbo-supercharging or simply turbocharging, was tested by Buchi's firm, then shelved when success proved elusive.
But the pursuit of efficiency prompted engineers to give turbochargers another chance. The typical piston engine converts only one-third of the energy from its fuel to useful work. Another third is squandered to friction and cooling-system losses, and the remainder is spit out the exhaust pipe as waste heat. A turbocharger could recover some of that exhaust energy.
In the United States, Sanford Moss, a 22-year-old mechanical engineering student at the University of California at Berkeley, had an inspiration during a class on thermodynamics and hydrodynamics: Why not combine the best aspects of internal combustion and steam turbines? A British scientist had patented the same brainstorm a century earlier, but that didn't diminish Moss' enthusiasm for spinning heat into horsepower. In his master's thesis, Moss proposed replacing a locomotive's thumping piston engine with a smoothly humming gas turbine. (He simply picked the wrong vehicle; today many warships are powered by turbines.)
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