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.)
In 1901, he began studies and research at Cornell Uni-
versity, and after working for a year, persuaded a spherical combustion chamber to deliver a continuous source of flaming hot energy. Gas-
es from the chamber were directed against a five-inch-diameter turbine wheel; Moss wrote that it was probably the first time a turbine had ever been driven by combustion gases in the United States—or perhaps anywhere.
Moss’ 1903 doctoral thesis, “The Gas Turbine, an Internal Combustion Prime Mover,” caught the eye of General Electric executives. Formed in 1892 as the amalgamation of the leading AC and DC power companies, GE was responding to mounting consumer demand by constructing ever larger electricity generating and distribution systems. Moss came to GE at the very time when reciprocating engines for power generation were being replaced by more efficient steam turbines.
He worked for four years at GE on gas-turbine reliability, but he couldn’t crack the efficiency nut. His best turbine consumed four gallons of kerosene per hour for every horsepower produced, versus only one gallon per horsepower-hour for the day’s best reciprocating engines. The materials available in 1907 could not withstand the high temperatures needed to achieve better efficiency in a turbine, so GE shelved its research. But back in Europe, the war was heating things up nicely.
The advent of the Great War focused intense interest on packing more air into engines to gain altitude performance. In Germany, Mercedes, Maybach, and BMW took a brute-force approach, building bigger engines that also squeezed the air-fuel mixture to a greater degree during the piston’s compression stroke. Power had to be limited at sea level or the engines would fail structurally, so BMW’s 19.1-liter engine, with a compression ratio of 6.4 to one (4 or 5 to one was more typical), had three throttle levers. If all three were opened at sea level, the engine would destroy itself, so the throttles were opened progressively as the airplane climbed. The first-stage throttle delivered 185 horsepower for take-off at a modest 1,400 rpm. The other two throttles were opened in succession above 6,500 feet, permitting higher engine speeds without the usual loss of power because the engine’s high-compression design squeezed more energy out of the thinner air. Rumpler C.IVs equipped with 260-horsepower “over-dimensioned” Mercedes engines and oxygen for their crews flew reconnaissance missions above 20,000 feet, well beyond anyone’s reach. The Germans were slower than the British and French at sea level, but at altitude they ruled.
The British government’s Royal Aircraft Establishment experimented with reciprocating-piston air compressors and Roots blowers, which have intermeshing vanes, but the trials bore no fruit. So the RAE concentrated instead on high-speed centrifugal blowers. A BE2C biplane powered by a 537-cubic-inch air-cooled eight-cylinder engine took 35 minutes to climb to 8,500 feet. With an experimental gear-driven supercharger, it climbed 3,000 feet higher. But engine maestro Sam Heron at the Royal Aircraft Factory had doubts, which he expressed in muted terms: “The observer sat forward with his feet under the fuel tank and over the supercharger’s gear drive. The gears were quite inadequate and the pinion failed in flight, producing showers of sparks and a feeling of distinct concern.”
August Rateau, an enterprising inventor, engineer, and industrialist in France, dusted off a 1909 idea of Alfred Buchi’s for a turbocharger and fitted it to SPAD, Breguet, and ALD types with some success. One turbocharged Renault engine improved the rate of climb at 14,000 feet by 15 percent and boosted top speed from 104 to 120 mph. The British evaluated Rateau’s equipment, noting a 23 percent improvement in the rate of climb, but suspended research after a catastrophic turbine failure at 13,500 feet.
Rateau’s turbocharger caught the eye of the U.S. Army Air Service’s technical experts stationed in Paris, and soon investigations were under way at McCook Field in Dayton, Ohio, with an experimental unit running on a Liberty engine. Excessive heat caused persistent failures. A parallel effort initiated in November 1917 by William Durand, chairman of the National Advisory Committee for Aeronautics, was more fruitful. Earlier, Durand had been at Cornell, where he first learned of Moss and his research. He was also well aware of GE’s prominence in steam turbines and centrifugal compressors. Durand promptly petitioned GE’s president for Moss’ assistance.
Engineering drawings of the Rateau device were available, but Moss’ GE team had its own ideas. By June 1918, GE’s Lynn Steam Turbine Department in East Lynn, Massachusetts, had shipped a prototype to the War Department’s Airplane Engineering Division at McCook Field for adaptation to a Liberty 12 aircraft engine. It was a teenage marriage: an untried turbocharger wed to an engine that one year earlier had been just a glimmer in the War Production Board’s eye.
Moss’ first turbo consisted of two 10-inch-diameter wheels mounted on a steel shaft turning at 20,000 rpm and supported by bearings at each end that were lubricated by engine oil. One wheel had surfaces that absorbed the blast of the engine’s exhaust gases and thereby spun the steel shaft. At the other end was another wheel that drew air in from the atmosphere and compressed it so it could be routed to the engine intake. Engine coolant circulated through a jacket surrounding the bearing on the hot side, where the engine exhaust gas flowed, in order to carry away the heat. Large welded-steel manifolds gathered the exhaust and delivered it to a nozzle box that directed the exhaust at the turbine wheel.
Moss mounted the turbo at the front of the engine so the propeller’s slipstream would cool the compressor housing, the nozzle box, and the exhaust manifolds. Valves called waste gates, located at the rear of the exhaust manifolds, could be opened to modulate the flow of exhaust gas to the turbine and thereby control the amount of boost generated.
But in the low-altitude flatlands of Ohio, McCook Field sat at the very bottom of the atmosphere. To test the turbocharger at McCook would mean compressing air that was already thick. The engine wasn’t built to tolerate loads imposed by air at high pressure, and every test risked destruction. Then Moss got his idea about climbing a mountain with the test engine mounted on a truck. And he found a mountain with a road all the way to the top—Pikes Peak, in Colorado.
Together with McCook Field engineer C. P. Grimes and a small crew of technicians, Moss assembled a mobile laboratory: the turbocharged Liberty engine with a huge propeller to absorb the power, a dynamometer to gauge the torque produced, and various support systems. The whole thing was mounted on a Packard motor truck and looked like a carnival contraption.
After a month of preparation and a week-long, 1,300-mile train ride, the mobile lab arrived in Colorado Springs. The crew fired up the Packard, which chugged its way 28 miles up the Pikes Peak Auto Highway to a rocky flat 100 yards in diameter at the top. On September 10, 1918, Moss and his team finally got to work.
By the time they were done four weeks later, they had made 25 test runs with the turbocharged Liberty. And they had surprisingly few problems: clogged carburetor jets, leaks in exhaust manifold joints, a leak in the compressor housing attributed to casting flaws, some broken turbocharger thrust washers, and some failed stay bolts that were supposed to keep the exhaust manifolds from warping in the heat. The crew performed minor repairs in a small shack at the summit; for major jobs they had to trundle the whole works back down the mountain to Colorado Springs. Before they left the mobile lab every night, they covered it in a canvas overcoat. On many mornings the crew arrived to find their equipment frozen and snowbound. In spite of the wintry conditions, Moss was stoic: “There were many pleasant days when the testing work could be carried on with facility,” he noted dryly.
With the supercharger in operation, the nozzle boxes glowing bright red, and the Liberty on the ragged edge of detonation, Moss measured a maximum horsepower of 377—better than the 354 they had achieved at McCook. On the mountaintop and with the turbocharger shut down, the best they could crank out was only 230 horsepower. In his notes from the Pikes Peak test series, Moss conceded that the 377 figure could be held for only 30 seconds; after that the spark plugs failed. The turbocharged Liberty also withstood a four-hour endurance run at 313 horsepower. (Differences between the power measured during these tests and the 400-plus horsepower at which Liberty engines were normally rated could be attributed to propeller losses.)
Moss left no record in his notes of any celebrations the team may have held after the trip down the mountain, but all who participated certainly deserved one. General George Kenney, later an air force commander in the Pacific, boasted in 1942 while touting two frontline fighters with turbochargers, “At high altitudes the Lockheed P-38 and the Republic P-47 can lick anything. There are only two honest 400-mile-per-hour planes in the world, and we’ve got both of them.” Moss and his turbocharger had begun to change aviation history. And GE’s expertise with gas turbines left little question as to which U.S. firm should be selected to develop the Whittle turbojet.
On the 50th anniversary of powered flight in 1953, U.S. Air Force Lieutenant General James Doolittle commemorated Moss, who had died in 1947, with a monument atop Pikes Peak. Doolittle cited Moss as an aviation giant, the gas turbine as his brainchild, and the advent of the turbocharger as the birth of true high-altitude flight.
And a final footnote: The current holder of the overall record for the Pikes Peak International Hill Cimb is Rod Millen, who set it in 1994 in an unlimited-class race car that made the run in 10 minutes, 4.06 seconds. Millen’s engine was turbocharged.