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.