Higher Altitudes, Lower Power

Among the comments on my recent piece about Octane Number (ON), Dante Fiero writes to mention the effect of density altitude. Let’s start with a formal definition: Density altitude is that altitude above sea level which, at standard atmospheric conditions, would give the observed air density. Aircraft operators must take this into account because as density altitude rises, oxygen becomes less plentiful (atmospheric pressure falls with altitude), and so unsupercharged engines produce proportionally less power. (There’s also the matter of aerodynamic lift—more on that later.)

Looked at another way, the reduced air density at altitude is like involuntarily operating an engine on part throttle. As you reduce the throttle opening (given constant fuel-air ratio), you also reduce an engine’s tendency to detonate. That being so, gas stations at higher altitudes can safely offer lower ON fuel, and in such places you’ll often see 85 ON pump gas being sold instead of 87.

Rider Richard Schlachter and I were definitely operating on part throttle one night as we crested the Continental Divide at some 12,000 feet. Wanting to stretch our legs, we parked the van and got out to run up to a nearby monument. We weren’t even close before we both had to stop, huffing and puffing; each breath was bringing us about half the oxygen normally available at sea level.

During World War I, pilots noticed the loss of power with altitude (their engines were unsupercharged). As an example, at 13,780 feet above sea level, atmospheric pressure is half of what it is at sea level—no wonder Schlachter and I were gasping. Engineers realized there was an easy way to boost engine power at high altitude: with less air pressure available there, they could safely raise the compression ratio far above what would cause knock at sea level. The various proposed variable-compression-ratio auto engines all exploit this same effect. At part throttle they switch to high compression because it extracts more energy from each pound of fuel burned. Yet as the operator opens the throttle to accelerate, the system reduces the compression ratio to prevent the detonation. The overall effect is reduced fuel consumption.

But the high-compression aircraft engine faced a problem: How could it take off in denser sea-level air without destroying itself with knock? The easy answer was to limit throttle opening, thereby keeping engine manifold pressure low. In practice, this meant opening the throttle by degrees during climb. On June 17, 1919, in an aircraft powered by a BMW IV engine (designed by the same Max Friz who four years later would give us BMW’s first motorcycle powerplant), Franz Zeno Diemer climbed to just over 32,000 feet using this scheme of throttle scheduling.

During the 1920s and ‘30s, supercharged aircraft engines would overcome the loss of air density at altitude; builders of racing cars and bikes were working along the same lines. Supercharging pumps a greater volume of air through an engine than can be simply drawn into a cylinder by its pistons. If a pump can force twice as much fuel-air mixture into an engine as it can breathe on its own, you can potentially double its horsepower at any given rpm. Diemer’s engine at 32,000 feet was inhaling only 23 percent as much air as at sea level, but was still making enough power to fly. His climb to that altitude took an hour and a half.

Air density also affects the lift that an aircraft’s wings can generate, whether those wings are fixed or rotary. This is why helicopters generally can’t fly as high as other aircraft. Air density also limits a fixed-wing aircraft’s options when taking off from airports that are “hot and high” (the two prime sources of lost air density), such as Albuquerque, New Mexico.

When in mid-1972 I drove to Boston’s Logan Airport to buy a barrel of 115-145 aviation gas (the purple stuff), the nervous young man pumping the fuel (I suspect he pocketed the money I handed over) kept referring to it as “helicopter gas.” I realized that was because the only aircraft still needing this fuel were helicopters with piston engines; jets had replaced all the DC-7 and Constellation piston airliners. But a long-serving Sikorsky S-58 with an R-1820 in its bulbous nose still needed all the knock resistance it could get to take off with normal margins. That purple gas was a soothing balm for my hot-running air-cooled Kawasaki H2R.

There is even a third cause of reduced oxygen availability. In late 1944-45, extreme humidity caused B-29 bombers operating from the Marianas Islands 1,500 miles south of Japan to lose roughly 100 takeoff horsepower per engine. West Coast riders competing at the old AMA roadrace national at Loudon, New Hampshire, were less experienced than locals when it came to dealing with moisture-saturated air. Near the end of motorcycling’s carburetor era, race teams carried gauges reporting air density and relative humidity. I would often see them, placed carefully in the shade under team trucks.

On today’s production bikes and racebikes, digital fuel systems continuously optimize fuel mixture just as race mechanics used to do by changing jets and metering-needle heights.

Who knows? Perhaps soon in our drive to make life safer by replacing human responsibility with machines we will choose to automate our personal lives. “Alexa, which one should I ask to marry me—Susan or Sarah?”