Boosting Your Knowledge of Turbocharging
(Part 1 of a 2 part Series)
By Randy Knuteson
A short 15 years after Orville and Wilbur made their historic flight at Kitty Hawk, General Electric entered the annals of aviation history. In 1918, GE strapped an exhaust-driven turbocharger to a Liberty engine and carted it to the top of Pike's Peak, CO — elevation 14,000 feet. There, in the crystalline air of the majestic Rockies, they successfully boosted this 350 hp Liberty engine to a remarkable 356 hp (a normally aspirated engine would only develop about 62 percent power at this altitude).
An astounding altitude record of 39,700 feet was achieved three years later.
This new technology began immediately experiencing a rapid evolution with the full strength of blowers being tested during WWII. The B-17 and B-29 bombers along with the P-38 and P-51 fighters were all fitted with turbochargers and controls. Turbocharging had brought a whirlwind of change to the ever-broadening horizons of flight.
Much of the early developments in recip turbocharging came as a result of demands from the commercial industrial diesel engine market. It wasn't until the mid-1950s that this technology was seriously applied to general aviation aircraft engines. It all started with the prototype testing of an AiResearch turbocharger for the Model 47 Bell helicopter equipped with the Franklin 6VS-335 engine. Their objective was not to increase power, but rather to maintain sea level horsepower at altitude. They succeeded. In the process, a new altitude record for helicopters of 29,000 feet was achieved.
Shortly afterward, the Franklin Engine Company entered receivership and in 1961, Bell ended up with a production helicopter powered by a Lycoming TVO-435. Coinciding with these developments were Continental's efforts to develop their TSIO-470-B (Cessna 320) and GTSIO-520 (Cessna 411).
Concurrently, efforts were also being made by TRW and later Rajay to provide 65 STCs to retrofit engines and airframes for approximately two dozen aircraft. Early OEM installations of these systems included the factory installed Rajay in Piper's Commanche and Twin Commanche. Other original equipment installations included the Piper Seneca, Turbo Arrow, Enstrom Helicopter, Mooney 231, and Aerostars.
Turbo-normalized or ground-boosted?
Distilled to the most basic of definitions, a turbocharger is simply an air pump powered by the unused heat energy normally wasted out the exhaust. This "air pump" (or more accurately, compressor), is capable of supplying the engine intake manifold with greater than atmospheric air pressures. A collateral benefit is derived as the turbo also provides air for the cabin pressurization of certain aircraft.
Some confusion persists as to the difference between an airplane that is "ground-boosted" as opposed to one that is "normalized." Simply put, turbocharging serves one of two purposes: either it directly increases (boosts) the power output of the engine, or it assures that sea level horsepower performance is maintained (turbo-normalized) to higher altitudes, thereby increasing the plane's potential service ceiling.
A "normalized" turbo installation like the Rajay system in no way increases the normal engine RPMs, loads, or BMEP limits already established as safe for the engine. Instead, it merely assures that sea level performance is maintained at altitude without the customary diminishment of power. An engine that sustains but does not exceed 29.5 inches of manifold pressure at altitude is said to be normalized. "Critical altitude" is that point above which the turbocharger can no longer maintain maximum rated manifold pressure. However, just because an engine maintains 29.5 inches of MAP and redline rpm, does not necessarily mean it is developing sea level power. Depending on the application, compressor discharge air at critical altitude may be as hot as 250 to 300 degrees Fahrenheit. An increase in induction air temperatures of 6 to 10 degrees Fahrenheit decreases horsepower by roughly 1 percent. So, an airplane with a critical altitude of 25,000 feet may be producing only 80 percent power even though sea level manifold pressure is indicated at that altitude. An intercooler serves the purpose of a heat exchanger to bring these temperatures down and recapture some of this power loss.
They are both expensive, and the death of one can lead to the death of the other.