Reprinted with permission from Aircraft Maintenance Technology, July 1999 BoostingBoosting YourYour KnowledgeKnowledge ofof TurbochargingTurbocharging (Part 1 of a 2 part Series) By Randy Knuteson short 15 years after Orville fully boosted this 350 hp Liberty engine to a strength of blowers being tested during and Wilbur made their his- remarkable 356 hp (a normally aspirated engine WWII. The B-17 and B-29 bombers along toric flight at Kitty Hawk, would only develop about 62 percent power at with the P-38 and P-51 fighters were all fit- General Electric entered the this altitude). ted with turbochargers and controls. Aannals of aviation history. In 1918, GE strapped An astounding altitude record of 38,704 Turbocharging had brought a whirlwind of an exhaust-driven turbocharger to a Liberty feet was achieved three years later by Lt. J.A. change to the ever-broadening horizons of engine and carted it to the top of Pike’s Peak, Macready. flight. CO — elevation 14,000 feet. There, in the crys- This new technology began immediately Much of the early developments in recip talline air of the majestic Rockies, they success- experiencing a rapid evolution with the full turbocharging came as a result of demands Aircraft Maintenance Technology • OCTOBER 1999 2 Recip Technology from the commercial industrial diesel engine market. It wasn’t until the mid-1950s that this Percentage of HP Available At Altitude technology was seriously applied to general avi- 100% ation aircraft engines. It all started with the pro- totype testing of an AiResearch turbocharger for 90% Turbocharge the Model 47 Bell helicopter equipped with the Franklin 6VS-335 engine. Their objective was 80% not to increase power, but rather to maintain 70% sea level horsepower at altitude. They succeed- ed. In the process, a new altitude record for 60% Non-Turbocharge helicopters of 29,000 feet was achieved. 50% Shortly afterward, the Franklin Engine Company entered receivership and in 1961, 40% Bell ended up with a production helicopter 30% powered by a Lycoming TVO-435. Coinciding with these developments were Continental’s 20% efforts to develop their TSIO-470-B (Cessna 320) and GTSIO-520 (Cessna 411). 10% Concurrently, efforts were also being made 25 50 75 100 150 200 250 300 350 by TRW and later Rajay to provide 65 STCs to retrofit engines and airframes for approximate- Altitude (X100) ly two dozen aircraft. Early OEM installations * Percentage based on full throttle, maximum RPM, standard day temp. of these systems included the factory installed Rajay in Piper’s Commanche and Twin Commanche. Other original equipment instal- This graph illustrates the dramatic disparity between turbocharged lations included the Piper Seneca, Turbo and non-turbocharged aircraft engines. Arrow, Enstrom Helicopter, Mooney 231, and Aerostars. maintained at altitude without the customary to bring these temperatures down and recap- diminishment of power. An engine that sus- ture some of this power loss while suppressing Turbo-normalized or ground- tains but does not exceed 29.5 inches of man- detonation. boosted? ifold pressure at altitude is said to be normal- An engine relying on manifold pressures Distilled to the most basic of definitions, a ized. “Critical altitude” is that point above greater than ambient on a standard day is con- turbocharger is simply an air pump powered by which the turbocharger can no longer maintain sidered a “boosted” system. These engines the unused heat energy normally wasted out the maximum rated manifold pressure. However, require manifold pressures ranging from 31.0 to exhaust. This “air pump” (or more accurately, just because an engine maintains 29.5 inches of 45.0 inches HgA. Installations incorporating compressor), is capable of supplying the engine MAP and redline rpm, does not necessarily intercoolers demand an additional 2 to 3 inch- intake manifold with greater than atmospheric mean it is developing sea level power. es HgA to compensate for the pressure loss in air pressures. A collateral benefit is derived as Depending on the application, compressor dis- air flow through the intercooler. Common the turbo also provides air for the cabin pres- charge air at critical altitude may be as hot as installations include TCM’s TSIO-520s in the surization of certain aircraft. 250 to 300 degrees Fahrenheit. An increase in Cessna 210 and Lycoming’s TIO 540 and 541 Some confusion persists as to the difference induction air temperatures of 6 to 10 degrees installed in Navajos, Turbo Aztecs, and Dukes, between an airplane that is “ground-boosted” Fahrenheit decreases horsepower by roughly 1 to name a few. These engines require reduced as opposed to one that is “normalized.” Simply percent. So, an airplane with a critical altitude compression ratios (to provide wider detona- put, turbocharging serves one of two purpos- of 25,000 feet may be producing only 80 per- tion margins) since they produce more than es: either it directly increases (boosts) the cent power even though sea level manifold normal sea level manifold pressures while in power output of the engine, or it assures that pressure is indicated at that altitude. An inter- the take-off and climb configuration. sea level horsepower performance is main- cooler serves the purpose of a heat exchanger Basically, the ultimate goal of turbocharging tained (turbo-normalized) to higher altitudes, is to gain more power or to increase the thereby increasing the plane’s potential service efficiency of the engine without enlarg- ceiling. “Distilled to the most basic of definitions, ing the powerplant. A “normalized” turbo installation like the a turbocharger is simply an air pump Rajay system in no way increases the normal Design differences engine RPMs, loads, or BMEP limits already powered by the unused heat energy Turbochargers manufactured by Rajay established as safe for the engine. Instead, it and Garrett are very similar. Perhaps the merely assures that sea level performance is normally wasted out the exhaust.” most striking difference is their compara- Aircraft Maintenance Technology • OCTOBER 1999 3 Recip Technology tive size. Rajay units weigh 12 pounds while the Garrett turbos weigh from 15 to 43 pounds. These radical size variances are associated with engine size and applications. For instance, the TAO4 Garrett and the Rajay Turbos are typical- ly installed in 180-230 hp engines. Compressor diameter in these models vary from 2.755 inch- es to 3.0 inches. In the Aerostar, twin turbos of this size are used. Both TCM’s earlier model 310P in the Malibu, and the Lycoming powered Surge Line Mirage, also use this dual turbo arrangement. The TEO6 turbocharger is used to boost the per- formance of the 520s and 540s in the 275 to 350 Area of Peak horsepower category. While the 340s, 414s, and Efficiency Navajos rely on a larger compressor wheel of the TH08 turbocharger to provide the addition- al bleed air for cabin pressurization. Internally, the bearing design of the Rajay is that of a “semi-floating” journal bearing. While the Garrett design incorporates dual RPMS bearings that turn at half of the turbine wheel speed. Instead of a ductile cast-iron bearing housing, Rajay’s housing is manufactured from aluminum. Both turbo lines depend on the Turbocharger compressor performance map for Orenda OE600A Rajay (formerly Garrett AiResearch) valves and engine — 600 hp at sea level take-off. This compressor can provide pressurized controllers to monitor turbo discharge and to airflow for 500 engine horsepower at altitudes in excess of 25,000 feet. The left side determine manifold pressure. There are of the map is referred to as the “Surge Area” where pressure and flow is unstable. exceptions; however, most noticeably TCM’s The center “Island” represents the area of peak efficiency. As a rule, the broader “fixed” wastegate in their Seneca and Turbo the range of the compressor, the lower the peak efficiency. Arrow models, and the Rajay manual waste- gates and controllers. engine is directly proportionate to the mass of gases are normally wasted, little power is air being pumped into the engine. On a non- robbed from the engine to drive the turbine. A Increased efficiency in a rarified turbocharged engine, an increase in airflow is centrifugal compressor is connected to the tur- atmosphere achieved by changing the throttle angle to bine wheel by a common shaft, enabling it to At sea level, the atmosphere in which we increase MAP, or prop-pitch to increase RPM. rotate at the same speed as the turbine. The live and breathe is continually under a pres- The atmospheric limitations of density alti- impeller draws in filtered air, compresses and sure of about 29.92 inches of mercury (Hg). tudes coupled with the mechanical limitations delivers it to the cylinders where each pound of At 1,000 feet, this “free air” drops in pressure of the engine and propeller pose as restric- fuel is mixed with about 14.8 pounds of air to about 28.86 inches Hg. Air becomes pro- tions to engine speeds. Some means of force- (stoichiometric or peak EGT). In reality, how- gressively less dense at all altitudes above feeding the engine additional air is required to ever, proper engine management dictates that sea level. Because of this, all naturally aspi- overcome these limitations. most engines be operated 100 to 125 degrees rated engines experience a reduction of full- rich of peak EGT, thereby altering this ratio. throttle, sea level power output as they Re-claiming wasted energy Wheel configuration and size, housing size, and increasingly gain altitude. On a “standard As much as one-half the total heat energy shaft speed all determine the pressure and vol- day,” atmospheric pressure at 10,000 feet from the engine is lost through the exhaust as ume of air delivered to the engine.