Boosting Your Knowledge of Turbocharging (Part 1 of a 2 part series)

Boosting Your Knowledge of Turbocharging (Part 1 of a 2 part Series) By Randy Knuteson July 1999 A short 15 years after Orville and Wilbur made their historic flight at Kitty Hawk, General Electric entered the annals of aviation...


An engine relying on manifold pressures greater than ambient on a standard day is considered a "boosted" system. These engines require manifold pressures ranging from 31.0 to 45.0 inches HgA. Installations incorporating intercoolers demand an additional 2 to 3 inches HgA to compensate for the pressure loss in air flow through the intercooler. Common installations include TCM's TSIO-520s in the Cessna 210 and Lycoming's TIO 540 and 541 installed in Navajos, Turbo Aztecs, and Dukes, to name a few. These engines require reduced compression ratios (to provide wider detonation margins) since they produce more than normal sea level manifold pressures while in the take-off and climb configuration.

Basically, the ultimate goal of turbocharging is to gain more power or to increase the efficiency of the engine without enlarging the powerplant.

Design differences
Turbochargers manufactured by Rajay and Garrett are very similar. Perhaps the most striking difference is their comparative 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 typically installed in 180-230 hp engines. Compressor diameter in these models vary from 2.755 inches 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 Mirage, also use this dual turbo arrangement. The TEO6 turbocharger is used to boost the performance of the 520s and 540s in the 275 to 350 horsepower category. While the 340s, 414s, and Navajos rely on a larger compressor wheel of the TH08 turbocharger to provide the additional 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 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 Rajay (formerly Garrett AiResearch) valves and controllers to monitor turbo discharge and to determine manifold pressure. There are exceptions; however, most noticeably TCM's "fixed" wastegate in their Seneca and Turbo Arrow models, and the Rajay manual wastegates and controllers.

Increased efficiency in a rarified atmosphere
At sea level, the atmosphere in which we live and breathe is continually under a pressure of about 29.92 inches of mercury (Hg). At 1,000 feet, this "free air" drops in pressure to about 28.86 inches Hg. Air becomes progressively less dense at all altitudes above sea level. Because of this, all naturally aspirated engines experience a reduction of full-throttle, sea level power output as they increasingly gain altitude. On a "standard day," atmospheric pressure at 10,000 feet hovers at only 20.5inches Hg. In these conditions, a naturally aspirated engine is unable to sustain full power performance due to the lack of air density at these higher altitudes. Without the aid of a turbocharger, a loss of 3 to 4 percent of power or approximately 1 inch of MAP per 1,000 foot gain in altitude seems to be the rule.

With the fuel to air ratio remaining constant, the amount of power developed by an engine is directly proportionate to the mass of air being pumped into the engine. On a non-turbocharged engine, an increase in airflow is achieved by changing the throttle angle to increase MAP, or prop-pitch to increase RPM. The atmospheric limitations of density altitudes coupled with the mechanical limitations of the engine and propeller pose as restrictions to engine speeds. Some means of force-feeding the engine additional air is required to overcome these limitations.

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Re-claiming wasted energy
As much as one-half the total heat energy from the engine is lost through the exhaust as a natural by-product of the combustion process. A portion of this heat energy is recovered by harnessing the hot expanding exhaust gases and re-routing them through the turbocharger. These gases enter a radial inflow turbine housing, striking the blades of the Inconel turbine wheel and exiting the turbine housing outlet. This flow of gas provides the necessary thrust that enables the turbine wheel to rotate at high speeds. Since these exhaust gases are normally wasted, little power is robbed from the engine to drive the turbine. A centrifugal compressor is connected to the turbine wheel by a common shaft, enabling it to rotate at the same speed as the turbine. The impeller draws in filtered air, compresses and delivers it to the cylinders where each pound of fuel is mixed with about 14.8 pounds of air (stoichiometric or peak EGT). In reality, however, proper engine management dictates that most engines be operated 100 to 125 degrees rich of peak EGT, thereby altering this ratio. Wheel configuration and size, housing size, and shaft speed all determine the pressure and volume of air delivered to the engine.

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