Boosting Your Knowledge of Turbocharging

F-DIESEL TURBO & FUEL INJECTION SYSTEM 26/6/2014 9:55:14 pm

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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 38,704 feet was achieved three years later by Lt. J.A. Macready.

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 groundboosted?

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 while suppressing detonation.

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 fullthrottle, sea level power output as they increasingly gai

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