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.
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.
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.
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.
Reams of data compiled by the engine manufacturers outline their desired parameters for the turbochargers and control systems to be used on their reciprocating engines. Design engineers then analyze this information, basing performance predictions on expected temperature and pressure changes in the manifold, the induction and exhaust ducting, as well as the pressure drop across the throttle and through the intercooler. The placement and orientation of the air intake, the exhaust by-pass valve, and cabin air-bleed must be factored in. The parameters of BHP (brake horsepower), turbine inlet temps, bypass flow and compressor discharge temps are only a few of the elements plotted against altitude for selected manifold pressures.
Finally, flight-testing verifies or invalidates the assumptions of expected engine performance. Continued analysis is made, errors are corrected, assumptions are cross-checked, and the accuracy of the data is refined with a high probability of obtaining the desired aircraft performance.
The goal is to use little or none of the engine's horsepower to drive the turbocharger. Consequently, when proper engine installation and matching is done, mechanical loads imposed on the engine to power the turbo are minute. Most of the work required to drive the turbine is recovered from the exhaust gases. Otherwise, if the turbocharger was not in the system, this portion of the gas energy would be lost with discharged exhaust gases.
Maintenance of the turbocharger
The turbocharger itself is subjected to an extremely hostile environment. Turbine inlet temperatures reach a scorching 1,650 degrees. Some are hotter still. TCM's liquid-cooled Voyager is rated at 1,750 degrees continuous with the capability of 1,800 degrees for 30 seconds while establishing peak EGTs. Turbine speeds range from 0 to 120,000 rpm. The T36 in the Malibus and the Lancair 4P is capable of 125,000 at 1,650 F. That's a screaming 2,083 revolutions per second! Pulsing exhaust gases, engine vibration and temperature variations all add to this hellish mix.
The turbo assembly is made of the strongest of alloys available to withstand these rigorous conditions. However, these tortuous duty cycles can shorten the life of a turbo if proper maintenance is not performed. It is of the utmost importance to follow the maintenance and inspection criteria spelled out in the applicable service bulletins and airworthiness directives.
Oil contamination, oil supply problems, FOD damage, and abrupt temperature changes are the chief contributors to premature turbocharger failures. There can not be enough emphasis placed on the importance of keeping the oil clean. Remember that the oil used to lubricate and cool the turbocharger is the engine oil. At high rpms, dirty oil that results from combustion by-products and the carbon residue from coking can dramatically shorten the life of a turbo. Although engine manufacturers recommend oil changes at 50-hour intervals, many engine overhaul shops suggest a more conservative interval of 25 to 35 hours with turbocharged engines.
Restrictions in the oil supply to the turbo result in a reduction of oil flow and subsequent overheating of the bearing(s) or center housing. Oil starvation can be caused by bad gaskets, restricted oil flow or improperly positioned orificed T-fittings. A classic example is the restricted T-fitting in the oil inlet line on Cessna's 210, 206, 207, and 337s. The restricted side of this fitting is meant to feed the oil pressure gauge, not the turbo. The turbo side of this fitting is unrestricted. Unfortunately, it occasionally gets plumbed incorrectly.
At all 100-hour inspections, remove the induction air supply duct to the compressor and the separate the exhaust outlet ducting from the turbine side. Check the compressor and turbine wheel blades for potential foreign object damage. Also, examine the outer tips of the blades and adjacent housing surfaces for any evidence of drag or rubbing.
Turn the wheels by hand while exerting an end and side-load. There should be no rubbing or binding of the wheels against the housings and they should be free to rotate. Be certain to check the turbine housing for cracks and the security of the exhaust housing bolts, the lock tabs and the condition of the "V" band clamps. Special attention should be given to the manufacturer's recommendations for proper "V" band installation procedures and appropriate torque values.
A preventative maintenance procedure that should be practiced universally is to allow the engine to sufficiently warm up before applying full power, and to avoid pulling power abruptly. It's also advisable to allow the engine to idle an additional three to five minutes prior to shutting it down. This allows the turbo to cool down and equalize temperatures before going to idle cut-off. These simple measures will reduce the possibility of oil residue coking in the hot turbine housing and should prolong the life of the turbo.
It's only as you consider the operational demands and the exacting details that factor into the analytical performance predictions in designing these systems that you gain a true appreciation for aircraft turbocharging.
Stay tuned. Part 2 in this two-part series will deal with the turbocharger control systems (valves and controllers) - both manual and automatic.