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.
They are both expensive, and the death of one can lead to the death of the other.
Boosting Your Knowlege of Turbocharging Part II -Valves and Controllers By Randy Knuteson October 1999 All normally aspirated aircraft engines gain altitude at the expense of...