I always say that the PT6 engine relies on efficiency. This especially applies to the compressor, which I have always said is the heart of the engine. Maybe it would be more accurate to say it is the engine’s lungs. When we discussed engine washes, we discussed how a dirty compressor could affect efficiency and, thereby, engine performance. Let’s look a little closer at the compressor and what the engine does with all the air from the compressor.
To begin with, the compressor is constructed with a series of rotating disks with blades between stationary vanes that direct the air path as they slow and compress the air. The small PT6 has three sets of these disks and blades. The large PT6 engine adds the fourth disk. These disks and vanes grab the air initially and move it axially through the compressor. The final stage of the PT6 compressor, regardless of engine size, is a centrifugal impeller. The impeller moves the compressed air outward for distribution to the rest of the engine. The compressor is housed between the inlet case and the gas generator case. It rides on the #1 bearing, a thrust ball bearing, and the #2 bearing, a roller bearing.
At engine start and low power settings, the compressor makes more air than the engine needs. The PT6 has a bleed-off valve (BOV) to dump the unused air overboard. If the BOV cannot remove the excess air quickly enough, the compressor can stall. You may have heard the big bang of a compressor stall when the BOV malfunctions. As the engine comes up to speed, the BOV begins to close and fully closes somewhere around 90% Ng. 90% Ng is a ballpark number for our discussion. If you have concerns about the operation of your bleed valve, there is detailed information in the maintenance manual for your specific engine model.
As the air leaves the impeller, it is directed through diffuser tubes in the gas generator case. These tubes slow the air, which increases the pressure. Some air is pushed into the combustion chamber, where fuel is added and burned. That hot compressed gas is sent through the turbines. Approximately 25% of the air made by the compressor is used for the combustion process. Once the compressor turbine is spinning and the combustion process is started, the engine will remain running as long as there is fuel. What about the other 75% of the air?
The combustion process takes place within the combustion chamber liner. These liners are made of extremely thin metal. To keep the whole engine from melting, 65% of the air the compressor makes is used to cool during combustion. Look at a combustion chamber liner and observe the holes on the inside and outside, as well as the rings inside the liner. Cooling air can be used in the vane rings, compressor turbine blades, and disks. There are many passages for cooling air to be directed and used by these parts. The PT6 wants to maximize the use of the hot air but also wants to protect the parts using it.
That leaves us the last 10% of the air. Half of it is used for airframe services like cabin heat. The other half is used by the secondary air system within the engine. This secondary air system includes bearing compartment sealing. You may have heard people speaking of air seals. Air pressure is sent to some of the bearing cavities to seal the oil from coming out of the engine. There is typically a rotating air seal and a stationary air seal with a minimal gap between them. The #1 and #2 bearing have these air seals. They allow the compressor to spin and the bearings to still have ample oil supply to keep them safe. The air supplied to these areas is then carried out of each bearing area in the oil. An air/oil separator in the oil tank removes the air from the oil and vents it overboard through the breather.
Air and its efficient use are what you want from your PT6. If you begin to lose that efficiency, you may notice Ng creeping up. That means the compressor has to work harder to get all the air the engine requires, and you will notice that you are not getting the torque out of the engine that you need to work the airplane. That is our clearest indication that we need to investigate the air system. Please call your maintenance provider or me if you have any questions about what I discussed in this article.
Robert Craymer has worked on PT6A engines and PT6A-powered aircraft for the past three decades, including the last 25+ years at Covington Aircraft. As a licensed A&P mechanic, Robert has held every job in an engine overhaul shop and has been an instructor of PT6A Maintenance and Familiarization courses for pilots and mechanics. Robert can be reached at email@example.com or 662-910-9899. Visit us at covingtonaircraft.com.