Saturday, November 21, 2009

Turbos, oil temp, and overboosting

The owner of a Florida-based 1977 Bonanza V35B with a turbonormalized TCM IO-520-BA engine contacted me with concerns about fuel flow. He described a recent instructional flight involving a takeoff on a "coolish day" that showed 31" MP, 2700 RPM, and 31 GPH fuel flow. Later in the flight, a full-throttle go-around was executed, and the owner that fuel flow was 3 GPH below red line on the factory analog fuel flow gauge, and only 27 GPH on the Shadin digital fuel flow gauge. After landing, the owner described this situation to his mechanic and suggested that the fuel flow be adjusted up. The mechanic was concerned that if he adjusted the fuel flow, it might be excessive on takeoff.

I explained to the owner that his turbonormalized engine has a hydraulic wastegate actuator and controller that uses engine oil as its hydraulic fluid. A byproduct of this is that regulation of upper-deck air pressure (UDP) is not precise, and the system delivers higher UDP when the oil is cold than when it is hot.

I further explained that engine has an aneroid-compensated fuel pump whose output pressure is a fuction of two variables: engine RPM and UDP. Therefore, if RPM is held constant at 2700 RPM (red line), the fuel pump will put out more fuel flow when the oil is cold (and the UDP is high) than it will when the oil is hot (and the UDP is lower).

For safety's sake, I explained, the turbo controller should be adjusted so that it gives full red-line MP when the oil is hot -- so that, for example, if a full power go-around or missed-approach is executed, full rated power is actually available.

If the system is adjusted in this fashion, then when full power is applied with cool oil (e.g., first takeoff of the day), there will be some "overboost" (MP above red line) by an inch or two. This is okay. Momentary overboost will not harm the engine, and is a quickly self-correcting condition as the oil warms up during the takeoff roll. I suggested that the owner NOT try to manually limit the MP to red line under these conditions, and simply accept the brief overboost. (TCM has a SB that states that momentary overboost up to 3" is a non-event.)

Under such brief overboost conditions (MP over red line), if the system is adjusted properly, fuel flow will also be over red line momentarily until the oil warms up and the MP comes down, since the fuel pump is compensated to follow UDP. This is the way things are supposed to work. If the engine is momentarily producing a bit more than 100% power, then it needs a bit more than 100% fuel flow. The system knows what it's doing, so the pilot shouldn't try to help it.

The turbo controller should be adjusted so that it will give full red-line MP with hot oil. Making this adjustment will automatically increase fuel flow as well, because higher UDP commands higher fuel flow. Once the turbocontroller has been adjusted, full red-line fuel flow should be seen when full power is applied with hot oil. If fuel flow falls short of red-line, the high unmetered fuel flow (the adjustment screw on the fuel pump aneroid) should be adjusted to achieve full red-line fuel flow.

On takeoffs with cool oil, there will be 1" or 2" initial overboost above MP red line, and momentary FF above red line. All this is good.

It is essential that the turbocontroller and fuel pump adjustments be made when the engine and oil are hot.


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