Boosting Your Knowledge of Turbocharging: Part II - Valves and Controllers

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 horsepower. Approximately three percent of horsepower is...

Graphic Air density is a function of both pressure and temperature. Therefore, air temperatures influence the upper deck and manifold pressures. As you recall, as air is compressed, it increases in temperature, resulting in a reduction of air density and consequently a loss of engine power. A natural reduction of air density also occurs at higher altitudes and at temperatures above standard, which further exacerbates this problem. The compressor is forced to work harder to feed a sufficient amount of air to the cylinders. Excessive heat becomes the undesirable by-product of this exchange. Generally, there is a loss of one percent of horsepower for every 6 to 10 degrees increase in induction air temperature. Remember, engine power is influenced not only by rpm and manifold pressure, but also by induction air temperature. In certain installations an intercooler acts as a heat exchanger to cool down this discharge air and recapture lost power.

The need to control
Intake air pressures and temperatures must be controlled since they have such a direct influence on engine performance. The Controller's purpose is twofold: to sense changes in the upper deck and intake manifold pressures and to keep engine power constant as ambient conditions vary. Some engines rely on more than one controller to perform this task. Controllers may be found in multiples or combined in a common housing. Regardless of the arrangement, they operate in parallel, even while responding to differing stimuli. Depending upon their installation, controllers may be plumbed directly into the upper deck air-stream or remotely mounted and referenced by means of hoses.

Automatic controllers
The automatic controller monitors deck, manifold, and/or ambient pressures by means of a pressure differential across the diaphragm or by using a bellows calibrated to a predetermined absolute pressure. As the controller senses pressure changes, it seeks to modulate turbo output by regulating the amount of oil allowed to bleed past a poppet valve also housed within the controller. The movements of the diaphragm or the expansion and contraction of the bellows within the controller alters the position of this oil metering poppet valve in relation to a seat. As this poppet closes, oil flow back to the sump is restricted. Oil begins to dam upstream and exert a force against a piston in the wastegate actuator. This piston is mechanically linked to the wastegate valve and begins to close the valve. As the controller poppet moves, it becomes an adjustable orifice regulating the buildup of oil pressure that is exerted against a piston in the wastegate actuator. By altering the position of the wastegate valve, it controls the amount of exhaust gases either routed to the turbo or diverted out the exhaust, thus maintaining a pre-selected manifold pressure setting (see diagram 1, previous page.). In this manner it schedules the appropriate amount of air to the engine at any given altitude or power setting.

The controller(s) could be appropriately thought of as the brains or command center for the entire charge-air system. As engine power, speed, or altitude is changed, the controller constantly strives for equilibrium within the system. The exhaust bypass valve (wastegate assembly) simply acts as a slave to the dictates of the controller. Engine oil becomes the muscle of the system, providing the power to actuate the movements of the wastegate assembly.

Design differences
Lycoming engines utilize four basic styles of controllers: the Differential, the Density, the Variable Pressure and the Slope Controllers. The density controller is the only controller capable of sensing changes in temperature. It relies on a bellows charged with dry nitrogen to accomplish this purpose.

Density controllers are found in the Piper Navajo and Chieftain and a handful of helicopter applications. In the Piper, the density controller modulates the wastegate movement at wide open throttle while a differential controller keeps deck pressures from exceeding manifold pressures by more than a specified amount at part throttle settings. Density controllers are extremely sensitive to in-field adjustments. A 1/16th turn of the adjustment screw will result in a 2-in. change in manifold pressures. A thermocouple probe referenced to deck temperature is required when setting up these systems (see Lycoming S.I. No. 1187J).

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