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...


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 lost or traded for every 1,000 feet of altitude gained; as altitude increases, performance wanes. The obvious advantage of any turbocharged aircraft is its ability to compensate for this loss and provide maximum rated power at altitude. This advantage gives the pilot the ability to over-climb inclement weather, achieve optimum altitudes and attain maximum speeds with relative ease. To fly high and fast in a pressurized cabin while experiencing smooth air and favorable winds is very desirable. Reduced drag, increased range, and improved fuel economy are just a few of the benefits turbocharging offers.

In our previous discussion (July issue of AMT), we focused solely on the turbocharger itself — that component which at altitude supplies additional power to otherwise anemic performing engines. We determined that pulsing exhaust gases provide the requisite force to drive the turbine wheel and consequently, the compressor, to speeds of up to 2,000 revolutions per second. The end result — exhaust energy is converted to manifold pressure. We also discovered that the turbocharger is subjected to the extremes of temperatures and duty cycles and that consistent oil changes, cool-down practices and maintenance are essential to its continued performance, health and operation.

Turbochargers are indeed a remarkable feat of engineering ingenuity. And when coupled with their supporting wastegates, controllers, relief valves, oil lines and piping they can seem both confusing and intimidating. Occasionally, even the seasoned mechanic becomes frustrated to the point of literally "throwing in the wrench" on these systems. However, armed with just a bit of knowledge, charge-air systems can be reduced from extremely daunting to simply challenging. Perhaps the following information will equip you to feel up to the challenge as we attempt to clarify some of the annoying and at times confusing nuances of turbocharging systems. Our discussion will focus on the individual components coupled with an explanation as to how they interrelate to each other.

Pressures to Perform
As their names suggest, Valves and Controllers both regulate and control turbocharger discharge pressure. This is an exacting science due to the fact that there is a disproportionate air/fuel ratio that must be tailored to the ever-changing demands of the engine as well as atmospheric density changes. For these reasons, a means of managing this forced air-flow to the cylinders is necessary to prevent the onset of detonation or overboost. The control portion of the turbocharging "system" was designed for this purpose. Valves and controllers provide the desired flight envelop, while keeping engine intake manifold temperatures and exhaust manifold pressures as low as possible.

Deck Pressure and Manifold Pressure
A basic knowledge of the air pressures associated with turbocharging is necessary for a more thorough understanding of these control systems. Air upstream of the throttle plate is often referred to as "Upper Deck Pressure." Deck pressure is measured between the compressor discharge and the inlet to the fuel injector or carburetor. Pressure downstream of the throttle is referred to as "Manifold Pressure" and is referenced between the throttle plate and the cylinder intake. On average, there is a pressure drop of approximately two inches of mercury across the throttle plate depending upon throttle position. Deck pressure always exceeds manifold pressure. At wide-open throttle, the air pressure drop across the throttle plate is at its minimum. And conversely, as the throttle is closed, this pressure drop increases. These pressures change both in response to throttle movements and variations in air density and temperatures.

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