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the intake manifold with up to 2.5 times more air than the engine’s natural displacement. It uses two wheels, an exhaust turbine and a compressor wheel, attached to opposite ends of the same shaft. The exhaust turbine is driven by exhaust gas that flows from the turbine’s outer circumference towards the center of the wheel. The compressor wheel draws air in through the center and discharges it into the compressor housing that surrounds the outer circumference of the wheel. the intake manifold or separate boost Traditional high-flow turbocontrol from engine speed/load conchargers generate their maximum ditions. Throttling reduces efficiency, boost pressure at high engine speed/load conditions. Over-boost is and most engine makers have opted prevented with an exhaust bypass or for all-speed boost control using “waste gate” that limits the exhaust small, highly efficient quick-response gas energy reaching the turbine. turbochargers that generate boost just However, during partial-load operaabove idle rpm. To control boost at tion, these large turbochargers supply low speed, they utilize a variable far less boost, and boost control is geometry turbine (VGT). extremely limited. The newest diesel engines are equipped with exhaust gas recirculation (EGR) to control oxides of nitrogen (NOX). To make exhaust gas flow into the intake manifold, exhaust pressure must be higher than intake manifold pressure, so in a turbodiesel, boost must be controlled during partial-load operation. There are two different ways to do this: Throttle


The VGT has a set of movable vanes in the turbine housing, and they control boost by controlling exhaust turbine inlet pressure. At low engine speeds when exhaust flow is low, the vanes are partially closed. This increases the pressure of the exhaust pushing against the turbine blades, making the turbine spin faster and generating more boost. As engine

Looking into the exhaust turbine housing, the hollow vanes are mounted to pivot at their inner ends. Pins on the plate behind the vanes project into the slots. When the plate rotates, the vanes pivot and the size of the opening between them changes. Notice the actuator piston to the left.

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March 2005

Photos: Richard McCuistian


A closer view of the turbocharger shows the crank behind the rotating plate. It is operated by oil pressure acting on the piston in the left of the picture and a return spring.

speed increases, so does exhaust flow, so the vanes are opened to reduce turbine pressure and hold boost steady or reduce it as needed. By reading a manifold pressure sensor, the Powertrain Control Module (PCM) can adjust turbine inlet pressure to control boost at any speed/ load and to limit boost at full load. Advanced materials and precision engineering are needed to keep these moving parts operating smoothly in the hottest part of the turbocharger, and there are two different basic designs. The most common design

rotates the vanes like slats in a window blind to open and close the flow area. The vanes are mounted in the turbine housing so they can pivot about one end. A plate with pins projecting into the center of each vane is rotated, causing the vanes to rotate together about their pivot points. Another type of variable turbocharger commonly used on larger engines has fixed vanes to guide the exhaust flow, and a sliding piston controls the size of the exhaust turbine inlet. This design is a bit less efficient but much simpler and a lot less expensive.

Variable Geometry Turbochargers were originally developed for automotive gasoline engines in cars about 20 years ago. Their cost and complexity, coupled with improvements in more traditional turbo designs, has kept them on the shelf until now. However, they provide big improvements in diesel engine efficiency and emissions, and it looks like their time has come. We can expect to see them on many light- and medium-duty truck engines and a surprising number of passenger car diesel engines over the next three years.

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