298 Diesel Technology Copyright Goodheart-Willcox Co., Inc. One of the most common methods of limiting turbo- charger boost and speed on a CGT turbocharger is with a waste gate, or turbine-bypass valve, Figure 13-31. It can be either a butterfly valve or a diaphragm. The valve may be operated manually, by intake manifold pressure, or by a servo motor. If a servo motor is used, it can be controlled manually or by a device that senses turbocharger speed, gauge pressure, absolute pressure, density, or air flow. Turbocharger speed can also be sensed and controlled electronically. Variable Geometry Turbocharger Operation A variable geometry turbocharger varies the amount of exhaust gas used to drive the turbine which will increase or decrease the boost pressure. This is done by varying the pitch of a series of vanes in the turbine housing. The vanes are mounted on two pins inside a unison ring one pin fixed the other in a slot. A piston controlled by the ECM rotates a crank pin that causes the unison ring to rotate. As the vanes change pitch, the usable area of incoming gas will increase or decrease changing the efficiency (speed) of the turbine. When the unison ring rotates and opens up the vanes, the efficiency is reduced and there is less turbo- charger boost. When the unison ring closes down the vanes, efficiency is increased and turbocharger boost is increased. Note: A VGT is essential for engines that are using EGR technology. Boost pressure must be lower than exhaust pressure in order to reintroduce exhaust gas back into the air intake system. Turbocharger Advantages A turbocharged engine has better fuel economy, fewer emissions, and more power than a naturally aspirated engine. The turbocharger also offers a distinct advantage to a diesel engine operating at high altitudes. The turbo- charger automatically compensates for the normal loss of air density and power as the altitude increases. Turbocharger Lubrication Lubricating oil for the turbocharger is supplied under pressure through an external oil line extending from the cylinder block to the top of the center housing, Figure 13-32. From the oil inlet in the center housing, the oil flows through drilled oil passages in the housing to the shaft bearings, thrust ring, thrust bearing, and backplate or thrust plate. Gravity forces the oil to the engine oil pan through an external oil line extending from the bottom of the turbo- charger’s center housing to the cylinder block. When the turbocharger is operating, the exhaust pres- sure behind the turbine wheel and the air pressure behind the compressor wheel are greater than the pressure inside the bearing housing. To prevent pressurized exhaust gas and air from entering the bearing housing, sealing rings are used. These rings fit snugly in the bearing housing and do not rotate with the shaft. A heat shield and, in some cases, an insulating pad are used on the turbine end of the bearing housing to prevent the turbine exhaust gas tem- perature from transferring into the bearing housing. Two- or Four-Stage Turbocharging To increase both the torque range and the mean effec- tive pressure to an even greater degree, some engines may employ either two or four turbochargers and charge-air coolers (one for each exhaust manifold, or use two turbo- chargers in series and a charge-air cooler), Figure 13-33, or use four turbochargers in series and two charge-air coolers. The series of turbochargers operates at lower-than-normal pressure ratios and reduced turbine speeds. Waste gate valve Figure 13-31. The waste gate valve is used to control the speed of the turbocharger. (Garrett Group, Inc.) Inlet oil line V-band coupling Gasket, outlet oil line to turbocharger center housing Outlet oil line Nut Turbocharger assembly Exhaust manifold gasket To oil filter adapter Figure 13-32. The turbocharger uses engine oil for lubrication, usually supplied by one or more lines that thread into the block. (Garrett Group)