Turbocharger Systems PDF

TURBOCHARGER SYSTEMS A drawback of gear driven superchargers is that they use a large amount of the engine's power output for the amount of power increase they produce. This problem is avoided with a turbosupercharger, or turbocharger, because turbochargers are powered by an engine's exhau...

TURBOCHARGER SYSTEMS A drawback of gear driven superchargers is that they use a large amount of the engine's power output for the amount of power increase they produce. This problem is avoided with a turbosupercharger, or turbocharger, because turbochargers are powered by an engine's exhaust gases. In other words, a tur-bocharger recovers energy from hot exhaust gases that would otherwise be lost. The components in a turbocharged induction system are similar to those in a normally aspirated system with the addition of a turbocharger and its associated controls. The turbocharger itself is located between the air intake and the fuel metering device.  A typical turbocharger consists of a single rotating shaft with a centrifugal compressor impeller mounted on one end and a small radial turbine mounted on the other end. Both the impeller and turbine are surrounded by individual housings that are joined by a common bearing housing. The bearing housing contains two aluminum bearings that support the center shaft. In this configuration, as exhaust gases spin the turbine, the impeller draws in air and compresses it. [Figure 5-9]  In addition to the friction caused by high rotation speeds, turbochargers are heated by the exhaust gases flowing through the turbine, and the compression of intake air. Therefore, a continuous flow of engine oil must be pumped through the bearing housing to cool and lubricate the bearings. Approximately four to five gallons of oil per minute are pumped through a typical turbocharger bearing housing to lubricate the bearings and take away heat. Once the engine oil passes through the bearings, it flows out a large opening in the bottom of the bearing housing and back to the engine oil sump. Some turbochargers may utilize an additional oil scavenge pump to ensure reliable oil flow from the turbocharger back to the engine oil sump.  Since the temperature of a gas rises when it is compressed, turbocharging causes the temperature of the induction air to increase. To reduce this temperature and lower the risk of detonation, many turbocharged engines use an intercooler. An inter-cooler is a small heat exchanger that uses outside air to cool the hot compressed air before it enters the fuel metering device. TURBOCHARGER CONTROL SYSTEMS If all the exhaust gases were allowed to pass through the turbine of a turbocharger, excessive manifold pressures, or overboosting would result. On the other hand, if the amount of exhaust gases allowed to flow to a turbocharger were limited, the turbocharger would be excessively limited at higher altitudes. Therefore, turbochargers are designed to allow control over the amount of exhaust gases which pass through the turbocharger's turbine. To control the amount of exhaust gases that flow past a turbocharger turbine, a valve known as a waste gate is used.  When a waste gate is fully open, all of the exhaust gases bypass the turbocharger and pass out the exhaust stack. However, when a waste gate is fully closed, all of the exhaust gases are routed through the turbine before they exit through the exhaust. The position of a waste gate can be adjusted either manually or automatically. [Figure 5-10] MANUAL CONTROL SYSTEMS One of the simplest forms of turbocharger control uses a manual linkage between the engine throttle valve and the waste gate valve. For takeoff at low density altitudes, the throttle is advanced until the engine develops full takeoff power as indicated on the manifold pressure gauge. At this point, the waste gate will be fully or nearly fully open. As the aircraft gains altitude, engine power decreases requiring the pilot to advance the throttle forward a little to partially close the waste gate. As the waste gate is gradually closed, the manifold pressure increases proportionally and the engine produces its rated horsepower. This process is continued as the aircraft climbs to its critical altitude. Once at its critical altitude, the throttle will be advanced all the way forward and the waste gate will be fully closed.  A second type of manual control system allows you to set the position of the waste gate using a control in the cockpit. With this type of system, the engine is started with the waste gate in the fully open position. Then, just prior to takeoff, the throttle is advanced full forward and the waste gate is slowly closed using the cockpit control until full engine power is developed. Once the aircraft takes off and begins climbing, the pilot must monitor the engine performance and close the waste gate as necessary to maintain the desired power output.  The final type of manual waste gate controller utilizes an adjustable restrictor in the exhaust section that bypasses the turbocharger. The amount the restrictor is threaded in or out of the exhaust pipe determines the amount of exhaust gas that is forced to flow through the turbocharger. With this type of system, no adjustments to the restrictor can be made from the cabin. [Figure 5-11] To provide additional protection against an overboost when temperatures and pressures are below standard, a pressure relief valve is typically installed in this type of system. In this case, when manifold pressure rises to within approximately one inch of its rated pressure, the relief valve begins to off-seat. This way, by the time maximum manifold pressure is reached, the pressure relief valve is open enough to bleed off excess pressure.  Figure 5-11. On turbocharging systems equipped with an adjustable waste gate restrictor, the amount the restrictor is threaded in or out determines how much of the exhaust bypasses the turbocharger. AUTOMATIC CONTROL SYSTEMS As the name implies, an automatic turbocharger control system automatically positions the waste gate so the engine maintains the power output level selected. To do this, these systems use a combination of several components including a waste gate actuator, an absolute pressure controller, a pressure-ratio controller, and a rate-of-change controller. WASTE GATE ACTUATOR The waste gate in an automatic control system is positioned by a waste gate actuator. With a waste gate actuator, the waste gate is held open by spring pressure and is closed by oil pressure acting on a piston. Oil pressure is supplied to the actuator from the engine's oil system. ABSOLUTE PRESSURE CONTROLLER On Teledyne-Continental engines, the waste gate actuator is controlled by an absolute pressure con troller, or APC. An APC consists of a bellows and a variable restrictor valve. The bellows senses the absolute pressure of the air before it enters the fuel metering device. This pressure is commonly referred to as upper deck pressure. As the bellows expands and contracts, it moves the variable restrictor valve to control the amount of oil that flows out of the waste gate actuator. [Figure 5-12].The rate at which the oil flows through the APC and back to the engine is determined by the position of the variable restrictor valve. As oil pressure builds, the waste gate begins to close and direct some of the exhaust to the turbocharger. This process continues until the upper deck pressure builds enough to compress the APC bellows and open the restrictor valve. Once the restrictor valve opens, oil is allowed to flow back to the engine. PRESSURE-RATIO CONTROLLER The purpose of a pressure-ratio controller is to monitor both the ambient and upper deck pressures and prevent the turbocharger from boosting the upper deck pressure higher than 2.2 times the ambient pressure. [Figure 5-13] As a backup to the pressure-ratio controller, most turbocharger systems incorporate a pressure relief valve. A typical pressure relief valve consists of a spring loaded pop-up valve that is mounted to the upper deck near the compressor output. In most cases, the relief valve remains seated until the upper deck pressure exceeds its maximum rated pressure by 1 to 1.5 inches. RATE-OF-CHANGE CONTROLLER A rate-of-change controller is installed in parallel with the absolute pressure controller and pressure-ratio controller, and prevents the upper deck pressure from increasing too rapidly. Under normal conditions, the rate-of-change controller remains seated; however, if the throttle is advanced too abruptly and the upper deck pressure rises too rapidly, the rate-of-change controller unseats and allows waste gate actuator oil to flow back to the engine. In most cases, a rate-of-change controller is set between 2.0 and 6.5 inches per second.