Turbochargers: Variable Geometry

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1 Created by Gus Wright Centennial College 1 Turbochargers: Variable Geometry Types of Turbochargers A variety of turbocharger designs have developed in response to increasing demands for improvements to engine performance and emission reduction. Fixed geometry This refers to a turbocharger without boost pressure controls. The housings and components have unchanging dimensions. Compound turbochargers This turbocharger configuration uses two turbochargers which are connected in series. Unlike paralleled turbochargers where two turbo s are mounted on the exhaust manifolds(s), to supply air to the intake, series turbochargers feed the output of the first turbochargers compressor into the inlet of a second turbocharger which has a turbine also mounted on the exhaust manifold. Caterpillar ACERT engine, C13, C15 and C16 engines are examples of series turbochargers. One turbocharger is designed for low speed low load operating factors while the other is designed for high load and speed factors. This set-up provides a maximum airflow over as wide an engine operating range. Below: ACERT Compound Turbos Wastegated These turbochargers have an exhaust bypass valve allowing exhaust gases to bypass the turbine leaving some exhaust energy unused. Allowing exhaust gases to bypass the turbine wastes some of the exhaust gas energy thus wastegated turbocharger. A single large turbo can make as much horsepower as two small ones, but the advantage of a dual system is that they respond faster and produce greater airflow at lower engine speed and load conditions.

2 Created by Gus Wright Centennial College 2 Above: VGT Variable Geometry Turbo Below Variable Nozzle Turbo VNT

3 Created by Gus Wright Centennial College 3 Variable geometry (VGT) & variable nozzle turbochargers (VNT). Both these turbochargers perform identical functions but have different operating mechanisms. VNT s and VGT s share the capability of changing boost pressure independent of engine load and the amount of exhaust gas energy. This design allows for very fast turbocharger response time and ideal matching of airflow to combustion requirements. VGT and VNT s are commonly used on engines equipped with exhaust gas recirculation (EGR) since they can also increase exhaust backpressure, which is necessary to force exhaust gases back into the intake manifold. (See section on EGR) In this section, variable geometry turbocharging is discussed. Above & Below: A/R ratio

4 Created by Gus Wright Centennial College 4 Matching turbochargers to engines Turbochargers must be precisely matched to a particular engines fuel rate setting and engine displacement to achieve the correct boost pressure for the engines design points. This means every horsepower rating and torque rise profile in an engine family uses a uniquely sized turbocharger. The necessity of matching the turbocharger to engine ensures the airflow from the turbocharger will match the engine fuel rates. On most engines, maximum boost pressure from the turbocharger will correspond to the engines peak-torque spot with minimal exhaust backpressure. Incorrectly sized turbochargers Too small turbine housing which does not allow enough exhaust gas flow at high engine loads and speeds. This results in excessive exhaust backpressure producing low power complaints and engine overheating. Too large a turbine housing will produce inadequate boost pressure at low speed and load operating conditions. Similarly, the compressor wheel and housing need to be properly matched to produce adequate airflow across all engine speed ranges without over or under boosting. An incorrectly matched turbocharger can lead to either too little or too much boost for the engine. Problems of fixed geometry turbochargers Fixed geometry turbocharger design is a compromise to produce boost pressure and airflow rates over a wide range of speed and load conditions. Generally, the designs can result in either too little boost at low engine RPM and low loads while supplying high boost at high speeds and loads. The opposite condition can also happen; high boost at low load and engine speeds at the expense of adequate boost during high speed and high load operation. This later situation can result in undesirably high exhaust backpressure and temperature at higher rpm and load conditions. Wastegated turbocharger operation address these deficiencies but without the precision and feedback to the engine control system. Turbo lag is another variable affected by turbocharger design. Larger turbochargers needed to produce greater airflow for better performance and emissions take longer to spool and causing extended turbo lag time. In comparison to fixed geometry turbochargers VGT/VNT s have an even greater impact on minimizing engine size while increasing power output. This turbocharger design provides better performance across the entire operating range of the engine, improves fuel economy, and is

5 Created by Gus Wright Centennial College 5 effective at reducing exhaust emissions. VGT s can reduce the number of turbocharger options carried by a manufacturer, and provides users the ability change engine power levels without requiring turbocharger replacement. VGT s/vnt s have been designed since the 1980 s but their complexity and cost has prevented their widespread application. Improved fixed and wastegated turbocharger designs during that time also diminished the need to bring these designs to market. VGT and VNT turbocharger operation Variable geometry turbochargers are built along two designs. What are commonly referred to as VGT are turbochargers which control the width of the nozzle directing exhaust gas onto the turbine wheel. By narrowing the width of the nozzle opening, the turbine speed increases as does exhaust backpressure. This has the same effect as holding your finger over the end of a garden hose to increase the pressure of water forced from the hose end. An electric or air type actuator operates a sliding nozzle ring to regulate the nozzle opening. As the actuator closes the nozzle to narrow its width, turbine speed and boost pressure rapidly increase. This effect can be accomplished with minimal engine load or exhaust energy. So. at low speed and load operation, relatively higher boost pressure is achieved than with using fixed geometry or even wastegated turbochargers. Opening the nozzle produces the opposite effect, more exhaust gas flow will take place across the turbine but turbine speed will decrease and exhaust backpressure drops. By varying the width of the nozzle opening with an actuator, turbine power can be set to provide just sufficient energy to drive the compressor at the desired boost level wherever the engine is operating. Above: VNT uses moving airfoils Above Sliding nozzle VGT

6 Created by Gus Wright Centennial College 6 VGT/VNT Advantages: 1. Electronic control of air delivery Ideally, a turbocharger is required that can quickly produce significant boost pressure at low speed and load conditions without the turbine causing high exhaust backpressure at high speed and load conditions. To create a turbocharger capable of controlling boost pressure independent of the engine load or exhaust energy is the purpose of variable geometry turbochargers. Also known as variable nozzle turbochargers (VNT), these devices can simulate the air flow and boost profiles of dozens of different turbocharger onto a single engine. VGT/VNT turbochargers are electronically capable of controlling the amount of air supplied to an engine for maximum performance, while maintaining the lowest exhaust emissions and fuel consumption. Since the turbocharger is electronicly monitored and actuated, airflow is also controlled over a wide range of ambient temperatures and pressures. This means engine and turbocharger wear factors can be compensated to maintain original performance and low emissions over a longer engine lifecycle. 2. Minimal turbo lag When the engine is rapidly accelerated VGT/VNT turbochargers spool far more quickly than conventional turbochargers virtually eliminating turbo lag and improving vehicle driveability. In fact, in the same model of vehicle, the newest diesel optioned passenger vehicles using VGT/VNT turbochargers are able to out accelerate even larger displaced gasoline powered models. For example, km/hr times for the 2005 C- Class Mercedes Benz using a 3.0 litre diesel engine is 6.9 seconds. This is 0.2 seconds faster than the 3.2 litre gasoline engine in the C320 sport. The Grand Cherokee using a European 3.0 litre V-6 diesel engine accelerates 0-110km/hr in 9.0 seconds 0.5 seconds faster than the 4.7 litre gasoline engine. The advantage of VGT/VNT over a wastegated turbocharger is that all the exhaust gas flows through the turbine wheel to increase net turbine power output. During partial-load operation, wastegated turbochargers supply less boost, and boost control is limited in comparison to VGT/VNT s Transient emissions which may be observed during engine acceleration of a turbocharged diesel engine

7 Created by Gus Wright Centennial College 7 3. Faster response time Minimal spool time of the VGT/VNT achieves important emission reductions during hard accelerations. Older fixed geometry turbocharged engines would spew black smoke during acceleration since significant quantity of emissions are produced during the interval when the engine is over-fuelled and inadequate air is present to support good combustion. 4. Electronic control of exhaust backpressure Newer exhaust gas recirculation (EGR) systems take advantage of the VGT/VNT turbochargers capability to control exhaust back pressure. Since turbo boost pressures are normally higher than exhaust pressure, some mechanism is necessary to force exhaust gas into the intake air. Some engine models will use an electronic throttle plate to draw EGR gas but this limits engine efficiency. Electronic control of exhaust backpressure allows VGT/VNT turbochargers to assist in building exhaust pressure and pushing exhaust gas into the air intake system over all engine speed and load conditions. 5. Improved engine braking Engine using compression release brakes will achieve improved braking efficincy using VGT/VNTs due to higher exahsut back pressure. EGR mode The VGT actuator is adjusted to increase exhaust backpressure if required by the EGR system. The exhaust backpressure forces more EGR gas flow into the EGR circuit where the EGR valve will ultimately regulate EGR mass flow. To achieve improved engine braking, the actuator can narrow the nozzle opening and increase exhaust backpressure if engine-operating algorithms are designed to produce this effect. Boost and compression pressure increase accordingly creating higher pumping losses and consequently greater engine drag VNT operation Variable nozzle turbochargers are the most common used type of VGT turbochargers in heavy duty applications. Mack, Detroit, Volvo, GM Duramax, and International Engine ccoampany are just a few manufacturers using this style of VGT. VGT with EGR

8 Created by Gus Wright Centennial College 8 VNT unison ring and vane operation VNT operate in a manner identical to the VGT. The most common design rotates airfoils or vanes arranged like slats in a window blind around the turbine wheel. These vanes are moved to regulate the quantity of exhaust gas flow through the turbine. The vanes are mounted in the turbine housing with one end pinned to the housing. The other end of the vane is connected through a pin to a plate called a unison ring. Rotation of this unison ring causes the all the vanes to revolve around the fixed pivot point. Movable vanes control exhaust gas velocity and pressure to turbine wheel Actuator mechanisms Fast response of the turbocharger to engine speed and load changes is the objective of the actuator mechanism. Several devices are used to regulate either the position of the sliding nozzle of a VGT or the vane position in a VNT. In the VGT turbocharger an electric motor or air operated device serve as actuators. Incorporated into the electric actuator is a position sensor to provide feedback to the ECM about actuator position. Rotation of the vanes changes boost pressure by controlling exhaust turbine inlet pressure. At low engine speeds when exhaust flow and energy is low, the vanes are partially closed. This increases the exhaust pressure which in turn pushes harder against the turbine blades. Thus, the turbine spins faster generating higher boost pressure. As engine speed and load increase increases, so does exhaust flow and energy. Under these conditions, the vanes are opened to reduce turbine pressure against the turbine and hold boost steady or reduce it as required. On some VNT s, the vane position actuation is performed using an engine oil operating against a hydraulic servo piston. A pulse width modulated (PWM) vane position control solenoid valve uses engine oil pressure and an ECM signal to move the turbochargers unison ring. To do this a hydraulic piston will move a geared rack mechanism, which in turn, rotates a cam-shaped pinion gear thereby articulating the vanes. The vanes are fully opened when no oil flow is commanded to move the servo piston and decrease opening as oil pressure increases through the vane position control solenoid valve On Duramax engines using this turbocharger, an analog position sensor with a movable tip rides on the vane actuator cam and measures the vane

9 Created by Gus Wright Centennial College 9 position to provide feedback to the ECM. Integrated in the sensor harness is a module converting the analog signal to a digital signal supplied to the ECM. Below: Detroit Diesel VPOD functioning to control vane position VPOD = variable pressure output device On Powerstroke 6.0L engines and VT-365 engines, the end of VGT control valve has a mechanical cam follower to provide positional feedback to the valve. Detroit Diesel uses VPOD s variable pressure output devices and an air operated actuator to control the unison ring.

10 Created by Gus Wright Centennial College 10 Turbocharger sensor functions Speed, actuator position and pressure sensors located on the turbocharger or intake system provide feedback information to the engine control module (ECM) in order to achieve optimal operational conditions. The engine control module (ECM) continually monitors the position of the actuator mechanism, engine operating conditions and driver demands. Based on ECM programmed software instructions, the ECM signals the actuator to move the actuator control rod to a new position which slides the nozzle ring (VGT) or blades (VNT) to achieve the optimal required position. OBD II diagnostic procedures allow the technician to monitor the operation of these components. Turbo-speed sensor The turbocharger speed sensor located in the center housing is a variable reluctance type providing turbine speed data. This data is critical to prevent turbocharger over speed conditions. By way of comparison, the speed sensor is the substitute for the wastegate used to prevent over speed conditions. If over speed is detected, the ECM controls will increase the nozzle width or vane opening in the VNT to slow turbine speed. The OBD diagnostic system will monitor turbo operation using this sensor. When diagnostics tests

11 Created by Gus Wright Centennial College 11 Above: Sliding nozzle ring operation of Cummins VGT

12 Created by Gus Wright Centennial College 12 are performed on the turbocharger of EGR system, turbine speed is monitored to measure the speed increase of decrease in response to actuator opening and closing. Cooling VGT/VHT center housings The bearing housing of the turbocharger and the variable geometry actuator usually contain engine coolant passages. The purpose of the coolant is to reduce the operating temperature of the housings, in order to increase the reliability of the position sensor and DC motor. During the hot soak period, when engine temperatures rise after the engine is switched off, coolant circulates to remove excess heat using convection currents. VNT/VNT diagnostics The OBD system continuously monitors the operation of the VGT/VNT turbocharger operation to detect any condition which could lead to potential increases in exhaust emissions. DTC are set for any associated sensor or actuator electrical problems, under or over boost conditions, and boost control position. Exhaust backpressure sensor An exhaust backpressure sensor may be used to provide data regarding exhaust manifold pressure. The VNT/VGT pneumatic type actuator will change position in response to exhaust backpressure data. This sensor is also important to EGR system calculation for exhaust mass flow. The OBD system will use the exhaust backpressure data to check the rationality of data from other sensors. This means that a comparison may be made between boost pressure, turbine speed or actuator position to ensure correct system operation. Other sensors A compressor inlet temperature sensor and intake boost pressure sensor are used to calculate air mass to optimize combustion efficiency by matching fuel rates and EGR gas mass flow to air inlet air mass. The excess air ratio is accurately maintained by adjusting boost pressure using this data. Sensor operation is monitored by the OBD system. The technician can initiate actuator performance test used manufacturers diagnostic software. This tests measure the speed change of the turbocharger when the actuator closes and opens the vanes or, moves the nozzle ring. Speed values included in operating software will determine whether the control mechanism is properly functioning. A turbocharger that does not change speed when the actuator test is performed likely has a defect associated with the turbocharger or control circuits. Glossary of terms introduced in this section A/R ratio: Refers to the dimensions of a turbocharger giving it flow characteristics. A refers to the inlet area where R is the radius of the volute. A/R is the area divided by the radius. Exhaust backpressure: The amount of pressure exerted by the exhaust system or turbocharger against upward piston movement on exhaust stroke. Exhaust back pressure lowers fuel consumption and diminishes airflow through an engine driving-up exhaust temperatures.

13 Created by Gus Wright Centennial College 13 Expansion ratio: The difference in the volume of air and fuel mixture compared to its expanded volume after combustion. Centrifugal pumps: Centrifugal pumps have a shaft mounted circular fan or turbine-shaped component called an impeller. Air or a liquid medium entering this turbine is thrown with centrifugal force outwardly into the housing enclosing the turbine. These pumps increase efficiency or, output volume per revolution as pump speed increases. At low speeds they will not produce as much output volume per revolution as they will at high speed. Coking: Formation of a carbonaceous abrasive material caused by baking fuel or lubrication oil. Diffuser: Is the slot in the compressor housing where area centrifugally thrown from the compressor wheel enter the compressor housing. Dynamic balance: Balance achieved on a rotating component so it does not vibrate when in motion. Hot or high soak period: Refers to the time before an engine has cooled down after being heavily loaded. The heat produced after an engine has been to operating temperature and heavily loaded penetrates many components with heat not normally experienced during running condition. Induction hardening: A process used to harden a metal surface with heat. A coil of wire is wrapped around the component and high amperage A/C current is used to induce current flow on the surface of the component through mutual induction. Supercharge: The use of a device to pressurize the intake air supply to the engine combustion chambers at higher than atmospheric pressure. Supercharger: A device used to pressurize the intake air supply to above atmospheric pressure. Rootes blowers or turbochargers are two common types of superchargers. Naturally aspirated: Refers to engine designs which are not supercharged i.e. the air intake has no turbocharger or supercharger. Peak torque: The point in an engines power band where the maximum torque is produced. This corresponds to when the maximum amount of fuel is injected into the cylinders per power-stroke. Positive displacement pump: A pump having an output volume which is the same for every rotation. For example, one revolution would produce 1 litre of volume. 100 revolutions produces 100-litres. Turbo lag: Generally observed to be the time delay between hitting the throttle and the turbo providing full boost. It is more accurately the time required for a turbocharger to spool or accelerate to a speed sufficient to produce maximum boost for the engine load factor. Thermal efficiency: Refers to the comparison between the heat energy produced by burning fuel and the amount of energy converted into mechanical energy available at the flywheel. Volumetric Efficiency: Comparison between theoretical cylinder volume and actual volume filled during operation Transient emissions: Particulates and other by products of poor combustion caused by turbo lag and engine over fueling during acceleration. Scavenging: Refers to the purging of exhaust gases from the cylinder at the end of exhaust stroke. Spool: The acceleration of a turbocharger from low to high speed rotation. The amount of time a turbocharger takes to spool is referred to as turbolag.

14 Created by Gus Wright Centennial College 14 Valve overlap: Refers to the period at the end of exhaust stroke and the beginning of intake stroke when both the intake and exhaust valves are open. Large overlap promotes better scavenging of exhaust gases and improved cylinder charging. Vibration sorting rig (VSR): A device used to dynamically balance a turbocharger. Compressed air is pushed into the turbocharger discharge to spin the turbocharger and determine where material needs to be remoeved from the turbine and or compressor wheel. Volute: Refers to the scroll or snail shell shape of turbine and compressor housings. Nozzle: Is the slot or opening of the turbine volute or housing directing exhaust gases onto the turbine wheel. The diffuser is the equivalent term for the compressor housing. Wide Open throttle (WOT): When the throttle is depressed fully for maximum engine speed. Below: Sliding nozzle open and Closed - Holset Above: Vibration sorting rig The VSR Bench is designed to spin the complete turbo assembly at real-life operating speeds, graph the loads on the turbine shaft and display the angle at which the imbalance is occurring. This allows the shaft to be re-balanced without disassembly and ensures a precise knowledge of the state of the turbo being reinstalled.

15 Created by Gus Wright Centennial College 15 Actuator mechanism for unison ring of VNT turbo Electric actuator motor for Cummins B series VGT turbo Above Airfoils and unison ring of VNT turbo used on DT-466