Week-6 : Turbocharger Modelling

Turbochargers

Turbochargers are centrifugal compressors driven by an exhaust gas turbine and employed in engines to boost the charge air pressure. Turbocharger performance influences all important engine parameters, such as fuel economy, power, and emissions.

A turbocharger consists of a compressor wheel and exhaust gas turbine wheel coupled together by a solid shaft and that is used to boost the intake air pressure of an internal combustion engine. The exhaust gas turbine extracts energy from the exhaust gas and uses it to drive the compressor and overcome friction. In most automotive-type applications, both the compressor and turbine wheel are of the radial flow type. Some applications, such as medium- and low- speed diesel engines, can use an axial flow turbine wheel instead of a radial flow turbine.

Types of Turbochargers

1. Single-Turbos:-They are a cost-effective way of increasing engine power and efficiency, and as such have become increasingly popular, allowing smaller engines to increase efficiency by producing the same power as larger naturally-aspirated engines, but with a lower weight. They do however tend to work best within a narrow RPM range, and drivers will often experience ‘turbo-lag’ until the turbo starts to operate within its peak revolution band.

2.Twin-Turbo:-Twin-turbos mean adding a second turbocharger to an engine. In the case of V6 or V8 engines, this can be done by assigning a single turbo to work with each cylinder bank. Alternatively, one smaller turbo could be used at low RPMs with a larger turbo for higher RPMs. This second configuration (known as twin sequential turbocharging) allows for a wider operating RPM range, and provides better torque at low revs (reducing turbo lag), but also gives power at high RPMs. Unsurprisingly, having two turbos, significantly increases the complexity and associated costs.

3.Twin-Scroll Turbo:- Twin-scroll turbochargers require a divided-inlet turbine housing and exhaust manifold that pairs the correct engine cylinders with each scroll. independently. For example, in a four-cylinder engine (with a firing order 1-3-4-2), cylinders 1 and 4 might feed to one scroll of the turbo, while cylinders 2 and 3 feed to a separate scroll. This layout provides more efficient delivery of exhaust gas energy to the turbo, and results and helps provide denser, purer air into each cylinder. More energy is sent to the exhaust turbine, meaning more power

4.Variable Geometry Turbocharger (VGT):- VGTs include a ring of aerodynamically-shaped vanes in the turbine housing at the turbine inlet. In turbos for passenger cars and light commercial vehicles, these vanes rotate to vary the gas swirl angle and the cross-sectional area. These internal vanes alter the turbos area-to-radius (A/R) ratio to match the engines RPM, and so give peak performance. At low RPM, a low A/R ratio allows the turbo to quickly spool up by increasing exhaust gas velocity and. At higher revs the A/R ratio increases, ther4eby allowing increased airflow. This results in a low boost threshold reducing turbo lag, and provides a wide and smooth torque band

5.Variable Twin-Scroll Turbocharger (VTS):- VTS turbocharger combines the advantages of a twin-scroll turbo and a variable geometry turbo. It does this by the use of a valve which can redirect the exhaust airflow to just a single scroll, or by varying the amount the valve opens can allow for the exhaust gases to split to both scrolls. The VTS turbocharger design provides a cheaper and more robust alternative to VGT turbos, meaning it is a viable option for petrol engine applications

6.Electric Turbochargers:- An electric turbocharger is used to eliminate turbo lag and assist a normal turbocharger at lower engine speeds where a conventional turbo is not most efficient. This is achieved by adding an electric motor that spins up the turbo’s compressor from start and through the lower revs, until the power from the exhaust volume is high enough to work the turbocharger. This approach makes turbo lag a thing of the past, and significantly increases the RPM band within which the turbo will efficiently operate.

Examples from GT suite

-GT POWER 3.4L V6 Twin/Parallel Turbo GDI Engine

-Fast running 4 cylinder Diesel Engine with a variable geometry turbocharger

- 4 Cylinder Turbocharged Gasoline Direct Injection engine with turbocharger wastegate controller and predictive 1D intercooler

- Fast running 4 cylinder Diesel Engine with a variable geometry turbocharge and high pressure EGR

-4 Cylinder Turbocharged Gasoline Direct injection engine with turbocharger wastegate contiller and semi predictive intercooler

- Elementary gas phase reaction mechanism to oxidize the unburned fuel vapor in the exhaust manifold

Tutorial 6 and 7

- Tutorial 6

This model consists of 6 cylinders with a Turbocharger. Turbocharger comprises of 2 inlets to the turbo , as can be seen from two inlets to the turbo at the right and compressor outlet at the bottom. Twin turbine inlets are used for more than 3 cylinders. Outlets of different cylinder were combined with help of flow split template. Inputs for turbine and compressor are absolute pressure, temperature and composition of air.

-Tutorial 7

In this model the actual turbocharger setup is implemented, where the turbine is connected with compressor impeller. There is a turbine map and a compressor map for the inputs that can be customized depending on the type of turbine and compressor. Inputs such as Rotational speed, Mass flow rate, ratio and efficiency can be inserted.

Operating points on compressor and turbine maps

- Compressor

Pressure vs Mass Flow Rate

-Pressure Ratio vs Speed

-Pressure vs Volumetric Flow rate

Turbine

- Reduced Mass Flow vs Pressure Ratio

-Mass Flow Flt vs Data

-EfficiencyFLT vs Data

In which application Variable Geometry Turbine is beneficial?

The variable geometry turbine allows significant flexibility over the pressure ratio across the turbine. In diesel engines, this flexibility can be used for improving low speed torque characteristics, reducing turbocharger lag and driving EGR flow.

The benefits of variable geometry turbines are

• no throttling loss of the wastegate valve;
• higher air–fuel ratio and higher peak torque at low engine speeds;
• improved vehicle accelerations without the need to resort to turbines with high pumping loss at high engine speeds;
• potential for lower engine ΔP (the difference between exhaust manifold and intake manifold pressures);
• control over engine ΔP that can be used to drive EGR flow in diesel engines with High Pressure Loop (HPL) EGR systems;
• a better ability to cover a wider region of low BSFC in the engine speed–load domain;
• ability to provide engine braking;
• ability to raise exhaust temperature for aftertreatment system managemen

In many designs, a variable geometry turbine does not include a bypass so the turbine must be capable of handling all of the exhaust flow from the engine while avoiding overboost and overspeeding the turbocharger. For a given engine power rating, this would imply a larger turbine swallowing capacity than that required by a wastegated fixed geometry turbine and comparable with that used for a fixed geometry turbocharger with no bypass

The fundamental difference between a fixed geometry turbine and a variable geometry turbine is illustrated in the below figure. Compared to a fixed geometry turbine, the variable geometry turbine allows significant flexibility over the pressure ratio/flow relationship across the turbine and by extension, the engine ΔP. This flexibility can be used for improving low speed torque characteristics, reducing turbocharger lag and in diesel engines, driving EGR flow.

Fixed Geometry

Variable Geometry

The peak efficiency of a variable geometry turbine occurs at about 60% nozzle opening. It is usually comparable to or a few percent lower than that for a fixed geometry turbine. However, efficiency drops off rather quickly as nozzle opening is reduced or increased from a mid-vane opening position

Diesel VGT EGR

Working

- This is a 4 cylinder 16 valve engine with EGR cooler and VGT

- The ambient air is sucked through the inlet and air is passed through air filters where impurities can be removed

- The intercooler reduces the temperature of air, thereby increasing the density.

- Air passes through throttle valve

-From throttle valve air is passed through two inlet poerts into cylinder where combustion takes place during power stroke.

-During exhaust stroke, exhaust gases and air passes through manifold. A portion of air goes through Exhaust Gas Recirculation to reduce NOx

-Turbine and compressor connected to a single shaft, turbine drives the compressor and aids in compressing air.

Detailed description of the model is given below

Model Description

-The Engine

The engine is a 4-cylinder, four-stroke, direct injection, 2L diesel engine. The engine runs in "speed" mode, where the engine speed is entered by the user and the engine torque is calculated. Combustion is modeled by the DIPulse method, a predictive combustion model using the detailed injection profiles. In-cylinder heat transfer is modeled using the Woschni model. A FE model is used to predict the cylinder, head and piston temperatures. This simulation will run until it reaches steady-state convergence for six different engine speeds at full load, three part load operating points, and an additional idle point (total of 10 cases)

- The Intake system

The intake system consists of the air piping upstream of the compressor inlet, and contains a simple model of an air box. The air box is modeled as a group of interconnected flowsplits that represent the total volume of the air box. At the inlet and outlet of the air box, "bellmouth" orifice connections are used to model smooth transitions.

-The Intercooler
The intercooler is found downstream of the compressor outlet. This intercooler is an air-to-air intercooler that is modeled using a "semi-predictive" method based on heat exchanger effectiveness.
The intercooler is modeled using a pipe part. The Number of Identical Pipes attribute is used to model a bundle of 25 identical pipes of 7.0 mm diameter each (giving a large surface area for heat transfer). The pressure drop across the intercooler is calibrated using the Friction Multiplier in the in the pipe. Flowsplits are required to model the inlet and outlet of the intercooler since pipes cannot connect to pipes with the Number of Identical Pipes attribute greater than one. The desired intercooler outlet temperature is calculated by a simple control system and is imposed by actuating the wall temperature of the intercooler pipe. The Heat Transfer Multiplier in the pipe is increased to a large enough value so that the outlet temperature matches the wall temperature. The desired outlet temperature is calculated in a 'IntercoolerEff' part from the effectiveness table of the intercooler.

-The Intake Manifold
The intake manifold is modeled as aluminum using heat conduction objects to calculate the wall temperatures. This is important in turbocharged models since wall temperatures may change substantially at different operating points, thus influencing the cylinder inlet temperature. The orifices that connect. The initial conditions are different from ambient conditions. The initial temperatures and pressures are specified at the expected boost pressure in order to decrease the convergence time.

-The Fuel Injection System
The fuel is injected directly into the cylinders. Both the injection timing and the injection profile are looked up using an 'RLTDependence' as a function of engine speed and injection rate. There is a simple control system that determines the total injected fuel mass (mg/stroke). This control system begins with a 'SignalGenerator' part that imposes an accelerator pedal position input. The "requested fuel mass" is then looked up as a function of engine speed and pedal position. In some cases, the requested fuel mass may produce excess smoke if the boost pressure is not high enough to provide enough air. Therefore, the maximum allowed fuel mass is looked up as a function of the engine speed and the boost pressure. A 'MinMax' part is used to take the smaller of the requested fuel mass and the smoke limited fuel mass. This value is then imposed on the injectors using an actuator.

-The Exhaust System
The exhaust manifold is modeled as cast iron using heat conduction objects to calculate the wall temperatures. This is very important in the exhaust manifold since wall temperatures change substantially between full load and idle condition, thus influencing the turbine inlet temperatures and turbine power. The orifices that connect the exhaust ports to the runners do not allow heat to be conducted between the pipe walls of the adjacent parts. This is necessary because the wall temperature is calculated in the exhaust pipes, while it has been imposed in the port (the imposed value is artificially high to account for heat transfer from the valve). The initial conditions are different from ambient conditions. The initial temperatures and pressures are specified at the expected backpressure in order to decrease the convergence time. The turbine part references a map file from which map data is read.

-The EGR System
The model includes an exhaust gas recirculation (EGR) system to transport exhaust gases from the exhaust manifold back to the intake manifold. The EGR flow is driven by a positive pressure difference between the exhaust manifold and intake manifold and is controlled by an orifice connection (EGR-valve). The EGR-valve diameter is imposed by an actuator, and is determined automatically by an 'ControllerEGRValve' part to achieve a target EGR fraction. The desired EGR fraction is mapped as a function of engine speed and the requested BMEP. EGR rates in the model are as high as 40% at some operating points. Because of the high EGR rates, it is desirable to cool the EGR gases using a heat exchanger (EGR-cooler). When the pressure drop across the EGR system is not sufficient to achieve the target EGR rate, a throttle in the intake system, controlled by a PID controller, increases the pressure difference. This controller is only active, if the EGR-valve is fully open.

The EGR cooler is modeled using 'HxMaster' and HxSlave' parts. Coolant side boundary conditions are imposed. The heat exchanger model inputs include geometry along with measured heat transfer and pressure drop performance data. During model pre-processing, the measured data is used to generate non-dimensional correlations for heat transfer that allow the model to accurately predict heat transfer rates at any operating condition. The pressure drop data is use to automatically calibrate physical characteristics (friction multiplier, orifice Cd) of the heat exchanger so that the data is well matched.

-The Turbocharger and Boost Controller
The turbocharger in this model contains a fixed geometry compressor and a variable geometry turbine (VGT). For the VGT, there are 5 different sets of turbine map data entered at each of 5 VGT rack positions. The VGT rack position is determined by a control system to achieve a target boost pressure upstream of the intake manifold. The boost pressure is sensed and is then filtered using a 'MovingAverage' part to produce an average pressure signal. The model based controller compares the sensed boost pressure to the target boost pressure and then adjusts the VGT rack position accordingly. .

-The Exhaust System
A full vehicle exhaust system is modeled downstream of the turbine outlet. The geometry of a Diesel Oxidation Catalyst (DOC) is modeled using a 'CatalystBrick' part. In this example only the pressure drop across the DOC is of interest, so no chemical reactions are modeled in the DOC. The inlet and outlet of the DOC must be modeled using flowsplits because the DOC is effectively a bundle of very small pipes.

Just downstream of the DOC is a Diesel Particulate Filter (DPF) using a 'DieselParticFilter' part. In this example, only the geometry attributes are specified so that proper pressure drop across the filter is modeled, but there is no soot accumulation or regeneration modeled. Similar to the DOC, the inlet and outlet of the DPF must be modeled using flowsplits.

Downstream of the DOC is a muffler. This muffler was modeled using GEM3D, which enables complex muffler geometry to be entered in a CAD tool and automatically discretized into a GT-POWER model. The muffler in this example is a tri-flow type muffler with two baffles. The center chamber between the two baffles is filled with wool, and various internal pipes and baffles include perforation. The .gem file is included in the directory and can be opened in GEM3D to see the muffler. The output of the GEM discretization is stored within an internal subassembly to keep the main project map clean.