Week-6 : Turbocharger Modelling

The following report will try to achieve the following objectives:

• Briefly explain the working of TurboCharger.
• List different types of Turbochargers by providing examples from GT-Power
• Exploration of the tutorial examples 6 and 7 available through the GT-Suite application
• Describe the results of the tutorial case 7 in the form of compressor and turbine maps
• Provide examples of engineering applications where Variable Geometry turbocharger is used.
• Exploration of the GT-Suite example of VGT-EGR and summarise the details about the modelling part.

Section 1: Turbocharger

Turbocharger is a forced-induction device which is driven by a turbine and its purpose is to increase the efficieny and power of the Internal Combustion Engine. This equipment consists of a turbine and compressor.

Compared to the natural aspirated piston engines, the compressor of the turbocharged piston engine can force more air (as a result more fuel) by increasing the density of air, thus packing more mass in the combustion chamber.

This increased density will lead to generation of more power per engine cycle by increasing the amount of fuel-air mixture or makes the system more fuel efficient for generating same amount of power.

To compress the ambient air, the compressor requires power. This energy requirement is sourced by recovering waste energy from the exhasut gases by allowing them to flow through a turbine.

The illustration of a turbocharger and the working principle behind it is provided below: Source

Section 2: Types of Turbocharges and their GT-Power models

The most commonly used turbochargers in the automotive industry are as follows:

• Single-Turbo
• Twin-Turbo
• Twin-Scroll-Turbo
• Variable Geometry Turbo
• Variable Twin Scroll Turbo
• Electric Turbo

Single Turbo:

These devices are popular because they are a cost-effective method to increase engine's efficiency and power, especially for smaller engines. They generally work best in a narrow spectrum of RPM. Despite of its advantages, a major drawback of this device is that the vehicle will experience 'turbo-lag' till the it reaches the band of its operating range.

Twin-Turbo:

Adding an additional turbocharger to a turbocharged engine is called as Twin-Turbo Engine. There are various configurations of twin-turbo engines. For engine with six to eight pistons, each cylinder block can provide exhaust gases to its corresponding turbocharger. Another popular twin-turbo setup is to use a smaller turbo for operating at lower RPMs and similarly larger one for the higher RPM. This type of configuration is known as sequential turbocharging and enlargens the range of operating RPM.

Twin-Scroll Turbo:

To ensure that the turbine operates smoothly, without any disruptions due to the firing order of the engine cylinders, twin-scrool turbochargers are used. This turbocharger configuration consist of separated inlet-turbine and exhaust manifold housing. This layout provides a continuous and balanced supply of exhaust gas to the turbine thus ensuring that compressor performs optimally to deliver compressed air.

Variable Geometry Turbo:

As the name suggests, the VGT include a ring of vanes which are optimized for aerodynamic performance. These vanes can rotate which can lead to variation in the gas swirl angle and cross-sectional area. The vanes are added so that they can change the turbo's area to radius ratio so that it matches the engine's RPM and provides peak performance. The RPM and A/R ratio directly propotional providing higher exhaust gas velocities at low A/R ratio and low boost threshold for high A/R ratio due to increased airflow.

Variable Twin Scroll Turbo:

This layout combines the benefits of two types of turbo geometry: The large operating range of variable turbocharger and the balancing effect on the turbine of the Twin Scroll turbocharger.

Electric Turbocharger:

The advantages of using an electric turbo is to eliminate the phenomenon of turbo-lag and operating turbocharger at low RPM where mechanical turbine is not efficient and suitable. The working principle behind the electric turbo is to aid the spinning of the compressor when it is at rest till the point where power from the exahaust gas is enough to power the compressor alone. This drastically widens the range of operating values of RPM as now the compressor can be sufficiently powered. Although several benefits are provided by the electric turbocharger, its cost and complexity, particularly relating to the sufficient cooling are still a topic of heavy research.

Section 3.1: Tutorial 6

In this tutorial model, we observe that the engine has 6 firing cylinders. As a result, since the engine has more than 3 combustion chambers, twin scroll turbine is used, where each inlet of the turbine is connected with the exhaust manifold of the 3 cylinders. For reducing the computational complexity, the rotating shaft is not modeled, instead the values are provided for the turbine inlets and the compressor outlet. Although it increases the speed of the simulation, the results obtained are not representative of the actual operating conditions.

The one-dimensional model of the tutorial 6 problem is as follows:

Section 3.2: Tutorial 7

Identical to the previous model, the tutorial 7 model is also a 6 cylinder diesel engine with a twin scroll turbocharger. The difference between the previous and the current setup is that the turbocharger is actually modelled instead of fixing the output values, as in the previous case. To properly replicate the effects of exhaust gas on the turbine and compressor's ability to increase density of the inlet gas, we use a turbine and compressor map for respective tasks. This information is plot between the pressure ratio and mass flow rate, and is obtained from the turbomachinery manufacturer. Additionally, rotational speed and efficiency is provided.

The one-dimensional model of the tutorial 7 problem is as follows:

The turbine map used for tutorial 7:

The compressor map used for tutorial 7:

Section 4.1: Simulation results of tutorial 6:

We run the tutorial setup with parameters of friction multiplier set to 1 and engine speed set to 3600 RPM. Since this tutorial only provides end effects of using a turbocharger without modeling a compresor or turbine, we provide summary of the engine and cylinder operational performance. We can observe that the volumetric efficiency is > 100% because of the compressor outlet and turbine inlets.

Section 4.2: Simulation results of tutorial 7:

For this case, we choose the same parameteric value for friction multiplier as that of the previous simulation .Since, we simulate the compressor and turbine along with the rotating shaft in this study, we can plot the operational values turbocharger on the compressor and turbine maps.

The maps for the turbocharger are colored by either the RPM of the rotating shaft or their efficiency. These plots are further shown with reduced and corrected mass flow rate and also corrected volume flow rate. For this study, we will only use the mass flow rate on the x-axis.

Compressor Plots:

Speed Map - Reduced Mass flow rate

Efficiency Map - Reduced Mass flow rate

Speed Map - Corrected Mass flow rate

Efficiency Map - Corrected Mass flow rate

Turbine Plots:

Speed Map

Reduced Mass Flow Map vs PR and Speed

Efficiency Map

Efficiency Map vs PR and Speed

From this maps we deduce the following observations:

• The rotating shaft the joints the compressor and turbine is rotating at almost the maximum allowable value. This is not a favorable situation as it can lead to damage to the turbocahrger.
• From the compressor efficiency maps, we can see that the current operating point is far away from the heart region of the map and is intimate to surge line of the chart.
• Turbine speed maps also are inline with the observations from the compressor speed i.e. the operating point is at its maximum speed limit and thus not very efficient as it can be.
• The turbine efficiency charts show that current operating point lie in the region of maximum turbine efficiency. This illustrates that tweaking the performance of a turbocharger is a difficult process.

Section 5: Applications of Variable Geometry Turbine

Initially, VGT turbocharger were used for particularly diesel engine applications as they were operational only on low exhaust gas temperature. But, due to the advancement in the materials which have high thermal resistance, the VGT has been increasingly used in the gasoline engines.

The engineering applications where VGT have been a highly successful solution are:

• Passenger cars
• Race vehicles
• Light commercial vehicles

These applications have varied requirements at different engine speeds and VGT provides them the suitable solution as it provides high pressure boost at low speeds and lower turbo lag compared to its counterparts. Furthermore, at high engine speeds, it prevents overboosting thus has a direct impact on fuel economy. From this, we can conclude that VGT are an appropriate choice for applications which operates on long operating scale.

Section 6: Exploring the VGT-EGR example case

To understand how to use the modeling libraries provided by GT-Suite, implementation of engines of varying complexity are provided by the software developers. In this section, we will explore one such case of implementation of 4 cylinder diesel engine with a Variable Geometry turbocharger and exhaust gas recirculation system.

The author of this example wants to display that you can model an engine rig with such a complexity with ease using their software and also save them a significant amount of time and costs by not performing experimental optimization.

The Diesel VGT EGR model is as follows:

The modeling part of the Diesel EGT VGR is as follows

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).

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.

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. Note that if heat exchanger performance data is available, it is generally preferred to model the heat exchanger using 'HxMaster' and 'HxSlave' parts (which will use the available data to create non-dimensional Nusselt correlations defining heat exchanger performance). This approach will generally extrapolate better than an assumed effectiveness to conditions beyond those present in the heat exchanger data. However, if heat exchanger data is not available (or if the simulation will be run at conditions where HX effectiveness is well known), the semi-predictive effectiveness method is appropriate.
• 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.

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.

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.

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.

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.

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.

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.