Modelling of Turbocharger and Supercharger in GT Power

INTRODUCTION ON TURBO/SUPER CHARGING:

Peformance of any engine is restricted by amount of air entering the cylinder. More the air available, more the fuel can be burnt and higher efficiency can be achieved. Naturally aspirated engine takes in air from the ambient and may face trouble in combustion and result in high emissions.

To increase the amount of air entering the cylinder, a turbocharger or a supercharger can be used. They are basically a compressor which compresses the ambient air before sending it to the engine. More air will be available in the engine when sent in compressed form.

Turbocharger is a compressor which takes energy from the exhaust kinetic energy of the engine, makes it into usable from with the help of a turbine and uses this energy for the action of compression of ambient air. When the compressor directly takes energy from the engine for compression, then it is termed as supercharger.

In GT-Power, we define the compressor and turbine object with reference temperature and pressure. To supply compressor / turbine data in GT, a standard SAE data file can be input. This standard SAE data file will be given by the supplier. Also every comperssor/ turbine has a performance chart data also called as compressor map or turbine map which has parameters such as RPM, Mass flow rate, Pressure ratio and efficiency. This performance chart should also be given as input. This data as well is given by the supplier. A sample of compressor map is shown below. The horizontal lines represent the compressor speed lines (RPM) and the vertical lines represent the efficiency lines. The compressor is always to be run in the heart region.

DIFFERENT TURBOCHARGER TYPES IN GT-POWER

1) TC with Power turbine

In this type of turbocharger, along with the exhaust kinectic energy from the engine, additional power is taken from the enigne via a power turbine, for more efficient air compression.

2) TC with VGT:

The turbocharger has a variable geometry turbine that controls the amount of kinetic energy entering the the turbocharger. This can help in preventing mechanical failure by exceeding the max pressure in the cylinder.

3) TC with WG Conttroller

This turbocharger contains a waste gate controller. The waste gate opens up to the ambient. When the pressure attained is more than the set value, the extra kinetic energy is diverted to the waste gate.

4) Twin Turbo

This configuration is used for bigger engines. In this model, three cylinders use air charge from one turbocharger and remaining three cylinders use air charge from another turbocharger.

5) Twin Scroll Turbine

Turbocharger takes exhaust air from cylinder. To prevent interruption of exhaust air charge from different cylinders due to different firing order, the turbine is given two different entry points. Hence it is called Twin scroll turbine.

TUTORIAL 6 ON TURBOCHARGING:

Model Information:

In this tutorial, the model has been given the properties of compressed air values as the ambient temperature and pressure. Hence, instead of having a separate reference model for a compressor or turbine, the temperature of the compressed and expanded air can directly be imposed as the end environment. This approach can be used when modelling of turbine and compressor is not important. The imposed temperature and pressure conditions are shown below:

Compressor-In Conditions:

Turbine-In Conditions:

Outputs post run:

TUTORIAL 7 ON TURBOCHARING:

Model Information: In this tutorial, the compressor and turbine model has been explicitly defined. The compressor module takes ambient air as input and is driven by a turbo-shaft which in-turn is rotated by a turbine that is powered by the exhaust kinetic energy of the engine. During low speeds of engine, there might not be enough kinetic energy to rotate the shaft. Hence, there is a delay in the efficeint output of the turbocharger from the start fof the engine to the efficient operation. This delay is called the turbo-lag.

Compressor Definition:

The type of compressor is chosen for modelling. The reference pressure and temperature can be defined either manually or a standard SAE file containing the model details of the compressor can be loaded. This standard SAE file can be obtained from the compressor supplier.

Apart from this, a compressor map (data points of the performance map of the compressor) has to be loaded. Since the compressor used in this tutorial is of fixed geometry configuration, only one rack position is filled with the compressor map object.

Turbine Definition:

The reference pressure and temperature for the turbine can be defined either manually or a standard SAE file containing the model details of the turbine can be loaded. This standard SAE file can be obtained from the compressor supplier. The turbine configuration can be chosen - whether it is a waste-gate turbine or not.

The waste gate turbine (WGT) configuration helps to bypass any extra external kinetic energy from the engine exhaust to the ambient. This acts as a protective measure against high pressures which can lead to the mechanical failure of the turbine.

Turboshaft Definition:

The rotational speed of the turbocharger can be around 2,00,000 RPM. Therefore, it is of utmost importance to design the shaft connecting the turbine and compressor properly. The most important parameter is the moment of inertia which gives the mass distribution of the shaft w.r.t. the axial line. The recommended values of this MOI of turboshaft is given by the supplier.

Outputs post run:

1) Compressor Map

The compressor map is generated with the data that we enter via the compressor map object. The circular point shown below in the compressor map is called the operating point of the system. It can be seen that, the compressor is operating at a Pressure ratio of ~3.6 and the mass flow rate is around 0.78 kg/sec. But this operation is at ~5950 RPM which is high value of the compressor and not desirable. Hence further balancing needs to be done in this setting.

2) Turbine Map

The turbine map is generated with the data that we enter via the turbine map object. The circular point shown below in the turbine map is called the operating point of the system. It can be seen that, the turbine is operating at a Pressure ratio of ~3 and the mass flow rate is around 0.045 kg/sec.

2) Performance

Inference:

The second tutorial uses explicit approach of turbocharger. When the performance is compared with the previous tutorial, better results can be obtained in this method as all related parameters can be altered as per user's need.

EXAMPLE ON VGT TURBOCHARGER:

About VGT:

In fixed geometry turbocharger, there are no means to control the amount of exhaust kinetic energy that enters the turbine. But with the help of a Variable Geometry Turbine (VGT), the same can be controlled. The VGT has guiding vanes present on the periphery of the surface of inlet. These vanes can be adjusted with 0 denoting completely closed position of vanes and 1 denoting completely open position of vanes. By adjusting the vanes electronically, we can aquire infinite possibilities to control the amount of air entering the turbine.

Model Information:

In VGT turbocharger example, a turbine and a compressor is present connected by a turboshaft. GT can understand that the tubine is a VGT by giving inputs data to the rack positions of the turbine. For each rack position, a performance map is defined and given as input. The rack position is controlled by an external electronic module. This module takes in information of requirement of compression pressure and accordingly the rack position is set in the turbine.

Compressor Definition:

VGT Definition:

The VGT is defined by setting data for more than 1 rack position. The turbine data for different rack positions can be obtained from the supplier. A performance map for each rank position is predefined in the given data.

VGT Rack Contol:

According to the required air fuel ratio in the engine, the amount of pressure required at the inlet manifold is determined. This pressure is given as an input to the VGT. According to the pressure requirement, each rack opening is controlled by setting a value between 0 and 1.

EXAMPLE ON SUPERCHARGER:

Model Information:

In superchargers, there are no turbines, the work needed to run compressor is directly taken from engine. In this example, a compressor is present which is coupled to the engine using a gear system and a shaft. Superchargers run at relatively lower RPM compared to turbochargers (~10000 RPM). Hence a gear ratios of around 1.6 - 1.8 are used. The main advantage of supercharger is that, there is no lag in giving higher amount of air to engine as the compressor is directly connected to the engine.

Compressor Definition:

Similar to a turbocharger, a supercharger map is defined. The reference pressure and temperatures are given as inputs.

Compressor to Engine connection specification:

The supercharger is connected to the engine directly using a shaft and a gear system. The specifications of the same is shown below.

Supercharger Performance Map and Volumetric Efficiency:

The operating point of the system can be seen from the performance map of the supercharger. A pressure ratio of ~1.75 is achieved with a mass flow rate of 0.032 kg/sec at RPM of ~ 600.

From the results, it can be seen that, the cylinder volumetric efficiency is more than 100% and the manifold volumetric efficiency is ~105%

SUMMARY:

• Turbochargers/ Superchargers are used to provide more air to the cylinder for better combustion.
• Turbochargers take energy from exhaust kinetic energy of engine whereas superchargers take energy from engine directly.
• Turbochargers face turbo-lag due to lower RPM of engine. Hence turbochargers are often not comfortable for passenger vehicles in cities. Superchargers do not face any such issues.
• To control the amount of kinetic energy entering the turbocharger, many configurations such as Waste gate controller, Variable geometry turbine, etc are used.
• The compressor map or turbine map data is of vital importance and the operating range has to be carefully chosen accoeding to the requirement of power and torque. The data for the same is given by the supplier.
• Usually turbochargers are used in Diesel engines.

CONCLUSION:

Modelling of different types and configurations of Turbocharger was done. Modelling of supercharger was also studied.