Assignment 6

Objective:

• List down different TC and locate examples from GT Power
• Explore tutorial number 6 and 7
• Plot operating points on compressor and turbine maps
• In which application Variable Geometry Turbine is beneficial?
• Explore example- Diesel VGT EGR and understand the modeling part

Different types of Turbochargers:

There are a number of different types of turbocharger available currently in the market and are used in the automotive industry. They are as follows:

1. Single-Turbo
2. Twin-Turbo
3. Twin-Scroll Turbo
4. Variable Geometry Turbo
5. Variable Twin Scroll Turbo
6. Electric Turbo

Single turbochargers: These are what most people think of as turbos. By differing the size of the elements within the turbo, completely different torque characteristics can be achieved. Large turbos provide higher levels of top end power, whilst smaller turbos can spool faster and provide better low-end power. 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 rev band.

Twin-Turbo: As the name implies 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 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.

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. The system is however complex due to its complexity,requiring complicated turbine housings, exhaust manifolds and turbos.

Variable Geometry Turbo:

A Variable Geometry Turbo includes 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 . At higher revs the A/R ratio increases, thus allowing increased airflow. This results in a low boost threshold reducing turbo lag, and provides a wide and smooth torque band.

Variable Twin Scroll Turbo (VTS):

The VTS turbocharger combines the advantages of a twin-scroll turbo and a variable geometry turbo. A valve is used, 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

Electric Turbo:

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.Though the electronic turbos are the answer to all the negative characteristics of conventional turbochargers, however there are some disadvantages.

Disadvantages- The electric turbos are costly and complex. Also,the electric motor must be accommodated and powered and cooled to prevent reliability issues.

Explore tutorial number 6 and 7

Tutorial 6 : Six Cylinder Turbocharged Diesel Engine

The above model is a 6 cylinder Diesel Engine Direct Injection Model.The model does not include turbocharging elements such as 'Turbine' and 'Compressor' but the above model is supplied with input conditions obtained after compressing the air from inlet and exhaust gases sent to a turbine as shown.

Tutorial 7: Turbocharging Final Model

In this model, the turbine is driving the compressor by a intermediary shaft in between and compressing the inlet air. The intercooler is later cooling the compressed inlet air thus increasing the density of inlet air. The model is of a 6 cylinder Diesel engine with two banks of 3 cylinders coming together and they finally releasing the exhaust gases into a twin-scroll turbine since there are 2 inlets for the turbine.

Plot operating points on compressor and turbine maps

Compressor Map:

The compressor map gives us an estimate where the operating conditions of the engine enable the compressor to work and from the above map it can be seen that for a given fuel flow rate for 3600 rpm, the pressure ratio obtained is 3.654 and the mass flow rate is 0.4469 kg/s.

Turbine Map:

In the turbine map , the operating point lies at a pressure ration of 3.0437 and at a reduced mass flow rate of 0.04525 kg/s. The pressure ratio of turbine is inverse of the compressor ratio.

In which application Variable Geometry Turbine is beneficial?

Whilst VGTs are more typically used in diesel engines where exhaust gases are at lower temperature, until now VGTs have been limited in petrol engine applications due to their cost and the requirement for components to be made from special materials. The high temperature of the exhaust gases means that the vanes must be made from heat-resistant materials to prevent damage. This has restricted their use to applications within luxury, high performance engines.

Diesel VGT EGR :

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. .

Sub-Models of Diesel VGT EGR :

EGR Cooler:

Intercooler :

EGR Control :

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.

After-treatment :

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.

Air Box: