To perform 1D simulation on Turbocharger model with EGR configuration and explore various models available in GT-SUITE

Objective: To explore and understand the Turbocharger model use in automotive.

1. List down different TC and locate it into GT power
2. Explore Tutorial 6 and 7
3. Plot operating point on compressor and turbine maps
4. Benefits of VGT (Variable Geometry Turbine)
5. Explore Diesel VGT EGR and understand the modeling part.

Theory:

Turbocharger

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 the 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. The flow of gases through a typical turbocharger with a radial flow compressor and turbine wheels is shown in Figure above.

Different types of turbocharger:

There are different types of turbochargers used within the automotive industry.

1. Single turbo-
   It is having limitless variabilities. we can achieve completely different torque characteristics by differing the size of elements within the turbocharger. Large turbochargers provide a high level of top-end power and small turbocharger can spool faster and provide better low-end power. They are a cost-effective way of increasing the engine's power and efficiency. But it has a fairly narrow effective RPM range. Turbo response may not be as quick as alternative turbo setups.
2. Twin-turbo-
   As the name suggests twin-turbo means adding a second turbocharger to the engine, this can be done by assigning one turbocharger to one cylinder bank and second another bank. Alternatively, small turbochargers run at lower engine speed and bypass the large turbochargers for higher RPM. 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-
   It requires divided inlet housing and exhaust manifold that pairs the correct engine cylinder with each 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. Again, the cost involves more for addressing the complexity of a system requiring complicated turbine housings, exhaust manifolds, and turbos.
   
   
4. Variable Geometry Turbo-
   It includes a ring of aerodynamically shaped vanes in turbine housing at the turbine inlet. This turbocharger is generally used for passenger cars and light commercial vehicles. This vane rotates to vary the gas swirl angle and the cross-sectional area.
   
   
5. Variable twin-scroll turbo-
   As the name suggests it combines the advantages of both twin-scroll turbocharger and VGT turbocharger. The VTS turbocharger design provides a cheaper and more robust alternative to VGT turbocharger.
   
   
6. Electric Turbo-
   It is used to eliminate turbo lag and assist a normal turbocharger at a lower engine speed where the conventional turbocharger is not most efficient. This is achieved by a coupling compressor with an electric motor where the compressor runs from the start and through the lower revolutions until the power from the exhaust volume is high enough to work the turbocharger. This approach eliminates the turbo lag and helps to increase the RPM band significantly within which turbocharger efficiently operates. But the cost associated with it is very high.
   
   

Fixed Geometry Turbine with Exhaust side bypass (Wastegate):

In this simplest turbocharger design, the turbine and compressor geometry is fixed and the boost pressure is entirely determined by the K.E. of exhaust gas flow. An exhaust side bypass, or a wastegate, is a common means of achieving better boost pressure control with fixed geometry turbines. The wastegate can be built into the turbine side of the turbocharger or it can be a separate valve connected to the external plumbing. Pressurized pneumatic wastegate actuation has been common, but vacuum actuation and electric actuation is utilized in many newer designs.

For applications that experience a wide range of operating conditions and that must provide a good dynamic response, for example in passenger car applications, a fixed geometry turbocharger with no control of boost pressure is unsuitable. Two methods can be used to control boost pressure for turbochargers with fixed geometry turbines in these applications:

• Exhaust side bypass, also known as a wastegate, to bypass some of the flow from the turbine inlet, or
• Inlet side bypass to bypass flow from the compressor inlet.

Adding the exhaust valve that allows some exhaust gas to bypass the turbine is a more common means of achieving better boast pressure control with fixed geometry turbines. Adding a wastegate to the turbocharger with the smaller turbine can significantly improve boost at lower engine speeds while avoiding overboost and overspeeding of the turbocharger at higher speeds. The amount of improvement depends on how well the wastegate is controlled.

Variable Geometry Turbine (VGT):

The alternative to Fixed Geometry Turbine is Variable Geometry Turbine. The benefits of variable Geometry turbine over wastegate includes:

1. increase intake air boost pressure at low engine speed
2. improve the response time of the turbocharger during transient engine operation phases
3. increase the availability of the maximum engine torque
4. prevent over-boosting at high engine speed
5. reduce exhaust gas emissions and improve fuel economy

The fundamental difference between a fixed geometry turbine and a variable geometry turbine is illustrated in 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.

To locate it into GT-POWER:

Path for Wastegate TC:
`"Examples " ->"Engine 1D gas exchange combustion "->" Diesel"->" Diesel WGController"`

Path for VGT EGR TC:
`"Examples " ->"Engine 1D gas exchange combustion "->" Diesel"->" Diesel VGT EGR"`

Path for Fixed Geometry turbocharger with power turbine:
`"Examples " ->"Engine 1D gas exchange combustion "->" Diesel"->" Diesel 6cyl TC"`

Tutorial No.- 6:

In this tutorial, we have a fixed compressor outlet and the turbine inlet. The modeling of compressor and turbine with characteristics maps have been completely ignored. We need not worry about turbocharger matching since we have assumed a fixed value of compressor outlet and turbine inlet. The same boost pressure we are going to get for all range of RPM.

Tutorial No.- 7:

This tutorial is a step by step advancement of tutorial no 6, where some of the modifications are made at intake and exhaust side of the diesel DI 6 cylinder engine.
We are going to look for step by step advancement below:

STEP-1: In this step, engine configuration is very much similar to the engine configuration shown in tutorial 6. Fixed boost pressure of 2.4bar and temperature 400K we get at the outlet of the compressor.

STEP-2: Here, the compressor with a speedboundary of constant imposed speed 96200 RPM has been added with a compressor map which includes important operating characteristics such as pressure ratio, flow parameters, range of RPM, isentropic efficiency. The compressor is taking inputs from the environment such as pressure 1bar and temperature 298K.

STEP-3: In this step, the turbine along with a compressor has been added. The turbine is also limited to speedboundary of the constant imposed speed of 96000 RPM the same as the compressor and also defined by the turbine characteristic map.

STEP-4:

STEP-5: In this step, the compressor and turbine are coupled with a shaft.

Results:

Tutorial 6 and tutorial 7 we have run by considering all the parameters the same as the default. We run the cases for 3600 RPM and keep the same engine configuration.

Engine configuration:

Engine predictions:

After running a simulation for all steps we have summarised the results shown below.

The conclusions we can draw from the above results are as follows:

1. As we can see, there is a gradual change in power output from the engine, but at step 2 where the only compressor is attached power has increased to 303.3 HP but as we compressor is a work consuming device it will required some amount of power to run. In this case, 80.2 KW of power is required by the compressor for its run which then lowers the value of power output.
2. The volumetric efficiency of the engine with the same geometry has been increased to 265.5 % from 185.4% because by using the compressed air we are doing forced induction of air.
3. Also, we can observe a slight improvement in BSFC value where the only air flow rate is increased and A/F ratio has increased to 30.2
4. The boost pressure we get for this configuration is 3.56 bar.
5. As we can see below, power required by the compressor is now taken from the shaft connected between them by utilizing the high K.E. of exhaust gas coming from the exhaust port.

Operating Points:

On Compressor map-

Here, we can see the operating points of the compressor are on a little higher side. All we needed to make sure it will work under the 'heart' region of the map. The operation of the compressor is close to the surge line if this point shifts to the left of surge line even further then flow may stop or flow reversal may take place.

On Turbine map-

Diesel VGT EGR:

This model using various important modeling concepts which is used in automotive diesel engine such as variable geometry turbine, boost control, intercooler, EGR circuit with EGR rate control & EGR cooler, injection limiting for smoke control, and exhaust after-treatment device modeling.

1. EGR circuit:
   This model has employed EGR system in order to limit NOx formation by recirculating the fraction of exhaust gas coming out of exhaust port. It is driven by a positive pressure drop between the exhaust manifold and intake manifold and is controlled by a throttle connection i.e. EGR valve whose diameter is imposed by an actuator and is determined automatically by a controllerEGRvalve part to achieve target EGR fraction. The target EGR fraction is mapped by engine speed and target bmep. The EGR cooler also added if at some operating point EGR fraction exceeds 40% then it is necessary to cool the high-temperature exhaust gas.
2. Turbocharger and boost controller:
   In this model, we are using a fixed geometry compressor and variable geometry turbine (VGT). For VGT, 6 different turbine maps are defined under a single rack position and the rack position is determined by a control system to achieve a target boost upstream of the intake manifold. This model sensed to actual boost pressure and compare it with target boost pressure then adjust rack position accordingly.
3. Intercooler:
   The main function of the intercooler is to cool down the incoming compressed air, to increase air density. The intercooler is found downstream of the compressor outlet. This intercooler is modeled using the 'pipe' part.
4. Exhaust system:
   The complete exhaust system is modeled downstream of the turbine outlet. It consists modeling of Diesel Oxidation Catalyst (DOC) and Diesel Particulate Filter (DPF). The geometry of DOC is modeled using a 'CatalystBrick' part. We are very interested only in pressure drop across DOC, no chemical reactions are modeled in the DOC.
   After DOC there is Diesel Particulate Filter (DPF) which is being modeled using the 'DieselParticFilter' part. Both DOC and DPF the inlet and outlet have been modeled using flowsplits. Downstream of the DPF is the muffler. This Muffler is being modeled using 'GEM3D', which enables complex muffler geometry to be entered in CAD tool and automatically discretized into a GT-POWER model.

Results:

This model we have run by considering various parameters.

As we can see, we have defined our target bmep at specified RPM of the engine based on that fuel injection, boost pressure, air induction, A/F ratio, EGR fraction will be set by the controller.

Engine Performance prediction:

Some of the observations we can have looking at the results above:

1. Based on the application, at lower speed engine is producing high power and high torque because we had set bmep value accordingly.
2. We can see boost pressure or compressor outlet pressure is maintained to on an average 2.39 bar hence A/F ratio is maintained to 17 in all speed range and consequently, BSFC value get lesser for low RPM.
3. The turbine and compressor is running with a very high speed than the designed value hence it may lead to mechanical failure.

Operating Point:

On Compressor map at 4500 RPM:

We can see here, the operating point is near the choke region on the compressor map which exhibits the maximum mass flow rate at particular pressure ratio.

On Turbine map at 4500 RPM:

CONCLUSION:

In this way, we have successfully explored the tutorial 6 in which 6cylinder DI engine is configured with a fixed compressor outlet and two-way turbine inlet and also explored tutorial 7 where the same 6 cylinder DI engine is configured with fixed geometry compressor with a compressor map and fixed geometry turbine with turbine characteristics map connected by shaft between turbine and compressor.
Also, we have discussed engine performance in both cases.

We have successfully explored the Diesel engine with VGT and EGR configuration. We also discussed the advantages of using this configuration how it will be benefitted in engine performance. In VGT, different characteristics maps use for different rack positions based on bmep value and engine speed.