Week-6 : Turbocharger Modelling

Week 6 Challenge:

Turbocharger Modelling

Aim and Objectives:

• 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

Turbocharger;A turbocharger, (or turbo), is a turbine-driven forced induction device that increases an IC engine’s efficiency and power output by forcing extra air into the combustion chamber.This improvement over a naturally aspirated engine’s power output is due to the fact that the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure alone.

In naturally aspirated piston engines, intake gases are “pushed” into the engine by atmospheric pressure filling the volumetric void caused by the downward stroke of the piston (which creates a low-pressure area), similar to how liquid is drawn up into a syringe.The objective of a turbocharger is to improve an engine’s efficiency by increasing the density of the intake gas (usually air), thereby allowing more power per engine cycle.

The turbocharger’s compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the kinetic energy of the engine’s exhaust gases.A turbocharger may also be used to increase fuel efficiency without increasing power. This is achieved by recovering waste energy in the exhaust and feeding it back into the engine intake. By using this otherwise wasted energy to increase the mass of air, it becomes easier to ensure that all fuel is burned before being vented at the start of the exhaust stage.

Working of a Turbocharger:

There are a number of different types of turbocharger used within the automotive industry:

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

Single- Turbo:

• Single turbochargers 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.

In this below image we can see normal Turbocharged engine from GT power

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 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.

In this below image we can see the twin turbo from GT power

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. Again, there is a cost penalty for addressing the complexity of a system requiring complicated turbine housings, exhaust manifolds and turbos.

In this below image we can see the Twin scroll turbo from Gt power

Variable Geometry Turbo:

• Typically, 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.
• Whilst VGTs are more typically used in diesel engines where exhaust gases are 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 exotic materials. The high temperature of the exhaust gases means that the vanes must be made from exotic heat-resistant materials to prevent damage. This has restricted their use to applications within luxury, high performance engines.

  In this below image we can see the variable geometry turbocharger from Gt power

  

Variable Twin Scroll Turbo:

• As the name suggests a 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.

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. So far, so good. It appears that electronic turbos are the answer to all the negative characteristics of conventional turbochargers, however there are some disadvantages. Most are around cost and complexity, as the electric motor must be accommodated and powered, plus also cooled to prevent reliability issues.

Waste Gate Turbocharger:

• A waste gate is a valve that controls the flow of exhaust gases to the turbine wheel in a turbocharged engine system diversion of exhaust gases regulates the turbine speed, which in turn regulates the rotating speed of the compressor. The primary function of the wastegate is to regulate the maximum boost pressure in turbocharger systems, to protect the engine and the turbocharger. One advantage of installing a remote mount wastegate to a free-float (or non-WG) turbo includes allowance for a smaller A/R turbine housing, resulting in less lag time before the turbo begins to spool and create boost.

In this below image we can see the waste gate turbo from GT power

Tutorial-6:

• This model consists of 6 cylinders with the implementation of Turbo Charger i.e. 2 inlets to the turbo as we see in the form of two end environments at the right side and compressor outlet at the bottom in the form of end environment. Generally, Twin Turbine inlets are used with more than 3 cylinders. In this case, we have 6 cylinders where 3 outlets of cylinders share on the turbine inlet. We were able to combine the outlets of different cylinders with the help of Flow Split Template. Here, as there is no physical shaft connecting the turbine and compressor, we directly input the outlet pressure (Absolute) values for the turbine as well as compressor. Here the inputs for the turbine and compressor are absolute pressure, temperature and composition of air.
• In the below image we can see the 6 Cylinder engine

Tutorial-7:

• In this model again we have 6 cylinders’ diesel engine with the two inlets to the turbocharger which is a collection from 6 cylinders splitting into 2 inlets. This model is conceptually same as the above model in tutorial no. 6 with actual turbocharger setup implemented here, where the turbine is connected with the compressor impeller with the rotating shaft as seen in the image below. Here for the turbine and compressor inputs, we have a turbine map and compressor map which is a contour plot of Pressure ratio v/s Mass flow rate. This turbine and compressor map data consists of parameters like Rotational speed, Mass flow rate, Pressure Ratio and efficiency.

• In the below image we can see the 6 Dual twin Turbocharger Cylinder engine

And wwe specify the engine cylinder geometry ike bore dia ,stroke length and connecting rod etc.

PLOT OPERATING POINTS ON COMPRESSOR AND TURBINE MAPS:

These Compressor maps and the turbine maps are generally provided by the manufacturers in a standard SAE format which has an extension of .cmp and .trp respectively. An alternative option for these files is the direct input of attributes like speed, mass flow rate, pressure ratio and efficiency.

Below is the standard compressor map where thick blue lines indicate the speed in RPM, and the thin vertical rail lines indicate the efficiency of the compressor. The x-axis represents the Mass flow rate and Y-axis represents the Pressure Ratio. In the middle of the map, we where Heart is written suggests the highest efficiency of the compressor and as we move outwards, the efficiency decreases. Also, we see other zones named Choke zone, Surge zone and Excess Rotor Speed which our operating points should avoid.

COMPRESSOR MAP:

Model 7 is used to plot the operating points on compressor and turbine map. The points on the map represents the operating point.

TURBINE MAP:

In which application Variable Geometry Turbine is beneficial?

• VGTs tend to be much more common on diesel engines, as lower exhaust temperatures mean they are less prone to failure. Early gasoline-engine VGTs required significant pre-Charge Cooling to extend the turbocharger life to reasonable levels, but advances in technology have improved their resistance to high-temperature gasoline exhaust, and they have started to appear increasingly in gasoline-engined cars
• Typically, VGTs are only found in OEM applications due to the level of coordination required to keep the vanes in the most optimal position for whatever state the engine is in. However, there are aftermarket VGT control units available, and some high-end aftermarket engine management systems can control VGTs as well.
• VGT devices are designed to increase boost pressure at low speeds, reduce response times, increase available torque, decrease the boost at high engine speeds to prevent over-boosting, reduce engine emissions, improve fuel economyand increase the overall turbocharger operating range
• This design is mostly limited to small engines and light-duty applications (passenger cars, race cars and light commercial vehicles).

EXPLORE DIESEL VGT EGR:

This model contains EGR, VGT, intercooler, injection limiting, boost control and exhaust after treatment device modelling.

The layout represents a 4 cylinder, 16 valve direct injection diesel engine VGT with EGR, air enter at ambient conditions passes through air box and goes into compressor which run through shaft on which turbine is mounted. Turbine runs through the kinetic energy of exhaust gases. Compressed air passes through the intercooler which decreases temperature of air to increase density and goes into inlet manifold. Exhaust gas is cooled into EGR cooler and mixing of exhaust into inlet air is controlled by EGR valve.

There is VGT rack controller which controls blade positions and vary the speed of turbine and hence boost pressure and target BMEP into the cylinder.

VGT requires varying rack positions and also the different turbine maps for respective rack position.

Working of a Diesel VGT EGR:

• This is a 4 cylinder and 16 valve engine with EGR cooler and VGT.
• The ambient air is sucked through the inlet and then the air is passed through the air filters where dirty air and dust particles can be purified and pure air passed into the compressor from compressor the compressed air is passes through the Intercooler.
• In the intercooler the temperature of the air is decreases and increases in the density of the air and thus we will have higher airflow rate than the flow rate of the Ambient air.
• The air from Intercooler passed through the throttle valve where air is passed through this if the throttle valve open means the drive inputs is 90 and if the throttle valve Close means the drive inputs is 0.
• From the throttle valve the air is passed through the two inlet ports into cylinder where combustion takes places during the Power stroke.
• During the exhaust Stroke the exhaust gases and air passes through the exhaust manifold from the exhaust manifold some of the air went to the turbine and some of air went through the Exhaust Gas Recirculation (EGR) in order to reduce the emissions of the NoX.
• The turbine and Compressor are connected to the single shaft from the turbine the air is passed through the wall flow filter (where the diesel and Gasoline particles filters) from there it passes through the Muffler and gases went into atmosphere through it.

CONCLUSION:

The various turbocharger are modelled using GT power and results are provided.