Turbo-charger modelling using GT-SUITE

A turbocharger is a turbine-driven, forced induction device that increases an internal combustion engine\'s efficiency and power output by forcing extra compressed 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 (and for that matter, ram air intakes) alone.

Turbocharger lag (turbo lag) is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost which can also be referred to as spooling. Inertia, friction, and compressor load are the primary contributors to turbocharger lag. Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine.

Types of turbochargers:

1. Single turbocharger: 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.

2. 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.

3. Waste gate turbo: Vehicles with turbocharged engines feature the wastegate turbocharger. This technology helps to deliver optimum engine performance during peak operating conditions. In addition, the wastegate valve prevents the turbocharger from over-running. Particularly, it also avoids the engine from over-boost and prevents any mechanical failure.The waste-gate regulates the pressure of the relief valve. It, in turn, limits the boost pressure in the turbocharger system. This is helpful in preventing the engine from potential mechanical damages caused by the high pressure. Furthermore, the system automatically opens the waste-gate valve when the pressure reaches the pre-set levels. Then, it allows all the high-pressure exhaust gases to escape the turbine wheel and enter into the downstream/outlet. Thus, it prevents the exhaust gas pressure from rising more than required.

4. Twin scroll Turbo: Turbo is powered by exhaust gases that are redirected to spin turbine blades and force air into the engine. Now, an engine’s cylinders fire in sequence, meaning that exhaust gases enter the turbo in pulses. As you can probably imagine, these pulses can easily overlap and interfere with one another when powering the turbo, and a twin-scroll turbocharger solves this issue by using a divided-inlet turbine housing and a specific exhaust manifold that pairs the right cylinders to each scroll. In a four-cylinder vehicle, you can then have the first and fourth cylinders powering one scroll, and two and three powering another. This means that there’s less pulse overlap and less lag.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.

5. Variable geometry turbocharger(VGT): 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.

6. 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. 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.

Turbocharger model examples in GT suite:

1. Diesel_6cyl_TC model

In the above model we can observe that there are 2 turbines, one is connected to compressor to turbocharge the engine and the other(power turbine) is connected to engine via transmission system. This is an example for fixed turbo model.

2. Diesel_WGController model:

3. Diesel_VGT_EGR

Tutorials:

This tutorial helps us to build turbocharger setup.This model consists of 6 cylinder SOHC intercooled engine with end environment compressor outlet and turbine inlet. The pressure at end environments is 2.4 bar and temperature of 350 k.

In above tutorial, the compressor and turbine is modelled and connected via shaft. Performance of compressor and turbine can be viewed through maps.

Compressor map:

The compressor operating behaviour is generally defined by maps showing the relationship between pressure ratio and volume or mass flow rate. The useable section of the map relating to centrifugal compressors is limited by the surge and choke lines and the maximum permissible compressor speed.

Surge line: The map width is limited on the left by the surge line. This is basically \"stalling\" of the air flow at the compressor inlet. With too small a volume flow and too high a pressure ratio, the flow can no longer adhere to the suction side of the blades, with the result that the discharge process is interrupted. The air flow through the compressor is reversed until a stable pressure ratio with positive volume flow rate is reached, the pressure builds up again and the cycle repeats. This flow instability continues at a fixed frequency and the resultant noise is known as \"surging\".

Choke line: The maximum centrifugal compressor volume flow rate is normally limited by the cross-section at the compressor inlet. When the flow at the wheel inlet reaches sonic velocity, no further flow rate increase is possible. The choke line can be recognised by the steeply descending speed lines at the right on the compressor map.

Turbine map:

The turbine map is orientated differently than the compressor map with the pressure/expansion ratio on the horizontal axis and the corrected flow on the vertical axis; turbines are also known as expanders as the gas starts off at a high pressure entering the turbine and expand to a lower pressure as it exits the turbine. Therefore, when speaking specifically about turbines, the pressure ratio is also known as the expansion ratio. Back on topic to the turbine map, turbine efficiency also looks different as compared to the compressor map; there are efficiency lines which correspond to each compressor speed line with the efficiency values also plotted on the vertical axis.The turbine map also has corrected mass flow rates and corrected speeds just like the compressor map. However, one difference is how the turbine pressure ratio is calculated; the pressure ratio on the turbine side uses total pressure at the turbine inlet and static pressure at the turbine outlet.

Q. In which application is variable geometry turbine is beneficial?

Turbocharger A/R Ratio: An important geometric characteristic (parameter) of a turbocharger is the A/R ratio, where A stands for the turbine/compressor inlet cross-sectional area and R for the radius of the turbo centerline to the centroid of the area A. The A/R ratio (area divided by radius) applies both for the compressor and turbine, but the major impact on the turbocharger performance in linked to the turbine A/R ratio.

Significance of A/R ratio: The flow capacity of the turbine depends on the A/R ratio of the housing and has significant impact on the overall performance of the turbocharger.

A small A/R ratio will increase the speed of the exhaust gas as it enters the turbine wheel, the compressor will spin faster and provide an increase of intake air boost. A negative effect of a small A/R ratio is the tangential flow of the exhaust gas into the turbine wheel, which reduces the flow capacity of the turbocharger. The effect is an increased backpressure in the exhaust manifold at high engine speeds, which translates into difficult gas exchange (exhaust gas vs. intake air) of the engine and reduced peak power.

A large A/R ratio will improve the flow capacity of the turbocharger at high engine speeds, reducing the backpressure in the exhaust manifold. This will improve the engine’s capacity to “breathe” (exchange gas) at high speed and push the peak power towards higher values. The drawback is that, at low and medium engine speeds, the exhaust gas velocity will be lower (because of bigger flow area) and the increase of intake air boost will be slower (turbo-lag).

Compared with fixed geometry turbochargers, variable geometry turbochargers are designed to:

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

VGT applications: 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.

In trucks, VGTs are also used to control the ratio of exhaust recirculated back to the engine inlet (they can be controlled to selectively increase the exhaust manifold pressure until it exceeds the inlet manifold pressure, which promotes exhaust gas recirculation). Although excessive engine backpressure is detrimental to overall fuel efficiency, ensuring a sufficient EGR rate even during transient events (such as gear changes) can be sufficient to reduce nitrogen oxide emissions down to that required by emissions legislation

Diesel_VGT_EGR model:

In this model we can observe 4 cylinder engine with VGT turbocharger with EGR. under turbine modelling, we can observe VGT rack controller which controls vanes according to the engine rpm and fuel injected in RLTxyz template as shown below.The rack array is defined varying from 0.1 to 1 such that at each rack turbine performance changes.

As shown above for each rack position, turbine performance map changes accordingly.