Week-6 : Turbocharger Modelling

• List down different TC and locate examples from GT Power

Single-Turbos:

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.

Cross- section of turbo charger

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.

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.

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,thereby allowing increased airflow. This results in a low boost threshold reducing turbo lag, and provides a wide and smooth torque band.

Variable Twin-Scroll Turbocharger (VTS)

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.

• Explore tutorial number 6 and 7
• Plot operating points on compressor and turbine maps

This is a model of 6 cylinders and 12 valve engine, the circuit starts from the compressor output and moves to the intercooler as we know that the temperature drops in the intercooler as the air gets cooled and respectively the density of the air increases thus the burn rate increases.

this high-density air is passed through the cylinder and the exhaust gases are fed to the turbine in 2 ports as shown thus a twin-scroll turbine is used so there is no overlapping of the exhaust gases.

the above diagram shows the flow of the intercooler as we know that the compressed air should be passed through the intercooler to decrease the temperature of the air and increase the density of the air for better efficiencies and higher burn rate.

the above diagram shows a 6 cylinder 12 valve engine but here we can see that the ambient air is directly fed into the compressor and turbine is fixed at the end of the exhaust with twin-scroll conditions.

and both the turbine and compressor are connected with a shaft which indicates a constant angular acceleration.

operating points for compressor and turbine maps are done on example 7 for better results:

COMPRESSOR MAPS:

TURBINE MAPS:

• In which application Variable Geometry Turbine is beneficial?

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 economy and increase the overall turbocharger operating range. VGT have the ability to adjust flow conditions upstream of the turbine without altering the moment of inertia.

The performance of a turbocharger is commonly described by non-dimensional mass flow rate and speed, which can be plotted against expansion ratio in the case of the turbine. The flow range of a radial flow turbine (ṁT/p) is limited at high-pressure ratios by the choking of flow.

In VGT devices, the aspect ratio will determine the EGR flow, since it governs the pressure difference between the inlet manifold and exhaust manifold. EGR is more commonly found on turbocharged diesel engines than petrol variants since the exhaust gas temperatures are significantly lower; around 850 °C for diesel engines and 1000 °C for petrol engines.

• Explore example- Diesel VGT EGR and understand the modelling part

• This is 4 cylinders 16 valve engine with EGR cooler and VGT.
• now we can see the flow of the circuit ambient air is passed through the and the temperature of the air is increased in the process.
• so this compressed air is passed through the intercooler which will decrease the temperature of the air and thus increase the density of the air and we will have a higher airflow rate than we started with the ambient air.
• this compressed air is sent to the throttle body where the air will be passed through based on driver input 90 degree means the throttle is wide open and 0 degree means it is fully closed.
• the passed air now passes through the engine cylinder for combustion.
• after this air is passed through the cylinder the exhaust gases are generated after the expansion stroke, these exhaust gases are passed through the exhaust manifold and from the manifold are sent to the turbine to recover some energy from the exhaust gases and some portion of the gases are diverted to flow through the EGR.
• gases are sent to the Exhaust Gas Recirculation(EGR) to reduce the NOx emissions.
• the turbine and compressor is connected through a shaft