Week-6 : Turbocharger Modelling

Single turbo is the Cost effective way of increasing an engine’s 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.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 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. Single turbos tend to have a fairly narrow effective RPM range. This makes sizing an issue, as we have  to choose between good low-end torque or better high-end power.

Twin Turbo:

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. But, having two turbos, significantly increases the complexity and associated costs. For parallel twin turbos on ‘V’ shaped engines, the benefits (and drawbacks) are very similar to single turbo setups.

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. By using two scrolls, the exhaust pulses are divided.  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. More valve overlap is possible without hampering exhaust scavenging, meaning more tuning flexibility. It requires a specific engine layout and exhaust design.

Variable Geometry Turbo:

 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, therefor by allowing increased airflow. This results in a low boost threshold reducing turbo lag, and provides a wide and smooth torque band. It has Wide, flat torque curve. Effective turbocharging at a very wide RPM range. It Requires just a single turbo, simplifying a sequential turbo setup into something more compact. It is Typically only used in diesel applications where exhaust gases are lower so the vanes will not be damaged by heat.

 Variable Twin Scroll Turbo: