Most people associate turbochargers with single turbochargers. Different torque characteristics can be achieved by varying the size of the elements within the turbo. Large turbos produce more top-end power, whereas smaller turbos can spool faster and produce more low-end power. They are a cost-effective way of increasing engine power and efficiency, and as such have grown in popularity, allowing smaller engines to increase efficiency by producing the same power as larger naturally-aspirated engines while weighing less. They do, however, work best within a narrow RPM range, and drivers may experience ‘turbo-lag’ until the turbo begins to operate within its peak rev band.
Twin-turbos, as the name implies, refer to the addition of a second turbocharger to an engine. In the case of a V6 or V8 engine, this can be accomplished by assigning a single turbo to each cylinder bank. Alternatively, at low RPMs, one smaller turbo could be used, with a larger turbo at higher RPMs. This second configuration (known as twin sequential turbocharging) provides a wider operating RPM range, better torque at low revs (reducing turbo lag), and power at high RPMs. Unsurprisingly, having two turbos increases the complexity and associated costs significantly.
Twin-scroll turbochargers necessitate a divided-inlet turbine housing and an exhaust manifold that matches the appropriate engine cylinders with each scroll. independently. In a four-cylinder engine (with a firing order of 1-3-4-2), for example, cylinders 1 and 4 may feed to one scroll of the turbo, while cylinders 2 and 3 may feed to a separate scroll. This layout allows for more efficient delivery of exhaust gas energy to the turbo, resulting in denser, purer air in each cylinder. More energy is directed to the exhaust turbine, resulting in increased power. Again, there is a cost penalty for dealing with the complexity of a system that necessitates complex turbine housings, exhaust manifolds, and turbos.
At the turbine inlet, VGTs typically include a ring of aerodynamically shaped vanes in the turbine housing. These vanes rotate in turbos for passenger cars and light commercial vehicles to vary the gas swirl angle and cross-sectional area. Internal vanes adjust the turbo’s area-to-radius (A/R) ratio to match the engine’s RPM, resulting in peak performance. A low A/R ratio allows the turbo to quickly spool up by increasing exhaust gas velocity at low RPM. At higher revs, the A/R ratio increases, allowing for more airflow. As a result, the boost threshold is low, reducing turbo lag and providing a broad and smooth torque band.
While VGTs are more commonly used in diesel engines with lower temperature exhaust gases, VGTs have previously been limited in petrol engine applications due to their high cost and the requirement for components made from exotic materials. Because of the high temperature of the exhaust gases, the vanes must be made of exotic heat-resistant materials to avoid damage. This has limited their application to luxury, high-performance engines.
A VTS turbocharger, as the name implies, combines the benefits of a twin-scroll turbo and a variable geometry turbo. It accomplishes this through the use of a valve that can either redirect the exhaust airflow to a single scroll or allow the exhaust gases to split to both scrolls by varying the amount the valve opens. The VTS turbocharger design is a more affordable and durable alternative to VGT turbos, making it a viable option for petrol engine applications.
An electric turbocharger is used to reduce turbo lag and assist a conventional turbocharger at lower engine speeds where a conventional turbo is inefficient. This is accomplished by incorporating an electric motor that spins up the turbocharger’s compressor from start to finish, until the power from the exhaust volume is sufficient to operate the turbocharger. This method eliminates turbo lag and significantly expands the RPM range in which the turbo can operate efficiently. Thus far, so good. Although it appears that electronic turbos are the answer to all of the negative aspects of conventional turbochargers, there are some drawbacks. The majority revolve around cost and complexity, as the electric motor must be accommodated, powered, and cooled to avoid reliability issues.