For example, a manufacturer may have a large turbocharger that delivers high boost at high engine speeds. Opting for this would result in good top-end power but poor performance at lower speeds, as well as issues with response and delivery.
Alternatively, a smaller turbocharger that the firm has to hand may start producing boost at very low engine speeds - improving the car's manners at from a standing start - but deliver less manifold pressure than the bigger turbocharger, ultimately restricting peak power.
One way around this issue is to use both turbochargers, with them working together in what's called a 'sequential' configuration. This allows the manufacturer to meet its power targets without delivering a car that suffers from driveability issues.
How does sequential turbocharging work?
In a sequential turbocharger set-up featuring one small and one large turbocharger, the small turbo will primarily produce boost for low speeds while the other will be reserved for higher engine speeds. When the engine speed is low, the limited amount of exhaust gas available will be directed into the smaller turbocharger. This results in the rapid production of boost, granting a prompt response to the driver's accelerator input and improved engine output.
As the power demands and engine speed increases, the engine management system will start to bring the second turbocharger online. A flap in the exhaust manifold will begin to redirect exhaust gas into the second turbocharger, which starts spooling before it is called on in earnest. This helps avoid erratic changes in engine output.
With the second turbocharger now up to speed, all of the engine's exhaust can now be diverted into the large turbocharger; this then allows the engine to reach its maximum output. Depending on the configuration of the system, valves on the intake side may also open and close to the respective turbochargers to avoid pressurised air escaping through the bypassed turbocharger.
This particular arrangement, with one turbocharger being bypassed as another is introduced, is called series-sequential turbocharging. If the first turbocharger continues to be used throughout the entire rev range, however, then the set-up is what's known as a parallel-sequential configuration.
Do the turbochargers have to be a different size?
No - sequential turbocharging can be carried out using identically sized turbochargers, with only one being employed until sufficient exhaust gas is being produced to drive both properly. This marginally less complicated set-up grants similar benefits, improving response yet still permitting for significant power output. Many existing sequentially turbocharged cars - such as the Mk4 Toyota Supra - feature similarly specified turbochargers in a parallel sequential set-up, instead of a small and a large one.
The downsides of sequential turbocharging
The primary problem is complexity and cost. Besides having to contend with two turbochargers and all the required plumbing, the systems to control them are also often complicated and - when the car is older - difficult to maintain. Advances in turbocharging technology, such as the more flexible variable-geometry turbocharger, have subsequently made involved and expensive sequential set-ups redundant.
A brief history of sequential turbocharging
The first production car featuring sequential turbocharging was the Porsche 959, which was launched in 1986. Its sequential turbocharger set-up permitted its six-cylinder boxer to deliver an impressive amount of power in a smooth, controllable fashion, instead of the often spiky delivery experienced in conventional single- or twin-turbo configurations.
Other manufacturers followed this approach; Mazda made use of sequential turbocharging in the Eunos Cosmo, which arrived in 1990, and the same system was used in the 1992 RX-7. Toyota similarly etched its name in the history books in 1993, when it launched the sequentially turbocharged 2JZ-GTE in the Mk4 Supra. Sequential systems have been employed by companies such as Subaru, too, and they can also be found in industrial and diesel applications.