What is sequential turbocharging? PH Explains
Discover how manufacturers improve engine response and performance with sequential blowers...
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.
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Went back to a single for the N55, I believe, although that was badged 'TwinPower Turbo' in an effort to catch out as many journalists as possible.
There may be more and all 3 are new on the market (~1 year)
Audi 3.0l V6 Turbo Diesel - only in the highest power rating. Uses two conventional tubo's plus an electric compressor for transient response improvements
Mercedes 2.0l T-GDI - Produces ~300PS. One turbo plus electric compressor
Both these engines also have a 48 volt starter/generator as 48 volts is needed to power the electric compressor as the current draw is "too high" for a 12volt motor (efficiency is improved by higher voltage). The electric compressors can run either just for transient boost or in some cases can run constantly at low engines speeds (< 1500 RPM). They are not sized to support constant high RPM operation, that is the turbo's job once the e-compressors have helped to get the turbo spinning nicely
The Volvo 2.0l Diesel has a similar but total different solution. It has an air compressor and a compressed air storage tank. When needed compressed air is released from the tank. This solution does not need 48 volts but the time the extra boost can be supplied is less (dependong on storage tank size.....) Volvo call this their Power-Pulse system.
With the trend to "right size" for emissions (HC, NOx) it is not clear how widespread these boost assist devices will become. Very good for down-sizing but with the change from NEDC to WLTP & RDE, extreme downsizing as a way to achieve low CO2 is less likely.
Went back to a single for the N55, I believe, although that was badged 'TwinPower Turbo' in an effort to catch out as many journalists as possible.
Ferrari (and BMW, I think) has a series of patents for electrically assisted turbochargers, however, with motors integrated into the cores – which could also be used to recover energy. Nothing concrete on that front yet, though. Give it time...
https://www.carmagazine.co.uk/car-news/first-offic...
Imagine buying that with 200k on at 15 years old and diagnosing a boost issue/leak
A sequential turbo set up will have much better drive-ability, even if the headline numbers look the same
A sequential turbo set up will have much better drive-ability, even if the headline numbers look the same
Good article.
The sequential turbos on my RX-7 are really nice when they're working; great low- and mid-range response, and then rather than running out of puff at high revs, it gets a second wind. The transition at 4,500rpm is noticeable, but not so marked that it really unsettles the car.
OTOH at 20+ years old, it needed a lot of parts replacing to work properly. Beyond the vacuum control lines, one further issue is that you end up with a rather substantial cast-iron manifold to house and feed the two turbos and flaps, which is then vulnerable to cracking with age / hard use.
'Going single' is one of the perennial debates among RX-7 owners; personally I find the sequentials really suit my driving style, and I love being able to go from stuck behind a dawdler to blasting past without needing a gearchange halfway through the overtake.
A sequential turbo set up will have much better drive-ability, even if the headline numbers look the same
https://www.borgwarner.com/news-media/press-releas...
The sequential turbos on my RX-7 are really nice when they're working; great low- and mid-range response, and then rather than running out of puff at high revs, it gets a second wind. The transition at 4,500rpm is noticeable, but not so marked that it really unsettles the car.
OTOH at 20+ years old, it needed a lot of parts replacing to work properly. Beyond the vacuum control lines, one further issue is that you end up with a rather substantial cast-iron manifold to house and feed the two turbos and flaps, which is then vulnerable to cracking with age / hard use.
'Going single' is one of the perennial debates among RX-7 owners; personally I find the sequentials really suit my driving style, and I love being able to go from stuck behind a dawdler to blasting past without needing a gearchange halfway through the overtake.
There is a noticable crossover between the small and the large turbo on mine.
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