Twin-turbo
The use of twin-turbocharger is a question of both efficiency and packaging. A small engine is of course better to use a single turbo, because it does not produce sufficient exhaust gas to drive 2 turbos efficiently. For larger engines, it is better to use a pair of small turbochargers instead of a big one, because small turbines result in less turbo lag.
But how large an engine needs to go twin-turbo? It depends. When I wrote this article for the first time in 1998, Subaru used twin-turbo on its 2.0-liter boxer engine. Many other car makers like Mitsubishi and Nissan used twin-turbo on engines displacing 2.5-liters or above. As the technology of turbochargers progressed, today's turbochargers achieve much lower inertia and turbo lag than the past. Consequently, the threshold between single and twin-turbo is also lifted. BMW, for example, changed its mainstream 3.0-liter straight-six from twin-turbo to single-turbo in 2009. Apart from a slight compromise in top-end power, it showed no deterioration in response and refinement. However, for performance versions like 1-Series M, BMW still keeps using twin-turbo to deliver the necessary horsepower.
Packaging also governs the use of twin-turbo. For V-shape and boxer engines, it could be a headache to connect all cylinders to a single turbocharger. Twin-turbo can easily avoid the problem. As one turbo needs to serve only one cylinder bank, it can be put in close proxmity to the bank. As a result, the turbo piping can be greatly shortened, saving a lot of space in the engine compartment. Moreover, the shorter pipes lead to less turbo lag. Therefore, almost all turbocharged V-shape and boxer engines on the market employ twin-turbo.
Generally speaking, there are 3 types of twin-turbo arrangement: Parallel, Sequential and 2-Stage Variable. Let's see how they work:
Parallel Twin-turboThe simplest twin-turbo arrangement is Parallel Twin-turbo. Both turbos work independently at the same time. Most twin-turbos on the market are this type.
Nissan VG30DETT engine on the last generation 300ZX had each turbo feed the opposite cylinder bank. This formed a feedback loop and automatically balance the power between the two banks. Most early twin-turbo engines, like Ferrari F40's and Lotus Esprit V8, had the same design. Modern engine management system can do the balance job by altering ignition, so the cross-feed arrangement is no longer necessary.
Maserati was renowned for being the first to mass market twin-turbo, or in its own word, Biturbo. This picture shows its early 2.5-liter Biturbo V6. Each turbo was supplied by the exhaust gas from the nearby cylinder bank. Compressed fresh air from the two turbos joined in a common intake plenum and supplied all six cylinders. This simple arrangement is still being used by the majority of twin-turbo engines today.
BMW's N54 twin-turbo straight-six has each of its turbo supplied by 3 adjacent cylinders. The compressed gas from both turbos joins and feeds all 6 cylinders. It's essentially the same as the Maserati design, just applied to straight engine.
Sequential Twin-turboTo reduce turbo lag, some manufacturers opt for sequential twin-turbo. At low engine speed, all the limited amount of exhaust gas is directed to drive one of the turbos, leaving another idle. Therefore the first turbo can spool up more quickly. When the exhaust flow reaches sufficient amount to drive both turbos, the second turbo intervenes and helps reaching the maximum boost pressure. The switchover is implemented by a bypass valve, which is controlled by engine management system. Cars employing sequential twin-turbo include Porsche 959, Mazda RX-7 Mk3, Toyota Supra (last gen) and the 1990s Subaru Legacy.
As modern technology has largely reduced turbo lag, sequential twin-turbo is no longer deemed to be necessary. Today, it has disappeared from production.
Audi's VVL-driven sequential twin-turboIn 2016, a very special kind of sequential twin-turbo system made appearance on the 4-liter diesel V8 of Audi SQ7. It uses Audi's Valvelift variable valve lift system to select either one or two turbos. In each cylinder, the 2 exhaust valves are hermetrically separated, each leads to its own turbocharger (both turbos are the same size). One of the exhaust valves is equipped with the Valvelift mechanism. At low to medium rpm, the Valvelift switches to a zero-lift cam lobe so to shut down that exhaust valve. In this way, all the exhaust gas goes through another exhaust valve to drive one turbo. At higher rpms where more exhaust gas is produced, the Valvelift switches to a normal cam lobe and reactivates that exhaust valve. Now another turbo joins the work to provide full boost.
The biggest benefit of this design is that it saves the complicated interconnection between the two turbos, thus making the engine smaller. On the negative side, the cylinder runs only 1 exhaust valve at low to medium rev, limiting breathing. However, since Audi uses it on diesel engine, whose breathing rate is much slower than that of petrol engine, it does not compromise output. I guess the technology will not be used on petrol engines.
2-Stage Variable Twin-turboIn recently years, turbo lag has been largely resolved on gasoline engines, thanks to technology like close-coupled turbochargers (some are even integrated with exhaust manifolds) and low inertia small turbines. However, the same cannot be said to diesel engines. Diesel engines may produce power comparable to their petrol counterparts, but that need higher boost pressure hence larger turbochargers. It goes without saying that large turbos result in more turbo lag. Moreover, diesel engines tend to work at much lower rpm than petrol engines. This means in normal usage they produce less exhaust gas to feed the turbos. As a result, the turbo lag problem is made even worse.
To deal with this problem, engineers developed a more sophisticated kind of twin-turbo specially for diesel engines. It is 2-stage variable twin-turbo.
2-stage variable twin-turbo made its first production appearance on BMW 535d in 2004. The system was developed by BorgWarner, although other manufacturers like Garrett-Honeywell also joined the party later on.
As shown in this picture, the turbo system on 535d was made very compact, engaging little space adjacent to the straight-six. It has very short pipes connecting between the two turbos.
The engine produced 272 hp and 413 lbft, far stronger than the single-turbo version's 218 hp and 369 lbft on 530d. Moreover, it generated 391 lbft of torque from as low as 1500 rpm, implying very quick spool up of turbocharger.
Unlike other twin-turbo systems, 2-stage variable twin-turbo employs different size turbos - a small one for quicker spool up at low rpm and a large turbo to take care of higher rev. They are connected in series so that the boost pressure from one turbo is further multiplied by another turbo, hence the name "2-stage". The distribution of exhaust gas is continuously variable, so the transition from small turbo to big turbo can be made seamless. Below is an example taken from an Opel system. Let's see how it works:
Below 1800 rpm
The exhaust flap is closed. All the exhaust gas drives the small turbo, which provides all boost pressure in this phase. The large turbo runs idle and does not contribute to compression.
1800-3000 rpm
The large turbo is now brought into action, so that both turbos run together. Depending on load, the exhaust flap opens increasingly and feeds exhaust gas to both turbos. The large turbo pre-compresses the air, which is then cooled in the intercooler and raised to higher boost pressure in the small turbo.
The check valve remains closed, since the large turbo's boost pressure is still lower than that of the small turbo
Only the large turbo compresses the air, because more air can flow through it than the small turbo. The exhaust flap is now completely open and the entire exhaust gas flows through the large turbo, which produces maximum boost.
The check valve is opened by the gas flow from large turbo. This bypasses the small turbo.
Twin-scroll turbo The first time I heard about twin-scroll turbo was in 1989, when the updated Mazda RX-7 Mk2 introduced this feature. It was employed to seperate the exhaust gas from the Wankel engine's two rotors in order to avoid interference. Anyway, twin-scroll turbo is also useful on 4-cylinder and 6-cylinder engines. Mitsubishi, for example, has been using it on its hot Lancer Evo since 1996. Renault used it on the 2.0 turbo engine of Avantime and Megane II Sport in the early 2000s. GM did the same to its 2.8 V6 turbo of Saab 9-3 Aero and Opel Vectra OPC in 2005. Then many manufacturers joined the camp. BMW is perhaps the keenest promoter of the technology. It used twin-scroll turbos on the 1.6-liter Prince engine of Mini (which also benefits countless of Peugeots / Citroens), 2.0-liter four-pot engine, 3.0-liter N55 straight-six and 4.4-liter V8. What makes twin-scroll turbo so attractive? The answer is quicker response and higher efficiency.
Let's see how it works. While conventional single-turbo arrangement has all exhaust manifolds connected together at the exhaust turbine, twin-scroll turbo splits into two separate paths. For example, in a typical 4-cylinder engine, cylinder 1 and 4 combines to one path, while cylinder 2 and 3 combines to another path. The two exhaust flows hit the turbine blades independently, as they are separated by a wall integral with the turbine housing. This prevents the two exhaust streams to interfere with each other.
But why is the avoidance of interference so important? Please see the following graph. It shows a typical exhaust pulse from one cylinder. When exhaust valves open, the hot exhaust gas rushes out from the combustion chamber and generates a high-pressure pulse. The pulse escapes quickly and pressure drops quickly as well. Tailing the pulse is a negative pressure (lower than atmospheric pressure) period, as we have explained in the Tuned Exhaust section. When the intake valve opens during the "overlap" period, the pressure dips again. Finally, the exhaust valve closed and the pressure in exhaust manifold gets stabilizing
This is only the case of one cylinder. Now suppose we have a 4-cylinder engine. If we add the exhaust pulses from all cylinders together, we will find a lot of interferences as below:
In particular, each positive pulse is partly offset by the negative pressure resulted from the overlap period. Consequently, the strength of the resultant pulse is reduced, and the turbine will take longer time to spool up. Therefore, interference is bad to the response of turbocharger.
Now if we use a twin-scroll turbocharger to separate the exhaust gas of Cylinder 1+4 from Cylinder 2+3, we will get two pulse streams with nearly no interferences:
As a result, the pulses are strong and capable to spool up the turbine earlier.
Because of the lack of interferences, it allows to use larger valve overlapping that is not possible on single-scroll turbo. Larger valve overlapping results in better scavenging effect - when both inlet and exhaust valves are opened, the exhaust flow helps sucking fresh air into the combustion chamber and driving away the residual exhaust gas. Therefore the combustion chamber is filled with colder, "higher quality" air and benefits volumetric efficiency.
Cross-bank turbocharging - BMW twin-turbo V8 as exampleIn 2008, BMW launched a new twin-turbo V8 with codename N62. The engine has an unusual intake and exhaust arrangement contrasting to conventional wisdom: the hot exhaust manifolds rest inside the V-valley, while the intake manifolds are located at either sides. This arrangement must have taken its engineers a lot of effort to solve the thermal insulation and cooling problems. It does make the engine more compact, at least in terms of width, if not height. However, the primary reason for the radical change is unlikely to be compactness, but the compatibility with its new cross-bank turbocharging technology. Here we are going to see how it works.
Unconventionally, the BMW V8 has its exhaust manifolds and twin-turbo mounted inside the valley.
The exhaust manifold is compact yet sophisticated. Note the turbos are twin-scroll.
An illustration to the connection of exhaust manifolds:
Cylinder 1 and 6 are connected to one of the scrolls of the first twin-scroll turbocharger. Another scroll connects to Cylinder 4 and 7.
Cylinder 2 and 8 are connected to one of the scrolls of the second twin-scroll turbocharger. Another scroll connects to Cylinder 3 and 5.
As you can see, each turbo is supplied by both cylinder banks, unlike conventional twin-turbo. This explains why we call it cross-bank turbocharging.
It is also why the exhaust manifolds and turbochargers have to be located inside the valley. If they were mounted outside the engine, the cross-bank exhaust manifolds would have to be too long and cumbersome, adding considerable weight and turbo lag.
Now we can use the same principles learned from the previous section of Twin-scroll turbo to analyse the engine. If you have not read that section, please read it first.
The firing order of a typical V8 engine is 1 - 5 - 4 - 8 - 6 - 3 - 7 - 2. If we combine the exhaust gas of all cylinders together, we will get a pulse train as below. It shows a lot of interference. Each pulse is partly offset by the negative tails of the preceding pulses. This reduces the strength of the resultant pulse thus hampers turbo response.
Of course, we won't equip a V8 engine with a single turbocharger, so the above graph is unrealistic. A fair comparison should be made to a conventional twin-turbo arrangement. In that case, Cylinder 1, 2, 3 and 4 connects to the same turbocharger. The resultant pulse train is as below:
Now we can see two problems:
1) There are still quite a lot of interferences between pulse 1 and 4, pulse 3 and 2, as well as pulse 2 and 1. Only pulse 3 and 4 are widely spaced enough to prevent from interference. As a result, the strength of resultant pulses is reduced.
2) The intervals between pulses are not constant, i.e. 180º - 270º - 180º - 90º. In other words, the pulse train does not run at a constant frequency. This does not help speeding up the turbine.
Apparently, to achieve a constant flow of exhaust stream, the turbo cannot rely on cylinders of the same bank. That's why the cross-bank concept gets into the picture. Consider BMW's arrangement with cylinder 1, 4, 6 and 7 connected together, we get the following pulse stream:
It has a constant frequency, but the interference is still too much, or actually the same as a conventional four-cylinder engine. Like the latter, we can use twin-scroll turbos to solve the problem. By splitting the exhaust gas into two groups, i.e. Cylinder 1+6 and Cylinder 4+7, we get:
The result becomes perfect ! with a constant frequency and nearly no interference to speak of !
No wonder the BMW cross-bank turbocharged V8 is renowned for good low-end response and high efficiency. Take the version on M5 for example, it produces 560 hp and 501 lbft of torque from 4.4 liters, and the peak torque is available from as low as 1500 rpm all the way to 5750 rpm. Such a combination of high specific power and broad spread of torque is unprecedented. Thanks must go to the cross-bank turbocharging.
Turbo + Supercharger - e.g. Volkswagen TwinchargerAs many people know, mechanical superchargers are good for low-end output but short of high-end efficiency, while exhaust turbochargers work strongly at high rev but reluctantly at low rev. For decades, engineers dreamed of combining supercharger and turbocharger together. In 1985, Lancia put such a complicated system into its Group B rally special, Delta S4. Despite of its motor racing success, Lancia had never put the technology into series production. It was the Japanese who did that first. In 1988, Nissan produced 10,000 units of March Super Turbo, whose tiny 1.0-liter engine utilized a Roots supercharger and a turbocharger to produce a respectable 110 ps. Nevertheless, it was never followed up since then.
In 2005, Volkswagen resurrected the idea with the help of supercharger maker Eaton. Its 1.4-liter TSI direct injection engine was fitted with a turbocharger + supercharger system called "Twincharger". It produced 170 hp and 177 lbft of torque, about the same as a 2.3-liter naturally aspirated engine but consumed 20 percent less fuel. The engine was used extensively on Golf, Polo, Scirocco, Skoda Fabia, Seat Ibiza and Audi A1. This is the first really mass production of turbo + supercharger.
The construction of Twincharger is quite simple. It has a Roots supercharger and a turbocharger connected in series. The supercharger can be bypassed through an alternative path, or disengaged completely by an electromagnetic clutch.
At low rev, the supercharger provides most of the boost pressure. The pressure it built up also helps spooling up the turbocharger so that the latter can run into operating range more quickly.
At 1500 rpm, both chargers contribute about the same boost pressure, with a total of 2.5 bar. (Had the turbocharger worked alone, it could provide only 1.3 bar at the same rev)
Then the turbocharger – which is optimized for high-rev power – started taking the lead. The higher the rev, the less efficient the Roots-type supercharger becomes. Therefore a by-pass valve depressurizes the supercharger gradually.
By 3500 rpm, the turbocharger contributes all the boost pressure, thus the supercharger is disconnected by the electromagnetic clutch to save energy.
The Twincharger is quite an achievement. It delivers excellent power and tractability yet the package is surprisingly compact. The only disadvantage is high cost. Predictably, it costs significantly more than a turbocharging system. As modern turbocharging has improved low-end response a lot, the significance of Twincharger is inevitably reduced. This prevents it from further penetration into the mass market.
Electric Supercharger - e.g. Audi e-boosterFor a long time engineers have been dreaming of electric superchargers. A conventional mechanical supercharger is rather heavy and power consuming. The best way to use it is, like the Volkswagen Twincharger as described above, pair it with a turbocharger, using the supercharger to provide instant boost at low rpm and switching over to turbo once the latter gets up to speed. However, the installation is quite cumbersome, as you need gear or belt drive to connect the crankshaft to the supercharger, and an electromagnetic clutch to disengage it at higher rpms. If the supercharger is powered by an electric motor, then it can be installed anywhere and saving the accessories. The loss of mechanical drives also reduces vibration and noise a lot, which is probably the most criticized weakness of superchargers. In 2016, Audi SQ7 4.0TDI SUV became the first road car to adopt electric supercharger. The unit, called "e-booster" by the company, is supplied by Valeo:
The e-booster uses a turbine like centrifugal supercharger or turbocharger. Thanks to its low inertia motor, it is capable to spin from rest to its maximum 70,000 rpm in just 250ms. This means it can provide full boost in merely a quarter of a second! In contrast, a mechanical supercharger spins proportionally to the engine crankshaft thus full boost only appears at higher rpms. Therefore, the e-boosted engine should show unprecedented response at the bottom end. In fact, that is exactly the case of the SQ7, which produces 664 lbft of torque at merely 1000 rpm!
The e-booster is connected in series to a sequential twin-turbo system in the 4.0 TDI V8. It is positioned downstream of the intercooler as, unlike exhaust gas turbocharger, it does not generate much heat. At very low rpms, the e-booster is solely responsible for providing boost. It also helps spooling up the smaller turbo. When the small turbo gets up to speed, a bypass valve opens and drives the air flow away from the e-booster while the latter is turned off. At higher rpms, the big turbo joins the work to provide full boost. In other words, the function of e-booster is to fill the torque gap at the bottom end. It is not designed to work permanently, since exhaust turbocharging is more energy efficient.
This e-boosted 3.0 TDI V6 was experiented in Audi RS5 prototype. It works similar to the SQ7's V8, just with a single turbo.
On the downside, the e-booster consumes 7kW (9.5 hp). As conventional 12V electrical system is not powerful enough to supply it, it calls for a an additional 48V battery system, including a boot-mounted lithium-ion battery, 48/12V converter and a large alternator, which adds not only weight but also considerable costs. This explains why other German car makers have yet to follow. When 48V electrical system becomes standardized, electric superchargers will get more popular.
Turbo Lag reduction - Volvo PowerPulseElectric superchargers might be the ultimate solution to turbo lag, but they are not compatible with the 12V electrical system currently used on all cars. Volvo opted for another solution that it called PowerPulse. Put it simply, PowerPulse uses compressed air to help spooling up the turbo more quickly. It adds an electric-driven pump and a 2-liter gas tank to the engine. The pump compresses air to 12 bar and store it at the gas tank. When the driver press the throttle pedal, the compressed air will be released to the exhaust manifold immediately. Within 0.3 sec, the air pulse spins the turbocharger to its operational range at 150,000 rpm, vastly reducing turbo lag.
Compared with e-booster, the PowerPulse is nearly as effective, but it costs less to make and is easier to be installed on current cars. Volvo equipped it first on the S90/V90 D5 sequential twin-turbo diesel four-cylinder, on which it is capable to double the power at 1 second after standing start. The only concern is its influence to the exhaust gas mixing, which may complicate the aftertreatment strategy.
Copyright© 1997-2011 by Mark Wan @ AutoZine
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