Cross-bank turbocharging - BMW twin-turbo

V8 as example

In 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.

Turbocharging

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.

2-Stage Variable Twin-turbo

In 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.

Sequential Twin-turbo

To 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.

Parallel Twin-turbo

The 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.

Turbocharging auto

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.