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.


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.
	Above 3000 rpm 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.