[GoingTooFast]

The S55 engine's 2-stage variable biturbo employs different size twin-scroll turbos - a small ELECTRIC one (see above pictures) 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. The distribution of exhaust gas is continuously variable, so the transition from small turbo to big turbo can be made seamless. The complete biturbo arrangement with a 4-cylinder engine example is as follows:

LOW RPM RANGE

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.

MEDIUM RPM RANGE

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.

HIGH RPM RANGE

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

Because of the lack of interferences of the two separate exhaust paths, 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.