Sequential turbochargers

Sequential turbocharging is an approach to forced induction designed to solve one of the oldest compromises in turbocharging: the trade-off between low-speed responsiveness and high-speed power. A single turbocharger sized for strong top-end output tends to be lethargic at low engine speeds, where there is too little exhaust energy to spin it quickly, producing the familiar sensation of lag. Sequential systems address this by using two turbochargers of differing characteristics and bringing them into action one after another rather than together.

In a typical arrangement, a small, low-inertia turbocharger handles the lower part of the rev range. Because it is light and easily spun up, it provides crisp throttle response and useful boost almost from idle. As engine speed and exhaust flow rise, a system of control valves progressively brings a second, larger turbocharger into play. The larger unit can flow far more air and therefore sustains high boost at elevated revs, where the small turbo would otherwise run out of breath and become a restriction. The hand-over is managed so that boost remains continuous, giving the engine a broad, flat torque curve that feels both eager off the line and strong at the top.

The appeal to the driver is a near elimination of turbo lag combined with the outright power of a large turbo, a combination that a single fixed turbocharger struggles to deliver. This made sequential systems attractive for performance engines where both tractability and high specific output were demanded, with the twin-turbo rotary Mazda RX-7 and several Japanese performance diesels among the better-known examples.

The approach should not be confused with simple parallel twin-turbo layouts, where two identical turbochargers each feed one bank of cylinders and operate simultaneously. The defining feature of a sequential system is the staged, role-based hand-over between turbos of different sizes, governed by an often intricate network of control, wastegate and bypass valves.

That complexity is also the system's chief drawback. The valving and control logic add cost, weight and potential failure points, and the transition between turbos can be felt as a step in delivery if not carefully calibrated. For these reasons, sequential turbocharging has largely been superseded in modern engines by the variable-geometry turbocharger, which achieves a comparably wide operating range with a single unit whose adjustable vanes alter the effective size of the turbine on the fly. Twin-scroll designs offer another route to broad, responsive boost. Sequential turbocharging nevertheless remains an instructive milestone in the evolution of forced induction and is still found on some twin-turbo configurations.