Cummins Turbo Technologies has developed a range of technologies including two-stage and modulated two-stage turbocharging. They allow us to provide the extra boost needed by engine and vehicle manufacturers, whatever their EGR strategies, control systems and packaging constraints.

Conventional two-stage turbocharging employs two turbochargers working in series at all times. True sequential turbocharging switches turbochargers in and out of use as required but they are normally connected in parallel. A modulated two-stage system brings some of the benefits of each of these two approaches. At low engine speeds it works as a two-stage system, delivering high boost pressure despite the low engine speed. At high engine speeds it bypasses the small, high pressure (HP) turbocharger allowing the bigger, low pressure (LP) turbocharger to work on its own and produce the higher flows at high engine speed.

Conventional two-stage turbocharging is most beneficial for applications that need high altitude capability because the low pressure turbocharger can compensate for the loss in ambient pressure without over speeding. Sequential systems are good for extending the flow range but do not deliver a higher boost pressure. These systems are ideal for passenger cars or turbocharging gasoline engines but are not appropriate for high output diesel engines. Modulated two-stage systems offer the benefits of both high boost pressure and wide flow range, mainly due to the fact that two compressors are used. Using two compressors replicates the effect of a variable compressor without the need for a complex mechanism in the compressor housing. The modulated two-stage system can have a high pressure turbocharger far smaller than that of a conventional two-stage system, improving transient performance by reducing the turbo lag that affects both driveability and emissions.