Overview
The engine usually has a Roots blower to provide air for scavenging and this is arranged to deliver excess air so that air follows the exhaust gases into the exhaust manifold. Some of this air is then forced back into the cylinder by a rise in pressure in the exhaust manifold resulting from the exhaust pulse from another cylinder.
For additional pressure charging a turbocharger may be fitted, in series with the Roots blower, but a turbocharger cannot be used alone because it would not provide enough air for scavenging at low speeds.
Read more about this topic: Exhaust Pulse Pressure Charging