As turbocharged engines became prevalent in mainstream cars and trucks over the past two decades, the technology has advanced substantially. More and more, automakers are looking for ways to package turbochargers more efficiently, encouraging engineers to provide the benefits of larger, more complex systems in smaller packages. 

In a very fundamental sense, a turbocharger is just two tubes with impellers in them connected by a common shaft. Air flowing through the exhaust tube spins its impeller, rotating the shaft and in turn spinning the impeller in the intake tube, which compresses the incoming air. The more air you can get into an engine, the more fuel you can burn with each ignition cycle. The more fuel, the more power. 

The ability of a turbocharger to compress air is dependent on several things, but the key is having the right amount of exhaust flow to spin up the intake compressor. The "right" amount depends on the size of the engine, the diameter of the exhaust piping, and the target amount of boost you want to build. 

Enthusiasts should be familiar with sequential turbocharger systems, in which a compact turbocharger operates at smaller throttle openings and builds initial boost before "handing off" to a larger turbocharger downstream which only operates efficiently at larger throttle openings, but builds way more boost. 

Over the years, engineers have found ways to realize the benefits of this sort of system without using multiple turbines. One of the more obscure solutions is the two-shaft turbocharger. What is it and how does it work? For that, we turn once again to Jason Fenske of Engineering Explained

A two-shaft turbocharger essentially combines a small and large compressor into one physical unit. Rather than a single shaft running between the exhaust impeller and the intake compressor, there are two shafts seated concentrically, meaning one is hollow and rotates independently around the other.

On either end of the inner shaft is a fan-shaped axial impeller. These have less surface area and mass than the larger radial compressors, and thus spool up more quickly. On the outer shaft, you have two large disc-shaped radial compressors. These are your workhorses, but they can only build boost once engine speeds climb and exhaust pressure increases. 

The smaller axial impellers spin up first, which increases pressure going into the radial compressor on the intake side, allowing them to spool up faster than they would with only natural incoming airflow. As this low-end boost feeds the engine and its speed increases, the larger radial impeller on the exhaust side takes over, turning the intake-side compressor and building toward peak boost. 

Two-shaft turbos can benefit from many of the same technologies that make modern single-shaft turbochargers more efficient than their ancestors. Fenske notes that there are no known implementations of two-shaft turbocharger technology thus far, but it was patented by Honeywell which builds turbochargers under the well-known Garrett brand. Perhaps, then, it's only a matter of time before you drive home in a car with a two-shaft turbocharger under the hood. 

Check out the full video for Fenske's in-depth explanation of this fascinating tech.