Since the dawn of the internal combustion engine, man has always strived to make each explosion larger, more efficient and ultimately cleaner to meet the constant demands of the automotive industry. And during the era of Group B rallying in the eighties, forced induction technology came to a head through the art of something called twin-charging.

Nearly every new car produced these days features a turbocharger, using exhaust gases to spin an impeller that forces air into the inlet manifold to be ingested by the engine's cylinders. A supercharger does the same thing but with the forced induction being driven by a belt that runs in time with the crankshaft via a pulley system.

There's nothing quite as mechanically-aesthetic as a Roots supercharger

There's nothing quite as mechanically-aesthetic as a Roots supercharger

So imagine bolting both of those methods of forced induction onto one engine, creating a ballistic flurry of compressed air and furious combustion. That is exactly what twin-charging entails.

Introduced by Lancia through its Delta S4 Group B rally monster, twin-charging uses a supercharger to compress air from the car's intake system and sends it onwards to help spool-up the turbocharger. The vanes of the turbocharger are additionally rotated by exhaust gases siphoned from the exhaust manifold, as seen below.

The vanes of the turbocharger then further compresses the inlet air previously forced through the system by the supercharger. Once the combining of the forced induction is complete, the air is sent crashing through the inlet manifold and into each respective engine cylinder, ready to be met with fuel for combustion. This is called 'series' twin-charging. 'Parallel' charging is possible which means one or both compressors can be used, utilising a diverter or bypass valve to alternate between the two.

What are the advantages?

The main reason for twin-charging is to cut down on turbo lag. In heavily turbocharged vehicles, it takes a certain amount of time for the impeller of the turbo to spool up, gathering momentum and rotational speed as the exhaust gases do their thing. Superchargers have virtually an instant response to a change in throttle appliance, so the forced induction from the supercharger acts as an initial boost for the turbocharger to take over from once the engine reaches a higher rotational speed.

An underskirt shot of a Lancia Delta S4, showing the turbocharger (left), supercharger (right) and a set of intercoolers (top)

An underskirt shot of a Lancia Delta S4, showing the turbocharger (left), supercharger (right) and a set of intercoolers (top)

It also helps eradicate the shortcomings of a supercharger. As it is connected via a belt to the crankshaft, a supercharger inevitably saps power from the engine and does so at a greater rate higher in the rev range. However, once the engine is reciprocating at a high enough speed, the turbocharger will take over, using the larger flow of exhaust gases to compress inlet air into the cylinders as effectively as possible.

The compounding of boost also allows for extremely high levels of air compression, much more so than an individual turbocharger and supercharger rated to those crazy boost levels added together.

Disadvantages

The headline drawback of twin-charging is the sheer expense of the practice. Sure, wheeling both a turbo and a supercharger out for a bespoke rally car is all well and good, but for mass production? Possibly not the greatest idea.

Saying that, Volkswagen produced its 1.4-litre TSI engine which was found in numerous cars like the Scirocco, Jetta and Touran. Volvo also produces a twin-charged powertrain for its T6 and T8 models, as well as Polestar variants along the way. But twin-charging is seen generally as an unnecessary method for creating large amounts of boost, with hybridisation coupled with turbocharging being the go-to route these days.

The Lancia Delta ECV or Experimental Composite Vehicle (above) was a development of the Delta S4 and used a twin-turbocharged engine tuned to 600bhp

The Lancia Delta ECV or Experimental Composite Vehicle (above) was a development of the Delta S4 and used a twin-turbocharged engine tuned to 600bhp

Another problem is that twin-charged engines have to run at a low compression ratio to cope with the large volumes of boost being applied to them. A low compression ratio means less mechanical energy is extracted from the air/fuel mixture entering the cylinders due to a decrease in thermal efficiency. So fundamentally, the actual internal combustion becomes less efficient.

From an engineer's standpoint, I am in love with the thought of a twin-charged car and I'd happily own a Polestar Volvo or a Stradale Delta S4, if only just to see how responsive and effective such a powertrain is. But for many of us, the twin-charging dream will be consigned to epic YouTube videos of Lancias tearing apart the rally stages of Europe. Like this.

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