Supercharging is the process of increasing the air density within the fixed cylinder volume during the intake stroke by forcing air using a compressor or a blower. To differentiate it from the turbocharger, the industry uses the term “supercharger” when referring to a mechanically driven charger. There are a few variations of the supercharger used in passenger cars, but in all cases, the device is driven by a toothed belt off a pulley on the crankshaft.
To this end, its location in the engine compartment is dictated by the necessity to align it with the crankshaft (like the alternator or air-conditioner compressor) to facilitate a belt-drive arrangement. The rest of the supercharger installation is somewhat less complicated than a turbocharger’s.
Not required is the complicated exhaust plumbing that is essential in a turbo system to drive the compressor. Hence, the exhaust piping in a supercharged car is no different to what you would find around any naturally aspirated engine. This also means that the charged intake air experiences a much lower rise in temperature, thereby allowing a more compact intercooler.
As for the supercharger itself, automakers currently rely on one of three different designs for road cars: Roots, screw-type or centrifugal. Each is chosen for its specific performance and packaging characteristics that suit the engine in question, rather than because of any specific comparative advantage. Whatever the variety, superchargers are high-precision machines with proven reliability.
The innards of a supercharger spin at between 12,000rpm and 50,000rpm (depending on type), which incidentally is nowhere close to the 150,000rpm of a turbocharger. This, coupled with the fact that the supercharger is placed relatively far from the exhaust system, means lower mechanical and thermal stresses.
The supercharger does have its drawbacks, such as additional cost. Most significantly, the power absorbed in driving the blower can be as high as 30 percent of the engine’s output. Even if the ultimate result is a nett torque gain of 20 to 30 percent, the energy consumed makes it a less efficient proposition than the turbocharger.
One of the most elegant solutions to harness both the effects of high-response supercharger response and high-volume turbo air flow is to combine the two in one assembly. Lancia, in 1985, was the first to develop such a system, which was installed in its Group B Delta S4 rally car. But Volkswagen was the first to put it into mass production for passenger cars, with its Twincharger engine.
Introduced in the 2007 Golf GT, the Twincharged 160bhp VW motor delivered exactly as promised. Boosted by a Roots supercharger at low speeds, thus totally obliterating any lag, and subsequently switching to pure turbocharged power, the little 1.4-litre produces a constant 240Nm between 1750rpm and 4500rpm.
It is perhaps logical to assume that the supercharger functions at low speed, then cuts out completely when the turbo is past its lag period, but the Twincharger system is cleverer than that.
Its Roots compressor is geared to run at an unconventionally high speed, so that high boost pressure is available at just over idling speed, and instead of pumping air directly into the cylinder, air is ducted into the intercooler via the turbine section of the turbocharger. So, while the intake manifold receives compressed-and-cooled air, the turbo also receives assistance to start spinning. A bypass valve regulates the quantity of air between the supercharger and the turbocharger. Beyond 3500rpm, a magnetic clutch on the Roots’ pulley disengages and the blower “passes” boost responsibilities to the exhaust gas-driven turbo. The transition is seamless. Maximum boost pressure is an unusually high 1.5 bar, roughly 50 percent higher than most turbocharged production cars.
The Twincharger’s efficiency, smoothness and throttle response are indisputable, but the truth is that turbocharged systems are now so sophisticated and well engineered, any turbo lag has been reduced to a point where most regular motorists will barely notice it.
Turbochargers will continue to be far more common than superchargers in the automotive world, but there’ll always be certain cars that need instant supercharged performance, whether direct from the factory or “indirectly” in an aftermarket workshop.
