Supercharging is a method to increase power output from a given engine without having to increase its displacement. As discussed in our earlier issue on Turbochargers, engines are basically big air pumps. They pump air in, add fuel, combust and then they pump the combustion products in the form of exhaust. Getting more air into the cylinders is the primary limiting factor behind how much power an engine of a given size can output. Forced induction is a way to force more air into the engine. More air = more fuel= more power. Since air is a gas, it can be compressed, increasing its density. Since the dimensions of the cylinders on the engine are fixed if you increase the pressure of air going into the cylinders, you increase the mass of air.
Both superchargers and turbochargers increase the pressure of the air entering the combustion chambers but unlike turbochargers, which use the exhaust gases created by combustion to spin the compressor, superchargers draw their power directly from the crankshaft. Most are driven by an accessory belt, which wraps around a pulley that is connected to a drive gear. The drive gear, in turn, rotates the compressor gear. There are many types of supercharger but all of them operate on the same premise: pull in atmospheric air, compress it and discharge it into the intake manifold.
Gottlieb Daimler, of Daimler-Benz fame was largely credited with patenting and popularizing supercharging an engine in the late 19th century. Mercedes-Benz produced the first production supercharged cars in the 1920s and were badged as “Kompressor”, a naming convention that continues today. Superchargers were used extensively in aircraft engines during World War 1 where the increased pressure of air entering the cylinders would enable fighters and bombers to fly at higher altitude without loss of power. Rolls-Royce was at the forefront of these developments but the solutions were expensive and complex since the superchargers were multistage (where the compression happens in more than one step) and were driven off their own gearboxes.
To pressurize the air, a supercharger must spin fast, much faster than the engine itself. Making the drive gear larger than the compressor gear causes the compressor to spin faster. Superchargers can spin at speeds in excess of 30,000rpm compared to over 100,000rpm for some turbo charger compressors.
Just like on a turbo, when the air is compressed it gets hotter, which lowers the density of the air entering the combustion chamber and increases the chances of detonation (knocking) The compressed air exiting the discharge unit must be cooled before it enters the intake manifold and, just like on turbo charger applications, Intercoolers are used to cool down the compressed air. Intercoolers are like a radiator, where the hot, compressed air enters the inlet of the intercooler and the cold air or coolant passing over the fins help heat to transfer out from the compressed air. This makes the compressed air both cooler and denser reducing probability of a detonation and increasing the mass of air in the cylinder.
There are predominantly two types of superchargers: positive displacement superchargers and dynamic compressors. The main differences between them is that in positive displacement superchargers deliver a fixed volume of air per revolution. So changing the amount of boost available comes down to changing the pulley ratios of the supercharger to change the amount of boost across all RPM ranges. Generally the pulley on the crankshaft will be larger than the pulley on the supercharger, enabling it to spin faster than the engine speed.
4 to 1 pulley ratio. Since the larger pulley is four times the smaller pulley, the smaller pulley will rotate four times faster
There are several types of popular superchargers. Roots type superchargers are probably the oldest design and are famous for sitting on top of muscle car engines with the intake sticking out of the hood. Lysholm superchargers (also known as twin screw superchargers) are also a popular, if expensive option. Both of the designs mentioned belong to the positive displacement supercharger category. One of the most common and easiest to install designs is the centrifugal supercharger, which is very similar to a turbocharger with the turbine section replaced with a pulley for the drive belt.
60’s Chevrolet Camaro with a large Roots type supercharger situated between the cylinder heads on the V8 and the intake sticking up over the hood.
Centrifugal Type Supercharger
A Lysholm (Twin screw) type supercharger
5 Illustration of how the three most common types of superchargers channel air
What about superchargers vs turbos? This question is hotly debated by car enthusiasts, but in general, superchargers offer a few advantages over turbochargers.
Since super chargers are driven directly off the engine, they do not have spool-up times and therefore have no lag. The response is immediate. Turbochargers suffer from lag because it takes a few moments before the exhaust gases reach a velocity that is sufficient to drive the impeller/turbine. Certain superchargers are more efficient at lower RPM, while others are more efficient at higher RPM. Roots and twin-screw superchargers, for example, provide more power at lower RPM. Centrifugal superchargers, similar to a turbocharger, become more efficient as the impeller spins faster, provide more power at higher RPM.
Installing a turbocharger requires extensive modification of the exhaust system, but superchargers can be bolted to the top or side of the engine. That makes them cheaper to install and easier to service and maintain.
So hopefully this was a helpful primer on supercharging an engine. Look out for more T&T Tech in upcoming issues.
Words: Kanchana Gunasekera