“There is no replacement for displacement.” This saying, which became popular during the muscle car era of the late ’60s, indicates a belief that the best way to improve the power of an engine is to add displacement by increasing either the size or number of cylinders powering a car. This approach, however, presents a bit of a challenge to the average car owner; it’s not possible to add more cylinders without an engine swap, and increasing cylinder displacement, through boring or stroking, is far from simple. Luckily, there is another option.
In a recent article explaining horsepower, I mentioned that air is the bottleneck when it comes to increasing the power output of an engine. It’s very easy to pump more fuel into the engine, but air must be drawn in. The downward movement of the piston on the intake stroke creates an area of low pressure in the cylinder.
When the intake valve opens, air (and usually fuel) is drawn into the cylinder to equalize the pressure. For most applications, this is fairly effective. But, what happens when pulling no longer provides enough air to meet your power needs? That’s when it’s time to give it a little push…
Supercharging: the Roots of forced induction
Supercharging was developed nearly 50 years before turbocharging, so we are starting there. A supercharger is a compressor that is driven by the crankshaft of an engine to increase the density of air that enters the cylinders. Increasing the density of air means that more fuel can be introduced and combusted, which increases the power of the engine. However, the fact that it is driven by the engine means it takes some power from the engine to run it — this is called a parasitic loss. But there is no need to fear this parasite as it more than makes up for any losses.
The capitalization in the above subtitle doesn’t mean that our editor is asleep at the wheel. “Roots” was actually the last name of the brothers, Philander and Francis Marion, who first developed the design for a supercharger in 1860. The first production cars to be fitted with a supercharger were introduced by Mercedes in 1921. They were the origin of the “Kompressor” name.
This is the design of a typical Roots-type supercharger, or “blower” (above), which is mounted on top of the engine. Two rotors mesh with each other, like the teeth on gears, and move air through the pump body. Early designs of the Roots-type superchargers had dual-lobe rotors, but it is more common to find designs with three or four lobes on modern superchargers, as this helps to smooth air pulses. A Roots-type supercharger is known as an external compression supercharger, which means that the air coming out of the supercharger is at the same pressure as the air going in.
The boost pressure (the pressure of the air measured in the intake manifold, which is boosted above the atmospheric pressure of 14.7 pounds per square inch, or PSI) is developed in the intake manifold because the Roots-type supercharger pumps more air in than the engine takes out. Unfortunately, this creates a lot of heat when attempting to develop high levels of boost, much like how it creates a lot of anger if you try to push lots of people into a crowded subway car.
There are two styles of superchargers that feature internal compression: centrifugal superchargers and Lysholm twin-screw superchargers. Both of these compress the air before it leaves the supercharger body. The animation below, which features a screw-type supercharger, will make it easier to understand the concept.
A Lysholm supercharger, much like a Roots-type, is a positive displacement pump. This means that the input and output are the same volume per revolution regardless of the speed of the pump, which allows for boost across the rev range. The boost in a Lysholm supercharger is created due to the shape of the twin-screws.
At the intake end of the unit, the space between the two screws (which is known as the inter-lobe volume), is large. As gas moves down the unit towards the outlet, the space shrinks, which reduces volume and increases pressure. This requires screws that are machined with very tight tolerances and that lead to a much higher price point than with a Roots-type. However, the internal compression makes it easier to add an intercooler.
Unlike the positive displacement style of the Roots and Lysholm superchargers, a centrifugal supercharger is known as a dynamic compressor. This is because the output of the compressor increases exponentially with the speed of the impeller. This produces a period of reduced power output at the low end of the RPM range, or lag, but provides extremely high output at redline. Centrifugal superchargers use the “fictitious” centrifugal force to compress air. Air is drawn into the center of the unit due to vacuum created by the rotation of the impeller.
This impeller rotates at high speed and ejects the air into the body of the supercharger (known as the scroll or volute) through a diffuser. Passing through the diffuser causes the speed of the air to decrease, which increases pressure (as determined by Mr. Bernoulli). This higher-pressure air then enters the volute, which is small at the point where the charge enters it and larger at the outlet. The change in pipe size will lead to a further decrease in velocity at the outlet, and a corresponding increase in the pressure of the air.
Centrifugal superchargers have more flexibility with how they are mounted, which makes them appealing in aftermarket applications. It also allows for an intercooler to be added downstream of the supercharger.
Stay tuned — our discussion of forced induction isn’t over. Check back for coverage on turbochargers, waste gates, and intercoolers!