7 Things Every Gearhead Should Know about Superchargers
Superchargers are one of the easiest and most effective ways to increase horsepower. Here are a few facts about superchargers you may just find interesting.
While many people associate superchargers with their first appearances in drag racing in the 1950s, the process is actually quite a bit older than that. Those drag racers were using blowers from dismantled GMC diesel engines that went back to the 1930s. And the original Roots-type blower design, still used by NHRA Top Fuel racers, dates back to 1860, invented as a compressor for blast furnaces.
The Lysholm or Rotary Screw Compressor was invented by Swedish engineer Alf Lysholm, a Swedish engineer, who patented the design in 1938. Its since found widespread use in a variety of industrial applications, both small and large (see image below).
One: Superchargers Typically Come In One of Three Flavors
are somewhat like a half a turbocharger. However, centrifugal superchargers are actually driven by a belt off the engine, while a turbocharger is exhaust driven. Centrifugal superchargers rely on an internal impeller to step up boost. Because boost depends on how fast the impeller spins, these units generally do not create as much boost at low revs (like turbo lag), because the impeller is not spinning fast enough. They do offer the advantage of being able to be mounted off the engine to reduce heat.
Roots or Blower-type Superchargers
are simply large air pumps. A Roots-type blower pulls in air and forces it into the engine’s cylinders. The speed at which the supercharger fills the cylinders depends on the the drive belt. The main drawback to Roots-style superchargers is the consistency of the air charge. While centrifugal superchargers provide constant airflow, the Roots blowers feed air into the engine in pulses, slightly reducing efficiency. It also requires a “dead end” like a cylinder with a piston into which it builds pressure. However, they do produce a flatter torque curve with linear power across the rpm range.
Lysholm or Screw-type Superchargers
are similar in appearance to Roots-type blowers, but they differ in one key way. Whereas Roots superchargers compress the inlet charge within the engine’s cylinders, screw-type superchargers use a set of interleaving internal rotors to draw in and compress air within the supercharger itself, so the air entering the cylinder is already at full pressure.
Two: How Superchargers Make Power
Simply, they pack the cylinders full of air. This allows a greater amount of fuel to be burned. The ideal mix for efficiency is an air to fuel ratio of 14.7:1 though peak horsepower typically comes in at a slightly richer blend. The fuel system needs to be adjusted so that the amount of gasoline entering the cylinder maintains the correct ratio, given the larger volume of air pumped in by the supercharger. When you (or the computer) gets it right, the engine burns more fuel in the same-size combustion chamber, creating more pressure on the top of the piston, pushing it down harder (through the connecting rod) to twist the crankshaft with more force.
Three: Superchargers heat the intake air
Air, like everything in the universe, is comprised of molecules. Molecules are always in motion, slower when it’s cold, faster when it’s warm. When these molecules get packed closer together, like when air is compressed by a supercharger, the molecules start bumping into each other, and transfer their energy into heat. This is a reason why the temperature of the intake air rises through any form of compression: supercharging or turbocharging. many manufacturers of supercharger kits have integrated an intercooler into their designs, to transfer the heat to passing air or engine coolant.
Four: Too large a supercharger can be as bad as too small
Too small a supercharger would have little effect on a lrage engine. You could increase the compression it creates by driving it faster, but then reliability becomes an issue. Too large of a supercharger can overstress the engine (i.e. create too much force at the top of the piston) or exceed the fuel system’s capability to deliver enough gas to main the proper air/fuel ratio, resulting in destructive pre-ignition. Supercharger manufacturers have created a very clever way to help customers select the proper size blower. They call it “effective compression ratio” and it combines the compression ratio of the stock engine with the amount of air that any particular supercharger can compress into the cylinder. It’s a great tool to select a blower that will provide the best match of reliability and power.
Five: Superchargers stress engines
As mentioned, power comes from the amount of pressure that the combustion charge creates on the top of the piston. Every engine component has its design capabilities and its normally multiple times the forces it would see in normal operation. However, if you exceed the manufacturer’s recommendations, you risk burning a piston, bending a rod, flattening the rod and crank bearings, and bending or even snapping the crankshaft. A great example are Top Fuel dragsters, engines that run right on the edge. When one of those engines lets go, the results are catastrophic. The best way to avoid these issues is to discuss your goals with the supercharger manufacturer and follow their recommendations.
Six: Superchargers consumer power
Yes they do, and don’t let the turbo crowd act superior because they get their extra power “for free” (they actually don’t, but the loss is much less than a supercharger). In reality an argument between the merits of a supercharger over a turbocharger are moot. Different horse for different course, as they used to say. For example, I know of no full turbo kit that can be installed as fast as a well-engineered supercharger kit. So yes, superchargers do suck some power (the supercharger of a Top Fuel dragster consumes over 900 hp so even a Hellcat couldn’t power it) yet it increases power by a factor of at least 20. Discuss with the manufacturer the practical net gain possible with your engine and make sure it meets your goals.
Seven: Electric Superchargers
What a great idea, you think. Why hasn’t someone thought of this earlier? Lots of folks did but the technology wasn’t there to support it. A standard 12 volt electrical system doesn’t have the oomph so another solution was needed. A special alternator was mounted to the back of the supercharger so that when the engine was coasting, it acted as an electrical generator. Power was transferred to a super capacitor (which is capable of both higher voltages as well as faster discharge rates than batteries), which in turn powers the electric centrifugal supercharger when the driver’s foot goes to the floor. Obviously engineers still need to develop the concept further to decrease recharge cycles, improve super capacitor performance, and supercharger efficiency, but it well could be the answer for small-displacement (1.0 L 3 cylinder) mini cars.
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