Though we vehicle builders like to think we’re more intelligent than the ordinary car guy, we all have a soft spot for excessive and gratuitous demonstrations of horsepower. A supercharger sticking out of the hood screams terrible extravagance. Also, bigger is better.
Still, it’s best to think twice before spending your hard-earned money on a supercharger that may or may not work effectively with your engine. This article will provide you with a rudimentary understanding of positive-displacement superchargers, which will help you plan your forced-induction construction.
Displacement Supercharger vs. VDP Supercharger
What distinguishes these two superchargers? The solution is easy. A positive displacement supercharger pump moves the same quantity of fluid or air with each revolution: No matter how fast the pump spins, one liter of air goes in, and one liter comes out. Conversely, a variable displacement supercharger pump transfers less volume at low speeds.
Superchargers: Not Just Blow Hards
Let’s compare the engine in your automobile to the blower motor in your HVAC system. Positive displacement pumps include piston engines. Whether you spin it at 6,500 rpm or turn it by hand, it should pump the same quantity of air. However, your heater blower motor does not work that way. It moves more air as it rotates faster. This is most obvious while trying to cool a hot automobile. We all know you need to turn up the fan to move the air.
Forced induction works the same way: a positive- or variable-displacement pump forces more air into the engine. These devices are known as superchargers on a street machine and are powered either by the engine or by exhaust gases via a turbine wheel in the exhaust system. The exhaust-driven variant is referred to as a turbocharger, which is short for turbosupercharger. Turbochargers are all variable displacement pumps; however, superchargers can be positive or negative. Positive displacement superchargers come in two varieties: Roots and twin-screw superchargers.
However, this post will not discuss centrifugal superchargers because we believe the industry is moving towards positive displacement superchargers. We also think a modern, efficient positive-displacement supercharger is the most cost-effective option to create extra power. Note that we mean the time it takes to install the system by time, as some kits are as simple as switching an intake manifold.
Positive-displacement pumps come in many shapes and sizes. As previously stated, a piston engine is one example. A piston pump likely presses the compressed air tank in your garage. Pistons in your air conditioner’s compressor move refrigerant. Another example is the Wankel rotary engine utilized in Mazda’s RX vehicles. Positive-displacement pumps are also found in your power steering system and automatic transmission oil pump.
But most of these pumps aren’t superchargers. They can’t move enough air to suit the demands of a vehicle engine while remaining compact. The Roots-style pump can move massive quantities of air, enough to fuel a ravenous V-8.
The Roots blower is the oldest and most widely used supercharger in cars. Nothing shouts horsepower like an enormous Roots supercharger. The Roots blower, which is used to power two-cycle GMC diesel buses, has a long history dating back before the vehicle era. The device was created by Philander and Francis Roots as an air pump for blast furnaces. It was immediately adapted to a wide range of uses, from mining ventilation to plumbing fluid pumping. During the Cold War, a Roots-style pump was also utilized to power the Federal Signal Thunderbolt air raid sirens.
Roots pumps are very basic machinery. A case housing two precision-machined rotors that rarely touch each other or the case. The rotors are linked by gears, and the crankshaft drives one of the shafts. This shaft drives the rotors in opposite directions. A hole in the top of the case allows air into the rotor cavity. The rotors then trap the air between themselves and the pump’s case, pushing it around, down through, and out the other side.
The Roots supercharger’s simplicity may also be its final flaw. No-pressure Roots pumps, according to Dustin Whipple of Whipple Industries. He maintains that they were never meant to pressurize the air. So the Roots supercharger is among the least efficient. Later on.
The other typical positive-displacement supercharger is the twin screw. It resembles a Roots blower but is internally different. Swedish engineer Alf Lysholm, like the Roots brothers, was constructing a pump for industrial applications. His design will later be used to make cars faster. The twin-screw supercharger is a genuine compressor since it compresses and pressurizes air inside its case, whereas a Roots supercharger does not.
The twin-screw design uses rotating rotors, but its shape is different. The male and female rotors have distinct lobe counts, usually three on the male and five on the female. The rotors never touch each other or the housing, but unlike the Roots superchargers, they turn toward each other. Air enters the casing from the back, between the rotors. As the rotors move, their meshing action forces the air through a narrower cavity, compressing it.
A supercharger forces more air into an engine than a normally aspirated engine can take in. Boost replaces the vacuum in the intake manifold by pressurizing it with more air. In simple words, a boost is an increase in air pressure above ambient pressure. The ambient pressure at sea level is 14.7 psi, although varies significantly depending on elevation and barometric conditions.
Because air expands at hot temperatures and contracts at low temperatures, extra pressure does not always indicate more air. In a closed system, heating air in a fixed volume increases pressure due to molecule expansion, but the amount of air or oxygen remains constant.
To demonstrate this, we used the air in our tires. We wanted to evaluate how hard cornering affected the tire pressure. We had 34 psi in the tires before driving. The tire gauge indicated 39 psi after a rip down one of our favorite canyon roads. Alternatively, the original 34 psi reading may be called our ambient air pressure. Heating the tires boosted the pressure by 5 psi. A boost gauge would show 5 lbs. of boost in our tires.
See what we mean? Air at 200° takes up more room than air at 80° in your intake manifold. But 200° air has less oxygen per volume and mixes with less fuel than 80° air. Because there is more air to burn, 80-degree air makes more power than 200-degree air. So what? The best supercharger can boost pressure with a minimal temperature increase.
TVS And H-Helix
As stated previously, Roots superchargers do not create pressure. They provide a boost by over-pressurizing the intake manifold with more air than the engine can use. They lose efficiency as boost increases because air leaks past the rotors and back up into the supercharger, generating turbulence. Turbulence increases molecule-to-molecule friction, and more friction means greater heat. That’s why Roots superchargers were inefficient. Roots superchargers in the 6- and 8-71 GMC design are fantastic for drag racing. They work best at low pressures (wide open throttle at high engine speeds), but efficiency drops at around 30% when the intake manifold pressure rises.
They are designed to have an advantage here. Turbulence and friction are decreased by twisting the rotors and guiding the air through the casing. Also, by compressing the air within the rotors, they provide high-pressure air into the intake manifold, preventing air blowback into the case. They are more efficient at higher boost levels and are appropriate for streetcars running at the part-to-medium throttle.
Eaton has introduced its Twin Vortices Series superchargers. Our supplier Eaton has a budget we can only dream about. The business incorporated a 160-degree twist to the traditional Roots design. TVS superchargers suck air from the back of the case and push it out the other end. The rotors still turn away from each other, and the air is pumped between the rotor and the case. Less leaking past the rotors with more twists than the earlier Roots design.
The Pay Off
Ultimately, this is a matter of efficiency. Which supercharger is best? We have numerous instruments at our disposal to help determine which is the best we can measure a supercharger’s efficiency. The efficiency numbers to consider are volumetric and adiabatic.
Volumetric efficiency compares how much a pump can move to how much it moves. We know this phrase from the vehicle engine. A 5.7L engine should pump 5.7 liters of air each crankshaft spin. But this is rare in reality. Unless you have a huge carburetor and super-flowing cylinder heads, a 6,000-rpm engine isn’t going to fill its cylinders. Atmospheric pressure can only force so much air through the intake, ports, and valves.
Superchargers work similarly. While superchargers work to force air into the engine, they rely on atmospheric pressure to do so. Airflow through the supercharger is affected by rough castings, rotors, and intake and exhaust openings.
The adiabatic efficiency rating concerns heat. Adiabatic efficiency measures how much a supercharger heats the air before delivering it to an intake port. But that’s too simple. The adiabatic efficiency evaluates the actual increase of heat compared to the ideal. Any machine that works generates heat, mainly due to friction. Friction is inevitable as bearings revolve and air molecules tumble inside the supercharger. All of these parameters may be measured to determine how much heat the machine will emit. Then you split the two numbers to get the adiabatic efficiency rating for that particular supercharger.