Boost ; Turbochargers and Superchargers

Want more power than what just bolt-ons can give you? Or maybe just massive amounts of sheer horsepower? If so, boost is a great option. Boost is an increase in pressure in the intake manifold above atmospheric pressure and is measured in PSI and Bar (In the States we use PSI). Boost is beneficial for power because it is forcing more air into the engine, which allows more fuel to be burned, which in turn produces more power. Within reason, you can make as much power as desired by cramming enough air and fuel into an engine. Using boost to get more air into the engine is a more very efficient way of gaining power, so let’s begin!

Note: You will require tuning going either of these routes. Getting the correct air to fuel ratio, ignition timing is very important.

Turbochargers:

Turbochargers are one of the most effective ways to gain power. They can be used for economy, pure power, or a nice mix of the two. They are incredibly efficient because they use the wasted energy of exhaust gases. Exhaust gases exit through the exhaust manifold and enter the “hot” side of the turbocharger. The gases spin a turbine on the exhaust or “hot” side which is connected to the compressor side. A compressor turbine is spun and ¬†air is compressed creating boost. Since compressing air increases temperature the air is routed through in intercooler, which can either be air/water or air/air. The use of an intercooler decreases temperatures, increasing the oxygen charge. The air is them dumped into the intake manifold and then used for combustion. (*Note* Exhaust gas is not compressed, ambient air is compressed, note the chart below.) In order to prevent the turbocharger from spinning too fast a wastegate is used to bypass exhaust gasses allowing the turbo to spin at a maximum speed. A blow-off valve (BOV) does the same task, but instead releases the air into the atmosphere instead of through the exhaust, this is what causes that wonderful whistling sound in turbocharged cars. A BOV can also recirculate air back into the manifold if desired. There is a type of turbocharger called a twin-scroll, this reduces the volume inside the turbocharger by splitting it into two chambers. This decreases the lag because of the reduced volume and still gives the same amount of boost. It is the same concept as adding a cylinder to an engine, another chamber to reduce the load of the others.

Here are some nice pictures from Garrett’s site:

Garrett also have a nice section with lots of information on turbochargers.

Superchargers:

Superchargers are another way of creating boost. They are much different from turbochargers, and we’ll cover the pros and cons of each in the next section. Superchargers differ from turbochargers by using the engine’s crank. A belt or chain is driven off of the crankshaft to spin the supercharger, the supercharger then spins in relation to RPMs and creates boost. There are three different type’s of superchargers and we will be covering all of them. First up, the Roots supercharger!

Roots:

The Roots supercharger creates boost by compressing air with meshing lobes. Air passes through these and is compressed between the meshing lobes. The air is then forced into the engine and used to create more power. The Roots supercharger is the least efficient of the three; air is delivered in bursts and they are heavier than the other models.

Twin-Screw:

Twin-Screw superchargers work much like Roots superchargers except air is compressed as it moves along the “screws”. The air pockets in between the lobes decreases as the air moves along, thus increasing pressure. A Twin-Screw supercharger is more efficient than a Roots supercharged but at a trade off. A Twin-Screw costs more than a Roots and makes a lot of noise.

Centrifugal:

Last we have the Centrifugal supercharger. This supercharger works just like a turbocharger. The pulley spins a compressor wheel, which then compresses air and forces it into the engine. This is the most efficient supercharger of them all and is much smaller than the other superchargers.

 

Turbochargers vs. Superchargers

There has always been a debate between the two, which route to take all depends on your goals for the car. Lets start with turbochargers, they are the most efficient way to make boost, using wasted energy of exhaust gases to produce boost. A superchargers have a small power loss associated with them, they are parasitic since they take power from the engine to run the supercharger. You can make much more horsepower with a turbocharger than with a supercharger.¬†Turbochargers come with a disadvantage though, they lag. Since it takes some time to spin the turbocharger fast enough to produce boost, there is a lag in the power. Superchargers do not suffer from lag (exception of a Centrifugal supercharger, which tiny amounts of lag have been reported) since their speed it relative to engine speed. Superchargers have a linear power curve with power evenly distributed through out the power band. Turbochargers have a non-linear power curve, more power is at the top end of the power band. Turbos are for top end power. Turbochargers also can have more maintenance than superchargers. A journal bearing turbo will need a turbo timer, what this does is it allows the engine to run for a short time after “shutting off” to allow oil to flow through the turbo. A turbo runs very hot and is cooled with oil or water, those need to keep flowing through in order to cool it off after running, otherwise oil can be “cooked” onto the bearing and cause damage. Turbochargers can also be cooled more efficiently with an intercooler than a supercharger. Since the air has time to travel into the manifold, there is an opportunity for it to be cooled. Since superchargers are on the intake manifold, there is very little room for them to be cooled. Small aftercoolers can be added to cool the air charge. After about 11 psi adding any additional boost may be useless since the air runs hot and heating it up any more can actually reduce the oxygen charge in the air. Hopefully this was helpful, leave any questions or comments in the ask box!

-KotS


Posted: Wed February 22nd, 2012 at 5:43pm
Tagged: TCC Write Up Boost Supercharger Turbocharger
Notes: 67

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