I've been thinking about writing this article for a while now. It seems like everyone is an expert on forced induction now a days... so a lot of this will seem elementary, but if you bear through it all and you might just learn a little something new.

In covering this topic, I shall relate the various performance gains each method characterizes, as well as how they relate to the ever emergent fuel economy trend.

First, there are three very common ways to find extra ponies and all of these methods REQUIRE more fuel to be burnt. No ifs ands or buts about that. The more power you want to put out, the more energy you are going to have to expend period. The real debate in regards to using these methods is which method gives you the best mileage for a given power level? This is not a simple answer as you will see shortly.

As most of you already know, turbo chargers use exhaust gasses to cram extra air into the engine. How exactly does it do this? The exhaust gasses are routed into a small turbine and the hot pressurized air flowing past the blades causes the turbine to spin at speeds well in excess of 100,000rpm. The actual efficiency and speed of the turbo charger varies depending on engine size, turbo bearing style, and turbine dimensions. On the other end of the turbocharger is another turbine, this turbine is what pressurizes air coming into the engine. Lets refer to the pressure as boost, and it is usually measured in psi.

Engines can be equipped with a single turbo charger, or multiple turbochargers. A single turbo charger has the advantage of reduced costs, and is generally recommended for low boost (low power gain) applications. Multiple turbochargers can be used in two ways; the can be used in parallel or in series.

A parallel turbo setup allows each turbo charger to be used for half of the engine. This allows the use of smaller turbochargers for a given amount of boost. Smaller turbochargers spool up (speed up) faster, which means the boost in power comes sooner after you step on the pedal which helps create a very responsive engine that people love to drive (although there are some people out there who get a thrill out of using a single excessively large turbo and waiting for the car to launch...).

A series turbo setup puts one turbocharger in front of another, usually they have staggered sizes and bypass valves so as not to overstress a/the smaller turbo(s). What this allows is a single smaller turbo to spool up quickly and provide boost at lower engine speeds and at partial throttle (basically when the engine is sucking less air). The larger turbo charger(s) is/are used to provide more boost/power at higher rpms/throttle positions. This setup is good because it provides the benefits of having a smaller quick spooling turbo, as well as having larger slower, more powerful turbo(s).

On a one last final note, before we move on. I'd like to mention the existence of variable geometry turbines which allow for flatter torque curves and better response!

Now onto superchargers!

Superchargers are like turbos in that they too are used to force extra air into an engine. However, unlike turbos they don't use exhaust gasses. They are driven directly off of the engine. I will cover three main types of superchargers; centrifugal, blower, and screw.

Centrifugal superchargers use a turbine much like a turbocharger, but they are hooked up to the engine instead of another turbine. They usually operate at slower speeds then superchargers, so to compensate the dimensions of the turbine are usually larger. The centrifugal supercharger (like other superchargers) enables the engine to have boost set in regards to engine speed alone, the turbocharger in contrast varies its boost in relation to throttle position as well. Unfortunately the centrifugal supercharger has a side effect also seen with most turbo applications. At low rpms the centrifugal supercharger does not spin fast enough to compress the air to any significant degree, and as such does not provide much of a performance gain.

Blower styled superchargers in the simplest sense act as a fan sitting on top of the engine (albeit with all sorts of weird and wonderful geometrically lobed cylinder shapes instead of a traditional bladed fan shape). It is important to note that even if a blower isn't spinning air can still make it passed the lobed blower blades. Because it isn't fully sealed, it is difficult for a blower to create much of a pressure difference. But it can speed up the air going into the engine, allowing the cylinder to fill faster, and with more air. Unlike the centrifugal supercharger, a blower allows the engine to have forced induction happen at lower rpms do to the geometry involved.

Screw styled superchargers are very similar to blower styled superchargers, but they are sealed, and their screw shape allows them to compress the air before allowing it to enter the engine. This is extremely beneficial especially in generating excess amount of low end boost. Unfortunately, since it's sealed they also have more friction and take more engine power to run. (depending on how you size them... blower style or screw style could net you more power overall)

As another side note, it is possible to combine turbochargers and superchargers, but generally this isn't done due to the costs involved.

Next, some thermodynamic implications.

One of the things that happens when you compress air... is that it heats up. The more you compress it, the more it heats up. Having your air heat up expands the air before combustion and lowers the power output of the engine. So while you still are getting more air into the engine it is rendered less effective since it's hotter and will expand less when ignited with fuel. In other words... if you have a 3.0liter engine, and you force the same amount of air a 6.0liter engine uses, it still won't produce as much power as the 6.0liter engine... unless... you up the complexity/cost of the engine and install an intercooler. By cooling the air before it enters the engine, you manage to regain some of this lost power. Both supercharging and turbocharging benefit from intercooling.

Now, before we start comparing fuel economy, I'd like to give a brief explanation over displacement and power.

A low displacement naturally aspirated engine will generally produce less torque then a higher displacement naturally aspirated engine. However, the overall power level is up to the designers. A smaller engine benefits from having smaller parts with less inertia, less inertia allows smaller engines to spin at much faster speeds. The inertia isn't directly what limits engine speed, what limits engine speed is the amount of force that inertia puts into the various engine components. If you use low quality steel components (i.e. connecting rods, crank shaft etc) if the engine spins too fast things will simply break do to the forces involved. If materials like titanium are used, the individual components can be made stronger, and can withstand the faster engine speeds. Spinning the engine fast means more power! Unless of course you lose torque at the higher speeds do to a multitude of other factors ranging from resonance patterns to flame propagation speeds. Resonance patterns are usually solved with adjusting intake/exhaust lengths, and flame propagation speeds are dealt with by decreasing the individual cylinder size... by adding more (smaller) cylinders...

Now, onto fuel economy!

Which is the best way to get the most performance without sacrificing fuel economy the most? Well... the jury is still out on that, but what I can do is give pros and cons for various scenarios.

First, the tiny high revving naturally aspirated engine.

Pro: Small displacement allows for less fuel to be used when operating with lower engine speeds.
Con: Higher engine speeds increase rotational friction and doesn't allow much time to get all of the energy out of the fuel you burn.
Pro: Lower torque requires more aggressive gearing which helps increase city fuel economy
Con: Lower torque requires a higher engine speed to maintain highway speed, which results in lower highway fuel economy.

Next, the large displacement lower revving naturally aspirated engine.

Pro: Large displacement allows for more power at lower engine speeds giving you gobs of torque when you want it
Con: Large displacement requires more fuel to be burned to keep engine from operating in a dangerous lean condition
Pro: Large displacement engines spins slower overall allowing them to be more efficient at comparable power levels, thus allowing longer gears and benefiting highway fuel economy.
Con: Lower engine speeds keep this engine from making potentially more power.

Next, the turbocharged and intercooled engine.

Pro: Lots of high rpm power when you need it, and virtually no effect at lower rpm means better fuel economy at lower rpms since you can use a smaller engine.
Con: Even high efficiency intercoolers will never get the temperature back down to ambient, so there will always be some thermodynamic loss. The back pressure created by the turbo will also net a small loss.
Pro: Gives you extra power without directly sacrificing engine power, thus allowing for extra efficiency (compared to supercharging)
Con: Uneven torque curves make car less desirable to drive (many technologies are available to help make this point minimal such as multiple turbos, variable geometry, etc)

Next, the supercharged and intercooled engine.

Pro: Lot of torque at every rpm allows gives a very nice torque curve
Con: Power is robbed directly from the engine thus lowering the efficiency
Pro: Low end torque boost allows you to use longer gears, and operate the engine at lower rpms thus saving you fuel
Con: As with the turbocharger, the incoming air will always be warmer then ambient, so there is some loss here as well.

As a final note. When comparing these engines, I'm picturing that they are all putting out the exact same amount of power. There is only one large displacement engine, and the forced induction engines are also small displacement engines.

Turbo Chargers, Superchargers, and Displacement

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