Engines 101: What does a turbocharger do?

What does a turbocharger do and why do we use them?

Very simply put and ignoring all the hard parts, pistons, rods, tuning, etc., engines need three things to run well: Fuel; Air; and an Ignition Source. The more fuel you can get in there, the more power an engine makes. That fuel however needs air to burn correctly. You can pump all the fuel you want into an engine but without the right amount of air, it's not going to perform well. 

Engines suck that air in on the "intake" stroke. As the piston goes down in the cylinder it draws fresh air in. So for an Naturally Aspirated engine (one without a turbo, supercharger or nitrous) you're limited to the amount of air that the suction from the piston going down, can pull in. If the cylinder is a liter in size, you'll never get more than a liter of air into each cylinder. Enter the turbocharger. Adding a turbocharger allows you to compress that air meaning more air can be forced into the cylinder, and therefore more fuel to match it. 

How It Works

Invented by Alfred Büchi some time around 1905, the modern turbocharger utilizes two "wheels" to compress air. One wheel, the "turbine," sits in a housing that is fed exhaust from the engine. This exhaust spins the turbine. The turbine is connected to the second wheel through a shaft. This second wheel, called the "compressor" wheel, also sits in its own housing and sucks fresh air in, compresses it, and sends it into the engine. There are many different ways to turbocharge an engine. We'll touch on the most common designs here. 

On the left, the turbine side of a turbo. On the right, the compressor side. Note: The compressor side is missing the housing in this image to display the wheel more clearly.

Single Turbo

Most turbocharged engines use a single turbo. Single turbo systems are nice and simple, but unless it's a VGT system (more on VGT's in a bit) they do come with a little "turbo lag," which is a delay between when your foot presses the accelerator pedal and when the power actually comes on. This delay is due to the time it takes for the turbo to spool/spin up. As technology advanced, VGT turbos came into play to get rid of this lag. 

Variable Geometry Turbos, aka "VGT"

The bigger a turbo is, the more air it can pump. However, the bigger it is, the longer it takes to "spool" or spin up and start compressing air. This is the "turbo lag" we mentioned earlier. On the opposite end, small turbos spool quick but deliver less air than their bigger slower spooling cousins. Simply put, a VGT design allows a larger turbo to spool like a smaller one. Sounds great right? It is, and it isn't, at the same time. Not all VGT's are created equally, but as a general rule, VGT's have proven to be less reliable than non-VGT or "fixed geometry" turbos. VGT's are fine for most, but for those looking for more power or those who really beat on their engines, they lack reliability. Case in point, the 11'-14' Powerstroke used a turbo design that was notorious for failure, while the 15' and up worked amazingly well. 

A VGT, for the sake of this discussion, would be a larger turbo, capable of providing gobs of air all the way through the RPM range. They will pull hard from just off idle all the way to redline. They do this by directing exhaust gasses at the turbine wheel at different angles, increasing velocity. At low RPM, the vanes are angled in a way that increases the pressure placed on the turbine wheel; at higher RPM, these vanes are opened up a bit, as at high RPM, the engine produces sufficient exhaust flow to spool the turbo without the help of the vanes. It's the addition of these extra parts combo'd with increased lateral load on the turbo shaft (from the vanes pushing harder on the turbine) that causes these turbo's to not last as long as a fixed geometry turbo traditionally will. VGT's have their place, they're not horrible, very much so the opposite. They perform very well. They just need to be paired with the right vehicle usage.   

This is the main difference between a fixed geometry turbo and a VGT, the exhaust side. Under that plate are nine controllable vanes which redirect exhaust gas at the turbine wheel. 

Compound Turbos

Compound turbocharging is next and is sort of the best of all worlds. You get reliability, as well as fairly instantaneous power, assuming you don't go too big with your turbo choices. Compound setups use two turbos: a small turbo and a larger turbo blowing into it. There's more to this than we're going to get into here, but the simple explanation is that you get the quick spooling characteristics of a small turbo with the large amount of air the larger turbo provides. Many will add a turbo in addition to their existing turbo, to save costs. Generally compound setups consist of two fixed geometry turbos, although like previously stated, many will add a large fixed geometry turbo and keep their VGT. 

SPE's Death Stalker Compound Turbo System for the 6.7L Powerstroke 

In The End

Selecting the right turbo for you 100% depends on what you're using the vehicle for. Road course use, you should probably stick with a smaller single or smaller set of compounds. If it's drag racing you're into, a large single, or larger set of compounds would be the way to go. The options are endless, give us a call and we'll get you set up right.