How Compound Turbocharger Systems (Turbos) Work
Forced induction is a term car guys throw around: it just refers to a process that compresses air into the intake side of an internal combustion engine. How you go about doing this is actually pretty complicated, but the concept behind it is not. You want cooler, denser air to flow into your engine so that you can produce more power per unit of fuel. This compression of air flowing into your intake is referred to as boost.
There are many ways to go about converting your naturally aspirated engine into a forced-induction engine, or alterng the airflows in an already boosted power plant. I would like to focus on just one of these forced-induction processes right now: compound turbo systems.
Twin Turbo Vs. Compound Turbo: There's a Difference
Not every turbocharger system that utilizes two compressors is a twin turbocharger system. In a twin turbo system, the two turbochargers that compress the air are the same size, and they are set up to split the job of feeding air into the intake between them; the turbos are set up "in parallel."
In compound turbocharged systems, you again have two compressors, but unlike in the twin system, these turbos are different sizes, and they are arranged in series as opposed to in parallel. Instead of splitting the job of forcing air into your intake manifold (or intercooler), they work together, one after the other, to get the job done.
How Compound Turbo Works
Within a compound turbo system, you have a low-pressure turbocharger (the larger one) and a high-pressure turbocharger (the smaller one). Air from the atmosphere flows through the low-pressure turbo, from there into the high-pressure turbo, and from there into your intake manifold or intercooler. This "compounds" the boost effect, which is exactly what you want.
If you have trouble visualizing this effect, think about it like this. Air is a fluid, just like water is a fluid. When water freely flows from a large pipe to one that's significantly smaller, both the pressure and velocity of the water within that pipe greatly increase. The same concept is being applied here. Instead of water flowing through pipes, you have air flowing through turbochargers. Remember, more boost means extracting more power per unit of fuel and greatly improving the performance and efficiency of your platform.
Compound Turbochargers Reduce Turbo Lag
In a previous article on how anti-lag systems (ALS) work, I described the phenomenon known as turbo lag. While there is no way to fully overcome turbo lag in a turbocharger system, there are ways of making throttle response nearly instantaneous.
As it happens, reducing turbo lag happens to be one of the best attributes of a compound turbocharger system. Adding a second, smaller, high-pressure turbo provides boost in the lower end of the RPM range without causing a delay (that you can perceive) between the time you depress the accelerator and the time you accelerate. The high-pressure turbo will continue to provide most of the boost into your intake until enough exhaust is produced to spool the low-pressure turbo. Once the low-pressure turbo is spooling, the amount of boost being fed through the intake is dramatically increased.
Again, to better understand what's going on, it helps to think about this in terms of water in pipes. Before, I mentioned that when water is freely flowing from a large pipe to one that's significantly smaller, both the pressure and velocity within the smaller pipe are significantly higher. Well, now picture the water as RUSHING through the large pipe. You can imagine that if the water in the smaller pipe was already flowing pretty well, that now it's just ridiculously pressurized and flowing VERY well! That's exactly what's going on within a compound turbocharger system. The level of boost it can produce is sinister!
In short, a well-designed compound turbocharger system provides just about everything you want from a turbo setup without any of the normal drawbacks. Excellent throttle response, a dramatic increase in platform performance levels, and a great time in the driver's seat. The only real drawback is a complicated design, and a lot of head-scratching when it comes time to tune the fuel and ignition maps.