Turbocharging 101

You’re not one of those poor misinformed people who thinks that big V8s are the only path to power, are you? If so, let us educate you: There’s more than one passage to big dyno numbers and low track times. Another option? Turbochargers.

Let’s talk about turbochargers — spooling, rev limiter hitting turbochargers!

What is a Turbocharger Anyway?

A turbocharger consists of:

  • A compressor housing
  • An exhaust turbine housing
  • A bearing housing

On the basic level, a turbocharger is a turbine driven compressor. Why do people get so worked up over them? Because it’s the most efficient way to increase power in an engine.

Turbo diagram

In a normal internal combustion engine, the actual percentage of combustion energy used to power the engine is around 25 percent — the rest gets lost as heat, most of which goes out the exhaust pipe. With a turbocharger, some of that wasted heat is recovered and used to power a compressor that pumps air into the engine.

One half of the turbocharger – the turbine side – is mounted in the exhaust system so that the hot exhaust gas causes the turbine to turn. While this turbine restricts exhaust flow, the benefits outweigh the negatives. This is because the turbine is connected to a shaft that turns a compressor on the engine’s air intake. As the engine produces more exhaust gases, the compressor on the air intake becomes more powerful. The more compression you have on the intake side, the more oxygen you have entering the combustion chamber. More oxygen means you can burn more fuel, and more fuel means more horsepower!

Depending on the design of the turbocharger and some various other attributes, the compression on the air intake can allow for extraordinary horsepower production even on a “small” engine:

YouTube Preview Image

The horsepower output of a turbocharged engine running at a high PSI can easily exceed the horsepower output of a big V8 that’s naturally aspirated (as you can see).

Note: The increased efficiency offered by a turbocharger is one of the main reasons that many automakers have begun selling turbocharged V6s instead of V8s, and turbocharged 4-cylinders instead of V6s. The increased efficiency of the turbo allows manufacturers to “downsize” an engine without losing much (if any) performance. This downsizing, in turn, helps automakers meet emissions and fuel economy regulations.

It’s A Feedback Loop

There are a number of configurations in turbochargers, but the function and general appearance are generally the same. The hot exhaust spins the blades of the turbine, which spins the compressor (on the same shaft as the turbine), which is essentially forcing “extra” oxygen into the combustion chamber of each cylinder.

Turbo loop

As the engine gains speed, there’s more exhaust flowing thru the turbine. More exhaust means more turbine speed, which means more compression, which means there’s an increase in the amount of “boost” on the air intake. The faster the engine produces exhast, the more power the turbocharger can add (at least until the turbo’s turbine reaches its maximum rotational speed, at which point the turbo is producing maximum boost and there are no more gains).

However, if there’s a problem with turbochargers, it’s that there’s a “lag” between the time the exhaust starts to spin the turbine and the time the compressor really starts to increase airflow into the engine. This problem – known as turbo lag – isn’t necessarily a big deal. Turbocharger designers can lessen turbo lag in a number of ways:

  1. Using two turbos instead of just one, where one turbocharger is designed for lower RPM ranges and the other is designed for higher RPMs. The lag is greatly diminished on twin-turbo vehicles.
  2. Using some advanced technology like variable vane geometry or twin scroll designs, the turbines inside the turbocharger can be designed to be more responsive to engine RPMs, lessening lag times AND increasing efficiency.

But the important thing to remember here is that turbochargers generate more power (aka boost) as RPM increases, with a lag time between the instant you put your foot on the gas pedal and when the turbo starts to do its thing.

Compression Creates Heat

Turbo intercooler

One of the last things to understand about turbochargers is that compressors – like the one that’s running on the intake side of a turbocharged engine – create heat. The higher the amount of compression, the more heat that is generated. On a standard turbocharged vehicle, this heat increases the temperature of the air that’s being pumped into the engine. As the intake air (aka intake charge) is heated, the efficiency and maximum horsepower output is reduced.

The solution? An intercooler. After the turbocharger compresses the intake charge, the air is driven past an intercooler, which is essentially a radiator for the air intake system. The hot compressed air “surrenders” some of it’s heat to the intercooler, which reduces the intake charge temperature.

Reducing the temperature of the intake air makes the air more dense, allowing more oxygen into the combustion chamber. More oxygen means more fuel can be burned, and more fuel means more power output.

Cooling the engine intake air also allows for more compression. A standard simple turbocharger often can’t increase the intake air pressure more than 10psi, as the air intake temperature becomes so high that it interferes with engine timing (the hot intake air causes pre-detonation). However, if a turbocharger is paired with an intercooler, the intake pressure can be increased considerably…15-20psi isn’t at all uncommon on stock vehicles, and modified vehicles have seen pressures as high as 40psi, 80psi, even 100psi (only there are problems with running at this extraordinary compression amount that we’re not going to cover here).

Suffice to say, an intercooler counter-acts the heat that’s generated during the compression of the air in the intake. Intercoolers allow for a more powerful turbocharger, which means more maximum horsepower.

What Happens When You Let Off The Gas?

As we discussed, turbochargers are a feedback loop, and there’s a lag between when you press the pedal and when the turbocharger starts to “kick in.” This can cause a problem when you suddenly lift your foot off the gas pedal. Here’s what can go wrong:

  1. Let’s say you’re cruising along at high RPM and your turbocharger is producing maximum compression. Intake pressure is upwards of 12psi. The engine is pulling hard and life is good.
  2. You start to take a corner, so you let off the gas. The engine is now injecting very small amounts of fuel into each cylinder (because you lifted the throttle). However, the air inside the engine’s intake manifold is still pressurized.
  3. This pressurized air forces its way into each cylinder, where there’s very little fuel, and you suddenly have an engine that’s running dangerously “lean” (too much air for not enough fuel). A lean engine gets very hot very quickly, and this heat causes damage.

The solution? Get rid of the compressed air whenever the throttle is lifted. Most automakers “get rid” of the compressed air that’s not needed using a wastegate. A wastegate releases excess pressure in the engine’s intake manifold whenever the throttle is lifted. This way, the air in the intake doesn’t try to forcing its’ way into the engine and causing a lean running condition. Crisis averted.

The wastegate makes a very distinctive “pffft” sound that many performance enthusiasts love:

NOTE: Some vehicles use a diverter valve or blow-off valve to release pressure. Instead of releasing the extra intake air to the atmosphere, a diverter valve sends the air back to the air box that’s “in front of” the compressor. Basically, a diverter valve sends compressed air back to the beginning of the system. This can be very efficient if the system is designed to support it.

Turbochargers vs Superchargers

No conversation about turbochargers is complete without a quick comparison between a turbocharger and a supercharger. Simply stated:

  • A supercharger uses some of the engine’s power output to compress intake air, typically by running off a belt that’s connected to the main crankshaft
  • A turbocharger utilizes the heat from the exhaust gases to compress intake air

Because a turbo is basically just using heat that would have been wasted, a turbo is more fuel efficient than a supercharger. However, superchargers do not have any sort of lag, throttle response is more precise, and they often generate more power overall than turbos (only it’s not cut and dry, as it depends on the boost level, engine design, supercharger type, etc.).

There are many different designs of supercharger design (Roots, Eaton, twin screw, and centrifugal), all of which have their own pros and cons. There are also maintenance and reliability benefits to superchargers (again, depending on design) over turbochargers.

However, when it comes right down to it, both turbochargers and superchargers compress air that goes into the engine. The main difference is how the compression is created (via the exhaust or by siphoning power from the engine).

Tying It All Together

Now that you’ve read thru the entire article, here’s a video that will help you pull it all together. We’ve included notes below to explain what’s happening at each point in the video:

0:10 – Air is sucked in by the turbine

0:16 – Exhaust gases powers the turbo

0:33 – Is where you’ll see the function of the intercooler

0:46 – You can get a close up of the intercooler at work

1:26 – Shows the function of the wastegate

…and the rest of the video shows how everything continues to function and run as it would on a Volkswagen!