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Turbochargers: The Basics

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This article is the first in a three part series aimed at introducing turbo technology concepts to new and advanced members. The reason behind this article is the general lack of an internal resource to link to and the recent influx of new members with basic questions.

If you have suggestions or corrections, please feel free to PM me and you will be properly credited for your additions. I have tried to make this as easy to understand as possible and factual as possible, but I am not perfect.


Turbochargers: The Basics
Last Updated: 02.04.11

Being a DSM owner means you will at one time or the other come across turbocharger discussion. Unfortunately, this also means you will come about a lot of really bad information. In an effort to help the members of this site understand their turbocharger and system, we are compiling articles that go from beginner to expert discussion.

Turbochargers 101: Basic Operation & Components

How does your turbocharger system work?

The amount of power that an engine can output is directly related to the amount of air and fuel can be packed into the cylinders. Naturally a larger engine can flow more air, which means it can burn more fuel and produce more power. Aside from swapping to a larger engine, the way our cars generate more power with-in it's limited displacement, is to force more air into the cylinders. The more air that enters the cylinders translates to the more fuel that can be introduced while maintaining a safe air to fuel ratio. This allows our engine to create a much larger power output, while staying relatively efficient.

The first step in maintaining your turbocharger system or in selecting a new turbo is to understand how the system accomplishes the above.

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From the diagram above, we will take a step-by-step look at how the turbocharger system works.

1. Ambient air is introduced through a filter or screen (not pictured in this diagram) into the intake pipe or directly into the compressor inlet (#1).

FAQ said:
Why do I need a filter or protection screen on the intake pipe or compressor inlet?

Many racing applications will not utilize a filter or screen because they will only see dyno pulls or run on a properly prepared and maintained track. There is little to no risk of them sucking in debris such as pebbles that bounce up into the engine bay. The average street driven car however runs a high risk of pulling in foreign material that could potentially hurt your turbo or even your engine.

Therefore it is always recommended that you utilize a screen or a filter to prevent such an occurrence.

2. The air is then compressed by the compressor wheel which raises the density of the air. From here the air exits from the compressor discharge (housing) (#2).

3. From here the air is introduced into the lower intercooler pipe and travels to the charged air cooler (intercooler)(#3) to be further cooled, which will also help to increase the detensity and make the air and fuel mix more resistant to detonation (premature ignition).

4. The air now travels through the upper intercooler piping and through the open throttle into the intake manifold (#4). Each cylinder will then pull in the charged air in from the intake valves to fill it's volume. The air being more dense than usual allows more fuel to also be introduced into the cylinder. When additional fuel is then combusted the engine is able to produce more power.

5. After the combustion stroke exhaust gasses are exited through the exhaust valves and through the exhaust manifold (#5) to which the turbo is bolted up.

6. As the exhaust gasses enter the turbo (#6) they flow directly onto the turbine (#7) which is directly connected to the same shaft inside the turbo housing that the compressor wheel is connected to. The gasses are what causes the turbine to spin that shaft, which allowed for the compressor wheel to spin and charge the initial intake air. The turbine wheel also creates the exhaust system backpressure that ensures the exhaust gas is higher than atmospheric pressure.

7. The rapid pressure and temperature drop of the exhaust gas causes it to expand, which ultimately provides the power needed to spin the compressor.


What are the components of a turbocharger?

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Turbochargers 102: Wastegates

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A wastegate will always be on the exhaust side of a turbocharger system. The purpose of a wastegate is to allow excess exhaust gas to bypass the turbine to help control the amount of boost being introduced in the system. This is done by either an internal wastegate or an external wastegate.


Internal Wastegate

An internally wastegated turbo has a flapper inside the exhaust housing on a turbo, visible from where the O2 housing bolts up. The flapper is connected to a wastegate actuator, which is nothing more than a boost actuated spring (boost pressure causes the spring to depress) that pulls on a rod that is connected to the flapper, causing it to open.

While in many applications this method works well, you will find that the inefficient design on most MHI turbochargers does not allow this flapper to open up wide enough. This causes not enough exhaust to bypass the turbine which causes the turbocharger to produce more boost than desired. This is known as "boost creep."


External Gate

An externally wastegated turbo functions on the same premise. A boost source is connected to the wastegate which actuates a spring that opens the valve. Because the valve is not limited to the movement of an actuator arm or rod, it can open more fully and more effectively bypass exhaust. This leads to better boost control and, in many cases, elimination of boost creep.

A wastegate is not properly equipped to see vacuum. The hose that is connected to an internal or external wastegate needs to be a boost source only and should run as close to the compressor housing as possible.



Turbochargers 103: Blow-Off (Bypass) Valves

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When the driver let's off the throttle and the throttle plate shuts, the turbocharger is often still producing compressed air. The compressed air can no longer get in through the throttle and is left with no place to go but return to the compressor housing. The compressor is spinning in one direction and the air wants to spin it in the opposite direction. This can and will lead to premature thrust bearing failure in the turbocharger. This is more commonly known as compressor surge.

To combat this killer of turbos, we utilize a bypass (blow-off) valve. The blow-off valve is connected to the intercooler piping leading to the intake manifold, the closer to the throttle body the better. The blow-off valve is actuated by both spring pressure and assisted by a boost and vacuum source. The spring inside the blow-off valve is designed to be compressed easily by excess boost pressure and by the BOV maintaining the atmospheric condition of the intake system by the vacuum hose connected to the intake manifold or close to it.

Under boost the BOV is helped to stay shut by the boost being fed by the vacuum hose. If boost is being lost still, your spring may be too loose. Under vacuum the spring is pulled open and assisted by the excess boost pushing on the underneth of the blow-off valve. This allows the backflow of charged air to escape before it gets to the turbocharger. If you are still getting compressor surge, the spring is probably set too tight.

You MUST use a dedicated vacuum source on your blow-off valve that is not tee'd off of or into any other line or component. This is to ensure proper operation of the unit.



Turbochargers 104: Intercoolers

As we have seen above, compressed or "charged" air comes out of the compressor discharge which is directly connected to components that see exhaust gas temperatures. We know that the cooler the air, the more density it maintains. From being so close to such high temperatures the charged air is rather hot. In order to reduce these temperatures, we utilize an intercooler setup.

Our vehicles come stock from factory with an air-to-air intercooler. However there is a separate intercooler setup for more race specific vehicles known as an air-to-liquid intercooler.


Air-to-Air Intercoolers

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This is the factory setup our car utilizes. It simply means we use ambient air temperatures from outside the car to absorb heat from the charged air that passes through the intercooler, or, air-to-air.

As a vehicle moves along the road or track, air from outside passes through the forward facing fins of the intercooler as charged air passess through chambers in-between those fins. The chambers that the charged air passes through also contain fins inside. The fins will transfer heat from the air passing through to the exterior of the intercooler, which is cooled off by the air passing through the forward facing fins, which lowers the internal flowing airs temperature. By creating a cooler internal temperature you increase the density of the air going to your intake manifold which allows for more oxygen to increase the fuel used during combustion.

FAQ said:
What is the best placement for the air-to-air intercooler?

There are multiple different locations that you may see an intercooler, but only one is recommended. You have probably heard of these locations referred to as side mount, front mount, top mount and in some rare cases, bottom mount. From factory we come with a side mount setup, but that is misleading. This simply refers to how our car has the intercooler placed towards the passenger side of the engine compartment. In reality it is still a frontward facing intercooler.

The problem with our stock location is that it is in an area of restricted air flow and does not allow for larger sized intercoolers.

As stated above, air-to-air intercoolers cool the air from heat transfer with their fins. This means the ideal location would be an area that maintains the highest flow of air from around the vehicle. The highest pressure area (flow) will be towards the front ofthe vehicle. This provides for the most optimal flow of air.

FAQ said:
Why is the fin count so important?

Original Posted by: Kevin Jewer, NEDSM - Edited by: SpawnedX

The internal fins that you see in the eBay and China cores do no work well. The ones in the Garrett core, which look all smashed up, are offset on purpose to promote turbulence and to present more surface area to the tumbling air. The difference is huge. I've run several different intercooler designs and have posted temperature performance as logged with an air intake temperature sensor for most of them. On average, you're looking at a 50-100 degree rise on a drag run with a cheaper core, and up to 50 degrees even on a smaller Garrett core, while a 4-5" Garrett core will pick up 0 degrees through the first 3 gears and about 10-15 degrees total on a full drag run. Due to the large size they can soak up more heat and are effectively 100% efficient for at least the first half of the run on a medium sized turbo. The bigger the turbo and higher the boost the sooner the temperature rise starts, but you get the idea.

The Garrett cores are worth the price premium, especially on pump gas. Race gas will make a less effective intercooler usable from a knock standpoint, but you'll still pay in other ways. The cheap cores can also have a much higher pressure drop than the good cores, but I only have other people's logged data to go on, not my own. However, 4 to 5 PSI is not uncommon, even with a fairly large crappy core on larger turbos. When you're trying to max out a turbo, that will set you back.

Some of the best things you can do for pump gas performance are good intercoolers, big turbine sides, and more displacement.


Air-to-Liquid Intercoolers

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These intercoolers operate in a similar fashion, only that there is an external pocket of liquid, usually water, that absorbs the heat from the air passing through the intercooler. The water is typically cooled off through it's own dedicated radiator. This setup is generally reserved for race specific applications. This will be covered more in-depth in a future article.
 
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