Turbos, a begginers guide.

[Demi]

This thread is part of a larger section im working on, but I have been so busy with the holidays its not off the ground yet. Ive been seeing a lot of posts asking questions that this answers recently so i thought i would go a head a move it over here.

The idea for this thread is to shed light on the basic workings of turbo chargers for those who have no experience with them. There is always much more to learn and this isnt intended to be an all encompassing thing, but to give you somewhere to start with your own reading. If you are not familiar with basic 4-stroke engine function, stop here and read this first: HowStuffWorks "How Car Engines Work"

Whats a turbo?
First a turbo charger is basically a glorified air compressor. Overly simplified, the more air and fuel you can cram into your engine the more power it can potentially make. Turbos have two main sections the compressor (intake) side and the turbine (exhaust) side, as can be seen here from this nifty photo.



Dont confuse turbocharges with superchargers. Superchargers work in much the same fashion but are not driven by exhaust gases and are instead driven by a pulley as seen below. Much of this stuff can be applied to understand both.



Both create boost which is what is the pressurized air charge is called. Boost is measured in either PSI (pound per square inch) or Bar (barometric pressure). 1 bar is roughly 14.7 psi. Here is a handy converter. PSI BAR Converter
For the most part in the states we measure boost in PSI. It should be noted that since turbos come in all shapes and sizes that 14psi on one turbo is not the same as 14psi on another. This is because of CFM or cubic feet per minute, or the volume of air being moved. Think of it this way, 14 psi of water in a garden hose is not the same amount of water as 14psi of water in a sewer pipe. Bigger turbos flow more CFM than smaller ones at the same PSI.

What a turbo does
The exhaust side is of the turbo is connected to the exhaust (or turbo) manifold off of the engine. Here is a picture of a turbo bolted on to a manifold.



The manifold channels the exhaust gasses into the turbine exhaust inlet and spins the turbine. The turbine is connected to the compressor via a shaft. So as the exhaust spins up the turbine, it in turn spins the compressor, sucking in air and compressing it, before its sent to the intake side of the engine. Here is a similar picture that shows it in a bit more detail.



Pre-ignition and Detonation
Awesome right? Well yes, but there is a bit more to it too. When you compress any gas you generate tons of heat, and heat is not a turbo'ed engines friend. Since your engine also compresses air, sending hot compressed air into it can lead to detonation or pre-ignition if the heat is not properly managed. This is again this is overly simplified but, the fuel air mixture can combust with out a spark due to high heat.

The difference between pre-ignition and detonation is the timing. Detonation occurs when the remaining air fuel mix spontaneously combusts after the spark plug fires. The charge in the piston chamber is supposed to burn in a neat orderly fashion called a "flame front". It starts at the plug and travels out, expanding the gases as they burn and pushing the piston. When detonation occurs the even flame front is disrupted by the the fuel igniting in other areas of the chamber. This leads to a sudden sharp spike in both pressure and heat, causing the piston to reverberate like you had hit it with a hammer. This is why its also called "knock", and in many cases you can actually hear it. Knock can destroy engines by pitting pistons and further degrading the combustion process and increasing wear.

Pre-ignition is even more damaging to an engine. Its very similar to knock, but as the name implies, it happens before the spark plug fires, and happens for the same reason, compression and heat ignite the mixture or the heat has built up in a component and the heat from that is causing combustion. (think glowing hot spark plug tip). This can be very dangerous for an engine because as the piston is coming up to compress the charge and the mixture burns its slams all the force that should be spining your engine , the opposite way! It increases engine load immensely and pre-igniting engines dont last long.
Of course air fuel ratios, compression ratios, and octane ratings play a big part in this but that is another thread in itself.

Here are some examples to illustrate the point. Healthy (new) piston, pre-ignition piston, detonation piston and detonation spark plug respectively. This is by no means the limit of what can happen.


Heat management
Great, so heat is an issue. How can we control it? There are countless ways to go about this, from larger amounts of oil in the system, oil cooler, upgraded radiators and cooling systems, to methanol/water injection, because its not just the heat of the air charge into the engine, but the temperature of the engine as well. We are going to focus on the charge however and talk about intercoolers, which are found on most turbocharged applications.

An intercooler works the same way a radiator does, except instead of cooling water and antifreeze, it cools the air charge from the turbocharger. Intercoolers work by exposing as much surface area as possible and allowing air from outside the vehicle to cool the air the turbo has compressed. Im not going to harp on it too much because there is already a great write up here for it , but here is a picture to help illustrate the point.



That should give you a pretty good idea of the basics of how a intercooler works. The turbo compresses air and pushes out the compressor air discharge from picture 1, through the inter cooler and the air in front of car passes over it cooling off the charge before it is sent into the intake side of the engine. Again, and I cant stress this enough, this is just the tip of the iceberg on how much there really is to know, so be sure to read the intercooler write up. I just want to get the basic idea across.

How it all goes together
Before moving on I want to make sure everyone knows whats going where. Refer all this to picture 1. The air going to the ambient air compressor inlet, comes from your intake, or stock air box (where your air filter is). Once compressed its coming out of the compressor air discharge on its way to your intercooler. Once cooled there into the intake manifold through the throttle body. On the "hot side" of the turbo, the exhaust gases being pushed out of you exhaust manifold from the engine, are going into the turbine exhaust gas inlet where it spins the turbine and travels out the turbine exhaust gas outlet into your exhaust system, through your muffler and out the back of your car. Clear as mud? Here.





Wastes gates and controlling boost
Great so now we have our exhaust driving our turbine, which drives our compressor, and gives us boost, but we need to regulate that boost. Without some sort of control over our turbo, the turbo woulds spins as fast as the engine could drive it, which would give us as much boost as the turbo could possibly produce. Not a good thing for either the turbo or the engine. Over speeding a turbo (faster than its rated efficiency) creates gobs of heat, increased metal fatigue, and produces lots of wear on the components. Over boosting an engine (more boost that it can mechanically handle or has been tuned for) will lean out the air fuel mixture which, as we discussed, can lead to knock and pre-ignition. So we control both with the use of a wastegate. A wastegate is just a diverter valve, and there are two types, internal and external. The job of both is to divert exhaust gases around turbine instead of through it, to control how fast it spins. A internal wastegate is actually integrated into the turbo housing. When the valve opens the exhaust gases take the path of least resistance and travel out the flapper valve.





The opening and closing of this valve is controlled by the wastegate actuator. When the desired boost is hit the wastegate actuator pushes on the control linkage (people call this all kinds of stuff but as long as you know what it is) which opens the wastegate valve and diverts the exhaust. Here is a video that shows the action to help understand what im talking about. Worry less about point of the video and more on how the wastegate works, but by this point Im sure you can figure out why the problem in the video is so serious.

YouTube - Turbocharging 101 - Wastegate Basics

YouTube - Turbocharging 101 - Wastegate Basics

Here is a great picture of an external wastegate. You can see the valve inside, and it operates the exact same way as its internal counterpart, the valve raises and exhaust is diverted.



The difference is where they are mounted and what they do with the exhaust. External wastegates are mounted on the exhaust manifolds and can be configured a number of ways as far as what it does with the exhaust its bypassing around the turbine. Internal waste gate typically dump right into the exhaust via the downpipe. Here is a good picture of a downpipe for an internal wastegated turbo.



This is called a divorced downpipe because it has a separate exhaust channel for the wastegate exhaust and the turbine exhaust to reduce back pressure. Something similar can be done with external waste gates. Here is a picture of an external with its exhaust welded into the turbine exhaust.



Another very popular way is called a "howler" or "screamer" pipe. This is when the wastegate exhaust is routed by itself and doesnt go through the catalytic converters or muffler, hence the name. The sound amazing because once the target boost is achieved and the valve opens, the car is essentially running an open manifold. Go to a local 1/4 track on race night and you will know one when you hear it.



Great video of a screamer pipe. It shows how the wastegate only opens when desired boost is achieved.

YouTube - Nissan 200SX S13 with screamer pipe

YouTube - Nissan 200SX S13 with screamer pipe

Blow off valves and bypass valves
Questions about blow off valves (BOV) and bypass valves come up a lot so hopefully this will help clear some of those up. The job of a blow off valve and a bypass valve is the same, the difference, like with internal and external wastegates, is what they do with the gas they are diverting. Of course BOVs and bypass valves deal with the compressed air charge from the turbo and wastegates deal with exhaust gas. Both BOV and bypass valves are mounted on the "cold side" following the intercooler, before the throttle body.



When the turbo spools and compresses air, its pushed through the intercooler and into the throttle body, and the engine gobbles it up. When you let off the throttle, the throttle body plate closes. Think of it like a door at the end of a hallway, the door is the throttle plate, and the hallway is intercooler piping. Now you still have a large amount of compressed air with no where to go, this is where the BOV comes in. When the throttle plate closes it creates a vacuum on the intake manifold and this is routed to the BOV or bypass valve, which opens the valve to release the built up compressor charge. Without it the compressed air would force its way back to the turbo which is called compressor surge and it can destroy turbochargers.



Above are a blow off valve and a bypass valve respectively. Blow off valves vent the charge into the engine bay, which is referred to as Vent To Atmosphere (VTA) where bypass valves recirculate their charge back into the intake side of the engine. This is the only difference between the two. Most people enjoy the BOV sound, and most turbo applications can be made to work VTA.

Here is a video showing a BOV running VTA.

YouTube - Eclipse GS-T Greddy Type S BOV VTA

YouTube - Eclipse GS-T Greddy Type S BOV VTA

Here is a video showing what compressor surge sounds like. Ignore the Prodigy sound track, these guys apparently think surge is cool.

YouTube - SR20DET Compressor Surge

YouTube - SR20DET Compressor Surge

Boost Creep and Boost Spike
Lastly two terms that warrant an explanation and dont really fit anywhere else. Boost creep is when, after the target boost is achieved, the boost continues to rise with rpm. This can stem from numerous problems, like insufficient sized wastegate. If the wastegate is too small then not enough exhaust gases can be diverted away from the turbine, causing the boost to "creep" up even though the wastegate is fully open. Improperly tuned electronic boost controllers can do this also.

YouTube - EJ225 (205/22T) Boost creep

YouTube - EJ225 (205/22T) Boost creep

Boost spike is when the target boost is passed and then settles back down. Say your target boost was 15psi and you hit 18 for a second before it settled back down to 15, this would be boost spike, and it can also be caused by a host of problems like an slow wastegate solenoid.

http://www.youtube.com/watch?v=kDfhyF7jAFE

http://www.youtube.com/watch?v=kDfhyF7jAFE

Turbo Timers

Turbo timers idle the car for a preset amount of time before shutting off the engine by holding the 12 signal from the ignition switch after the key has been removed. Usally this is triggered by a switch installed on the emergency brake. The ebrake is pulled, the keys are are removed and the engine keeps running for set amount of time and then shuts off. This is to prevent the oil passing through the turbo from "coking". This happens when oil flow through the turbo stops and the oil sitting in the turbo begins to burn due to heat. This leads to build up and can damage the turbo. This is only the case however in OIL COOLED turbos. Meaning that oil from the engine (or sometimes a seperate system) is flowing through the turbo for cooling purposes. Idling the engine keeps the oil moving long enough to allow the oil to cool. With the MZR engine however, this is a non issue, due to the tubro being water cooled and water doesnt burn.


Extra reading....
HowStuffWorks "How Superchargers Work"
HowStuffWorks "How Turbochargers Work"
TurboByGarrett.com - FAQ's

I hope this helps, and feel free to post any questions! Please keep in mind that this is just scratching the surface of what there is to discuss but i hope it gives you somewhere to start. Also there are a lot more experienced guys on here, to you guys, if I got anything wrong please chime in so I can keep the info as accurate as possible.