How does a turbo work?

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idletask

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Some more explanations!

First, the goal of the turbo is to increase the quantity of air into the combustion chambers thus allowing more fuel/air mixture to be built, hence the increased power. Of course, all the mobile stuff (pistons, cranks, crankshafts) have to be strengthened.

Second, no, no unburnt fuel is used at all in the turbo. However, competition cars (WRC in this case) use a system by which fuel is injected in the exhaust pipes during engine idling (between gears), thus creating some more back pressure and allowing the turbine to continue spinning. The road legal Evo doesn't have this for pollution reasons, and that's why the engine suffers turbo lag.

Also, the bigger the turbo, the harder it will blow, BUT the more inertia it will have as well. When it starts to run though, it will imprint the seat's shape into you back (who said STi ? :mrgreen:). Smaller ones will start to spin faster, but will provide less pressure in the intake pipe.

In order to overcome the inertia, there are mainly three ways:

* twin scroll: in this system, and in the example of a four pot, exhaust gasses from cylinder 1 and 4 on one side, and 2 and 3 on the other, arrive separately into the turbine. It allows to minimize the back pressure caused by "non working" cylinders, therefore making the turbo spin sooner;
* variable vane turbocharging: the wings (?) on the turbine are mobile, their inclination can be changed according to the conditions. The inclination can either be changed mechanically, using back pressure, or electrically. It therefore reduces initial inertia, allows for more blow sooner, and also allows to scrap the waste gate. But the materials used by these VVT don't cope too weel with high temperatures, so these turbos are only in use now on turbo Diesel engines, on which the exhaust gasses are 100?C cooler.
* dump valve: this valve connects the intake pipe to the exhaust pipe. While in "non producing" mode, the valve will open, feeding air from intake to exhaust, right in front of the turbine, therefore minimizing the effect of back pressure.

And finally, increasing air intake pressure means that its temperature will raise, which is not good. But then this is why you have intercoolers.
 

andyhui01

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idletask said:
competition cars (WRC in this case) use a system by which fuel is injected in the exhaust pipes during engine idling (between gears), thus creating some more back pressure and allowing the turbine to continue spinning.
I had this anti-lag system installed on my previous subaru and after I installed it, I realized that because of that, my turbo screwed up in less than a month because even when I was a heavy traffic at low rpm's, the turbo would still be spinning and I would constantly have flames going out of the exhaust. :lol: ... it was pretty funny, in traffic jams, cars stayed like 20feet behind me
 

bone

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there's a third way to get rid of the inertia, it's not that old, and don't know if it has been commercially released.
it this system, the turbo is placed in the back of the car, behind the muffler. makes the turbo on rpm most of the time, and cools down the air before it enters the engine

i'll see if i can find the article again
 
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