Boost - what is it - how is it measured - why does it give power gains - how do turbos make boost? - how is it controlled



Ok, lets start at a very basic level here.


What is boost?
Boost is simply pressurised air present in the intake of an engine.
The fact that the air is pressurised means that it is denser, and therefore any given volume of it contains more air molecules than if it were not "boosted" up to a pressure above atmospheric.


Atmoshperic pressure is 1bar or 14.5 psi (actually a bar is 14.7 and people often confuse the two, but its not relevant to this dicussion really anyway!), therefore if we have no boost at all, we still have 1 bar of pressure in the inlet.

Why does it give more power?
Now assuming we produce .5 bar of boost, that means that potentially we have 50% more air going to enter our engine (feel free to ask questions on why its only potentially and not actually, but heat is basically the enemy here!)

Ok, so we know that boost is a cool thing to have cause its going to make our engine more effiecient at consuming air, which in turn means we can burn more fuel, so therefore make more power!


How do turbos make boost?
so where does this boost come from on a turbo car?

As an engine runs it generates a lot of exhaust gas, this is forced out of the engine under pressure.
This pressure is collected in a manifold and then forced through a turbo, as it goes through the turbo its made to spin a wheel (turbine) this wheel is then attached via a shaft to another wheel (compressor) so as the exhaust gas leaves it ends up spinning this compressor wheel.
The compressor wheel is housed in a sealed part of the turbo, and the input to this housing comes from the air filter (we dont want stones in our turbo!) and goes to the engine (via an intercooler in most cases)

So what happens is that the spinning of this wheel (its like a fan) produces boost in the inlet tract for the engine.



How is it controlled?

Inside the turbo there is a "wastegate", this is basically a circular bypass valve (typically the size of a 2p piece or so on smaller turbos), when this gate is opened it allows some of the exhaust gas to completely bybass the turbine housing and instead exit straight down the exhaust.

This valve remains shut initially, and is operated by a pressure controlled device called an actuator, basically pressure pressing on the actuator overcomes a spring and allows it open the wastegate, either partially or fully.

So in a very simple system, this is the only control that is present, it allows boost pressure to rise to a point, then it is held there via this mechanism.


So how do we control it on a more performance related application where we want to be able to raise boost levels?

Well basically nearly all boost control devices work in fundamentally the same way, they allow boost to leak from the actuator so that it doesnt get enough pressure to open.

So if we have a 10psi actuator, and we want 15psi of boost, we "bleed off" 5psi of pressure so that the actuator doesnt open until 15psi instead.




Future chapters still to come
Right, thats the very basic stuff, i will pop back and detail a bit more about:
wastegate creep
bleed valves
amal valves
external wastegates
manual boost controllers


But if in the meantime before i get a chance, someone else wants to have a pop at one or more of those chapters, then that would be fantastic