Turbine Theory

[JBrady]

http://www.turboclub.com/turbotech/TurboFun2.htm

Alright, we determined that a turbo was a device that could be used to get useful work out of otherwise wasted energy, now we will discuss how that happens in more detail.
It is a common misconception that the exhaust turbine half of a turbo is driven purely by the kinetic energy of the exhaust smacking into it (like holding a kid's tow pinwheel behind your tailpipe) While the kinetic energy of the exhaust flow does contribute to the work performed by the turbo, the vast majority of the energy transferred comes from a different source.

Keep in mind the relationship between heat, volume, and pressure when we talk about gasses. High heat, high pressure, and low volume are all high energy states, low heat, low pressure, and large volumes are low energy states.

So our exhaust pulse exits the cylinder at high temperature and high pressure. It gets merged with other exhaust pulses, and enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level.

As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly, it expands, cools, slows down, and dumps all that energy - into the turbine that we've so cleverly positioned in tho housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate. Presto! We've just recovered some energy from the heat of the exhaust, that otherwise would have been lost.

This is a measurable effect: Stick an EGT upstream and downstream of the turbo, and you see a tremendous difference in temperature.

So, in real world terms, what does this tell us?

All else being equal, _The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet_ (in English, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.

Raising the inlet pressure is possible, but tough. Lowering the outlet pressure is easy - just bolt on a bigger, free flowing exhaust. I've seen a couple of posts from people who added aftermarket exhausts, who report "my turbo spools up faster now" Well, that's because by lowering the outlet pressure, you increased the pressure differential, and now the exhaust gas can expand more, and do more work. That increased work pushes harder on your turbo, and it spools up faster. You should also see less boost drop at redline, because if an exhaust system is flow-limited, once you pass the flow limit of the system, any additional gasses you try and force through it only raise the outlet pressure. Higher outlet pressure, lower pressure differential, less work, less boost.

[Note that the compressor side comes into play here too - that's another story]

 

[stevechumo]

Very nice explanation, John. :arms: I'd also like to add a detail outside of that scope is the faster turbine spool also depends on the pipe structure. We all know generally smaller pipe flows faster than larger pipe or shorter pipe flows faster to the turbine so I'll skip that. The bad pipe structure could create more turbulence and slow down the exhaust gas. So a smooth and evenly distributed pipe structure will have less turbulence and have faster velocity.

 

[JBrady]

Very nice explanation, John. :arms: I'd also like to add a detail outside of that scope is the faster turbine spool also depends on the pipe structure. We all know generally smaller pipe flows faster than larger pipe or shorter pipe flows faster to the turbine so I'll skip that. The bad pipe structure could create more turbulence and slow down the exhaust gas. So a smooth and evenly distributed pipe structure will have less turbulence and have faster velocity.

Correct. All work requires energy. Inefficient design costs energy. Changes in volume and direction, resistance, thermal considerations and more all can and should be considered before the required compromises of design and budget are applied. In context to your good point an example would be an intersection such as between the manifold and the turbine housing (or head and manifold) where the two do not match. While a missmatch can actually help some designs it will require either an increase in velocity (going from large to small) or a decrease in velocity (going from small to large) as well as protrusions creating turbulence which effectively reduces flow capacity. Very similar to the art of maximizing carburetor CFM without changing the butterfly diameter. Not all holes are created equal even.

BTW, the above is copied and is not my work although it is more or less the way I would describe things. The link has the other 4 sections and descriptions. The site is dated but basic turbo theory is important to learn and even review.

 

[JustenGT8]

Glad someone spent the time to write this out in nice easy language...save me having to try and explain it time and time again...."see link"