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Turbo Fundamentals:-3 / 4.

Having covered what a turbo is, and how the exhaust turbine works,
we now turn our attention to the compressor side of the turbo.
 
By: Dennis Grant dg50@chrysler.com
June 1997
Subject: Turbo Fundamentals.
PAGE 3/4
 
The Compressor
 
Page 1
FREE Energy?
 
Page 2
Turbine Theory
 
Page 3
Compressor Side
 
Page 4
InterCooler Wastegate & BOV

 

  If you can extract work from an expanding gas via a turbine, then it stands to reason that you can compress a gas by driving the turbine shaft with a power source. In other words, the compressor side is just the turbine side driven backwards. The exact same physical lays apply, just now in reverse: we take a low pressure, low temperature gas, do work on it with the compressor vanes, and get a high pressure, high temperature gas at the outlet.

That temperature increase is unfortunate, and will cause us problems later on - and we'll come back to it in a bit.

While the turbine and compressor sides of the turbo are essentially the same, they are _not_ mirror images of each other, and the reason why is due to the chemistry of combustion. A given volume of air will burn an exact amount of fuel, in a ratio of air:fuel about 14:1. The volume of exhaust produced is much greater than the volume of the air used to create it, and the resulting exhaust pressure is much higher than the boost pressure will ever be, so the wheel and housing designs are completely different.

Which leads us to turbine/compressor design.

Turbines are wonderful devices. They are light, and _very_ efficient, but they also tend to suffer from a limited RPM range. That is, a turbine/compressor is very efficient at a certain RPM/flow capacity, but if you vary the shaft RPM very much, the efficiency drops. Run too fast, and the turbine blades cavitate and (aerodynamically) stall, and flow drops. Run too slow, and the blades aren't getting enough "bite", and flow drops.

Here's an example. The M1A1 Abrams tank weighs about 55 tons, most of it in armour. (Steel and depleted uranium) It has a gas turbine engine that produces 1800HP at the wheels... er, tracks, which is enough power to move that beast at about 70 MPH. The turbine is amazingly small, and while I don't remember exactly how much it weighs, it seems to me that it's on the order of 300-500lbs. Compared to the weight of the rest of the tank, the engine might as well not be there!

However, the design of the turbine was optimised for WOT operation. At WOT, the turbine gets better gas mileage than an equivalent diesel at the same power point, but at idle, the turbine efficiency drops, to the point where gas mileage (per minute of operation) is **lower** at idle than it is at WOT!

Turbines are fantastic power plants for vehicles that can run at a constant RPM all day - like tanks, boats, airplanes, Indy Cars, etc. For vehicles that need to be run at different engine speeds, they don't work so well. (although if somebody invents a good infinitely- variable-ratio transmission, look out!)

So, getting back to turbochargers, what does this mean?

Well, a turbo is really a single speed device. We're only producing enough exhaust to generate boost at WOT, and we have boost-limiting devices to keep the turbo running at a constant speed (once it gets there) so, if we know how much boost we want to produce at WOT, and we know how much air we are consuming at WOT and full boost, then we can select a turbo (really, we're selecting a compressor wheel and housing combo) to maximise the turbine efficiency at that flow point.

Well what does _that_ get us?

A smaller turbo.

That is better, because the smaller the turbo, the less rotational inertia you have to overcome, and the faster the turbo accelerates to it's WOT speed (and the associated boost level) The time delay between opening the throttle and the production of full boost is commonly referred to as "turbo lag" and is the single most hated "feature" of turbo's. Ever wonder why the turbo on the 2G is so small? It's been exactly matched to the air consumption of the engine for the driving style of Joe Public - who rarely, if ever, exceeds 4500RPM.

Reducing lag has another important side effect though. If you have a data logger, and plot the boost curve of your vehicle, the area under that curve determines your transitional power band. Do a little calculus, and you find that increasing that area - even without increasing the peak boost point - increases the torque available to accelerate the car by a large amount. One of these days, one of our tuner guys is going to get a flow bench, and a dyno, and work out the air consumption of his motor at a certain boost point, and select a compressor wheel and housing combo that maximises efficiency at that point (describing how is beyond the scope of this post - in a nutshell, you compare pressure maps) and go really, really fast.

Next ;- Wastegates and Intercoolers and BOV's

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