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- Page 1
- FREE Energy?
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- Page 2
- Turbine Theory
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- Page 3
- Compressor Side
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- Page 4
- InterCooler Wastegate & BOV
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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. |
