posted October 01, 2000 11:19 PM                

The Compressor

Having covered what a turbo is, and how the exhaust turbine works, we now turn our attention to the compressor side of the turbo.

Side Note: If you thought the previos posts were a little verbose, just wait until you see this one.

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 laws 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 fifty-five 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 seem 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 (wide-open-throttle) operation. At WOT, the turbine gets better gas mileage than an equivelent diesel at the same power point, but at idle, the turbine efficiancy drops, to the point where gas mileage (per minute of operation) is LOWER at idle than it is at WOT!

Turbines are fantastic powerplants 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 infinately-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 combination) to maximize the turbine efficiency at the flow point.

What what does that get us?

A smaller turbo.

That is better, because the smaller 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 turbos.

Reducing lag has another important side effect though. If you have a datalogger, 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 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 combination that maximizes efficiency at that point (describing how is beyond the scope of his post - in a nutshell, you compare pressure maps) and go really, really fast.

If the transmission stays together, that is.

Next up, wastegates, intercoolers, and blow-off-valves, OH MY!