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Turbo Fundamentals:-1 / 4

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By: Dennis Grant dg50@chrysler.com
June 1997
Subject: Turbo Fundamentals -
Authorised Exerpts of text from the DSM Digest.
How Turbos Work
(or: The Closest Thing to a Free Lunch)

PAGE 1 / 4

The Ideal Gas Law.
Page 1
FREE Energy?
Page 2
Turbine Theory
Page 3
Compressor Side
Page 4
InterCooler Wastegate & BOV

Before we start, we have to take a second to review a little grade 10 physics -

The Ideal Gas Law.

In short, gas temperature, pressure, and volume are all related. Compress a gas (reduce the volume) and pressure and temperature goes up. Let it expand, and temperature and pressure go down. Increase the temperature, and the pressure goes up (in an enclosed space) or the volume goes up (it expands). Finally, gases want to flow from a high pressure area to a low pressure area, and the greater the difference, the bigger the push. (Pop a baloon, little bang. Pop a welding O2 cylinder, big bang)

OK, a 4 stroke engine produces work by expanding a gas in a confined space where the high pressures created can push against a piston. Furthermore, that gas is heated by the process of creating it (unlike a steam engine) so you get even higher pressures - and more power. Unfortunately, most of that heat (which is the same as energy) is dumped overboard in the exhaust before we get any chance to use it. It's just not in the cylinder long enough to transfer all that heat into mechanical energy, and it's not practical to make cylinders "tall" enough to extract every last bit of work from that hot expanding gas.

So, what can we do about it? well, we can point the tailpipes out the back to try and get some thrust - except that aside from some very rare circumstances, the gas volume isn't high enough to get any worthwhile push. (A few older IndyCars actually created a couple of pounds of thrust from their exhausts, but that's not enough to be really useful)

OK, how about sticking some sort of auxillary engine in the exhaust flow? Steam engines did this for years...

Enter the turbocharger, a turbine fed by exhaust gasses, connected to a compressor via a shaft that compresses intake air into the engine. More air in the cylinder means more fuel can be burnt per power stroke, more burnt fuel means more hot gas, more hot gas means more power - and more boost too.

This is the closest thing to a free lunch you'll find in engineering, because you're taking heat (energy) that would otherwise be wasted and getting usable work out of it, with almost no tradeoffs. You gain a little complexity, and added manufacturing costs, but there is no real performance hit from adding a turbo.

"But doesn't the turbo increase exhaust backpressure?" Under boost conditions, no. Here's why: when the exhaust valve opens, the pressure inside the cylinder is much much higher than the pressure at the turbo inlet. That cylinder pressure "blows down" very quickly, but we're on the exhaust stroke - the cylinder volume is decreasing very rapidly, and from the Ideal Gas Law, that tends to keep the cylinder pressure higher than the turbo inlet pressure. Finally, when the exhaust stroke is nearly done, and the pressures are nearly equal, the intake valve opens, the intake pressure (we're under boost here!) "blows down" into the cylinder, and presto! we have a higher cylinder pressure again.

(I'll discuss backpressure - I _hate_ that term, it's misleading - in greater detail later )

Next: what goes on at the turbine, and how to make it work better.




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