By: Dennis Grant (dg50@daimlerchrysler.com)
Overview:
I posted this about two months ago for someone in the "Newbie" forum, but people rarely go there for information, so I am going
to post this information here. This information is also posted in the "Performance" forum, as to the fact that I am hardly ever in
the "Got Boost?" forum. I hope this helps someone out in the future.
How Turbos Work
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 along with 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 (i.e. Pop a ballon, little bang. Pop a welding oxygen cylinder, big bang).
Okay, a four stroke engine produces work by expanding a gas in a confined space where the high pressure can push against a
piston. Futhermore, 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.
Side Note: A few older Indy cars actually created a couple of pounds of thrust from their exhausts, but that's not enough to be
really useful.
Okay, 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" that you will find in engineering, becuase you are 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 once again.
Side Note: I'll discuss backpressure - I hate that term, it's misleading - in greater detail in another post.
I hope this gives you a general idea of how a turbo works, thanks for taking the time to read.
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Turbo Fundamentals: How Turbos Work
