Overview: Supercharger vs. Turbocharger
The intention with this article is to shed some light on the theories behind turbocharging
the modern engine, and the wonderful advantages it gives us in both
speed and basic engine design issues. This post is also to try and give
people a better understanding of supercharger systems and how
they work to make such a simple thing as an engine even simpler
(speaking in terms of tuning). I personally am tired of seeing people
bicker over the differences between the two basic designs, and would
like everyone to note the vast similarities among them instead. So
without further delay... I will start with this simple statement:
"It makes no sense to build a naturally aspirated engine if what you are looking for is good power output."
To
understand why this is so obviously true, we need to break the concept
of an engine down into its simplest parts. There are 4 main parts to an
engine that concern airflow:
-the induction system
-the cylinder head(s)
-the exhaust system
-the camshaft(s)
Now
if you think of the engine in terms of airflow and forget about fuel
for a minute, it becomes a very simple matter really. What we want to do
is best flow air through the cylinder head, from the induction system
to the exhaust system and then out into the world again. This is best
& most naturally accomplished by pressure variation, because as
almost anyone with a high school education knows, air naturally flows
from areas of higher concentration (pressure) to lower concentration
(pressure). Now let's assume for a minute that we are talking about an
engine at or near sea level, well we can just forgo the exact physics of
things and say that at both the induction system's inlet & the
exhaust's outlet we have equal pressure (just under 15psi absolute
pressure). So in order to flow air into this system we must always be
working a balancing act between the three fundamental sections of the
engine, which are exposed to each other only through the camshaft's
orchestration of the valves. So forget everything else you know about
engines and start thinking of what's under your hood in this way for the
rest of this post :).
NA ENGINES (naturally aspirated)
These
must work within a maximum pressure variation of 0psi (which is really
hard to create without massive pumping losses) and 14.7psi (maximum
atmospheric pressure @ sea level). To add to the basic problem of how to
flow air into and out-of this system, both ends of the system start out
at the same pressure, meaning air doesn't naturally want to go IN or
OUT. This can be accurately termed as a 損ain in the ass . Now engineers
and enthusiasts alike have long been fascinated with how to make power
from this setup, but I am talking specifically about supercharged
engines here, and as I already stated "It makes no sense to build a
naturally aspirated engine if what you are looking for is good power
output." So forget about how you can best accomplish this through piston
movement and it's effects on cylinder pressures, and understand that
it's just a whole lot easier to get an engine to work if it's
supercharged.
FI ENGINES (forced induction)
From a pure engine
design standpoint, it makes MUCH more sense to pressurize the intake
system than to run NA. When only the intake system is running under
pressure well above atmospheric, it becomes perfectly obvious that air
is going to want to flow through the engine exactly the way we want it
to, and both cam timing & exhaust sizing becomes much less important
to getting the system to work right (as it was before in NA setup). The
air will naturally want to flow into the cylinder head, and then after
the very strong power stroke (thanks to all that air) it will naturally
want to flow out into the lower pressure exhaust system afterwards.
Everything in the engine will be working at pressure above atmospheric
and the pressure differences will be greatest in the induction system,
so all air will want to exit out the tail pipe quickly and efficiently.
One other thing should be said here: turbos technically ARE
superchargers. A supercharger is ANY device that pressurizes the intake
to above atmospheric pressure, and turbos do this exactly like
superchargers do. The only difference is in how a turbo gets the energy
necessary to perform it's job, and also that the turbo contributes to
supercharging the exhaust system (or more accurately a portion of it,
the exhaust manifold).
THE CASE FOR SUPERCHARGING
Since a
crank driven s/c (s/c = supercharger) is what people are normally
talking about when they use the term supercharger, I will no longer say
"crank driven" to make the distinction between it and a turbo. Now using
a supercharger makes a ton of sense simply because it only has a direct
effect in pressurizing the engine on the side we want it to, the
induction side. Since pressures will always be higher here than in any
other part of the system (except of course during the engine's power
stroke, but that's always sealed off from the rest of the system so we
can forget about that complexity), it's very easy to make this
combination a powerful one. NA engines often use large amounts of valve
overlap to get the whole system to work properly at higher RPM, which
has obvious drawbacks in that it's possible for the intake system and
exhaust systems to interact in a negative way (since they operate at
similar pressures). It's sometimes just as easy to get air flowing
backwards through the system as it is to go forwards in an NA setup,
which is one reason camshaft choice is so important to where in the RPM
band best power will be produced. And here is where the beauty of
supercharging is; neither valve overlap amounts nor perfect exhaust
system designs are completely essential to keep everything flowing in
the right direction. No matter how long the exhaust is exposed to the
intake system through valve overlap, air should NEVER pass backwards
through the system unless the supercharger stops working.
THE EVIL OF SUPERCHARGING
The
evil of supercharging is that some of the power we finally get from
combusting the air/fuel mixture must go back into powering the
supercharger. So here we have designed this whole system that works so
well, yet we have to power it with some of our hard earned torque. This
is not a good thing, but then again nothing so simple is ever going to
come for free. Do superchargers work? Of course they do, which is why
many racing engine uses the technology unless the rules prohibit it. The
net result is more total power from the system, but a portion of this
power must be sapped from our output to make it all work.
THE CASE FOR TURBOCHARGING
This
section is easy to write, because it's exactly the same thing as the
supercharger portion. We have all of the same advantages, except for one
major benefit. That benefit is that turbocharging runs off what is
largely wasted energy, so that damn drawback of needing to power the
system with some of our hard earned torque is removed. In this way, a
turbocharger addresses the one main drawback to using a supercharger,
but as you will see in a second the supercharger addresses the one main
drawback of turbocharging.
THE EVIL OF TURBOCHARGING
Hopefully
you now understand why it makes so much sense to forgo designing
engines for NA use and just supercharge the sucker instead, at least
when we are talking about how to best make power. And if you have been
following what I have said, you will also understand the bad effect
turbos have on our little perfect world of pressure variation.
A
turbo is an ingenious little design that harnesses the wasted kinetic
energy we dump out through the exhaust system to actually force more air
into the engine. This is good for the same reasons that supercharging
is good, but it has one major drawback: it of course increases the
pressure within a portion of the exhaust system. While turbocharging a
motor increases the amount of air that can be flowed into it, it has a
negative effect on how easily we can flow it back out again. This
weakens our positive pressure difference between these two fundamental
sides of the engine, and causes both cam timing & exhaust system
design to again become extremely important to making good power. This is
most certainly not a good thing, but can a turbo overcome this drawback
with the other inherent good it possesses? It certainly seems so,
because in most current forms of racing where the rules don抰 probihit
the use of tubos or slap restrictions on their use, the turbo reigns
supreme in terms of engine power output.
By Brian Ferrari
http://www.automotivearticles.com/
Turbocharger TO4B S6D110
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