..............................................................
Turbochargers
How Turbochargers Work
BY KARIM NICE
When
people talk about race cars or high-performance sports cars, the topic
of turbochargers usually comes up. Turbochargers also appear
on large diesel engines.
A
turbo can significantly boost an engine's horsepower without significantly
increasing its weight, which is the huge benefit that makes turbos so popular!
In
this article, we'll learn how a turbocharger increases the power output of an
engine while surviving extreme operating conditions.
We'll
also learn how wastegates, ceramic turbine blades and ball bearings help
turbochargers do their job even better.
Turbochargers
are a type of forced induction system. They compress the
air flowing into the engine.
The
advantage of compressing the air is that it lets the engine squeeze more air
into a cylinder, and more air means that more fuel can be added. Therefore, you
get more power from each explosion in each cylinder.
A
turbocharged engine produces more power overall than the same engine without
the charging. This can significantly improve the power-to-weight ratio for the
engine.
In
order to achieve this boost, the turbocharger uses the exhaust flow from the
engine to spin a turbine, which in turn spins an air pump.
The
turbine in the turbocharger spins at speeds of up to 150,000 rotations per
minute (rpm) -- that's about 30 times faster than most car engines can go.
And
since it is hooked up to the exhaust, the temperatures in the turbine are also
very high.
Keep
reading to find out how much more power you can expect from your engine if you
add a turbocharger.
Turbochargers and Engines
One
of the surest ways to get more power out of an engine is to increase the
amount of air and fuel that it can burn.
One
way to do this is to add cylinders or make the current cylinders bigger.
Sometimes
these changes may not be feasible -- a turbo can be a simpler, more compact way
to add power, especially for an aftermarket accessory.
Turbochargers
allow an engine to burn more fuel and air by packing more into the existing
cylinders. The typical boost provided by a turbocharger is 6 to 8 pounds per
square inch (psi).
Since
normal atmospheric pressure is 14.7 psi at sea level, you can see that you are
getting about 50 percent more air into the engine.
Therefore,
you would expect to get 50 percent more power. It's not perfectly efficient, so
you might get a 30- to 40-percent improvement instead.
One cause of the inefficiency comes
from the fact that the power to spin the turbine is not free.
Having a turbine in the exhaust flow
increases the restriction in the exhaust.
This means that on the exhaust
stroke, the engine has to push against a higher back-pressure. This subtracts a
little bit of power from the cylinders that are firing at the same time.
Turbocharger Design
The turbocharger is bolted to
the exhaust manifold of the engine. The exhaust from the
cylinders spins the turbine, which works like a gas turbine engine.
The turbine is connected by a shaft
to the compressor, which is located between the air filter and the
intake manifold. The compressor pressurizes the air going into the pistons.
The exhaust from the cylinders passes
through the turbine blades, causing the turbine to spin. The more
exhaust that goes through the blades, the faster they spin.
On
the other end of the shaft that the turbine is attached to, the compressor pumps
air into the cylinders.
The
compressor is a type of centrifugal pump -- it draws air in at the center of
its blades and flings it outward as it spins.
In
order to handle speeds of up to 150,000 rpm, the turbine shaft has to be
supported very carefully.
Most
bearings would explode at speeds like this, so most turbochargers use a fluid
bearing. This type of bearing supports the shaft on a thin layer of oil
that is constantly pumped around the shaft.
This
serves two purposes: It cools the shaft and some of the other turbocharger
parts, and it allows the shaft to spin without much friction.
There
are many tradeoffs involved in designing a turbocharger for an engine. In the
next section, we'll look at some of these compromises and see how they affect
performance.
TOO MUCH BOOST?
With air being pumped into the
cylinders under pressure by the turbocharger, and then being further compressed
by the piston there is more danger
of knock.
Knocking happens because as you compress air, the
temperature of the air increases. The temperature may increase enough to ignite
the fuel before the spark lug fires.
Cars with turbochargers often
need to run on higher octane fuel to avoid knock. If the boost pressure is
really high, the compression ratio of the engine may have to be reduced to
avoid knocking.
Turbocharger Parts
One
of the main problems with turbochargers is that they do not provide an
immediate power boost when you step on the gas.
It
takes a second for the turbine to get up to speed before boost is produced.
This results in a feeling of lag when you step on the gas, and then the car
lunges ahead when the turbo gets moving.
One
way to decrease turbo lag is to reduce the inertia of the
rotating parts, mainly by reducing their weight.
This
allows the turbine and compressor to accelerate quickly, and start providing
boost earlier. One sure way to reduce the inertia of the turbine and compressor
is to make the turbocharger smaller.
A
small turbocharger will provide boost more quickly and at lower engine speeds,
but may not be able to provide much boost at higher engine speeds when a really
large volume of air is going into the engine.
It
is also in danger of spinning too quickly at higher engine speeds, when lots of
exhaust is passing through the turbine.
Using Two Turbochargers & More
Turbo Parts
Some
engines use two turbochargers of different sizes. The smaller
one spins up to speed very quickly, reducing lag, while the bigger one takes
over at higher engine speeds to provide more boost.
When
air is compressed, it heats up; and when air heats up, it expands. So some of
the pressure increase from a turbocharger is the result of heating the air
before it goes into the engine.
In
order to increase the power of the engine, the goal is to get more air
molecules into the cylinder, not necessarily more air pressure.
An intercooler or charge
air cooler is an additional component that looks something like a
radiator, except air passes through the inside as well as the outside of the
intercooler.
The
intake air passes through sealed passageways inside the cooler, while cooler
air from outside is blown across fins by the engine cooling fan.
The
intercooler further increases the power of the engine by cooling the
pressurized air coming out of the compressor before it goes into the engine.
This
means that if the turbocharger is operating at a boost of 7 psi, the
intercooled system will put in 7 psi of cooler air, which is denser and
contains more air molecules than warmer air.
A
turbocharger also helps at high altitudes, where the air is less
dense. Normal engines will experience reduced power at high altitudes because
for each stroke of the piston, the engine will get a smaller mass of air.
A
turbocharged engine may also have reduced power, but the reduction will be less
dramatic because the thinner air is easier for the turbocharger to pump.
Older
cars with carburetors automatically increase the fuel rate to match the
increased airflow going into the cylinders. Modern cars with fuel injection will
also do this to a point.
The
fuel-injection system relies on oxygen sensors in the exhaust to determine if
the air-to-fuel ratio is correct, so these systems will automatically increase
the fuel flow if a turbo is added.
If
a turbocharger with too much boost is added to a fuel-injected car, the system
may not provide enough fuel -- either the software programmed into the
controller will not allow it, or the pump and injectors are not capable of
supplying it.
In
this case, other modifications will have to be made to get the maximum benefit
from the turbocharger.
|
| Where the turbocharger is located in the car. |
 |
| How a turbocharger is plumbed in a car |
|
| The turbocharger system of the Mitsubishi Lancer Evolution IX. |
 |
| Inside a turbocharger |
|
| A Mazda RX-8 rotary coupe fitted with an aftermarket turbocharger system. |
 |
| Turbochargers provide boost to engines at high speeds. |
No comments:
Post a Comment