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.
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.
How a turbocharger is plumbed in a car.
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
Inside a turbocharger.
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 (see How Car Engines Work for a
demonstration), 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
plug 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
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.
A large turbocharger can provide lots
of boost at high engine speeds, but may have bad turbo lag because of
how long it takes to accelerate its heavier turbine and compressor.
Luckily, there are some tricks used to overcome these challenges.
Most automotive turbochargers have a
wastegate, which allows the use of a smaller turbocharger to reduce lag
while preventing it from spinning too quickly at high engine speeds.
The wastegate is a valve that allows the exhaust to bypass the turbine
blades. The wastegate senses the boost pressure. If the pressure gets
too high, it could be an indicator that the turbine is spinning too
quickly, so the wastegate bypasses some of the exhaust around the
turbine blades, allowing the blades to slow down.
Some turbochargers use ball bearings
instead of fluid bearings to support the turbine shaft. But these are
not your regular ball bearings -- they are super-precise bearings made
of advanced materials to handle the speeds and temperatures of the
turbocharger. They allow the turbine shaft to spin with less friction
than the fluid bearings used in most turbochargers. They also allow a
slightly smaller, lighter shaft to be used. This helps the turbocharger
accelerate more quickly, further reducing turbo lag.
Ceramic turbine blades are lighter than
the steel blades used in most turbochargers. Again, this allows the
turbine to spin up to speed faster, which reduces turbo lag.
Using Two Turbochargers & More
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
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.
by Karim Nice
Images courtesy : Garrett