There are two main ways to get more power from a car's
. The first
(and until recently the most popular) is to increase the capacity of the
engine. The second is to increase the amount of
/air mixture going into the
Generally, the more fuel/air mixture going into the cylinders, the more
power the engine will produce. So part of the solution is to tune the
to allow the engine to `breathe' more
freely, but there are limits to how much power can be extracted from an engine
by these means while at the same time maintaining the engine's reliability and
An alternative way of getting more fuel/air mixture into the cylinders is
What is a turbo?
A turbocharger is basically a
driven by the exhaust gases passing out
. The unit consists of a wheel with vanes - the turbine
inside a housing in the
. From this turbine a short
runs to a similar vaned wheel called the
feeds into the engine's air intake.
When the exhaust gases flow
from the engine, they spin the turbine, which in turn spins the drive shaft
to turn the compressor. So, when the engine is running, the exhaust gases drive
the turbine which makes the compressor pump air into the engine.
A fixed amount of fuel is automatically sucked in with the air if the engine
has a carburettor. If the engine has
, the computer control unit
is programmed to suit the boost pressures.
The faster the engine is running, or the larger the throttle opening or
both, the faster the turbocharger will spin. The faster the turbo spins, the
more pressure, or boost it develops and the more air it
into the engine
to create more power.
Although the turbo is designed to pressurize the mixture going into the
engine, too much pressure would be dangerous because it can lead to 'knocking'
(pre-ignition) and put too much strain on the internal components of the
engine. Therefore the maximum boost pressure that the turbocharger can produce
has to be limited by a
known as a wastegate.
The wastegate is a relief valve, located in the turbocharger, that opens to
let some of the exhaust gases
the turbine and flow directly into the
exhaust system. If the boost pressure is getting too high, the wastegate is
activated by a pressure-sensitive
which senses the pressure being
produced by the compressor.
Compressing the air causes problems of its own. When the air is compressed
it heats up, which tends to make it expand. Because the purpose of the turbo is
to get as much fuel/air mixture into the cylinder as possible, this hot air
needs to be cooled down.
To do this, most turbocharged cars are fitted with an intercooler. This
looks like a small
, and cools the compressed air that leaves the
turbocharger. As the air cools down, its
shrinks, so the amount of
fuel/air mixture fed to the engine - and hence the power output -
The turbo unit is plumbed in to the exhaust system as near to the engine as
possible. This helps to keep it compact and also helps prevent turbo lag. If
there was a long length of
between the engine and the turbo, there
would be a time delay between the
being pressed down, the engine
speed increasing, and the turbo accelerating. The effect would be like having
an elastic throttle cable.
Therefore, the turbo is often bolted directly on to the exhaust manifold.
The exhaust outlet is in the centre of the turbine housing and leads off to the
On the inlet side, the pressurized air leaves the compressor housing via a
large-bore pipe. This runs through the intercooler (if fitted), and then to the
inlet manifold, or occasionally plenum chamber, where the fuel is added by
injection before the air enters the engine.
The high speeds at which the turbine can spin create lubrication and cooling
problems. In some turbochargers the turbine can spin at up to 200,000rpm, and
the hottest parts of the turbo will be at or near the temperature of the
Most turbo units have the central drive shaft
fed with oil from the
engine. The turbocharger's lubrication system is specially designed to cope
with high temperatures.
The oil drain pipe is of large diameter to ensure that the oil, which
develops a creamy consistency after going through the turbocharger, will drain
back to the
under gravity. If there were a restricted flow in this pipe,
it would cause a build-up of pressure around the bearing in the centre housing
that would result in oil leaks on the turbocharger.
Some turbos have a water-cooled centre bearing to reduce heat still further.
The advantage is that, because the water is still being warmed by the engine,
it continues to circulate and take heat away from the bearing for a few minutes
after the engine has been stopped.
Early criticisms of turbo engines were their poor performance off-boost -
when the engine was not turning fast enough to spin the turbine quickly - and
the amount of time it took for the turbocharger to start boosting once the
accelerator was pressed.
The poor off-boost performance was because road-going turbo engines do not
usually have a very high
. Forcing a lot of pressure into the
cylinders is equivalent to raising the
so, if the engine
started with high compression, at high boost the pressures inside the engine
problems, or '
', which would result in serious
As a rough guide, every three pounds of boost are equivalent to increasing
the compression ratio by a factor of one. So if an engine with a compression
ratio of 8:1 had a turbo which could deliver nine pounds of boost, the
effective compression ratio would be about 11:1. An average family car has a
compression ratio of 9:1.
Better engine and turbo control is the answer - almost all turbo systems now
use some form of engine management which looks after the electronic
and fuel injection systems, retarding the ignition slightly if the engine
starts to knock. Saab's APC (Automatic
Performance Control) system goes one step further. Not only does it reduce
boost pressure to a safe level, it also allows the engine to be run on any
grade of fuel because the management system automatically compensates -
although you get the best performance only with the highest grade.
Early turbo engines suffered turbo lag, partly through poor engine
management and partly because the lack of suitable turbo units often meant that
the engines and turbos were not ideally matched to each other - a large turbo
on a small engine will give good top-end power but will lack flexibility. Lag
is almost inevitable because a small engine would take time to `spin up' a
large turbo unit. A small turbo on a large engine gives good mid-range power
with little or no lag, but ultimate power is compromised.
These problems have been minimized by better matching of the turbo and
engine sizes, and by using lighter materials such as ceramics and new designs
such as variable flow
(see sideline overleaf).
The obvious benefit from a turbocharged engine is that of increased
performance combined with economy - a turbocharged two-litre engine gives
similar performance to an unturbocharged three-litre one, without burning much
more fuel than a two-litre.
It's often simpler for a manufacturer to turbocharge an existing engine than
to design and develop a new, larger one. Adding a turbo to an engine does not
usually significantly increase fuel consumption unless the enhanced performance
is used to the full.
Stop wasting time on YouTube and get serious!
The Ultimate Car Mechanics video course
Learn everything about modern cars from our new video series.