We discussed in part 1 that turbocharging lately has become
very advanced due to the governmental requirements to achieve higher gas
mileage ratings. We now are finding turbochargers installed on V-6 engines
which either necessitates some special designing to use only one turbo, or the
use of twin-turbochargers. With twin
turbos on a V-6 engine, each manifold has a turbocharger installed on it and
both feed into a single plenum on the intake manifold. This system is also used
on boxer or flat engines such as Subaru uses.
There are
also manufacturers who are using twin-turbochargers in series to create higher
boost at higher road speed, but eliminate turbo lag at low speeds. To
accomplish this, a small turbo charger is installed first which will spool up
quickly at low speeds. Then, there are specially designed piping leading to a
second larger turbocharger for road speed. This type system is most commonly
used on diesel engines, but some exotic car builders also use it.
Another
design is the twin-scroll turbocharger where there are two exhaust inlets in
one turbocharger, with a smaller angled one designed for quick response and a
second less angled larger inlet for peak performance. Usually, these twin turbos
will pair cylinders 1 and 4 along with pairing 2 and 3 to more efficiently burn
the fuel mixture and to reduce engine manifold temperatures. It will also
greatly reduce turbo lag.
Variable
geometry or variable nozzle turbos adjust the amount of air entering the intake
side of the turbocharger with a set of adjustable vanes. This will cause the
turbocharger to operate at optimum pressure and efficiency based on the demand
placed on it. There is an actuator which is computer controlled to move the
vanes to increase or decrease airflow. By doing so, it will maintain the
correct exhaust velocity throughout the engine’s power range and limit turbo
lag.
The center
housing/hub rotating assembly (CHRA) is the most highly engineered and probably
the most important part of the turbocharger. This section contains the lubrication,
cooling and the turbine impellers and their mounting. The housing has ports for
engine coolant to run throughout, and also oil passages to the bearing system.
The bearings in most automotive turbochargers are either high-speed ball
bearings or thrust bearings. In older turbochargers, the oil would sometimes
become so hot that it would actually harden around the bearing, called coking,
and this would cause the turbo to fail. This risk has been greatly reduced with
better bearings, cooling designs and synthetic oils, which are more resistant
to heat.
One of the
technologies that has been most effective in improving turbocharger performance
is intercooling. The process of intercooling is basically forcing the air from
the intake side of the turbo through a radiator in an effort to cool it as much
as possible. The reason for this is that hot air is less dense than cool air
and that loss of density means loss of power. When you force air through the
turbocharger it builds up heat, plus it absorbs some from heat transfer from
the exhaust side, so by going through the intercooler, it gives the air a
chance to cool down before entering the engine.
Another application
that is used often by performance tuners is water injection where a spray of
water is injected into the air charge to further cool it. A variation of this
is to actually alter the air/fuel ratio by richening the mixture. The extra
fuel does not actually get burned, but by turning the fuel from a liquid to a
gas, it absorbs heat.
The final
add on feature to a turbocharger is a waste-gate. The waste-gate’s purpose is
to regulate the pressure built in the turbocharger by regulating the amount of
exhaust gas passing through the turbo. A pressure sensor sensing that the
engine is reaching optimum boost pressure does this. The sensor sends a signal
to the engine computer, which in turn sends a signal to a vacuum valve that
opens and pulls vacuum, opening the waste-gate and allowing the exhaust gas to
bypass the turbo.
