>From Berrima Diesel: via fj40dana@hotmail.com

Tech Sheet
'Turbo Basics'

Basics:
Air entering the engine first passes through an exhaust driven compressor. 
Compressed air results in a larger quantity of air being forced into the 
engine, creating more power.
The energy used to drive the turbo compressor is extracted from waste 
exhaust gasses. As exhaust gasses leave the engine they are directed through 
a wheel placed in the exhaust flow. The gasses drive the turbine wheel 
around, which is directly connected via a shaft, to the compressor wheel.
Increased exhaust gas drives the turbine wheel faster, this provides the 
engine more air, producing more power. A limit is met once a pre-determined 
boost pressure is achieved. At this point the exhaust gas is redirected away 
from the turbine wheel, thus slowing it down and limiting the maximum boost 
pressure. This redirection valve is known as the wastegate.
This extraction of energy, from exhaust gas, to improve engine efficiency is 
the device known as the turbocharger.
Turbochargers were once usually seen as power enhancements on performance 
cars, but today, turbochargers are becoming more regularly used to provide 
greater torque on small capacity engines. The advantages of using a turbo 
engine include improved fuel efficiency and reduced exhaust emissions.
 

Compressor housing and wheel:
A close-fitting compressor wheel with vane type blades revolves inside a 
contoured housing. Speeds of up to 150,000 rpm are achieved. Air is drawn 
onto the centre and exits around the rim of the housing. Made in aluminium, 
and machined to a high degree of accuracy. Pressure signal for the actuator 
is usually taken from this housing.
 
 

Bearing Housing:
This cast iron housing contains the journal and thrust bearings. Its 
function is to support the bearings and direct oil, via small internal 
passages, to the bearing surfaces. The bearings accurately locate the main 
shaft connecting turbine and compressor wheels. Some bearing housings 
incorporate a water jacket to reduce the transfer of heat.

Turbine wheel:
The turbine wheel is located inside the exhaust housing and is driven by the 
passing exhaust gas. It can reach speed of 150,000 rpm and temperatures of 
up to 1000º centigrade. The shaft is supported on two journal bearings, and 
connects to the compressor wheel at its opposite end. Heat transfer to the 
compressor end is kept to a minimum by passing oil over the surface of the 
shaft and usually with assistance of a water cooled housing.

Journal bearings:
Turbocharger journal bearings are made of a bronze alloy. Two bearings 
support the rotating shaft connecting the exhaust turbine to the compressor 
impeller wheel. Each bearing has an inner and outer load bearing surface, 
each machined to extremely fine tolerances. The shaft rotates within the 
bearing at speeds up to 150,000 rpm, whilst the bearing also rotates at 
around 80,000rpm. Under these conditions, lubrication is critical.
Turbochargers are lubricated by the engine's oil, supplied along a pipe to 
the top of the turbo. Many turbo failures are caused by this pipe becoming 
blocked or restricted, resulting is oil starvation. It results in deep 
scoring around the bearing surface. This can be caused in under 1 second.
The second most common cause of turbo failure is due to particles of dirt, 
suspended in the oil, entering into the turbo. Tiny particles, half the 
width of a human hair, can damage a journal bearing's surface leading to 
failure. Even particles of carbon, disturbed from within the engine, can 
cause turbo failure. It goes without saying that, whenever engine oil is 
changed or any work carried out on the turbo, the strictest cleanliness must 
always be observed.
Once a journal bearing is damaged, turbo oil will escape into the exhaust 
system producing large quantities of blue exhaust smoke.
 
 

Thrust bearings:
The thrust bearing controls the lateral movement, or end-float, of the main 
turbo shaft. It is particularly important for a turbocharger to have as 
little movement as possible, to ensure that clearance between the rotating 
blades and housings is maintained.
The efficiency of the compressor wheel, in relation to the housing 
surrounding it, will be adversely affected if this clearance is too large. 
If the clearance is reduced, contact between the housing and wheel will 
result in severe damage to both components.
Thrust bearings are easily damaged by oil contamination or by excessive 
boost pressure.
Hybrid turbochargers for modified engines are often fitted with a 360º 
thrust bearing. These have a larger contact face which can withstand the 
greater pressure exerted upon them.

Turbine housing:
Usually attached to the engine's exhaust manifold, the housing directs the 
hot exhaust gas onto the turbine wheel for maximum efficiency. Some turbine 
housings also incorporate the wastegate mechanism. This internal passage 
redirects the exhaust away from the turbine wheel thus limiting boost. The 
housing is made from a thick walled cast iron to prevent metal parts 
escaping should the turbine wheel break. It is this housing which glows red 
hot whilst the engine is under load.

Wastegate:
Either an integral or remote type wastegate may be fitted. The wastegate 
redirects exhaust gasses away from the turbine wheel to control the boost 
pressure.

Actuator:
A spring diaphragm device for opening the wastegate. Many variations, 
similar in appearance, are produced to create the correct boost 
characteristics required by each engine application.