This article discusses turbo chargers, diagnostic, and
repair processes.
Turbo charges now come standard on all but the smallest of marine
diesel engines. This is because a turbo charger can reduce fuel
usage, provide better performance and even cleaner exhaust.
New restrictions on exhaust emissions have been a driving factor
for the increased use of turbo chargers. "Without the added
efficiency the turbo charger provides it would be very difficult
to meet the new European emission requirements." says Craig
Mills from Turbo Chargers NZ Ltd.
Turbo chargers increase engine efficiency and performance
A turbo charged engine is smaller, lighter and uses less fuel
for the same horse power as a non turbo charged version of the
same engine, but why is this? Lets take a quick look at the logic
behind why a turbo charger increases engine efficiency.
The obvious first efficiency gain is frictional losses are reduced
for a given horse power. For example Yanmar produces a 50hp engine
that can be turbo charged and intercooled to produce over 100hp.
This means nearly the same number of moving parts are now producing
twice as much horse power. This translates into slightly better
fuel economy.
A second reason turbo chargers make an engine more efficient is
they force more oxygen molecules into the combustion chamber through
higher boost pressures. This increase in available oxygen means
less chance of unburned fuel exiting the engine thus more fuel
energy is dispersed inside the combustion chamber producing horse
power rather than undesirable high exhaust gas temperatures.
The big reason turbo charged engines produce a large increase
in fuel economy lies buried in the efficiency ratios of an internal
combustion engine. A typical diesel engine converts less than
a third of the diesel fuel it burns into rotational energy (or
usable torque). The other two thirds of the energy contained in
diesel fuel is wasted through heat in the exhaust system and heat
dispersed by the water cooling system. In other words, we lose
the majority of our diesel fuel energy through the engine cooling
system and hot exhaust gasses.
A turbo recaptures much of the lost exhaust heat. Pre-turbo exhaust
gas tempters can range between 6-700 degrees C, while after turbo
temperatures can run in the 550 Celsius range. This 150 C temperature
drop across the turbo directly correlates into energy introduced
back into the engine.
Reasons Turbo Chargers Fail
Modern turbo chargers are designed to last the life of the engine.
Still we occasionally hear about turbos failing. Lets take a look
at the four most common causes of turbo failure
.
Impact damage-This common cause of turbo damage occurs when
foreign material enters the air intake and collides with the high
speed turbo blades. This could be particles as small as gritty
dust, or maybe a loose nut or bolt. It only takes a small blade
nick to cause an imbalance that will soon ruin the turbo bearings.
The second possible reason a turbo blade might be damaged is due
to an engine failure that causes metal bits (IE a broken valve)
to exit the engine and collide with the impeller. This is important
as after any engine failure it's essential to inspect the turbo
and exhaust manifold for damage and cleanliness.
.
Insufficient oil supply-Turbos need plenty of high quality clean
oil. Common causes for lack of oil are :
Turbos that spool up after a rebuild or long shutdown where the
bearing is still dry. This can even happen after an oil change
where the oil filter takes time to fill before oil pressure is
built up.
-A disconnected or cracked oil feed pipe (also a fire hazard)
-Low engine oil pressure
-Gasket sealant left over after service causing a downstream blockage
-Lack of oil in sump
.
Dirty or low quality oil-Turbo charged engines need a steady
supply of clean, fresh, high-grade lubrication oil. --Common causes
of turbo failures due to dirty oil are:
-Clogged oil filter
-Malfunction of oil bypass valve causing low oil pressure
-Thinned lubricating oil due to diesel intrusion into the engine
oil (ie; cracked lift pump diaphragm)
.
Carbon build up in an oil film burned onto the bearing surface-
The exhaust turbine is mounted on the same shaft as the turbo
bearing. The exhaust turbine is extremely hot during operation
(6-700 C) and needs time to cool after use (2-3 minutes). If the
engine is shutdown too quickly after heavy running, the heat from
the exhaust turbine can transfer down the turbo shaft heating
the bearings. The residual heat can cook the remaining bearing
oil onto the bearing surface. This cooked film contains the abrasive
carbon (a byproduct of combustion) and can cause damage to the
bearing and seal. This film also adds another layer of insulation
preventing heat transfer and causing more oil/carbon to build
up after the next hot shutdown. To prevent carbon build up in
the bearings:
let the engine cool for a few minutes after running at speed and
follow the oil change recommendations of the engine manufacturer.
Frequent oil changes help prevent carbon loading the lubricating
oil. This failure is often seen on local sport fishers that run
full speed to a fishing spot only to immediately shut down on
arrival.
Signs the marine turbo has failed
Turbo chargers have become so reliable they are often ignored
for years till something goes wrong. The boat operator might notice
a lack of performance or you might hear the operator say "This
boat does not go like it should." The operator may suspect
a dirty prop, fouled boat bottom, or maybe other causes. The question
is what might we experience to point us in the direction of the
turbo charger needing attention?
Black smoke is a dead give away something in the air system is
amiss. Black smoke means unburned fuel is exiting the boat costing
our wallet in increased fuel use. Black smoke might also manifest
itself in a dirty, black soot stained hull. When we see black
smoke it's time to inspect the air supply system, air filter,
and boost pressure.
Blue smoke may mean the turbo seal has failed thus engine oil
is being fed into the air intake system. The thought of engine
oil feeding into the air intake makes many operators think of
a possible engine run away, but this is rare these days. In today's
high quality manufacturing processes the seal gap is typically
not large enough for a full blown engine runaway. Still blue smoke
may mean the turbo is leaking oil and the cause must be tacked
down. Inspect the intake manifold after the turbo for oil contamination.
The DIY turbo test
Testing a turbo charger can be accomplished in a verity of ways
by the DIY.
The most basic inspection is the simple spin test. With the engine
off remove the air cleaner or ducting so you can see directly
into the turbo air inlet. Grab the center bolt holding the impeller.
You should be able to spin the shaft freely. Attempt to lift and
pull the shaft in all directions. You should not feel any play
in the shaft either side to side or up and down.
This same test can be completed more accurately using a dial indicator.
The side to side play should be less than .05mm and the radial
(up an down) play should be less than .5mm. Testing the radial
play may be difficult because of the space limitations inside
the turbo housing. Sometimes a turbo shaft radial play can be
measured through the oil drain hole. Simply remove the oil drain
tube and insert the dial indicator through the oil drain holes
to measure the shaft play.
A visual inspection can help, but since most of the impeller.
blade is hidden by the turbo housing you may miss worn impeller.
corners, or bent blades.
The most simple turbo test is to check boost pressure. Some engines
provide a boost reading at the engine gauges, while others may
need a pressure gauge inserted into the air intake manifold. Most
yacht and launch engines will have a boost pressure between one
and two bar, but be sure to compare the full turbo boost pressure
with the engine specifications.
Turbo maintenance
Turbo charges are designed to run for years with little or no
maintenance. Some manufacturers recommend a light mist of 2-3
squirts of soapy water into the air inlet while running at 80%
load. The idea is the misted soapy water will hit the impeller.
and lift the oil residue cleaning the blades. This effect is increased
because the oil and carbon residue left on the turbo blades is
hydroscopic. In other words a small amount of water is absorbed
by the film on the turbo blades helping to lift off the oil film.
Turbo Repair-For the DIY the turbo can easily be inspected, tested
and a determination made if the turbo is functioning properly.
Repair of a damaged impeller. or bearings quickly exceed the abilities
of even the best DIY boaties. The cleanliness of the work area
must be impeccable and the extreme rpm's of the turbo shaft means
the balance of the impeller. blades must be perfect. For this
reason it's typically best to make the determination if the turbo
needs repair then simply send the turbo and housing to a high
quality repair shop.
If you find yourself in the outback and have no other options
turbo cartridges can be ordered and an attempt made to replace
the internal moving parts. This has a relatively low success rate
in the long run due to the high tolerances and cleanliness of
the repair area needed.
Engine room air flow
Turbo charged engines need a lot of air flow. The boat designer
often calls for an engine room ventilation system to match the
original engine, but after a re-power the old ventilation system
may no longer be sufficient. The easiest method to test for sufficient
air flow is to use a manometer and measure engine compartment
pressure. At full load we should see less than 10" of water
column in the engine compartment.
Air filter
Turbo chargers need a clean dry air filter. Peter Williams of
Turbo Chargers NZ in Nelson recommends a cotton or foam filter
instead of paper often seen in land use engines. "Paper can
soak up moisture in the marine environment causing a restriction
of air flow" he warns.
Boy Racer Sounds
We have all heard the turbo hissing sound of a "boy racer"
car accelerating, but what exactly is causing that annoying whine?
How come a turbo charged boat engine does not produce the same
high pitched hiss?
The reason the petrol car makes this sound is during operation
is the turbo is spooled up producing boost pressure right to the
moment of shifting gears. When the driver lifts his foot off the
accelerator peddle the butterfly valve in the intake manifold
closes preventing further air passing into the engine. The turbo
pressure is still being produced but now forcing air against a
closed valve. The excess pressure must be released somewhere,
so it's spilled through a waste gate directly to atmosphere. The
reason a turbo charged boat engine does not make this same sound
is there is no butterfly valve located in the air intake manifold
as in the petrol engine, thus the waste gate can dump excess pressure
internally through the turbo housing.
Turbo charging a non turbo charged engine?
Installing a turbo charger on a non turbo charged diesel engine
can be done, but, of course is not recommended by any manufacturer.
Local engineers that have made this conversion on a variety of
diesel engines typically keep the boost pressure below a modest
half bar. They claim to receive a large increase in fuel efficiency
and a wider rpm torque curve. The more complicated valve timing
is not often modified.
Valve timing
Inlet valve timing is set differently on turbo engines than non
turbo engines. Valve timing is a function of the lobe locations
on the camshaft and can't be easily modified. Lets take a look
at why valve timing is different on a turbo charged engine than
a non turbo charged engine.
Consider a typical four stroke engine. The intake (suction) stroke
is the time when fresh air is introduced into the combustion chamber.
In a non turbo charged engine air is sucked into the engine by
virtue of the piston "pulling" a vacuum. This means
once the piston has reached the bottom of it's liner stroke no
more air will be sucked into the combustion chamber and the intake
valve can begin to close.
On a turbo charged engine when the piston reaches the bottom of
it's stroke the intake air is still charged, or has positive pressure
so air will continue to be pumped into the combustion chamber,
thus the intake valve can be left open till the piston begins
it's upward travel. This translates into about twenty degrees
more crankshaft rotation providing much more time for air to be
forced into the combustion chamber.
In other words, compared to a non turbo engine, a one bar turbo
boost can mean more than double the amount of combustion air available
to burn excess diesel fuel.
Marine turbo disasters
Turbo chargers are a safe and time proven engine attachment.
Rare as they are the two most common turbo disasters are burst
turbine/impeller. or broken oil feed line that can become a fire
hazard.
The turbo housing contains quickly rotating internal parts. When
these parts fail a lot of energy can be released quickly (IE flying
bits of impeller. blades). For this reason turbo chargers must
be limited to the manufacturer recommended rpm's and high speed
moving parts should be contained in high quality, well designed
turbo housings.
All large turbo manufacturers have designed their product for
the possibility of an impeller bursting by building thick turbo
housings. The problems can arise when turbo replacement times
comes along. Off brand turbos made in third world conditions are
available on the market for as little as half the price of a quality
replacement. These low grade turbo chargers may contain a one
two punch that can result in disaster.
First the impellers may not hold over the rated rpm's. Reputable
manufacturers typically design and test their impellers to sustain
over 150% of operating rpm's without damage. Off brand turbo chargers
sometimes "burst" near or just slightly over their rated
rpm's. A bursting impeller can be a hazard to the boat or crew
due to the release of metal bits at high speed.
The bursting impeller. is releasing a large amount of energy.
The thick turbo housing is designed to contain an impeller burst,
but the off brand turbo housings may not have the strength to
contain a bursting impeller. causing a hazard to those in the
engine room. In extreme cases it's possible for the impeller.
bits to pierce the hull.
Because of these risks most turbo charger rebuilding companies
recommend using replacement parts from a known quality manufacturer.
The second possible disaster is a rare event where a turbo oil
feed line cracks spraying misted oil onto the hot turbo housing.
This can become a fire hazard. For this reason most modern marine
turbo housings are water cooled thus the housing does not reach
the tempters needed to start a fire. Check your turbo charger
to see if it's water cooled and inspect the oil feed line for
cracked paint or other signs of movement that may indicate a coming
failure.
Future of Turbo Chargers
Turbo charger technology has advanced in leaps and bounds during
the last ten years. High tech materials such as solid milled titanium
impeller. blades have resulted in higher turbo rpm ratings thus
higher boost pressure. High speed computer control systems have
resulted in quick boost pressure corrections meaning faster acceleration
and less pollution. The ability of turbo chargers to pump up the
charge air has meant manufactures of all motors (petrol included)
are building lighter, more powerful, and less polluting engines.
The Mercedes Sprinter, one of the most advanced turbo powered
vehicles on the market now arrives with a two stage turbo charger.
A two stage turbo means one turbo is compressing the inlet air
of the second turbo charger. This increase in turbo efficiency
allows full engine torque starting at just 1200 rpm's. Amazingly
the torque curve remains flat till the engine is spinning over
2400 rpm's.
To the man on the street this means quiet, smoke free delivery
vehicles. To the driver, a wide flat torque curve means less shifting
of gears and the ability to climb long hills without leaving dark
soot stains on the vehicle. To the diesel truck race fans multi
stage turbo chargers mean engines as small as 12 liters are producing
an incredible 1200 hp.
Another turbo advancement is the milling of turbine blades from
solid blocks of titanium. This advancement means turbo rpm's can
increase without the risk of turbo blade bursting. Titanium turbo
chargers now obtain near supersonic speeds spinning over 180,000
rpm's and boost pressures exceeding 5 bar.
High speed electronics play their part in turbo advancements in
the form of variable geometry turbine (VGT) housings. VGT are
the latest advancement combining computer monitoring and high
speed electrical controls. A VGT housing means the turbine is
spooled up and producing boost pressure at very low engine rpm's,
but the VGT regulates boost pressure by use of a sliding a cover
over the turbine. This translates into a turbo control system
that "think" rather than simply react. As soon as the
operator moves the accelerator the VGT turbo increases the boost
pressure BEFORE increased fuel is injected. This prevents the
black smoke cloud typically associated with diesels during acceleration.
Imagine a city buss pulling away from a stop without emitting
a black smoke cloud.
New models of VGT turbo systems arrive with their own computer
control systems, up to 9 sense points, 250 error codes, and over
250 hrs of data logging. The turbo computer can interface with
a laptop computer for testing and data downloads.
Thanks to Turbo Chargers NZ Ltd for background information and
photos
Marine
turbo chargers, love them or hate them they are here to stay
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