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Yacht Charter Guide

Marine turbo chargers, love them or hate them they are here to stay.

By: Scott Fratcher - Marine Engineer/Captain

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


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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

 

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New marine diesels are turbo dependent

Compact marine turbo chargers make diesels smaller and more efficient

Turbo damage due to impact damage

This impeller was completely ruined due to impact damage

All that can be seen has been destroyed

 

Marine turbo parts in the trash

 

Articles originally published in
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