Help your engine breathe easier

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In these days of environmental awareness, your engine’s turbocharger is facing more attention than every before. After all, it’s where emission controls live and breathe.

How it works

Basically, a turbocharger’s operation involves a shaft with two-vane wheels at either end – similar to two fans at each end of a common shaft. But the beauty of a turbocharger is that there is no direct mechanical drive to rotate it.

Hot pressurized exhaust gases coming from the engine cylinders spin the turbine wheel at speeds in excess of 100,000 rpm. The compressor wheel at the opposite end of the common shaft allows filtered air to be placed under pressure or boosted into the engine cylinders.

The use of an aftercooler ensures that the hot pressurized air leaving the turbo is cooled before entering the engine, to provide the maximum energy from the supply of dense air mixed with injected fuel.

Every turbocharged engine is fitted with a specific model of turbo sized for the required air flow and boost pressures for its horsepower rating. And each has an ID plate attached to it, listing the model, part number and A/R ratio (the area over radius of the turbine housing). The letter A is the area of the exhaust gas inlet to the turbine wheel, while the R refers to the radius of the turbine housing’s spiral.

If you use a turbocharger with the wrong A/R, you’ll face a variety of power problems. But so too can improper operation lead to an expensive repair bill. In extreme cases, a disintegrating turbocharger can lead even to a major engine failure if the parts are ingested into the engine’s cylinders. But truck drivers and maintenance personnel can lengthen the life of the turbocharger by employing a number of proper operating procedures.

The most frequent causes of turbocharger failures can be traced to either a fault in the oil system or air intake system. It’s easy enough to prevent these troubles

Engine Start-up

First, avoid revving the engine towards the high end of the speed range when you first turn it over. Revving the engine immediately after you start will spin the turbocharger to a high speed when it’s not fully lubricated, burnishing the journal and thrust bearings.

Just touch the throttle and allow the engine to run at idle or fast idle when it’s controlled by the electronic control module (typically around 800-850 rpm in a cold start mode).

In cold weather, it can take between four and five seconds or longer for oil to reach the turbocharger, so let its parts warm up for a few minutes. Luckily, most electronically controlled engines are programmed to ignore any throttle inputs after a cold start-up, to allow enough lubrication to flow through the engine.

After service

Always prelube the turbocharger after major repairs by adding clean engine oil to the inlet side. This can be accomplished by removing the inlet lube oil line and slowly turning the compressor wheel by hand as you add about a half litre of oil into the supply hole found in the turbocharger’s centre housing.

If the engine has been rebuilt, pressure prelube the oil system, and then pour oil into the turbocharger through that inlet lube line.

Engine idling

One of the best ways to lengthen your turbocharger’s life span is to minimize idling. Under idling conditions, low pressures are generated inside the turbocharger. This can cause a mist of oil to leak past the seals and into the hot (exhaust side) and cold (intake side) end housings.

As a load is applied to the engine, temperature increases and this oil will burn off, emitting a blue smoke. The burnt oil can leave deposits on the hot turbine wheel vanes, and that can affect the balance of the rotating assembly over time.

Oil can also be forced into the aftercooler at the cold intake end, leading to plugging problems. If the oil enters the combustion chamber, it will act as an external source of fuel and cause ignition problems.

Engine shutdown

When an engine has been working hard, the turbocharger’s components become very hot. Prior to shutting the engine down, it’s best to idle the engine three to five minutes to allow it to cool down.

If you shut down the engine when it’s hot, you’re threatening the turbocharger’s service life. The lube oil within the turbocharger’s centre housing sump will soak up the heat, and can actually carbonize and eventually restrict the oil drain line that leads back to the engine oil’s sump. The main damage that comes with this carbonized oil comes in the form of a corroded bearing system and end seal rings, as well as grooves in the shaft and wheels.

Problems and causes

When a turbocharged engine lacks power and/or you see an unusual color of exhaust coming from the stack, a faulty turbocharger may not be directly to blame.

For the turbocharger to function correctly, it must be able to inhale fresh, clean air and vent exhaust gases through the exhaust piping and muffler, and the flow can’t be restricted in either case.

Anything that restricts the air flow will deprive the compressor wheel end of air, leading to a low turbocharger boost under load.

On electronic engines, the turbocharger boost sensor will advise the electronic control module of this condition and restrict the engine’s fueling rate to avoid black smoke at the stack – the end result is limited engine power.

Some engines are equipped with an electronic air restriction sensor that will cause the electronics to activate the dash warning lamp, and simultaneously log a diagnostic trouble/fault code for troubleshooting back at the shop.

An air filter restriction service indicator is found on many dashes to allow the operator to determine when a high AIR (Air Inlet Restriction) condition exists. Typically, the maximum restriction on naturally aspirated (non turbocharged) engines is 510 mm (20 inches) of water, while turbocharged engines are usually set to 635 mm (25 inches) when using a water manometer.

If you restrict the air intake too much, you’ll see black smoke billowing from mechanical engines that are under load, poor fuel consumption, low power and increased soot levels in the exhaust.

Other problems associated with running in high AIR conditions can be oil leaking into the intake ductwork/piping at the turbocharger’s compressor side.

Making some noise

Noise from the turbocharger can indicate a number of problems.

A bearing or belt squeal comes in the form of a high pitch that is always in exact proportion to the engine speed, but a turbocharger’s noise causes the pitch to change with both the engine’s speed and load.

A leaking intake manifold or exhaust gasket joint will produce a high-pitched squeal when the engine is operating under load.

Check brackets, baffles and heat shields since they can resonate and generate high-frequency turbocharger-like sounds if they are loose or damaged.

Most boost leaks are linked to charge-air-cooler joints (hoses and loose clamps). But also inspect the exhaust system muffler for loose baffles, etc.

The rattle of a turbocharger may be heard as a scraping or whining sound at idle speeds. Check for vane or blade damage inside the turbocharger, or the intake piping for contamination if you hear this.

Excessive turbocharger noise – particularly at higher speeds and loads – could be caused by worn bearings that are allowing the vanes to rub against the housing.

A turbocharger that makes a warbling noise at higher rpms and loads can be traced to an intake or exhaust that has a plugged filter, collapsed ducting, piping or elbows, or a plugged muffler.

In the end, if you watch the intake, exhaust and lube system and pay close attention to noise while operating the engine, as you should, the turbocharger should last for many miles before it needs a repair. n

– Bob Brady is the president of HiTech Consulting in Burnaby, B.C.

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