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Turbocharging Basics

If ever there was a mechanical marriage made in heaven, it is a diesel engine and a turbocharger. On the farm, this union is found in everything from a pickup truck to a combine.

There are two ways an engine can breathe.

1. An engine can consume air naturally via the differential in pressure in the cylinder bore vs. the atmosphere. This describes a naturally aspirated engine.

2. An engine can have air forced into its cylinders through the use of a turbocharger or supercharger (forced induction).

The two methods of forced induction differ in how they are powered. A supercharger is driven from the engine’s crankshaft and consumes power. A turbocharger, on the other hand, uses the exhaust gas exiting the cylinder to operate, and it requires no power from the engine. 

The turbocharger accomplishes two things. It fills the cylinder bore with more air, and it causes turbulence in the cylinder. This latter effect greatly improves combustion. Thus, a turbocharger makes a diesel more powerful, lets it run cleaner, and gives it the potential to use less fuel.

Volumetric efficiency

The standard used to measure cylinder fill is called volumetric efficiency (VE), and it is read as a percentage.

A naturally aspirated engine experiences around 80% VE. In other words, it uses 80% of its capacity in regard to cylinder volume.

By employing forced induction due to a turbocharger, the VE of an engine can improve to 100% and higher based upon the amount of airflow created and the pressure produced. 

That pressure, by the way, is read in the intake manifold as boost. The gauge on a dashboard reads this as pressure per square inch, but it is really the amount of pressure over the atmosphere.

If atmospheric pressure is 14.7 psi and the turbocharger is producing 14.7 psi (via the gauge reading), then the cylinder is actually seeing 29.4 psi. Thus, the effective size of the engine can be considered doubled for every 14.7 psi of boost.

In theory, a 12-liter engine (1 liter is approximately 61 cubic inches) when exposed to approximately 15 psi of boost in pressure is the equivalent of a 24-liter engine that breathes naturally. 

What is wonderful about turbocharging is that the power gain it offers is passive. In other words, the gain is only there when you need it, such as the times an engine is called on to work hard. When the load is low, the engine operates at its mechanical size. When more power is needed, the turbocharging helps the engine to respond as if it were larger in displacement. 

The intercooler

Many turbocharging applications also employ a heat exchanger, which is identified as either an intercooler or charge air cooler (CAC). The purpose of the CAC is to cool charged air, which, in turn, increases the density of the air being sent to the cylinders. 

The CAC also helps to reduce the heat caused by the act of turbocharging. Hotter air is undesirable to engine performance as it contains fewer oxygen molecules than cooler air. For every 10°F. change in charge air temperature, power is altered by 1%.

A turbocharger incorporates an exhaust-driven turbine wheel. That wheel is contained in a volute, which is a snail-shape housing. That wheel is also connected via a shaft to another volute that contains a centrifugal compressor wheel that sends the charge air to the intake manifold. The turbine side of a turbocharger is considered hot; the compressor side is considered cold.

The shaft that connects the wheels rides in a bearing housing that is fed engine oil under pressure. That oil flows through the housing draining back to the engine (usually back to the oil pan or timing cover). The shaft’s bearings are of the floating variety. Some turbocharger applications (especially earlier designs) may have used semi-floating or pressed-in ball or roller bearings.

Under high boost conditions, the shaft (and, therefore, the turbine and compressor wheel) can spin at speeds as high as 150,000 rpm. Due to the exhaust heat at the turbine, many models also send engine coolant through the bearing housing to improve service life and to reduce coking of the oil.

Seals are used in a turbocharger to keep the oil away from the exhaust and intake tracks. They also contain the spent gasses and boost pressure in the volute.

Turbochargers vary by the way they control their boost pressure. These controls can either be a wastegate valve or can consist of movable rings with vanes on the turbine side. A wastegate allows exhaust gas to bypass the turbine wheel and housing and, thus, limits its speed.

Function of load, not engine speed

The energy that turns a turbocharger’s turbine wheel comes from the hot exhaust gas leaving an engine’s cylinder. The turbocharger is passive since it responds to a much lesser extent to engine crankshaft speed than exhaust temperature. That is why you will hear the turbo spin up when the engine is loaded, even though there may be little to no increase in engine speed. 

As the load on the engine increases, so do the exhaust temperature and its velocity. When the exhaust leaves the port of the cylinder head, the inert gas experiences isentropic expansion. This means without temperature change.

The hot and expanding gases are forced into the turbine housing and act on the turbine wheel in the same manner the flow of a river would have had on an old grist mill. The compressor wheel then feeds air to the intake manifold under pressure. The result is an increase in VE, power, and reduced emissions.

Turbo Service Tips

A turbocharger is similar to a crankshaft in engines designed for forced induction in that the turbo is considered a main component meant to last the life of the engine if given the proper maintenance. This is not to say that turbos don’t fail. When they do, 90% of the time that failure can be traced back to either the turbocharger ingesting a foreign object or to poor maintenance.

Foreign objects will damage – if not destroy – a turbocharger. The best way to prevent such a disaster from happening is to faithfully change air filters as recommended by the engine manufacturer.

Changing filters also offers the benefit of preventing an excessive drop in air pressure and a vacuum at the turbocharger’s compressor oil seal. If a drop in pressure continues over time, the condition will challenge the oil seal. 

Another general turbo tip is to let an engine idle for a minute or so after being run hard. This lets the turbocharger slow down while cooling down. This is an old tried-and-true procedure that is often neglected today, but it pays dividends over the life of the equipment. 

Troubleshooting

The most common symptoms related to lack of turbocharger performance are lack of engine power, excessive exhaust smoke from excessive oil consumption, and (if applicable) the ingestion of coolant from a water-cooled bearing housing.

When dealing with a lack of engine power, first determine if the engine is operating correctly before blaming the turbo for causing the issue. If the engine runs well, then there is an excellent chance that the cause of the lost power or excessive exhaust smoke is in the turbocharger system. 

When setting out to determine what’s making the turbochanger perform badly, always conduct a physical examination of the component. Check all inlet connections for a tight fit from the turbocharger to the engine. Loose hose clamps or compromised hoses will let boost escape.

During your examination, look for telltale signs of an exhaust leak upstream from the turbine housing back to the engine.

Exhaust leaks limit turbo performance since not all the exhaust gas is flowing to the turbocharger’s turbine. This, in turn, greatly affects its ability to compress combustion air.

When looking at the system, be sure to check the integrity of the turbocharger’s intercooler (CAC). It’s possible for it to get a crack in a tank or a small pin hole in a tube (particularly a problem in over-the-road vehicles). It may be necessary to remove the CAC and pressure-check it as you would a radiator. 

If an oil seal on a turbo’s compressor does go bad, it will force lubricant into the CAC. Wash that lubricant out since it not only causes excessive exhaust smoke but also limits the thermal conductivity of the unit.

Stuck wastegate

If the turbocharger you are examining employs a wastegate device, confirm that gate is not stuck open. A stuck-open wastegate can starve an engine of power or cause it to slowly build boost.

During the examination, be sure to check the integrity of the line that goes to the turbo’s diaphragm, which senses boost. If that line is cracked or is leaking, this will cause the engine to overboost. 

If you’re checking out a turbo with variable vanes, look for carbon buildup in those vanes. Carbon accumulation causes the vanes to stick and, in turn, causes the solenoid operating the vanes to fail.

Check the oil lines

If the turbocharger you are examining has had a bearing or seal fail, be sure to confirm the integrity of the oil feed and drain lines. If the drain line is clogged with sludge, this will cause oil to build up in the shaft housing and work through the seal. 

When checking a turbo, look at its compressor inlet to see if the impeller is damaged. Also look for excessive oil film and smooth movement of the shaft. Keep in mind that if the turbo you are examining has a floating bearing, then its shaft will move up and down slightly. However, if that movement causes the fins to hit the housing, this is a sure indication of excessive shaft wear. 

If and when the time comes that the turbocharger needs professional service, it’s important to make sure the job is done properly. Failure analysis is the first step in determining what caused the breakdown. 

When hiring work to be done, insist that the service shop always use original equipment seals and bearings and that the assembly be balanced after it is repaired. Balancing requires special equipment that a low-dollar rebuilder will not have or will argue is a task that isn’t necessary. 

 

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