Whilst there are many and varied problems which will cause turbocharger malfunction, they can be split into five groups as follows:
1. Lack of lubricating oil or oil delay
As the turbo revolves at very high speeds, up to 100,000 rpm, the need for oil is of paramount importance. Oil is required at the correct flow rate and pressure to do the following:-
(a) Lubricate the thrust and journal bearings
(b) Stabilise the rotating shaft and journal bearings
(c) Act as a coolant before high turbocharger speeds are reached
As the turbocharger speed and the engine load increases, so the need for oil both as a lubricant and as a coolant increases. If there is a delay for a short period of time in the oil reaching the turbocharger then bearing failure will occur. Oil delay can be recognised very simply by the blueing of the bearings or the shaft. The blueing is simply the colour of the shaft when it has been exposed to excessive heat.
2. Foreign material or dirt in the lubricating system
Many operators assume, quite wrongly, that if they operate the engines with dirty or contaminated oil, the oil filters will remove any foreign matter before the oil reaches the engine and, in our case, the turbocharger bearings: It can be a very costly mistake. All of these materials can cause damage to the bearings when the amount is sufficient to cause bearing wear and bearing-housing bore wear or if the particles of dirt are large enough to block the internal oil passages of the turbo so causing the unit to become starved of oil. The turbo bearings are most likely to fail before the main engine bearings, simply because the turbocharger rotates at much higher speeds than the engine. A unit with 80,000 rpm will have a blade tip speed on the compressor wheel of 700 miles per hour. New designs of turbochargers have blade speeds of over 1,000 miles per hour.
3. Oil break-down
Diesel lubrication is a very important part of the engine and although modern oil technology has gone a long way in providing good oils, we still have two basic problems to contend with:-
(a) Oil deterioration: The high temperatures that are present in modern diesel engines can cause oils to crack or break-down. This action produces carbonaceous (tarry) materials, which stick to the engine rings and cause other troubles. Oxidisation is caused by the hydrocarbons in the oil mixing with the oxygen; this produces organic acids of which there are two main types: those with low boiling points and those with highly corrosive.
These products are responsible for several of the problems on diesel engines and turbochargers. If the acids are al-lowed to become concentrated, they will attack the bearings etc., causing pitting and subsequent failure. Also they react to the remaining oil to form sludge, this is then deposited throughout the engine, particularly in the filters aggravating the turbocharger oil supply. Heavier oxidation causes hard varnish to appear. Where sludge is allowed to accumulate in the oil systems, as this passes through the turbo it is thrown by centrifugal force from the rotating shaft against the walls and internal surfaces of the bearing housing where it can stick and impede the free oil flow. In time the build-up will cause problems with oil drainage, resulting in oil leaking from the turbine end of the unit. If this matter is allowed to accumulate on the turbine side, the heat will cause a baking to take place and the result is usually unbalance in the turbocharger system.
(b) Outside contamination: So far we have briefly looked at oil break-down caused by changes in the oil, usually caused by its exposure to heat and air. However, we must also consider other agents.
Amongst these are the products of fuel combustion, such as ash, soot, the un-burnt heavy ends of the fuel and water. All these elements cause oil contamination. The engine itself contributes to oil break-down in that tiny metal particles produced by wear and tear will either pass through the oil ways into the turbo or oxidise and hasten deterioration. Finally, foreign matter from outside the engine, such as dust and dirt, enters the cylinders through the air intake system.
4. Foreign material in exhaust or air-filtration systems
Any material, which enters these systems is, without doubt, going to damage the turbocharger and could damage the engine. As a turbocharger is a precision instrument its vulnerability will become instantly apparent the first time any particles go into the casings; damage will be to the wheels which could cause pieces of aluminium to go into the engine, resulting in engine piston, valve, liner and, possibly, crank shaft damage.
This type of material will vary tremendously from dust in the air system to engine valve fragments in the exhaust system. It should also be noted that if any foreign body stays in these systems, the turbo will react with possible loss of power, black smoke, excessive oil usage and leakage and damaged wheels.
5. Material and workmanship
Only quality assured materials are used and constant quality checks are made on both material and workmanship to meet the stringent OE specifications. The turbocharger is probably the most misunderstood component on the engine, and it is this lack of knowledge by the owner that makes turbocharger service a very difficult area to work in. As we have seen, a turbo will increase a given engine power by about 30%; it will not change any operating characteristics of the engine, it will only do what the engine tells it to do. The source of power for the turbocharger is exhaust gases produced by the engine and this power is controlled by flow, pressure and temperature. If there is an engine malfunction or an abnormal working condition in the engine, the turbocharger will not overcome this, it is more likely to emphasize the problem. From this it follows that replacing a faulty unit with a new one will not always solve the problem. If we are in any doubt at all, we should consult the original equipment engine manual.
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