Perhaps of interest to those of the group contemplating a "TURBOCHARGER" for their project.............
 

 

Turbo Oil Needs

Turbos rotate at very high speeds and their center bearings require a continuous supply of pressurized, clean and cool oil. Sleeve-type bearings need considerably more oil than ball-bearing turbos but in either case, if the oil supply is ineffective, the turbo will soon fail. The same importance also applies to the oil exit: if the oil cannot freely escape, it will force itself through the seals, causing smoke.

The specific oil needs vary from turbo to turbo, however, as an example, the manufacturer’s data sheet for the small Garrett GT12 shows the turbo requires a supply of engine oil:

• Filtered at a 20 micron level
• Fed to the turbo through a minimum internal diameter line of 3.2mm
• At a minimum pressure of 2 Bar (ie 29 psi) at peak engine torque
• Not less than 0.7 Bar (10 psi) at idle
• At a temperature below 130 degrees C

Ball-bearing turbos use a restrictor (normally positioned on the oil inlet) so that the amount of oil that flows through the turbo bearing is less than in a conventional turbo.

The oil drain from a turbo operates via gravity. That is, the pressure drop across the turbo bearing is nearly complete and it is only gravity that causes the oil to flow back to the sump. In fact, it cannot really be termed ‘oil’ as after it has passed through the turbo, the oil becomes an aerated foam – one reference suggests it looks like ‘dirty whipped cream’. It’s therefore important that:

• The drain line is much larger in diameter than the pressure feed line
• The connection to the sump is made above the oil level
• The drain pipe is kept as close to vertical as possible

If the sump drain joins at a level below the oil, the much lower density of the aerated oil will cause it to sit on top of the sump oil, gradually backing-up to the turbo.

In applications where the turbo is positioned too low relative to the engine for normal drain-back to occur, an electric or mechanical scavenge pump can be fitted. (This approach is adopted in many piston aircraft engines.) In this situation a small sump must be installed beneath the turbo allowing the oil to accumulate within it, with the pump then drawing from this sump. Turbos mounted remote from the engine (eg at the back of the car) also use a scavenge pump to return the oil to the engine.

The oil requirement of the turbo will have an affect on the engine oiling system. System oil pressure will drop a little, a major reason why factory turbo versions of naturally aspirated engines often use a larger oil pump. In aftermarket applications, the fact that the engine oil pump is normally a little over-sized is taken advantage of to allow for the extra supply to the turbo. However, in applications where a turbo hasn’t previously been fitted to the engine, system oil pressure measurements should be taken after fitment of the turbo to ensure that adequate oil pressure is maintained at the engine. Manufacturer’s oil pressure specs can be used to ascertain if this is the case.