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As we saw last week, excessive heat poses a threat to the components of a turbocharger. This, incidentally, is one of the reasons why turbos are better suited to diesel engines than to petrol engines - exhaust gas temperatures on a petrol engine are higher than on a diesel.
The turbine wheel has to be made of highly heat-resistant material, and research is continuing into the use of ceramics instead of special steel alloys. Even more problematic is the bearing assembly supporting the shaft which links the turbine and compressor wheels.
It is not uncommon for the wheels to rotate at 200000 rpm - that's over 3000 revolutions per second! Plain, but "fully-floating" bearings are used. In other words, the bearing is just a bush around the shaft, but pressure from the oil pump maintains a film of engine oil on both sides of the bush.
Such a bearing works well while the engine is running and the constantly flowing oil can carry away some of the heat. The crunch comes when the engine is suddenly shut off after a hard run.
The inertia of the turbine and compressor wheels keeps the shaft spinning for a while, but the engine's oil pump is no longer working and therefore the oil inside the shaft's bearings is no longer flowing. This can turn the oil into solid bits of carbon which will drastically shorten the life of the shaft and bearings.
That's why many manufacturers insist that, after even a moderately hard run, a turbocharged engine should be allowed to idle for a minute or two to cool the turbo before being shut off. It's also the reason why one should stick to the engine oil specified by the manufacturer of a turbo engine - some oils, notably synthetic oils, are simply better able to resist being carbonised.
Unwanted heat is responsible for another complication: the air drawn into the compressor side of the turbo unit is heated, both by conduction from the hot turbo housing and by being compressed. Warm air is "thinner" than cold air, precisely what you don't want if you are trying to increase the mass of air going into the cylinders.
For this reason, an intercooler is often fitted between the compressor housing and the inlet manifold. It is simply an air-to-air or air-to-water radiator in which the compressed air is cooled before entering the cylinders. Even though it places some restriction in the path of the incoming air, which lowers the manifold pressure, the net effect on a well-designed system is to increase the mass of the intake charge.
In theory a turbocharged engine should be lighter on fuel than an equivalent one that breathes normally - the higher the pressure reached in the cylinders just before ignition, without incurring pre-ignition, the more efficient an engine becomes.
In practice, however, a turbo's extra power and torque are so tempting that many owners drive a turbocharged vehicle harder than they would its naturally-aspirated counterpart which simply cannot deliver the same performance. And, inevitably, they pay the price for their exuberance at the petrol pump.
In a way, the wonderful "willingness" of a turbocharged engine, especially a turbo-diesel, is its own worst enemy. It will uncomplainingly continue to slog uphill at low revs long after a naturally-aspirated engine would have protested loudly. Think of a fully-loaded turbo-diesel bakkie climbing a mountain pass in top gear.
The speed, and revs, are dropping, but the engine is pulling so strongly that there seems to be no reason to downshift. Meanwhile, unbeknown to the driver, dangerous low-frequency torsional vibrations are causing serious stresses in the crankshaft. A strict minimum-rev limit under power will take care of this danger.