Feature: Caring for your turbocharger
With the widespread adoption of turbocharging on regular road cars, many motorists now own a turbocharged vehicle for the first time. Here are a few tips on how to take care of them
Until relatively recently, the fitment of turbochargers used to be limited to highperformance applications, many intended for race or rally cars. However, the introduction of legislation to reduce vehicle emissions has led to the widespread use of smaller capacity turbocharged engines in road cars. These smaller engines reduce fuel consumption and emissions while turbocharging maintains output levels.
In South Africa, the adoption of downsized turbocharged engines has been further embraced thanks to their minimal power loss when operating at higher altitudes compared to their normally aspirated equivalents. With more than 70% of vehicles in SA operating at above 1 300 m altitude – where normally aspirated engines experience power losses of more than 13% – turbocharged engines enjoy a significant performance benefit in these conditions.
A quick check of manufacturers’ model line-ups confirms the trend, with 57% of the petrol cars and LCVS currently listed in SA now featuring turbocharging. This rapid growth raises the inevitable questions regarding reliability, and whether turbocharged engines should be driven or maintained any differently from naturally aspirated engines.
Considering a turbocharger is an assembly of precision-machined components, some rotating at up to 300 000 r/min and required to withstand exhaust gas temperatures of up to 1 000 degrees Celsius, they are inherently reliable if engine oil changes are carried out as specified and basic precautions are followed.to appreciate the necessity of these measures, it is important to understand the operation of a turbocharger.
TURBOCHARGER OPERATION
In simple terms, a turbocharger consists of a turbine housing and a compressor housing mounted on either side of a centre bearing housing. A turbine wheel and a compressor wheel are mounted at either end of a shaft supported on bearings mounted in the centre housing. Exhaust gas flows through the turbine housing and over the turbine wheel, and a portion of its energy forces the turbine – along with the shaft and the compressor wheel – to rotate. The compressor side of the turbocharger receives clean, atmospheric air from the air-intake system, and compresses it through the rotation of the wheel in the compressor housing. The compressed air is then directed through an intercooler to cool the charge and increase its density before it reaches the intake manifold. Pressure in the intake manifold is thus higher than in a normally aspirated engine, resulting in more air entering the cylinder on the intake stroke. This, in turn, requires more fuel be injected, and the engine produces more power.
IMPORTANCE OF LUBRICATION
With the shaft spinning at 300 000 r/min, lubrication of the journal bearings – ball bearings in some applications – is critical. This lubrication is provided by the engine oil, which is why it is critic that the correct specification engine oil is used, the oil level is maintained and the oil is changed at the intervals specified by the manufacturer. For most operating conditions, these measures will ensure the turbo operates reliably; however, the incorrect oil specification, oil contamination, or any interruption to the oil feed will result in premature wear followed by terminal failure.
HOT SHUTDOWN
Exceptional operating conditions ideally require additional measures by the driver to ensure premature wear does not take place, optimum service life is achieved and reliability is maintained. These conditions typically occur after the engine has been driven hard – with high levels of boost pressure and consequently, high exhaust gas temperatures – and is then shut down without any opportunity for temperatures to cool off.
As already explained, the turbocharger relies on engine oil for lubrication and, while the engine is running, the circulation of oil is sufficient to keep its temperature below its flashpoint. When the oil flow stops, the oil lying stagnant within the turbo system is exposed to heat soak from the hot turbine housing and manifold. This is also known as “hot shutdown”, and results in the thermal load burning and coking up the residual oil in the system.
In addition to the oil inside the bearing housing, it is the oil in the steel oil feed line that is affected. The feed line connects to the bearing housing inlet and is typically routed approximately 20-30 mm away from the turbine housing and exhaust manifold, which results in temperatures of up to and beyond 650 degrees Celsius under normal operating conditions. With a relatively small volume of oil in the oil feed line, it doesn’t take long to increase its temperature to above the oil’s flashpoint. This is the lowest temperature to
which a lubricant must be heated before its vapour, when mixed with air, will ignite but not continue to burn. Its ame/smoke point is the temperature at which the oil will stop glistening and begin to burn/smoke.
The result is coking; carbon build-up inside the oil feed line. At the next engine start up – when the oil pump feeds ltered oil through the feed line – this build-up is forced through the turbocharger bearings causing polishing and scoring on the bearing and shaft mating surfaces. this gradual wear of these components will eventually result in excessive radial and/or axial tolerance, allowing contact between the wheels and the end housings and can lead to catastrophic turbo failure.
With exhaust gas temperatures of Euro 5 engines rising to 1 050 degrees Celsius at high engine loads, and the ashpoint of engine oil typically ranging from 120-150 degrees, how does one prevent the oil inside the oil feed line and bearing housing from reaching its ashpoint? The solution is straightforward; let the engine idle for a short period to allow the thermal loads to settle down and the under-bonnet temperatures to cool before shutting off the engine, especially after a spirited drive.
Ideally, driving the last few kilometres of a long, hard drive “off-boost” at low engine loads, will enable the air ow through the engine compartment to cool everything, and on arrival, the engine can safely be shut down without having to worry about the temperatures. If this is not possible at the end of a journey, let the car idle for a few minutes before shutting off the engine as this will allow temperatures to drop suf ciently.
At the other end of the temperature scale, there is also merit in warming up a turbocharged engine from cold and driving it steadily for a few kilometres to allow oil temperatures to reach 50-60 degrees and the turbocharger components to reach normal operating temperatures before subjecting them to maximum boost and temperatures.
ENGINE STOP/START
With many modern vehicles being tted with a stop/start system that shuts off the engine when the vehicle is stationary to reduce emissions, owners might reasonably ask how these turbo care recommendations apply to their day-to-day driving. Manufacturers have different approaches. Some use heat shields to reduce heat soak while others, particularly in high-performance applications, use turbos with a water jacket incorporated into the turbo’s bearing housing. On certain vehicles, an additional electric water pump is used to aid turbo cooling. Controlled by the ECU, it will be activated, if required, when the engine is shut off and continue circulating coolant until the turbo housing temperature has been reduced to an acceptable level.
While not always stipulated as an operating requirement by vehicle manufacturers, the simple precaution of a short cooling-off period before shutting down a hot engine is a minor inconvenience that could go a long way in preventing premature turbocharger failure and extending its working life.