The job of a diesel engine turbocharger is to supply compressed air to the engine. The air, heated by the compression, passes through a cooler that reduces its temperature and increases its density.
In the engine cylinder, the air mass is compressed to a high pressure. Fuel is injected and burnt. It is normal to burn a small portion of the cylinder lubricating oil during combustion. The exhaust gases produced, pass to the turbocharger’s turbine, which drives the compressor.
How does the turbocharger become polluted?
Particles in the air are deposited on the compressor and diffuser blades, in time forming a layer that reduces the compressor’s efficiency (Fig. 1). These particles have several origins. The air, taken by the compressor via filter from the ship’s engine room often mixes with oil vapours. Also, the engine room ventilation system sucks in air from outside, and this can be laden with dust, e.g. Ports that handle ores. Another factor is the salt in the sea air.
What to do?
Inject a certain amount of water periodically (once a week to once daily, as necessary) during full-load operation to counteract the drop in efficiency. This measure considerably lengthens the intervals between turbocharger cleanings requiring dismantling (Figs. 3 and 4).
In time, the air flowing through the cooler also causes a layer of dirt to form on the fins and tubes. As a result, the pressure loss across the cooler increases and the air supplied to the engine become hotter. This heat affects the behaviour of the compressor, in that the operating curve in the compressor characteristics moves closer to the point where surging would occur.
Depending on the quality of the fuel (MDO, HFO or mix) burnt in the engine and on the constituents of the lubricating oil, residues may be formed which pass together with the exhaust gases into the turbine.
The composition of these residues decides the deposition of a layer of dirt on the nozzle rings and turbine blades. Dirt layers can be moved by periodical cleaning (once a week to once daily, as required), allowing longer intervals between cleanings requiring a dismantling of the turbocharger.
Proven methods of cleaning are water washing of the turbine during part-load operation and cleaning by blasting with ground nutshells, etc. The combustion residues can contain hard and chemically corrosive components that can initiate erosion and corrosion of the turbine as well as the nozzle and cover rings. By selecting special materials or applying coatings to relevant parts, the service life of these components can be lengthened substantially.
The importance of cleanness:
The processes described above underscore the need to keep the turbine clean to ensure trouble-free operation. Pollution reduces the through-flow area and turbine efficiency. Consequently, compressor surging can also occur in the case of two-stroke engines.
Erosion and corrosion also enlarge the clearance between the moving blades and the cover ring, causing a drop in turbine efficiency as well as turbocharger speed. Providing the plant is properly looked after, i. e. the compressor and turbine are cleaned periodically during operation and the silencer filter is kept clean, the efficiency will diminish only very slowly. A normal figure today for the interval between two cleanings requiring dismantling is 8,000 to 16,000 hours, depending on the operating conditions and unit size.
A turbocharger that has been dismantled, cleaned and had its worn parts replaced, will have an efficiency “as good as new”.