TurbochargerTwo stroke crosshead engines must be supplied with air above atmospheric pressure for it to work. Although turbochargers were developed in 1925, it was not until 1950s that large two stroke engines were turbocharged. Pressurized air is needed to scavenge the cylinders of the exhaust gases and supply the charge of air for next combustion cycle was provided by mechanically driven air compressors (roots blower) or by using the space under the piston as a reciprocating compressor (under piston scavenging). This of course meant that the engine was supplying the energy to compress the air, which meant that the useful work obtained from the engine was reduced by this amount.

About 35% of the total fuel energy goes out in the exhaust gas. The turbocharger uses 7% of the total energy (20% of the exhaust gas energy) to drive a single row turbine. The turbine shaft drives a rotary compressor. Air is drawn and compressed. Due to compression, the air temperature rises. Hence it is cooled in a cooler to increase its density and then sent to the air inlet manifold or scavenge air receiver. At full power of diesel engine, the turbocharger may be rotating at > 10000rpm.

Power of a two stroke diesel engine = pm x L x A x N x no. of cylinders Where pm = mean effective pressure;L = stroke of the engine;A = cross sectional area of the cylinder; N = revolution per second of the engineThus to increase the power of the engine of given swept volume i.e. the power to weight and volume ratios of the engine we have to increase either mean effective pressure or the revolution per second of the engine. The approach of increasing power output by increasing speed is unattractive, due to rapid rise of mechanical and aerodynamic losses, and the corresponding fall in brake thermal efficiency.

For increasing the mean effective pressure, more fuel has to be burnt during the cycle of the engine, which requires higher quantity of air per cycle of the engine. The purpose of supercharging is to increase the mass of air trapped in the cylinders of the engine, by raising its density. A compressor is used to achieve the increase in air density.

Two methods of supercharging can be distinguished by the method to drive the compressor. If the compressor is driven from the crankshaft of the engine, the system is called mechanically driven supercharging or often just supercharging. If a turbine drives the compressor, which itself is driven by the exhaust gas from the cylinders, the system is called turbocharging. The shaft of the turbocharger links the compressor and the turbine, but is not connected to the crankshaft of the engine.

The advantage of the turbocharger, over a mechanically driven supercharger, is that the power required to drive the compressor is extracted from the exhaust gas energy rather than the crankshaft. Thus turbocharging is more efficient than mechanical supercharging. However the turbine imposes a flow restriction in the exhaust system, and therefore the exhaust manifold pressure will be greater than atmospheric pressure.

If sufficient energy can be extracted from the exhaust gas, and converted into compressor work, than the system can be designed such that the compressor delivery pressure exceeds that at turbine inlet, and the inlet and exhaust processes are not adversely affected. For a compressor pressure ratio of 5, allows to increase the specific power output of the engine by 400%.

Compressor characteristic and the surge limit

Centrifugal compressor characteristics are similar to those of centrifugal pumps. At a constant RPM, the characteristic would appear similar to the figure. At constant speed the discharge pressure first rises as volumetric flow increases and then drops off rather sharply. The compressor efficiency curve also rises to a peak, although at any constant this peak is to the right of the pressure peak. The power consumed by the compressor is related to the product of discharge pressure and flow rate.

In the region to the right of the peak in pressure curve, operation will be stable: in this region a momentary drop in volumetric flow rate, for example, perhaps brought on by a momentary reduction in engine speed, will be countered by a rise in pressure, with little or no effect on the turbine. In the region to the left of the pressure peak, a momentary drop in volumetric flow rate will be accompanied by a drop in discharge pressure and a reduction in compressor power consumption.

Operation in the unstable area to the left of the pressure peak may result in compressor surge. As the pressure at the compressor discharge falls below that downstream, the flow can reverse. The result can simply be a pulsation if the situation is not severe or of long duration, or the reversed flow can continue to the air intake and become audible, ranging in volume from a soft sneezing to a very loud backfiring sound.

Obviously, operation in the surge region should be avoided; consequently, turbocharger designers establish a line, called a surge limit, through the pressure characteristics slightly to the right of the peak. Similar data as previous figure are obtained at several constant speeds covering the range of operation, and plotted together on the same axes. The resulting compressor performance map is shown.

Cleaning Turbochargers in operationPeriodic cleaning reduces or even prevents contamination, allowing significantly longer intervals between overhauls. The proposed cleaning method, carried out periodically, will prevent a thick layer of dirt from forming. A thick layer of dirt can cause a drop in efficiency and increased unbalance on the compressor side of the turbocharger, which could influence the lifetime of the bearings.

The compressor wheel of the turbocharger can be cleaned during operation by spraying water into the air inlet casing. The dirt layer is removed by the impact of the injected water. Since the liquid does not act as a solvent there is no need to add chemicals. The use of saltwater is not allowed, as this would cause corrosion of the aluminium compressor wheel and the engine. Water is injected from a water vessel that holds the required quantity of water.

ProcedureThe best results are obtained by injecting water during full-load operation of the engine, i.e. when the turbocharger is running at full speed. The complete contents of the water vessel should be injected within 4 to 10 seconds. Successful cleaning is indicated by a change in the charge air or scavenging pressure, and in most cases by a drop in the exhaust gas temperature.

If cleaning has not produced the desired results, it can be repeated after 1 0 minutes. The interval between compressor cleanings will depend on the condition of the turbocharger suction air. It can vary from 1 to 3 days of operation. If a very thick layer has built up and it cannot be removed using the method described, it will be necessary to dismantle the turbocharger in order to clean the compressor side. Since the dirt layer is removed by the kinetic energy of the water droplets, the engine has to be run at full load.

Cleaning the TurbineThe combustion of heavy fuel in diesel engines causes fouling of the turbine blades and nozzle ring. The result of this fouling is reduced turbine efficiency and engine performance as well as an increase in the exhaust gas temperature, Experience has shown that the contamination on the turbine side can be reduced by regular cleaning in operation, and that such cleaning allows longer intervals between turbocharger overhauls.

Some of the deposits have their origin in soot, molten ash, scale and unburned oil, partially burnt fuel and sodium vanadyl-vanadat. Investigations have shown that most of the residues are caused by the calcium in the lube oil reacting with the sulfur from the fuel to form calcium sulfate during the combustion process.

The quantity of the deposits depends on the quality of the combustion, the fuel used, and the lube oil consumption. The frequency with which cleaning has to be carried out depends on the extent of the contamination on the turbine side.Two cleaning methods existWet cleaning (water injection)dry cleaning (solid particle injection)

Procedure for wet cleaningThe exhaust gas temperature before the turbine should be in the range of 200 to 4300 CThe boost pressure should be above 0.5 bar to prevent water entering the oil chamber on the turbine side.The quantity of injected water will depend on the exhaust gas temperature, water pressure, size of the turbo-charger and number of gas inlets.

Water should be injected for 5 to 10 minutes. Check if the water has entered the turbine parts by opening the drain of the gas outlet casing. Water flowing out provides assurance that enough water has passed the nozzle ring and the turbine blades. The interval between turbine cleanings will depend on the combustion, the fuel used and the fuel oil consumption. It can vary from 1 to 20 days of operation. Principle:The dirt layer on the turbine components is removed by thermal shock rather than the kinetic energy exerted by the water droplets.

Procedure for dry cleaningThe exhaust gas temperature before the turbine should not exceed 5800 C.Dry cleaning has to be carried out more often than water cleaning as it is only possible to remove thin layers of deposits. A cleaning interval of 1 to 2 days is recommended.To ensure effective mechanical cleaning, granulated dry cleaning media are best injected into the turbine at a high turbocharger speed.The quantity needed will vary from 0.2 l to 3 l, depending on the size of the turbocharger.

Experience has shown that the best results are achieved with crushed nut-shell or granulate.Principle:The layer of deposits on the turbine components is removed by the kinetic energy of the granulate causing it to act as an abrasive. Experience has shown a combination of the two to be very effective, especially in the case of 2-stroke engines.