• 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 engine

• Thus 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.

Turbocharger

• 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.

• If the compressor is driven from the crankshaft of the engine, the system is called ‘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%.

• 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.

Constant pressure turbocharging of two-stroke engines

The two-stroke engine is neither self-aspirating nor self-exhausting. It relies on a positive pressure drop between the inlet and exhaust manifold in order to run at all. The scavenging process, in which fresh charge is forced in and residual gas goes out, is the key to a successful two-stroke engine. It follows that the two-stroke engine is far more dependent on a reasonable pressure drop being developed across the cylinder than is the case with the four-stroke engine. The four-stroke engine will work with an adverse pressure gradient, the two-stroke will not.

• To obtain reasonable scavenging, it is essential to pass some air right through the system and to waste, since considerable mixing of air and combustion products takes place. The amount of ‘excess air’ required would depend on scavenging efficiency of the system being used and many other factors. The amount will vary from 10% to 40% (from uniflow to cross scavenging).

• The consequence to the turbocharger is twofold. Firstly, the gas in the exhaust manifold will be diluted with cool scavenge air lowering the turbine inlet temperature. Secondly, a penalty must be paid for compressing the excess air since, although it will be expanded through the turbine in due course, only part of the energy will be regained, due to compressor and turbine inefficiencies. Thus not only has the turbocharger have a more difficult job than on a four-stroke engine, since it must provide a positive pressure drop across the cylinder, but it is required to work under the adverse conditions mentioned above. It was for these reasons that turbocharged two-stroke diesel engines first appeared with some form of compressor assistance, and many now operating on the constant pressure system, still retain it.

• The exhaust temperature will be lower for two-stroke engines. Typical values of mean turbine inlet temperature on highly rated two-stroke and four-stroke engines at full load might be 4000C and 4500C respectively. For a pressure ratio of 2.5:1, a four-stroke engine will perform adequately if the turbocharger efficiency is 50% or more, whilst the two-stroke engine will require a turbocharger efficiency of over 55%.

• A large, well designed turbocharger can develop 60-65% overall efficiency, but some allowance of about 5% must be made for a reduction in efficiency in service (due to fouling- build up of dirt and carbon). In practice this means that a well designed engine having a good scavenging system, with little pressure loss, will run satisfactorily at full load provided that it has a well matched, efficient turbocharger.

• At part load, the turbine inlet temperature will fall due to the lower air-fuel ratio. Turbocharger efficiency will probably also fall since the turbocharger would usually be matched for optimum working conditions near full load. If the turbine inlet temperature drops below 3000C and the turbine efficiency below 55%, then the engine will stop. Thus all two-stroke engines using the constant pressure turbocharging system require some additional aid for scavenging for starting and part load operation.

• In the past, the underside of the piston was used in cross-head type engines, as compressor placed in series with the turbocharger compressor. As engine and turbocharger design have improved, the contribution of the scavenge pump to overall compression has reduced, so that simple electrically driven fans are used today. These are switched off once the engine load and speed are such that the turbocharger can provide the necessary positive pressure differential across the cylinders.

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.

OPERATIONAL CHECKS• Turbocharger Maximum RPM – within limitsTurbocharger Maximum RPM – within limits• High air temperature reduces efficiency • More than 100 mm water drop across air suction

filter indicates fouling• Low cooling water temperature – drop in thermal

efficiency and cold corrosion on gas side.• High cooling water temperature – hot corrosion on

gas side and reduced life of components• Reduced exhaust gas temperature drop across

turbine – fouling of nozzle and blades – hence drop in turbocharger efficiency

1. Level checked and maintained

2. Bearings supplied with luboil, specially after luboil renewal

3. Maximum allowable temperature not exceeded. (Luboil temperature depends on turbocharger rpm, suction air inlet temperature and cooling water temperature at turbine end).

4. Colour of luboil closely monitored – dark colour indicates exhaust gas leakage. Gas leakage to be rectified and luboil renewed at the earliest.

5. Foaming of luboil to be checked. (Foaming of luboil is caused by Contamination). Luboil to be renewed.

LUB OIL SYSTEM – Ball & roller bearings

LUB OIL SYSTEM- Plain sleeve bearing

1. Luboil pressure within specified range.

2. Luboil return line flow checked from the sight glass.

3. Level of the luboil in reservoir tank to be maintained.

4. Check for the non return valve in the lube oil supply

line to the turbocharger. Fit the non return valve in the

oil supply line after consulting the management office,

if not provided.

5. The lube oil system with plain sleeve type bearings are

normally fitted with an auto clean (motorized drive )

filter . Please check regularly for operation of the motor

and pressure differential ,clean filter regularly.

1. Closely monitor Turbocharger vibrations. If the vibration monitor is fitted, check the indicator for abnormal readings. Any indication of abnormality should be immediately attended and rectified by stopping the engine. Check the foundation bolts, all casing bolts and pipe connections for tightness.

2. Listen to any unusual noise and surging of turbocharger, erratic operation can hence be determined at an early stage, engine to be stopped and problem rectified.

VIBRATIONS AND NOISE

CLEANING TURBOCHARGER DURING OPERATION

1. Clean Blower side daily with fresh water to keep blower impeller clean.

2. Clean Turbine side with dry cleaning method if provided, at least once in three days at full load or once in a week with wet water washing method at reduced output.

3. Drain accumulated moisture from the gas outlet casing periodically.

4. The exhaust gas outlet casing drain valve to be kept in open position during the period when engine is stopped.

RUN DOWN TIME

• After the engine has stopped the rotor of the turbocharger keeps turning for sometime due to its moment of inertia. The run down time is indicative of the mechanical condition of the turbocharger. When rotor comes to a sudden stop, it is usually due to mechanical damage of bearings, or rubbing of compressor wheel or turbine wheel against stationary parts, or foreign matter having become wedged between moving parts.

ROUTINE CHECKS1. Check periodically the anti corrosion plugs in the cooling

water spaces of the turbine casings and replace if

necessary. Check cooling water spaces of the turbine

casing for contamination / deposits.

2. Inspect Turbocharger grating fitted in the exhaust

manifold on the turbocharger gas inlet side for fouling.

3. With integral feed oil, renew luboil every 500 hrs and not

longer than 1000 hrs. if mineral oil is used, (if synthetic

oil is used then renew after 5000 hrs). Ensure oil spaces

are cleaned.

4. In case of Mineral oil, oil sample to be landed for

analysis every 3 months along with other luboils.

PREVENTIVE MAINTENANCE1. Exhaust gas leakages and lube oil losses in the engine

room should be rectified to keep atmosphere clean.

Suction of contaminated air causes deposits on filter

meshes and blower thus reducing turbocharger

efficiency.

2. Poor combustion in engine can lead to erosion of

nozzle ring and turbine blades and formation of

deposits on the nozzle ring and blades. This will lead

to a drop in turbocharger efficiency and increased

vibrations due to rotor imbalance.

3. Avoid running engine on part loads for extended period

of time. Run the engine on full load for four hours for

every 24 hours of part load operation. During the four

hours of full load operation clean Blower side with fresh

water and dry wash turbine side, if provided.

4. Use clean recommended luboil, keep oil spaces clean.

5. Carry out major overhaul as per the intervals specified in

the manual. At every major overhaul turbocharger to be

dismantled, all parts to be cleaned and inspected,

bearings to be replaced. In case of turbocharger with ball

and roller bearings with integral oil feed, lube oil pump

assembly to be renewed.

6. Check axial play of the rotor as per instructions given in the

manual.

7. Check Nozzle ring and turbine blades, shroud ring for

erosion, mechanical damages and fuel / carbon deposits.

8. Check Blower wheel for deposits and mechanical damages.

9. Sealing air passages, balance passages and sealing

bushes must be kept clean and Sealing bushes must be

firm in their casing. Check labyrinth strips for damage.

Replace / repair if necessary.

10. Rotor to be dynamically balanced and balancing witnessed

by Chief Engineer during every major overhaul.

IMPORTANT NOTEIMPORTANT NOTE• NEVER TAKE UP MAIN ENGINE TURBO

CHARGER OVER HAUL BY SHIP’S ENGINEERS. INSTEAD MAKER SERVICE ENGINEER MUST BE PRESENT DURING COMPLETE OVERHAULS.