Scavenging system
Scavenging system
The Scavenge efficiency
The scavenge air receiver
Amount of air/exhaust gas flow through scavenging and exhaust ports
Tuned exhaust system
Increasing the power output of the engine
Supercharging or pressure charging
Arrangement of exhaust pipes in a constant pressure turbocharger
Pulse system of exhaust turbocharging
Quasi-pulse systems
Scavenge air Cooling
Effects of fouled air cooler
Drain cocks or valves on the airside of air coolers
Cleaning of air coolers
Boost pressure ratio and air density ratio
Scavenge fire
Scavenging system
The engine is supplied with scavenging air from one or more turbochargers, depending on the engine type and layout. The engine exhaust gas drives the turbine wheel of the turbocharger
and, through a common shaft, the turbine wheel drives the compressor wheel.
The compressor draws air from the engine room, through the air filters. From the compressor outlet, the air passes through the charging air pipe to the charging air cooler where the air is cooled down. The charging air pipe, with a compensator, is insulated.
The air cooler incorporates a water mist catcher, which is designed to separate condensate from the air.
When the air has passed the water mist catcher, it is pressed into the scavenge air receiver through non-return valves. The non-return valves opened by pressure from the turbocharger.
From the scavenge air receiver, the air flows to the cylinder through the scavenge ports, when the piston is in the bottom position. When the exhaust valves open, the exhaust gas is pressed into a common exhaust gas receiver, from where the gas drives the turbine of the turbocharger
with an even and steady pressure
The scavenge air receiver is a container having a large volume. The receiver is bolted onto the cylinder frame.
Scavenge air is collected in the receiver after the air has passed through the cooler, the water mist catcher, and the non-return valves. The receiver and the cylinder frame communicate through large openings. The scavenge air receiver is provided with manhole covers and a safety valve.
The engine is provided with two or more auxiliary blowers. The suction sides are connected to the space after the water mist catcher. The discharge sides are connected to the scavenge air receiver. Separate non-return valves are installed at the suction side or discharge side of the
auxiliary blowers, in order to prevent reversed airflow.
During the starting of the engine, and when the engine is running at a low load, the turbocharger is not able to supply enough air for the engine process. In these cases, a pressure switch will automatically start the auxiliary blowers.
When the auxiliary blowers are operating, they draw air from the engine room through the turbocharger's air filter and compressor side.
The non-return valves fitted after the water mist catcher have now closed as a result of partial vacuum and gravitation acting on the valve flaps.
There will be a lack of air supply if the non-return valves do not close. It is of the utmost importance that the non-return valves of the auxiliary blowers always function correctly and move easily. This can be checked either by moving the valves manually in connection with the regular scavenge port inspections or via locally placed inspection covers.
The non-return valves protect the blowers and engine during the startup of the auxiliary blowers and running with auxiliary blowers.
Starting the auxiliary blowers: Owing to the relatively high starting current, the blowers starting sequence, with 6-10 seconds in between. The non-return valve of the blower that has not yet started must be in a closed position to prevent the blower from rotating backward.
If an auxiliary blower fails to star, the non-return valve must be in the closed position. Otherwise, the operating blower will not be able to draw fresh air in through the turbocharger and air cooler. This is due to differences in the airflow resistance.
Running with auxiliary blowers: If an auxiliary blower fails during running, the non-return valve must close to ensure the continued supply of fresh air to the engine.
From the exhaust valves, the exhaust gas is led to the exhaust gas receiver where the pulsatory pressure from the individual exhaust valves is equalized and led to the turbocharger at a constant pressure.
The exhaust gas receiver is fastened to the seating by flexible supports. Compensators are inserted between the receiver and the exhaust valves, and between the receiver and the turbocharger. Inside the exhaust gas receiver, a protective grating is mounted before
the turbocharger. The exhaust gas receiver and the exhaust pipe are insulated.
The charging air cooler insert is of the block type. It is mounted in a housing which is welded up of steel plates.
The cooler housing is provided with inspection covers. The cooler is designed with an air reversing chamber which incorporates a water mist catcher. The water mist catcher is built up of a number of lamellas that separate the condensation water from the scavenge ait
during the passage of the airflow.
The separated water is collected in the bottom of the cooler housing from which it is removed by a drain system. It is important to check that the drain functions correctly, as otherwise water droplets may enter the cylinders. An alarm device for the high water levels in the drain system is installed.
Scavenge efficiency
Scavenge efficiency is the ratio of the mass of air in the cylinder at the beginning of compression i.e at the time of closing of the exhaust valve of ports to the mass of air
represented by the swept volume of the piston at the same pressure. The higher the scavenge
efficiency, the more will be the amount of air available and fuel that can be burned to produce more power.
The scavenge air receiver
It has an inner and an outer compartment with a set of non-return valves between them.
From the outer compartment, air enters the inner compartment. From the inner compartment, the scavenge air enters the cylinders when the piston uncovers the scavenge ports towards the end of its downward stroke.
An electric motor-driven auxiliary blower draws air from the outer compartment and delivers it to the inner compartment via non-return valves. It automatically operates upon the pressure drop and provides sufficient air to supply to run the engine at reduced load.
Amount of air/exhaust gas flow through scavenging and exhaust ports
Factors governing the amount of air or exhaust gas flow through scavenging and exhaust ports:
- The pressure differences across the ports, which in turn govern the flow velocity. (The pressure difference across the scavenge port is the difference between the scavenge pressure and the pressure in the cylinder; the pressure difference across the exhaust port is the difference between the cylinder pressure and the exhaust manifold pressure.) As the pressure difference increases, the velocity of flow increases.
- The area of the port. An increase in the area allows more gas to pass.
- The shape of the port entry. Rounded entry edges allow air or gas to pass through without turbulence.
- The degree of surface roughness in the port. Smooth, polished ports improve airflow.
- The period of that the ports are open.
Tuned exhaust system
When the exhaust valve of a diesel engine opens, the gases in the cylinder rapidly-expand, and gain velocity and kinetic energy as they pass into the exhaust pipe. The kinetic energy of the mass of exhaust gas carries it along the exhaust pipe and causes a pressure build-up ahead of the mass of gas and a partial vacuum behind it.
If two cylinders are connected to a common exhaust pipe, it can be seen that if the exhaust valve of the second cylinder is opened when the exhaust from the first cylinder has created a vacuum in the pipe, the exhaust from the second cylinder will be discharged more easily.
This principle is used in a tuned exhaust system. By making the exhaust pipes a suitable length and arranging for two or three cylinders with suitable exhaust-valve timing to exhaust into the same pipe, the partial vacuum created by the exhaust from one cylinder is used to help exhaust expulsion from the following, cylinder.
Increasing the power output of the engine
The power output of a diesel engine is limited by the weight of air in the cylinders when fuel is injected. Injecting more fuel in order to raise the power output without a corresponding increase in the weight of air would result in some of the fuel being incompletely burnt owing to lack of air. Incomplete combustion would cause the mean temperature of the cycle to be increased and lead to serious mechanical troubles, such as fouling of piston rings, inefficient
lubrication of the cylinder, burnt exhaust valves, and temperature stresses so great that cylinder heads, pistons, and other parts subjected to the higher gas temperatures might be fractured.
The power of a diesel engine is dependent on the amount of fuel used, and this is limited by the amount of air available to burn the fuel. If more air can be introduced into the cylinder, more fuel can be used and the power of the engine will be increased.
When contemplating the possibilities of power increase, care must be exercised because of the attendant problems of thermal stress which usually lead to early failure of engine parts subjected to high temperatures.
In four-stroke engines, more air can be introduced into the cylinder during the suction stroke by connecting the air inlet valve to an air supply above atmospheric pressure. In two-stroke engines, the same result will be obtained by increasing the scavenge pressure. In order to get the requisite air into the cylinder, the timing is altered to increase the length of the air inlet period.
If the pressurized air is obtained from a pump or blower driven by the engine, some of the extra power gained will be lost in driving the pump or blower. The mechanical efficiency of the engine is reduced but the considerable net gain in the power output can be achieved.
In four-stroke cycle engines, the air entering the cylinders under pressure pushes the piston downwards; this reverses the effects of pumping losses experienced in naturally aspirated engines where power is lost during the inlet and exhaust strokes.
The strength of the parts and the cooling must be adequate for the increased mechanical and thermal loads.
Supercharging or pressure charging
When the piston of an engine is beginning the compression stroke, the cylinder should be full of air at atmospheric pressure. If means are adopted to cause the pressure at this point of the cycle to be greater than that of the atmosphere the engine is said to be supercharged or pressure-charged.
Modern diesel engines are pressure-charged by utilizing the energy in the exhaust gases to drive a gas turbine connected to a rotary blower. The blower compresses the air so that it is delivered under pressure to the engine cylinders.
Because the air is under pressure, a greater mass can be contained in the cylinder and so more fuel can be burnt per stroke, which increases the power developed. Engines pressure-charged in this way with exhaust-gas turbo-driven blowers are often referred to as turbo-charged engines.
Arrangement of exhaust pipes in a constant pressure turbocharger
In constant-pressure exhaust-gas turbocharging systems the exhaust branch or pipe from each individual cylinder is led into a common manifold. As the exhaust gas blows into the manifold, eddies are set up which help damp out any pressure wave caused by the influx of the exhaust gas. The volume of the manifold must be large enough to accommodate the gas flow from individual cylinders without causing any localized pressure rise in the manifold as exhaust gas leaves the cylinder. The exhaust gas is led from the manifold into the exhaust-gas turbo-blower at constant pressure.
If the exhaust manifold is made too large the response of the turbine to engine load change is slowed up due to the manifold volume taking longer to bring to the higher exhaust pressure. The discharge pressure from the turbine must be higher than the exhaust manifold pressure. When the engine is operated with a low load the air discharged from the compressor may be insufficient; a separate electrically driven blower is then brought into operation.
Pulse system of exhaust turbocharging
In the pulse system, maximum use is made of the energy in the exhaust gas during the blowdown period, by designing the exhaust valves so that they have the bn designed to give the most rapid possible opening of the exhaust valve, When the exhaust valve opens, the
blowdown of the exhaust into the exhaust pipe causes a pressure wave or the impulse to pass down the exhaust pipe to the nozzle plate in the exhaust turbine.
The static pressure of the exhaust gas during the impulse period is greater than the air charging pressure; therefore no cylinder should be exhausting at the time the impulse passes in the exhaust pipe, otherwise, exhaust gas may blow back into the other cylinder. "
Engines turbo-charged on the impulse system must have the exhaust pipes carefully grouped according to the engine exhaust-valve timing. Each group of exhaust pipes has a separate entry into the exhaust turbine, and each entry leads to its own nozzle group.
Quasi-pulse systems
These systems utilize a system of exhaust pipes similar to the pulse system. The turbine nozzles are made larger and two groups of pipes will be joined together, the common pipe from the two groups being led to a group of turbine nozzles. A variant of this is to lead all the common pipes from each group to a single entry into the turbine and feed all the nozzles from the common entry point. The object of these arrangements is to increase the efficiency of the turbocharging system and reduce the size of the turbocharger.
Scavenge air Cooling
Air coolers are fitted on turbo-charged engines to cool the air after it has been compressed in the turbo-blower. Cooling the air reduces both the temperature and the volume, and the mass per unit volume is therefore increased, colder air also cools the internal parts of the cylinder more effectively during the scavenge period; a greater mass of air can therefore be present in the cylinder when compression commences.
Sea-water is the usual cooling medium. The temperature of seawater is much lower than the temperature of the freshwater used for cooling the engine and therefore has a better cooling effect.
The cooler consists of two tube plates with finned tubes welded or expanded between them. In one end there is a water box with a division wall separating the seawater inlet and outlet. On the other end, there is a cover that returns the water flow. Air flows on the outside of the tubes.
The tube bundle is fitted inside a fabricated steel framework mounted on the engine. The frame has passages for hot air from the blower to enter from the top to the outside of the tubes and exit from the bottom to deliver the cooled charge air to the scavenge trunk via a water separator
Normal air temperature after turbocharger blower is 60 1o 70°C. Cooling the air reduces
its volume and increases density. It is thus possible to pack the cylinder with more air
which in turn allows more fuel to be burnt and greater power to be developed.
Cooling the charge air reduces average cycle temperature, reduces thermal stresses
Cooling of charge air reduces liner wall temperature, improves cylinder lubrication
and reduces wear of the liner and piston rings.
Effects of fouled air cooler
When air coolers become fouled, less heat will be transferred from the air to the cooling water. This is shown by changes in the air and cooling-water temperatures. Changes will also occur in the pressure drop of the air passing through the cooler. The amount of change will depend on the degree and nature of the fouling.
The symptoms of air-side fouling are as follows.
Decrease of the air temperature difference across cooler,Increase of air pressure drop across cooler.Rising scavenge air temperature.Rising exhaust temperature from all cylinders.A smaller rise in cooling-water temperature across the cooler.
Fouling of the cooling-water side is shown by the following symptoms.
Rising scavenge temperature.Reduction in the difference of the air temperature across the cooler.Reduction in the temperature rise of the cooling water across the cooler if the fouling is general on all tubes.Rising exhaust gas temperature from all cylinders.An increase in the temperature rise of the cooling water if fouling or choking materially reduces the amount of water flow.
The temperature and pressure differences recorded should be compared with those obtained from the engine when on the testbed.
Drain cocks or valves on the airside of air coolers
The purpose of these cocks is to check the tightness of the air cooler against water leakage from the cooling side and to drain off any condensation that may occur.
If a leak occurs it can be found by continuing the circulation of water when the engine is stopped and leaving the drains open. If water is found to drain in any appreciable quantity, it indicates a water leak.
The cocks should be regularly used at sea and left open in port when the engine is shut down. The presence of water may indicate that condensation is occurring in the cooler. This may happen when the atmosphere is humid, and the sea-water cools the air below the dew-point temperature. This condition is most likely to be met when the air is warm (warm air holds more water vapor than cold air) and the seawater is cool. Condensation can be prevented by opening the cooling-water bypass on the cooler or reducing the amount of water passing, through the cooler.
Some engines are not fitted with a cooling-water bypass on the air coolers. If other heat exchangers are in a series circuit with the cooling water from the air coolers, care must be taken if the amount of water flow is restricted because it will also reduce the cooling-water flow to the other heat exchangers.
Cleaning of air coolers
The cooling-water spaces can be cleaned with the cooler in place. The connecting pipes to the cooler are removed, and the lower cooling-water branch is blanked off. The cooling-water space is then filled with the cleaning liquid.
Sometimes a standpipe is fitted on the upper branch to provide a pressure head on the cooler. The air spaces on smaller coolers can also be cleaned in place by fitting blanks and filling the air space with cleaning fluid.
It is of the greatest importance to keep the air coolers clean, and engines are designed with a view to facilitating this job as far as possible.
Air coolers on large engines are designed so that the tube stack or nest can be quickly dismantled and lowered into a cleaning bath. The bath is made of glass fiber reinforced plastic, which is light in weight and stores easily, After the stack is lowered into the tank, it is soaked in cleaning fluid to remove dirt. After the requisite soaking period the cooler should be rinsed clean. Care must be taken in the final examination to see that all the spaces between the tube fins are clear.
Note Engine builders usually give instructions for cleaning air coolers in the engine instruction book. The cleaning materials or chemicals that can be used without causing damage are usually also listed. If no instruction is given, the cleaning material must be carefully selected, since some cleaning materials may actively corrode some of the metals used in coolers. Cleaning fluids having any content that may create flammable vapors must be used discreetly and any chance of vapor entering the engine at the first start must be obviated. This is done by carefully rinsing away all traces of the cleaning fluid.
Care must be exercised if the coolers are cleaned in situ, as the supports for the cooler may not be strong enough to support the cooler when it is full of cleaning fluid.
Boost pressure ratio and air density ratio
The term boost pressure ratio (often known as pressure ratio) refers to the pressure rise across the air section of the turbo-blower.
The term air density ratio refers to the change in density that occurs when the temperature of the air rises when undergoing compression.
When graphs of boost pressure ratio are plotted against the mass airflow, the ambient conditions under which the compressor is working should be stated.
In other cases, when performance curves are plotted, two ordinates may be used; one gives the boost pressure ratio while the other gives the air density ratio. The abscissa on which the ordinates are plotted is the blower rpm or percentage of maximum rpm.
Scavenge fire
Due to improper combustion and prolonged blow past, there may be an accumulation of carbonaceous matters in the under piston space, Any excess cylinder oil may also be scraped down by the piston rings and deposit in this space. If the situation is allowed continue burning carbon (sparks) in the blow past gases may heat up and eventually set the deposits on fire. This is known as scavenge-fire. Ignition may also be caused by slow combustion of carbonaceous matter or by blowback of hot combustion gases through the scavenge ports owing to the wrong adjustment of exhaust valve or excessive back pressure in the exhaust system,
Scavenge fire - warnings:
Scavenge space fire alarm (usually set at 90°C)Increased exhaust temperature of affected units,Turbocharger surgingSmoke from the turbocharger air filterScavenge air box noticeably hotter, smoking near the seat of the fireSmoke, noise, slow down of engine in case of a large fire
Action to be taken if small or local fire:
Continue run at reduced speedCutoff fuet to the affected unitIncrease cylinder oil teedScavenge drains open (close if ire or sparks emanating)Keep clear of scavenging doors and crankcase doorsKeep close observation; the fire should die out in 15 - 30 minWhen temperatures are normal and no more signs of scavenge fire, restore fuel to the affected unit, raise fuel slowly and step by step keeping close observation.Reduce cylinder lubricating oil gradually to normalIf fire continues or spreads to other units, proceed as in case of extensive fire
Action to be taken if large or extensive:
Reduce speed, stop the engine as soon as possibleAuxiliary blowers offShut off fuel oil supplyShut off lubricating oil supplyCover turbocharger blower filtersKeep clearIntroduce CO, or other fire fighting mediumApply external cooling if necessaryWait until the fire is extinguished and the temperature falls below 80°C.Open scavenge doors, ventilate, coolWhen sate, enter space, thoroughly clean examine for any damages, repair as necessaryTry to ascertain the cause and remove cause if possibleRestart engine after necessary preparation, raise fuel slowly and step by step keeping close observation, revert to normal running conditions if all found in order
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