Turbo-Chargers/Scavenging and Supercharging Flashcards
With reference to main engine turbochargers:
a) Explain their function; (2)
b) State two advantage of fitting them; (2)
c) Explain how they are cooled and lubricated. (4)
a.) Turbocharger reuses the exhaust gases in order to increase overall efficiency of the engine. Turbocharger converts the waste energy of exhaust gas into useful work by supercharging the combustion process. They make use of and reduce the amount of waste energy from exhaust gas and provide a high pressure air charge for efficient scavenging.
b.) * By utilising a Turbo charger, it allows the injection of a larger amount of fuel for the cylinder size due to the greater density of air created. Therefore, producing more power.
- This increased power being produced by an engine will experience far less mechanical power loses compared to a larger naturally aspirated engine (an internal combustion engine in which air intake depends solely on atmospheric pressure so has no T/C), and the Specific fuel consumption will be reduced.
c.) t/c cooling
- Turbo chargers can be cooled by the engines HT jacket water, or a stand-alone cooling water system or air cooled depending on the engine manufacturer.
t/c lubrication
Most turbochargers don’t have a separate dedicated system. lubrication depends on type of turbo charger bearings used. the two types being metal journal type and ball bearing type. The lube oil is supplied from the main engine lube oil system through a gravity tank. the gravity tank connects through to a pipeline connected to the t/c bearing housing via a non-return valve. the gravity is supplied with oil from the main engine lube oil system through an orifice and one line from the tank goes back to the sump tank, thus maintaining the oil level in the gravity tank at all times.
a) Describe, with the aid of a sketch, the operational principle of a main engine exhaust gas turbocharger. (10)
b) State the function of each of the following turbocharger system components:
(i) Suction filters; (2)
(ii) Exhaust grids; (2)
(iii) Charge air cooler. (2)
a.) SEE ORAL/IAMI Sketch Pack for drawing
Exhaust gas directed towards turbine blades using the nozzle and kinetic force of exhaust gas rotates the turbine and compressor wheel. these two wheels turn together and the rapid spinning allows the compressor to suck in large amounts of ambient air and compress it. as a result, the air is very dense and has a higher temperature.
air is sucked in through filter and accelerated to a higher velocity and then slow pressurised using diffuser and volute casing.
This air then passes through a charge air cooler where it is cooled and gains a higher density before going to the engine. Once in the engine the compressed air allows the engine to burn more fuel efficiently and therefore making the engine operation better.
b.) i.) ensure that air entering in doesn’t have harmful debris that would then damage the engine components upon entry to the engine
ii.) protection device used for the front of t/c to prevent t/c blades being damaged from debris or small bits of metal by collecting the metal
iii.) reduce the temperature of the compressed air, increase the density and mass of air available within the cylinder for combustion.
a) State two reasons for fitting an exhaust gas turbocharger to an engine.
b) Describe the effect of dirty or restricted:
(i) Air filters;
(ii) Exhaust gas turbine.
c) Outline how the exhaust gas turbine is cleaned.
a.) t/c convert waste energy from exhaust gas into useful energy, by reusing this energy they are increasing air intake and the engine overall efficiency. allow larger amount of fuel to be injected into each cylinder because of the greater density of air created and therefore increasing engine power.
b.) i.) it will cause reduced engine power and combustion efficiency and scavenge pressure. debris matter may be able to enter the engine causing damage to engine parts. If basic filtration methods are not maintained or when the intake air quality is very bad, downfall in the turbocharger performance can be expected along with mechanical problems in compressor blades and nozzles. Moreover, if the air contains abrasive particles, it may even damage or break the turbine blades.
ii.) can lead to back pressure surging and excessive turbine vibration or potential sparks, resulting in breakage of turbine blades. If cleaning is not carried out properly, then the supply of air to engine will be reduced, resulting in lack of of air and improper combustion with black smoke.
c.) two methods can be used water washing and dry washing. When dry cleaning, engine should be running at high load as cleaning is more efficient. If water washing method is used engine speed is reduced until exhaust inlet temperature is below 420 degrees. Fresh water must also be slightly hot to avoid thermal stress. Water is injected through a regulating valve connected to exhaust gas turbine side. Both methods are used when engine is running to remove carbon soot and other exhaust by products.
With respect to main propulsion engine, state the immediate action to be taken as an EOOW in the event of the following, stating one reason for each: (4) marks
a) Excessive vibration from turbocharger at full sea speed;
b) Excessive vibration of air start branch pipe to one cylinder during manoeuvring
c) High Main Bearing Temperatures
a.) Contact the bridge and chief engineer, request the engine load to be reduced as much as possible until vibration stops. Caused by propeller being crashed into water in rough weather. often “turbocharger surging” could be an imbalance of exhaust pressure. It could be the engine starting to run on water if the exhaust temperature of the first cylinder starts dropping, followed by the other cylinders as the water replaces the fuel in the lines, find out where the water is coming from. If the worst comes to the worst, the rotor needs removing and sending ashore to check the balance if there’s nothing physically wrong with it.
b.) Inform the bridge and C/E, shut off fuel to the affected unit until manoeuvring is complete or if possible stop engine. this could be caused due to turbulence in the pipe due to irregular running of compressor. inspect compressor drain any oil and water moisture form it then inspect safety devices on air starting system.
c.) Contact the bridge and chief engineer, ask the engine loading to be reduced or stopped. Check lubrication oil flows, pressures and temperature.
With reference to large slow speed diesel engines,
a) State four possible factors leading to a scavenge fire. (4)
b) State four indications of a scavenge fire. (4)
a.) 1. if the piston rings have been damaged due to perhaps poor maintenance, this then allows hot combustion gases or sparks to blow past between the piston cylinder liner running surface and through the underside of cylinder liner area and to the scavenge space.
- large amount of unburnt carbon from combustion process could have accumulated and blocked the under piston scavenge drain. Due to poor maintenance of scavenge space
- over injection of fuel oil and poor combustion due to leaking fuel valves or fuel injectors and improper injection timing
- Blocked stuffing box drain or Insufficient or excessive cylinder lubrication.
b.) 1. Under piston scavenge space temperature high alarm goes off and or there’s a rise in cylinder exhaust gas temperatures
- turbocharger starts repeatedly surging and there is loss of engine power and reduction in rpm which happens because a back pressure is created under the piston space due to fire
- discharging of Sparks and black smoke from the scavenge drains cocks and scavenge air box and potentially the funnel due to incomplete combustion
- Paint blisters and hotspots on the scavenge drains, which can be seen on scavenge doors due to high temperature increase but occurs only in large fires
State two dangers of allowing a scavenge fire to exist. (4)
Scavenge fires can cause a loss in engine power to one engine cylinder unit which then would lead to irregular running. This puts a lot of strain on the rest of the engine cylinder units due to them having to cope with excessive load and causes various engine components to fail for example the piston and piston rod.
Irregular running will also lead to high temperature and then cause distortion in diaphragm which is sealing the scavenge space from the crankcase. if this happens hot combustion gases could possibly escape between any cracks or distortion in diaphragm and enter the crankcase. the high temperature would have also caused oil droplets to evaporate within the crankcase and mix with air so once the oil vapour reaches range of flammability and comes into contact with hot combustion gases then explosion will occur. explosion occurs when air fuel and heat is present.
State four actions to be taken by the EOOW on discovering:
(i) A scavenge fire in one engine unit only; (8)
ii) A scavenge fire in several engine units at the same time. (8)
Action taken in case of a scavenge fire depends on the type of the fire, whether its small or large. You can determine by what kind of fire it is because large fire will have signs easily visible like paint discolouration or blisters generated by the heat from the scavenge space inside the main engine. Also, turbocharger surging and large reduction in rpm and loss of engine power
i.) For Small Fire
- Inform the bridge, and start to reduce the speed of the engine & reduce it to slow or dead slow when navigationally safe to do so. This will reduce the amount of heat being generated
- Cut off the fuel supply to the affected unit by using fuel isolation valves. Cylinder lubrication of the affected unit should be increased to reduce heat.
- Coolant flow through jacket and piston should be maintained to reduce heat. Scavenge drains to be shut to prevent flow of spark and smoke in the engine room.
- Then, keep clear of the scavenge space relief valves to prevent human injury. Minor fire should in time burn it itself out.
ii.) For Large Fires
- Inform the bridge and C/E immediately to stop the engine immediately and reduce heat being generated.
- Stop fuel oil booster pump and use fuel isolation valves to cut off fuel supply. Open indicators cock, engage turning gear and turn the engine to prevent engine seizure.
- maintain normal engine cooling and lubrication. careful not to increase fire by having too much lubrication circulate through the cylinder. close Scavenge air duct flap valve to cut off air supply.
- Extinguishing the fire with a fixed firefighting system this can either be CO2 or steam used to smother the scavenge fire.
a) Explain the procedure to be carried out before the inspection of the scavenge space on a large 2-stroke diesel engine. (8)
b) State four reasons for carrying out a scavenge space inspection. (8)
a.) First work permit should be obtained and risk assessment must be carried out. Scavenge space is considered as a enclosed space so you must obtain a enclosed space entry form. This essentially allows you to enter the space and ensures that all entry checks are carried out. Prior to entry the space has to be ventilated for a period of 24 hours in advance. This ventilation can be done by using a portable fan to allow air to enter the space and all the toxic combustion gases and lowers the atmosphere toxicity to an acceptable level for entry as before carrying out this ventilation the oxygen level isn’t survivable. Its possible to use engine auxiliary blowers to ventilate the space. A crew member who is entering scavenge space should wear a atmosphere tester at all times so it can continuously measure the oxygen level and if it falls below 20.9% then alarm will go off to indicate that oxygen is too low and anyone inside should evacuate immediately. Starting air should be isolated by closing main air starting valve to prevent flow of air inside scavenge space as you are going to be entering through scavenge space receiver you cannot have air blowing through whilst your inside cleaning the space or for inspection. Then isolate fuel as well. Engage the turning gear for the main engine and then if possible person who is carrying out inspection after scavenge space cleaning is preformed should have the turning gear controller with them when entering scavenge space that way if an emergency occurs they can stop turning engine immediately also if you have someone outside who is turning the engine they don’t don’t know where the inspector is so an accident could easily occur. Before entry ensure all PPE is worn and before entering you have all equipment you need like hydrometer for checking piston rings, feeler gauge, Vernier calliper and rags. Have someone else outside space to ensure that crew members inside are okay that way they can quickly signal for help in an emergency. 2 people to enter and inspect scavenge space when its clean and use permit to work list to check through all required items like cylinder unit, scavenge air duct flaps, scavenge relief valves.
b.) 1. To check the condition of the piston rings. When the piston is below scavenging ports, inspect the piston crown and cylinder wall. Check for any frictional wear between the sliding surface of cylinder liner and piston. turn the engine, inspect piston rings and piston skirt through the scavenge ports.
- To check the condition of the cylinder liner and check for abrasion on the liner surface for remnants of mechanical wear, corrosion, and combustion.
- to check that the scavenge drains are clear of blockage and to Check for micro seizures on the piston’s rings and liner surface.
- Check the free movement of piston ring springiness by pressing the rings and inspect the piston rod.
with reference to turbochargers;
what is the purpose of U-tube manometer?
main engine air coolers and the turbochargers are equipped with U-shaped glass tubes filled with liquid. These tubes are called U-tube manometers, and they are used to measure the pressure difference between two points in a fluid system. A U-tube manometer is a simple device that consists of a U-shaped glass/plastic tube containing liquid, usually water or oil. The liquid level in each leg of the tube depends on the pressure applied to that leg. If both legs are exposed to the same pressure, such as atmospheric pressure, the liquid levels will be equal. However, if one leg is connected to a point of higher pressure, such as the inlet of an air cooler or a turbocharger, and the other leg is connected to a point of lower pressure, such as the outlet of an air cooler or a turbocharger, the liquid level in the high-pressure leg will drop, while the liquid level in the low-pressure leg will rise. The difference in liquid levels indicates the pressure difference between the two points. So, the principle behind their operation is simple: as the differential pressure across the component changes, the liquid level in one arm of the U-tube rises while the other falls, providing a visual indication of the pressure difference.
why are U-tube manometers used instead of pressure gauges?
U-tube manometers are used instead of pressure gauges for several reasons:
Direct Reading:One of the primary advantages of U-Tube Manometers is that they provide a direct reading of the differential pressure. Unlike pressure gauges that rely on mechanical elements, U-Tube Manometers offer a clear and instantaneous visual representation of the pressure difference, making them highly reliable. They are accurate and reliable, as they are not affected by temperature changes or mechanical vibrations.
Accuracy:U-Tube Manometers are known for their accuracy and precision in measuring pressure differentials. Pressure gauges may drift or require recalibration over time, while U-Tube Manometers maintain their accuracy as long as the liquid column remains stable. They do not require any external power source or calibration.They can measure both positive and negative pressures, as well as vacuum.
Durability:U-Tube Manometers are robust and durable instruments that can withstand harsh marine environments. They are less prone to damage compared to fragile pressure gauge dials and needles. Also, they are simple, cheap, and easy to install and operate.
why are U-Tube Manometers important?
U-tube manometers are important because they provide a visual indication of the pressure difference across the air coolers and the turbochargers. This pressure difference reflects the performance and efficiency of these components, as well as the condition of the engine. For example, the main engine air cooler is a heat exchanger that cools down the compressed air from the turbocharger before it enters the engine cylinders. This increases the density and oxygen content of the air, which improves the combustion process and reduces emissions. The U-tube manometer connected to the air cooler shows the pressure drop across the cooler, which is proportional to the amount of heat transferred from the air to the cooling water. A low pressure drop indicates a low heat transfer rate, which means that either the air cooler is dirty or fouled, or that there is insufficient cooling water flow. A high pressure drop indicates a high heat transfer rate, which means that either the air cooler is clean and efficient, or that there is excessive cooling water flow. U-Tube Manometers act as early warning systems. A sudden change in the pressure differential could indicate a problem with the air cooler or turbocharger, allowing engineers to take corrective actions before the issue escalates, potentially avoiding costly repairs and downtime. By monitoring the U-tube manometers regularly, one can assess the performance and condition of the air coolers and turbochargers, and take appropriate actions to maintain or improve them.
what maintenance is carried out on U-tube manometers
Maintenance of U-Tube Manometers
U-tube manometers require little maintenance, but proper maintenance of U-Tube Manometers is essential to ensure their accuracy and reliability:
Liquid Column Inspection:Regularly inspect the liquid column in the U-tube for signs of contamination, evaporation, or air bubbles. Any irregularities can affect the accuracy of the readings and should be addressed promptly.
Leak Checks:They should be checked periodically for any leaks, cracks, clogs, or contamination. Ensure that the connections between the U-tube manometer and the monitored equipment are leak-free. Leaks can lead to false readings and should be sealed immediately.
Calibration:Periodically calibrate the U-Tube Manometer to confirm its accuracy. This calibration process may involve adjusting the liquid column height to a known reference value.
what are some problems that occur with u-tube manometer.
When U-Tube Manometers are not functioning correctly, it can lead to inaccurate pressure readings. If there is any discrepancy between the readings of the U-tube manometers and other indicators of the engine performance, such as power output, fuel consumption, exhaust gas temperature, or emissions, one should investigate the possible causes and solutions.
Check for Blockages:Inspect the tubing and connections for any blockages or obstructions that might impede the movement of the liquid in the U-tube. If both legs of the U-tube manometer show equal liquid levels, it means that there is no pressure difference across the component connected to the tube. This could indicate that either the component is blocked or bypassed, or that there is no flow through the component. One should check the valves, pipes, filters, and pumps related to the component, and ensure that they are open, clean, and working properly.
Verify Liquid Integrity:Ensure that the liquid inside the U-tube is in good condition and free from contamination. Replace the liquid if necessary.
Recheck Connections:Confirm that all connections are secure and that there are no leaks. Tighten or replace fittings as needed. If one leg of the U-tube manometer shows a higher liquid level than the other, it means that there is a negative pressure difference across the component connected to the tube. This could indicate that either the component is leaking or damaged, or that there is a backflow or reverse flow through the component. One should check the seals, gaskets, flanges, and clamps related to the component, and ensure that they are tight, intact, and aligned correctly.
Verify Liquid Column Stability:If the liquid column is fluctuating excessively, it could indicate air bubbles or evaporation. Replenish the liquid and remove any trapped air. If the liquid level in the U-tube manometer fluctuates or oscillates rapidly, it means that there is a pulsating or unstable pressure difference across the component connected to the tube. This could indicate that either the component is vibrating or resonating, or that there is a surge or stall in the flow through the component. One should check the mounts, supports, dampers, and silencers related to the component, and ensure that they are rigid, secure, and effective.
what is the purpose of charge air coolers?
The turbocharger uses the exhaust gas from the engine to compress new air and charge the engine with a positive pressure higher than ambient conditions. Exhaust gas temperature convection and compression raises the temperature of the air and this cannot be delivered directly into the engine due to operational limitations exceeding.
As a result, the engine is equipped with a cooler that returns the air temperature to ambient relatively close levels.
Because hot air has a lower density, the mass of air charged into the engine is smaller than when the air is cold.
Thus, the charge air cooler increases the density and lowers the temperature of the charge air. The compressed charged air exiting the charge air cooler will have a temperature of around 40 to 50 degrees Celsius, down from approximately 150 degrees Celsius. At cold temperatures, the lower temperature of the air increases the density of the charge air. Increased charge air density increases scavenging efficiency and allows for more air to be compressed inside the engine cylinder, allowing for more fuel to be burnt inside the combustion chamber, resulting in increased power. Additionally, the engine is kept at a safe operating temperature. Compression temperature reduction alleviates stress on the piston, piston rings, cylinder liner, and cylinder head. Additionally, the charge air cooler lowers the exhaust gas temperature. If very cold air reaches the cylinder liner, a severe thermal stress occurs, resulting in cylinder liner failure. Charge air coolers are installed between the turbocharger compressor outlet and the engine intake or scavenging manifold.
What causes fouling of the air coolers?
When the air cooler become fouled, the heat transfer between air and cooling agent, which is normally fresh water, is decreasing and this is indicated by the raise in air temperature after cooler and a raise in the pressure drop across the cooler (U-tube manometer). Fouling of the air passages in air cooler, which is part of the engine turbocharging system, is usually due to oil film and oily-water films collected on the sides of the tubes and tube fins. Lint and similar material adheres to these films of oil or emulsion. The presence of oil may be caused by faulty air filters which allow the air to pass by the side of the filter element. Sometimes the oil is drawn from bearing at the blower end. The presence of moisture is usually the result of high humidity, when the engine is operating in warm air temperatures in conjunction with low sea water temperature. So, the signs of air coolers on air side are: higher pressure drop across the cooler, higher air temperature after the cooler, higher cooling water outlet temperature, higher under piston scavenging air temperature, higher exhaust temperature on all cylinders and in worst case scenarioturbocharger surging. If the fouling is on the water side of the cooler the signs are: higher air temperature after the cooler, higher cooling water outlet temperature, higher under piston scavenging air temperature and higher exhaust temperature on all cylinders.
how can fouling of air coolers be prevented?
As a remedy, fouling can be prevented through proper plan maintenance by cleaning the coolers at regular intervals. The air side of the cooler can be water washed and/or chemically cleaned. On most of the 2 stroke main engines there is a possibility of periodically cleaning the air coolers even during engine operation at low load. On some of the vessel there is designated tank available where a mixture of water and Air Cooler Cleaner chemical are mixed and recirculated through the air cooler for a certain period of time. It is very important to rinse with fresh water the air cooler very well after cleaning and blow with air if possible in order to prevent chemical corrosion of the cooler parts. For the water side a soft brush can be used to clean inside tubes and in case of hard deposits a long drill bit can be used, but with caution in order not to damage the tubes. However, if there is no improvement after cleaning then the cooler needs to be removed from the engine and dipped into a chemical bath for several hours. Usually this procedure, especially for main engines are performed during every vessel special survey (dry docking), when the coolers are dismantled and taken into yard facilities for proper cleaning into chemical ultrasonic bath.
explain why turbocharger surging occurs?
Turbochargers are designed to match the engine and balance the rate of air consumption over the whole working range and it should not fall into the surging limit area. Therefore when various engine part do not perform in synchronization with the turbocharger, this will lead to surging . Surging can have a multitude of reasons with which most of the engineers are more or less familiar, but sometimes finding the cause of surging can be troublesome and time consuming. Surging must be avoided as much possible as it reduces the turbocharger’s efficiency and performance and continuous surging can cause damage of its bearings and compressor failure.
The most common causes can be:
Rapid change of engine load – this often happens during bad weather when the vessel is pitching heavily and propeller comes out of the water and in this case the engine rpm must be reduced concomitantly with changing of vessel course in order to slow down the vessel pitching. Most of the engine governors have a rough sea mode that will slow the governor response in such cases in order to prevent turbo surging. Another reason of change in load can be a defective engine governor or high wear and lag in governor command link to the engine. This can be easily observed especially during good weather if sudden and frequent rpm change occurs in the engine.
Improper power distribution between engine cylinders. In this case one unit is producing more or less power than the others, thus leading to engine unbalance and variation on exhaust gas pressure. This is affecting mainly the impulse type turbochargers as the exhaust is pumped directly into the turbine. In order to remedy this issue a power card must be taken on the engine to determine and remedy the faulty cylinder. The fault in the cylinder may be: excessive wear of the cylinder, unit misfiring, leaking exhaust valve, improper adjusted valve timing and leaking injectors.
Fouled turbocharger parts like: dirty filters (can be observed visually or by checking the U-tube manometer installed on the compressor silencer), damaged silencer, dirty and clogged nozzle ring, dirty turbine impeller, worn out turbocharger’s bearings etc.
Dirty and clogged scavenging air cooler and/or water mist catcher prevents the air flow and creates back pressure into the compressor. This can be observed on the U-tube manometer installed on each scavenge air cooler. The value must be compared with the one’s from the engine shop trial at different loads. Hence the importance of having a fully functional U-tube manometer, although you can find plenty vessel where nobody pays attention to this. As a remedy the air cooler must be chemically cleaned, although this must be a frequent activity and part of the engine plan maintenance system.
Damaged or blocked scavenging air flaps.
Restriction in exhaust gas flow due fouling of economizer, nozzle ring dirty, obstruction of exhaust manifold, damage gratings inside exhaust manifold. As a remedy U-tube manometer readings on economizer must be checked and compared with the shop trial and economizer cleaned if necessary. Due nowadays engine slow steaming it is a good practice to soot blow the economizer twice a day using soot remover despite engine load, as long as the engine runs at constant rpm. Similarly, exhaust manifold must be inspected for any obstructions and gratings checked and repaired as found necessary.
Problems in engine fuel supply system like: leaking injectors, cold fuel which leads to improper injection, engine fuel starvation, seized or defective fuel injection pump etc. As a remedy fuel temperature must be checked and monitored and power card must be taken in order to check the performance of engine cylinders.
In the worst case scenario scavenge or exhaust fire.
These are the most common causes, but sometimes there is some unexpected reason for turbocharger surging. Few years back I’ve been on a vessel with prolonged turbocharger surging issue. The support from the office was really disappointing as they kept told vessel to open and inspect the air coolers for cleanliness. However, after several inspecting and cleaning of air coolers (although Δp was in very good range compared with shop trial), we continued our own investigation by checking the scavenging flaps, water trap, ME exhaust valves, ME performance and compression pressures, exhaust gas economizer Δp, exhaust gas manifold for any obstructions and everything found in good order. After presenting a full report to the office with our investigation and where we explained that something is wrong with the turbocharger (even though as per our records they have been overhauled by a third party company in close relations with superintendents) they decided to send a ME maker representative onboard as they didn’t trust our investigation. I don’t want to blame anyone but it seems that many of the superintendents are simply clerks with no deep engineering knowledge but good at office politics. However, the attending engineer have done the same investigation as we have and in their report specified that everything is in order with ME and the turbocharger have to be opened and inspected (imagine the reaction from the office). However, after long debates the office decided to sent someone else to investigate the turbochargers and we discovered that the nozzle ring’s fin blades were heavily worn (change in thickness and shape) and some of them with hard soot deposits. It seems that the nozzle rings were not replaced at last overhaul as we found few sheared bolts and most probably to remove it would have taken quite a lot of time and would have delayed the vessel. The wear and deposits on nozzle ring’s fins had a negative impact on turbocharger’s efficiency and functioning, as restricted the gas flow and changed the flow pattern, thus leading to turbocharger‘s continuous “barking” and surging.
How is maximum engine performance achieved?
For maximum performance to be maintained its essential that during the gas exchange process the cylinder is completely purged of residual gases at the completion of the exhaust phase of the cycle and a fresh charge of air introduced into the cylinder ready for the following compression stroke. However, even the most efficient systems still leave behind unburnt hydrocarbons from the previous cycle, the most fuel efficient engines are using techniques such as exhaust gas recirculation (EGR) which is an attempt to burn some of the unburnt gasses and reduce harmful emissions from the engine. In the case of four-stroke engines, purging the cylinder of the gasses from the previous cycle is carried out by careful timing of inlet and exhaust valves where, because of the time required to fully open the valves from the closed position and conversely to return to the closed position from fully open, it becomes necessary for opening and closing to begin before and after dead centre positions if maximum gas flow is to be ensured during exhaust and induction periods.
What’s the purpose of a valve overlap in 4 stroke engines?
Basic object of overlap, that is, exhaust and inlet valves opening together, is to assist in final removal of any exhaust gases from that cylinder so that contamination of charge air is minimal.
The overlap in the case of pressure-charged engines serves to:
further increase this scavenge effect,
provide a pronounced cooling effect which either reduces or maintains mean cycle temperature to within acceptable limits even though loading may be considerably increased. Consequent upon (b) it becomes clear that thermal stressing of engine parts is relieved and with exhaust gas turbo-charger operation prolonged running at excessively high temperatures is avoided. This latter process would have an adverse effect on materials used in turbo-charger construction and could also contribute towards increased contamination.
explain and compare the scavenging arrangement of 2 stroke engines?
The scavenging arrangement of two-stroke engines is either uniflow or longitudinal scavenge and loop and cross scavenge
All the latest large two-stroke marine engines currently under construction have been designed to work with the uniflow system that employs a poppet-style exhaust valve situated at the cylinder head. Very early versions of the uniflow system were opposed piston engines where the exhaust (upper piston) uncovered the exhaust port while at the top of its travel. The uniflow system is the most efficient system which is why it has been adopted for new engines.
In the uniflow system the charge air is admitted through ports at the lower end of the cylinder and as it sweeps upwards towards the exhaust discharge area, almost complete evacuation of residual gases is obtained. By suitable design of the scavenge ports or the provision of special air deflectors the incoming charge air can be given a swirling motion which intensifies the purging effect and also promotes a degree of turbulence within the charge which is required for good combustion when fuel injection takes place.
Both cross and loop scavenge systems have exhaust and scavenge ports arranged around the periphery of the lower end of the liner and in so doing eliminate the need for cylinder head exhaust valves or upper exhaust ports and the associated operating gear. This simplifies the engine construction considerably and, in the past, it might also have led to a reduction of maintenance. Due to a simplified cylinder head construction the cylinder combustion space can be designed for optimum combustion conditions. However, the scavenging efficiency is now so much lower than with the uniflow system due to the more complex gas–air interchange and the possibility of charge air passing straight to exhaust with little or no scavenging effect. Careful attention to port design did reduce this problem but not enough to stop its fall from favour.
explain the gas exchange process?
The gas exchange process itself may be divided into three separate phases:
- blowdown
- scavenge
- post-scavenge.
During ‘blowdown’ the exhaust gases are expelled rapidly – the process being assisted by generously dimensioned exhaust ports or valves arranged to open rapidly. At the end of this ‘blowdown’ period when the scavenge ports begin to uncover, the cylinder pressure should be at or below the charge air pressure so that the scavenge process which follows effectively sweeps out the residual gases without any resistance from a pressurised charge in the cylinder. With scavenge ports closed again the post-scavenge period allows completion of the gas exchange process. The engine design should ensure that the exhaust discharge mechanisms close as quickly as possible to prevent undue loss of charge air and maximise the trapped air ready for the beginning of compression, giving the highest possible density of charge ready for combustion. Although some loss of charge air is unavoidable it should be borne in mind that the air supply is in excess of that required for combustion and the cooling effect of the air passing through the system has the result of keeping mean cycle temperatures down so that service conditions are less exacting. The production of NOx during combustion, happens at the peak temperatures during the process. Therefore, if these peak temperatures are reduced then so is the volume of environmentally harmful NOx gasses. In the latest engines this is accomplished by using the ‘Miller’ cycle, which modifies the timing of the inlet and exhaust valves to ensure that there are no peak temperatures produced during the combustion process. This can only be done with an engine that has full control over the inlet, exhaust and the start and stop of the fuel injection. Also with this system the pressure charging is increased with the use of two-stage turbo-charging. The increased cylinder pressures encountered with modern turbo-charged machinery may result in exhaust opening being advanced so that sufficient time is given for cylinder pressure to fall to or below charge air pressure when the scavenge ports uncover. A complementary aspect of earlier opening to exhaust is the increased pulse energy obtainable from the exhaust gas which can be utilised to improve turbocharger performance. In many cases this is the main criterion which influences exhaust opening, since the loss of expansive working is more than offset by the gain in turbo-charger output. Obviously in the case of reversing engines there may be some slight penalty incurred if prolonged operation in the astern direction is considered.
explain what is pressure charging?
By increasing the density of the air, and therefore the mass of oxygen present in the cylinder at the beginning of compression a corresponding greater mass of fuel can be burned giving a substantial increase in power developed. The degree of pressure charging required, which determines the increase in air density, is achieved by the use of free-running turbo-chargers which are driven by the energy left in the exhaust gases expelled from the main engine. About 20% of the energy available in the exhaust gas is utilised in this way. In the past it was usual practice to employ some form of scavenge assistance either in series or in parallel with the turbo-chargers. This was accomplished by engine-driven reciprocating scavenge pumps, under piston effect or independently driven auxiliary blowers. Only the under-piston effect and auxiliary blowers would still be used on the engines of ships still in service today. The turbo-charger provides charge air at 70–95% of required pressure with underpiston effect or series pump making up the balance. There is an increase in temperature of air delivered to the engine since air cooling is carried out after the turbo-charger only. With parallel operation air supply to engine is increased by air delivery from pumps with proportionate increase in output resulting in greater exhaust gas supply to turbo-charger and improved turbo-charger performance
what are the advantages of pressure charging?
- substantial increase in power for a given speed and size
- better mass power ratio, that is, reduced engine mass for given output
- improved mechanical efficiency with reduction in specific fuel consumption
- reduction in cost per unit of power developed
- an increase in air supply has a considerable cooling effect leading to less exacting working conditions and improved reliability.
the manner in which the energy contained within the exhaust gases is utilised to drive the turbo-charger may be described in two ways:
1.The pulse system of operation
2. Constant pressure operation.
explain both of these systems?
Pulse operation
This makes full use of the higher pressures and temperatures of the exhaust gas during the blow-down period and with rapidly opening exhaust valves or ports the gases leave the cylinder at high velocity as pressure energy is converted into kinetic energy to create a pressure wave or pulse in the exhaust leading to the turbo-charger. For pulse operation it is essential that the exhaust leading from the cylinder to the turbine entry are short and direct without unnecessary bends so that volume is kept to a minimum. This ensures optimum use of available pulse energy and avoids the substantial losses that could otherwise occur with a corresponding reduction in turbo-charger performance. Of necessity, exhaust ducting must be arranged so that the gas exchange processes of cylinders serving the same turbo-charger do not interfere with each other to cause pressure disturbances that would affect purging and recharging with an adverse effect upon engine performance. With two-stroke engines the optimum arrangement is three-cylinder grouping with 120° phasing which gives up to 10% better utilisation of available energy than cylinder groupings other than multiples of three. Due to the small volume of the exhaust ducting and direct leading of exhaust to turbine inlet the pulse system is highly responsive to changing engine conditions giving good performance at all speeds. Theoretically, turbo-charging on the pulse system does not require any form of scavenge assistance at low speeds or when starting. In practice however the use of an auxiliary blower or some other means of assistance is employed to ensure optimum conditions and good acceleration from rest.
Constant pressure operation
In this system the exhaust gases are discharged from the engine into a common manifold or receiver where the pulse energy is largely dissipated. Although the pulse energy is lost, the gas supply to the turbine is at almost constant pressure so that optimum design conditions prevail since, under normal conditions, gas flow will be steady rather than intermittent. Furthermore, as the engine ratings increase, the constant pressure energy contained in the exhaust gas becomes increasingly dominant so that sacrifice of pulse energy in a large volume receiver is of less consequence. Figure 4.5 shows the results of tests carried out on a Wärtsilä two-stroke engine which indicates that up to a BMEP of around 7 bar the advantage lies with the pulse system but as the BMEP increases beyond this figure the constant pressure system becomes more efficient giving greater air throughout and some slight reduction in the fuel rate. Due to the much larger volume of the exhaust system associated with constant pressure operation the release of exhaust gas is rapid and earlier opening to exhaust is generally only necessary to ensure that cylinder pressure has fallen to or below the charge air pressure when the scavenge ports begin to uncover. With a possible reduction in exhaust lead expansive working can be increased which is a further contributory factor in reducing the fuel rate. A major drawback to constant pressure operation is that the large capacity of the exhaust system gives poor response at the turbo-charger to changing engine conditions with the energy supply at slow speeds being insufficient to maintain turbo-charger performance at a level consistent with efficient engine operation. Some form of scavenge assistance such as under-piston scavenging is often utilised. To offset this however the number of turbo-chargers required as compared to pulse operation can be reduced, a greater flexibility exists in the case of turbo-charger location and exhaust arrangement and no de-rating of engine need to be considered for cylinder groupings other than multiples of three. For this reason most large slow-speed two-stroke engines tend to be of the constant pressure configuration. Figure 4.6 shows the diagrammatic arrangement of the Wärtsilä RTA scavenge engine which operates with constant pressure supercharge. In normal operation air is drawn into under-piston space B from common receiver A and compressed on downstroke of piston to be delivered into space C so that when scavenge ports uncover purging is initiated with a strong pressure pulse. As soon as pressure in spaces B/C falls to common receiver pressure in space A scavenge continues at normal charge air pressure. For part load operation the auxiliary fan is arranged to cut in when charging pressure falls below a pre-set value. Air is drawn from space A and delivered into space F and this together with under-piston effect ensures good combustion and trouble-free operation under transient conditions. See figure 4.7 for the MAN arrangement.
Under conditions of high humidity precipitation at the air cooler can occur how is this prevented and why?
Carryover of this water to the engine can have a number of detrimental effects. Water contamination of cylinder lubricating oil may reduce its viscosity and hence its ability to withstand the imposed loads leading to increased cylinder and piston ring wear. Water contamination may also lead to corrosion of engine components. To prevent the carryover of water a water separator is fitted.
with the aid of a sketch explain the working principle of a water seperator?
see EK reed motor book scavenging/supercharging figure 4.8 for sketch
Figure 4.8 shows a water separator fitted on the outlet side of an air cooler. This separator utilises the difference in the mass of water and air. As the moist air flows into the vanes its direction is changed. Due to its lower mass the air is able to change direction easily to flow around the vanes. The water, however, because of its greater mass and, therefore momentum, is not able to change direction so easily and flows into the water trap to be removed at the drain. The water separator should also be sprayed with cleaning solvent when cleaning the air cooler. It must be noted that the vapour given off by cleaning solvents is harmful and by spraying into air coolers may contaminate the atmosphere throughout the engine. The air coolers should not be cleaned when personnel are working within the engine
Some engine manufacturers are introducing water injection into the combustion process. how is this different from water contamination?
This is different from the water contamination because it is a carefully designed system that has been developed following an extensive research and development programme where the effect on all the engine components and fluids will have been considered and any adverse effect will have been removed as part of the engine’s design. The reason for the water injection is to reduce the peak temperatures of combustion thus reducing the harmful NOx.
what’s the reason for not overcooling the charge air?
The temperatures of the inlet air, combustion and exhaust have all been calculated carefully by the engine’s designers. This is not only done so that the correct density of air can be achieved but also that the gasses do not fall to their ‘dew point’ where water will be formed from any steam in the system. The water of course will combine with any oxide of sulphur to form sulphuric acid which in turn will damage the engine or other components.
explain the importance of air cooling
air must be passed through a cooler on its passage to the engine in order to reduce its temperature and restore the density of the charge air to optimum conditions. Correct functioning of the cooler is therefore extremely important in relation to efficient engine operation. Any fouling which occurs will reduce heat transfer from air to cooling medium and it is estimated that the 1°C rise in temperature of air delivered to the engine will increase exhaust temperature by 2°C. Reduction in air pressure at cooler outlet due to increased resistance is also a direct result of fouling. It is therefore imperative that air coolers are kept in a clean condition. It is preferable that this is accomplished on a regular basis rather than changing a dirty cooler since progressive fouling will have an adverse effect on engine performance. Regular cleaning should be included into the ship’s routines and can be carried out by spraying with a commercial air cooler cleaning solvent.
what is the most common turbo charger used?
The majority of marine turbo-chargers is still single-stage axial flow turbine wheel driving a single-stage centrifugal air compressor via a common rotor shaft to form a self-contained free running unit.
State 2 important key turbo charger constructional features and characteristics?
Due to the high rotational speeds perfect dynamic balance is essential if vibrations are to be reduced. This is done by mounting the bearings in resilient housings incorporating laminar spring assemblies to give both axial and radial damping effects.
Another aspect to be prevented flutter or chatter at bearing surfaces when they are stopped so that incidental bearing damage is prevented. Lubrication of the bearings may be by separate or integral oil feed, but whatever arrangement is adopted it must be fully effective at a steady axial tilt of up to 15° and support a temporary tilt of 22½° as may occur in a heavy sea. The bearings themselves may be a combination of ball and roller bearings or separate sleeve (journal)-type bearings.
periodic replacement of ball and roller assemblies is essential if trouble-free service is to be maintained what is is the resons for this?
this is due to the fact that rapid and repeated deformation with resultant stressing causes surface metal fatigue of contact surfaces with the result that failure will occur. The effects of vibration, overloading, corrosion or possible abrasive wear, lead to premature failure which emphasises the need for isolation of bearings from external vibrations together with use of correct grade of lubricant and effective filtration. Plain bearings should however have a life equal to that of the blower provided that normal operating conditions are not exceeded. Ball bearings can end up with tiny indentations in the rolling surface caused by vibrations from the vessel when the turbo-charger is at rest for longer lengths of time.
what cooling media is used for turbo chargers?
Cooling media for cooled exhaust gas casings is generally from the engine jacket water cooling system although in some cases seawater has been employed. In both cases anti-corrosion plugs are fitted to prevent or inhibit corrosion on the water-side.
With water-cooled casings under light load conditions when low exhaust temperatures are encountered it is possible that precipitation of corrosive forming products – mainly sulphuric – will occur on the gas side of the casing. This results in serious corrosive attack which is more marked at the outlet casing because of lower temperatures. Methods of prevention such as enamelling and plastic coatings, etc. have been tried to alleviate this problem with varying degrees of success. A particularly effective approach to the problem is the use of air as the cooling media with the result that this particular instance of corrosive attack is virtually eliminated.
What’s the material used for the components in the high temperature gas stream and why?
The components in the high-temperature gas stream, that is, the nozzle ring, turbine wheel, blades and rotor shaft are manufactured from heat resisting nickel-chrome alloy steel to withstand continuous operation at temperatures in excess of 450°C. Some degree of cooling may be given by controlled air leak-off past the labyrinth seal, between the back of the impeller and volute casing, which flows along the shaft towards the turbine end.
What’s the function of the diffuser?
The diffuser functions to direct air smoothly into the volute casing which continues the deceleration process with further increase in air pressure. From here the air passes to the charge air receiver via the air cooler. The turbine end of the turbo-charger consist of casings which house the nozzle-ring turbine wheel and blading.
