Certification Assistance for Canadian Navigators
Canadian Ocean Navigator
EK Motors NotesBrought to you by www.dieselduck.net comments to [email protected]
Disclaimer
Transport Canada has ask us to advise users of this webpage to keep in mind that these questions are not the exact questions found in their exams. Martin's Marine Engineering Page - www.dieselduck.net is not affiliated with Transport Canada and these questions have been gathered from various sources.
Thank you Walt P. for submitting this material.
Internal Combustion Engines
In diesel engines air in the cylinder is compressed to a high pressure so that it attains a high temperature, the oil id injected into this high temperature air causing it to ignite immediately.
This is classed a compression ignition engine.
Diesel engines are either a four stroke engine or a two stroke engine.
The reciprocating motion of the piston is converted into a rotary motion at the crankshaft by connecting rods. The connecting rod has an eye at its top end and a gudgeon pin passes through this and the sides of the hollow piston to form a swivel connection between rod and piston, and the bottom the cylinder is open to allow for the swing of the connecting rod. The piston performs the additional function of a cross head and the side thrust at the top of the connecting rod is taken up on the cylinder wall.
In larger engines a piston and a cross head are fitted so that the side thrust is taken up by the cross head guide shoe on the guides.
Four-Stroke Diesel Engine
So named as it takes four strokes of the piston, or, two revolutions of the crankshaft to complete one working cycle of the engine.
INDUCTION STROKE- the air induction valve is opened and air in being sucked into cylinder by piston as it moves down wards. At end of induction stroke, cylinder is full of air and the air induction valve is closed.
COMPRESSION STROKE- as piston moves upwards, and all valves are closed, the air compresses and heats up to a sufficiently high temperature to ignite the oil being injected, to ignite and burn rapidly at the end of this stroke. The pressure of the air at end of compression stroke is in the region of 35 bar to obtain a temperature of 540 degrees C.
POWER STROKE- as piston is moving down, fuel is injected into the cylinder as a fine spray through the fuel valve, it mixes with the hot air and burns rapidly. Fuel is admitted a few degrees before top dead center of the crank to give it time to reach full combustion for the beginning of the power stroke, and the valve remains open for about on tenth of the downwards stroke. As the oil burns it heats the air which in creases its volume, or pressure. In pure diesel engines the admission of the oil is controlled so that the pressure of the gasses remains constant while the piston is moving down during the combustion period ( this is termed a constant-pressure cycle ), in practice pressure actually rises a little during combustion. With fuel shut off, the gasses continue to push the piston forwards ( down ) and the pressure consequently falls towards the end of the stroke, this is the expansion period of the power stroke. At the end of the fuel combustion , and beginning of expansion, the temperature of the gasses has risen to approximately 1,650 degrees C., when the pressure has fallen to be of little further use the exhaust valve opens.EXHAUST STROKE-with exhaust valve open, piston moving upwards, the gasses are being expelled from the cylinder. At the end of this stroke, exhaust valve is closed and induction valve opened, and cycle begins all over again. There is a certain amount of overlapping with regards to the closing of exhaust valve and opening of induction valve, for a short period they are both open together, the momentum of the exhaust gasses sweeping out through the exhaust valve has the effect of pulling air through the induction valve so as to assist in scavenging the combustion space in the cylinder, at the beginning of the induction stroke.
VALVE MECHANISM
The Air-induction, exhaust, and in some cases, starting air valves are opened by means of rocking levers fulcrummed about their center and actuated by cams fixed to the camshaft. Each cam has a peak, which, when it comes around to contact the roller on the end of the rocking lever, pushes this end up, and the other end is depressed to open the valve against a spring in the valve housing. Push rods may be used as distance pieces between cams and rocking levers. The cams are set in the correct position relative to the crank so that the valves open and close at the exact moment and for the required period in the working cycle. The timing of the opening and closing of the valves is relative to the position of the crank and direction of movement, separate cams fixed at different relative positions are required to actuate the rocking levers to run the engine in reverse. The fuel valve is usually opened by the pressure of the oil discharge by the fuel pump and closes under the action of a spring when the pressure is released. The timing of the beginning of the opening, period of opening, and closing of the valve is varied by the fuel pump plunger.
TWO STROKE DIESEL ENGINE
So named because it only takes two strokes to complete one cycle, or, one complete revolution of the crankshaft. Every down wards stroke is a power stroke, and every upwards stroke is a compression stroke. The exhaust of the burned gasses from the cylinder and the fresh change of air taken in during the late period of the downward stroke and the early part of the upward stroke. In the basic two-stroke engine, the exhaust gas pass through a set of ports in the lower part of the cylinder and the air is admitted through a similar set of ports. These ports are covered and uncovered by the piston itself, which must be a long one to have a skirt attached so that the ports are covered when the piston is at the top of its stroke. Air is pumped into the engine by a scavenge air pump, whose function is to ? scavenge ?, or clean out the cylinders by pushing the remaining exhaust gas out, and leaving clean air in the cylinder for the compression stroke.
There are no induction valves in the cylinder heads. However, air starting valves and exhaust valve may be found in some engines, as well as a relief-valve.
FUEL SYSTEM
Most diesel fuel systems are designed to use residual oil, which requires heating for normal running, and diesel oil for abnormal conditions, and sometimes maneuvering.
A typical, simplified fuel oil system, has oil pumped from there double bottom tanks to a settling tank. The settling tanks are in duplicate and fitted with internal heaters, so that when one tank is in use, oil in the other tank can be settling and impurities drained off. The settling process being sped up by moderate heating.
The oil passes from the settling tank in use, through a heater unit to centrifugal purifiers and then to the daily service tank. The daily service tank is also in duplicate so that one can be filling while the other is in service. From here the oil passes through a flow meter and into a buffer or circulating tank. A booster pump draws the oil from this tank and discharges it through a heater, viscosity controller, filter and finally to the engine fuel pump. The engine fuel pump delivers the oil to the cylinders via the fuel injectors.
SCAVENGING
Scavenge pumps may be either reciprocating or rotary, both types usually derive their operative power from the main engine.
A reciprocating scavenge pump may be driven direct by an additional crank in the engine crankshaft, this additional cylinder increases the size of the main engine. ( length ) or it may be operated by links attached to a cross head of the engine. Air is delivered by the pump unit at about 1.17 bar into the feeder trunking which feeds each engine cylinder through its scavenge ports in turn with the necessary air for scavenging and recharging at the correct time in the cycle.
Rotary scavenge pumps, also referred to as blowers are chain or gear driven from the engine crankshaft, these have the advantage of steady air delivery and compactness since their rotational speeds are high and constant during normal operation. These rotary pumps have a disadvantage in reversible engines, as their direction of rotation is geared to that of the engine.
Turbo-charging is another method of scavenging, hot exhaust gases drive the turbine portion of the turbocharger, which in return drives an air compressor which supplies air to the engine for scavenging and supercharging. Exhaust gases from the diesel engine enters the single stage gas turbine through water cooled cast iron inlet casing, expand in the nozzle thereby gaining in velocity. and pass through the turbine blades while driving the turbine rotor. The exhaust gases leave the turbine through the water cooled outlet casing and flow to the atmosphere, in some cases via a waste heat boiler.
Some times electrical blowers are fitted for use during starting and slow running of the engine, these are automatically turned off when engine attains operating speed.
SUPERCHARGING
The mass of fuel that can be burned in the cylinder depends on the mass of air present in the cylinder at the end of the compression stroke. By increasing the pressure of the scavenge air in a two stroke engine, and supplying air under slight pressure in a four stroke engine during the induction stroke instead of relying on drawing the air in by suction, a greater mass of air for compression can be supplied to the engine. More fuel can then be burned without causing excessive temperature during combustion, Burning more fuel produces more heat energy to be imparted to the piston, thereby increasing the power of the engine.
The air supply pressure in supercharged engines varies, but is often around 200 millibars above atmospheric pressure.
SCAVENGE FIRES
Accumulation of oil and dirt in the scavenge space of a diesel engine could be caused by such faults as excessive cylinder lubrication, slack or worn piston rings, uneven cylinder liner wear, damaged air inlet filters, and cracked oil-cooled pistons.
If flame from combustion blows past the piston and into the scavenge air trunking this accumulation of oil and dirt could be ignited, resulting in scavenge fire.
Indication of scavenge fire are excessive black smoke, high exhaust temperature, paint blistering and peeling from scavenge trunking. Normally this would be local, limited to only one cylinder.
Fuel should be shut off, to the effected cylinder unit, lubricating should be increased to minimizes the risk of seizing, and the engine slowed down.
Stopping the engine could cause excessive distortion of the overheated parts.
The fire would normally burn itself out, but in most serious cases it may be necessary to stop the engine and fight the fire. Some diesels have fire detecting and CO2 gas smothering systems for the scavenge space.
CRANKCASE EXPLOSIONS
Three things are required for this to happen.
Fuel, Air, Source of ignition.
In the crankcase of a diesel engine lubricating oil provides the fuel. air is always present, the final ingredient is a hot spot.
Overheated bearings) can provide the much needed hot spot, which vapourizes lubricating oil to form a mist, this oil mist with air in the correct ratio can ignite at the hot spot producing a primary explosion.
The pressure generated by the resultant flame speed will increase unless relieved.
Guarded non return pressure relieve valves are normally fitted to the crankcase of each cylinder and to the gear case.
After the primary explosion air enters the crankcase, due to the partial vacuum created by the rushing out of the hot gases, a secondary more severe and violent, as well as dangerous explosion may result with disastrous results. If no secondary explosion occurs, engine must be stopped, and allowed to cool off.
Under no circumstances should the crankcase doors be opened until the engine has cooled down, as the ingress of air may result in another explosion.
CRANKCASE OIL MIST DETECTOR
A fan draws a sample of oil mist through the rotary valve from each crankcase sampling pipe, then through the measuring tube and delivers it to atmosphere at the same time. In the event of overheating in any part of the crankcase there will be a difference in optical density in the two tubes, less light will fall on the photo cell in the measuring tube. The photo cell outputs will be different and when this difference reaches a predetermined value, an alarm signal is operated and the slow turning rotary valve stops, indicating the location of the alarm.
Alarm is arranged to operate at 2.5% of the lower critical point.-- assuming 50mg/l as lower explosive limit then warning at 1.25mg/l
COOLING OF DIESEL ENGINES
Parts close to the combustion space must be cooled to prevent the metal from overheating. These parts are, cylinder head cylinder and piston, and the exhaust valves if used. Fresh water is the most common cooling medium used,
PISTON COOLING
Cooling medium for pistons differ with different engine types; fresh water, distilled water, or lubricating oil.
In some opposed piston engines, lubricating oil is used in the bottom pistons and distilled water is used for the top piston. The cooling medium is led to and from the pistons by a system of telescopic pipes, this consists of two pipes attached to each piston, one for supplying coolant and one for return, these pipes oscillate in trombone fashion in larger diameter stationary pipes fitted with glands at every entry to prevent loss. A sight glass is included in the return pipe so that the flow of cooling medium can be observed. Temperature measurements are incorporated in supply pipes.
The temperature is controlled by passing coolant through a cooler ( heat exchanger ) most likely using sea water. The quantity of coolant is in header tank is checked against leakage on a regular basis.
LUBRICATION
Forced lubrication systems are always used. Lubricating oil is fed under pressure by means of a pump or gravity tank to the main lubricating oil supply line, from a sump, passing through filters on its way. Oil is led to each main of the crank shaft and the cam shaft bearings. A hole is drilled through the center of the crank shaft, through the webs and crank pins which allows oil to flow from the main bearings to the crank pins, from here up through a hole in the connecting rod ( or pipes strapped to the connecting rods ) to the cross heads and guides.
The lubrication of the inside wall of the cylinder liner on which the piston rubs, may be adequately effected by oils mist thrown up by the crank, or oils may be sprayed directly into the cylinder through two or more points, timed carefully to inject a few drops of oils on the piston rings at the moment the piston passes the oil holes.
Lubricating oil carries away a very great deal of the heat generated by friction at the various bearings, and must itself in return be cooled, at some stage in the circuit, between leaving the sump and returning to the sump. Oil coolers are used for this purpose, Sea water is circulated most commonly through these coolers.
WARMING THROUGH
Some engines are warmed through prior to starting, although not essential for all engines.
Warming through is done by heating the cooling systems. The cooling water storage tank is fitted with a heating element, usually steam, cooling water is heated to about 70 degrees C while it is circulated through engine; cylinders, cylinder heads, ect. by an independently driven pump.
Engines burning high viscosity fuel, jackets, pistons, the fuel valves and pipe lines are also warmed through before stating. Final warm through temperature depending upon the oil viscosity required.
Usually the cooling circuit for the fuel valves, which are different from the cylinder jacket system may use water or fine mineral oil. This system is fitted with heating elements as well. Some oil lines have steam jackets or a small bore heating line wrapped around them to heat the fuel oil standing in the pipes. A small auxiliary pump may be used to circulate part of the fuel system with preheated oil.
Preheating is a good practice, as it not only makes starting easier, but also minimizes corrosion, wear, thermal shock, and damage due to unequal expansion.
STARTING
In heavy marine diesels, the practice is to drive the engine on compressed air at about 20 to 40 bar pressure, this air is stored in staring air reservoirs.
Compressed air is injected into the cylinders through starting air valves when the piston is at top dead center and commencing what would be its power stroke, and remaining open until the piston has traveled part of this stroke.
The period of opening depends upon the number of cylinders and whether it is a two or four stroke engine. As on valve closes another opens, this carries on until the engine attains sufficient speed, the fuel pump and valves are brought into operation, and the staring air is valves is shut off.
Starting air is supplied to the reservoirs by two or three stage compressors, with sufficient air to do a minimum of 14 starts of the engine without replenishing from the compressor.
SPEED CONTROL
The speed of the engine is controlled by the quantity of fuel injected into the cylinders, from zero quantity ( stop ) to maximum for full speed. This may be affected by varying the discharge period of the oil from the fuel pump..
REVERSING
Two cams for each valve, one set so that its peak will lift the valve rocker lever to open the valve and keep it open for the correct period of the cycle when the engine is running in the one direction. The other cam alongside is set for running on the opposite direction. Reversing mechanism is arranged to bring either cam ahead or astern into line with the valve rocking lever to its working position.
You may find one setup, lifting the rocking lever clear of the cams, then the camshaft slides ( moves ) so that the other set of cams comes into line, and returning the rocking levers to their working position.
SAFETY DEVICES AND ALARMS
Some of these items are:
Turning gear interlock to prevent starting of the engine with turning gear engaged.
Governor to control the speed of the engine fitted with an over speed trip.
Cylinder relief valves to prevent over pressuring the individual cylinder.
Wrong way interlock and alarm to prevent the engine from going in the opposite direction to that of the demand.
Alarm for lubricating oil temperature and pressure. ( and gear if applicable )
Alarm for overheating of engine. ( and gear if applicable )Brought to you by www.dieselduck.net comments to [email protected]
Disclaimer
Transport Canada has ask us to advise users of this webpage to keep in mind that these questions are not the exact questions found in their exams. Martin's Marine Engineering Page - www.dieselduck.net is not affiliated with Transport Canada and these questions have been gathered from various sources.