Saturday, July 6, 2013

MARINE ENGINE Series: Fuel Supply, Injection and Control


Fuel Supply, Injection and Control

The following description applies to free standing fuel tanks, a multi element (in line) injector pump, and to engines on flexible mountings. There will be variations, especially if the fuel tanks form part of the vessel’s structure but the principles and safety features remain the same.
Vent pipe - is fitted to the top of the fuel tank at the highest point when the vessel is in normal trim. This is to prevent an air lock developing. An air lock is when the tank is being filled, air or vapours become trapped in the top of the tank, are compressed, and when the pressure exceeds the filling pressure, fuel is forced out of the vent or filling pipes and a spill occurs.
The smaller vent pipes terminate in a gooseneck, the end of which must be higher than the filling point.
The end of the vent pipe has an anti-flash wire gauze fitted to it. If the fuel vapours from the vent pipe ignite, the flames cannot penetrate the gauze and ignite the contents in the tank providing the size of the holes in the gauze are not too large.
(Before a combustible substance will take fire, its temperature must first be raised to its point of ignition, and, if after it has ignited the temperature is reduced in some way below this point, the flame will be extinguished. A moderate flame can be extinguished by passing a current of air over it, for instance, blowing out a candle.
The reason for this is that more air than is required for combustion is supplied to the burning gas, the surplus tending to cool the flame below its point of ignition. In a similar way, gauze, which is a good conductor of heat, prevents the passage of flame, since it looses its heat very rapidly, and the flame upon coming into contact with it, is cooled below the point of ignition; consequently, no flame appears on the other side of the gauze. A good example is placing a lighted match under the gauze. The flame will not penetrate the gauze).
The purpose of the vent pipe is to:
1.         allow the escape of air and vapours when the tank is being filled so it is not pressurised;
2.         allow air into the tank when fuel is being consumed so a partial vacuum is not placed on the tank thereby stopping the engine; and

4.         allow normal expansion and contraction of the fuel due to temperature change.
Filling pipe - is fitted to the top of the tank and it is preferable that it be piped continuously to deck level. It does not have to be piped to the deck, if in the event of an overflow, the fuel will not run onto a hot surface and ignite. The end of the pipe is to be fitted with a sealed cap or plug.
Drain valve - is fitted to the lowest part of the tank. Its purpose is to drain water or sediment from the tank. A plug or cap is fitted so, if the valve vibrates open, the fuel is not lost or causes a fire risk.
Water can be in the tank:
1.         via coming with the fuel supply;
2.         condensation due to the level in the tank being kept low for a lengthy period;
3.         through the deck fitting due to it not being secured and rain or a wave entering; and
4.         being mistaken for a water tank.
Fuel contents gauge - There are a number of methods in which to measure the amount of fuel in the tank. If the tank is fitted with a gauge glass, the cocks or valves must be of the self closing type. To take a reading, open the cocks or valves against a spring or lift a weighted handle and, on letting go, it will automatically close. If the glass breaks or the plastic tube perishes, it prevents all the fuel in the tank running into the bilges or in the case of a fire, prevents all the fuel in the tank feeding the fire.
If a sounding rod is used, a striking pad must be fitted to the bottom of the tank to prevent damage to the tank itself through repeated soundings.
Inspection opening - is fitted in a position or a number may be fitted to provide access to the whole tank. It allows the tank to be cleaned and inspected.
Baffle - They are fitted to prevent free surface effect. This affects the stability of the vessel and in extreme cases can cause vessels to capsize.
Fuel pick up - is fitted above the bottom of the tank. This is to allow a safety margin so as to reduce the amount of any water or sediment flowing to the fuel filter. A valve or cock must be fitted directly to the tank.

Emergency fuel shut off - This is fitted to allow the fuel to be shut off outside the engine room in the case of an emergency. It can be fitted anywhere in the metallic fuel line. It cannot be fitted after the flexible fuel line. Where fuel tanks are fitted outside the engine room and the fuel shut offs are easily accessible, emergency shuts offs are not required.
An extended spindle can be fitted to the fuel shut off valve so it can be operated from outside the engine room. The fuel shut off and the emergency fuel shut off are then the one valve.
Filter/water trap - They can be a combined unit or separate units. The unit provides a secondary means of filtering the fuel from sediment and impurities while the water trap removes any moisture or water. The fuel pump and injectors have very small clearances and any impurities or water in the fuel will cause them to seize. (The fuel acts as a lubricant). In addition, moisture could cause corrosion to those finely machined components.
Sometimes additional filters are fitted to the system.
Fuel return - Excess fuel from the injectors is returned to the tank. It is good practice to operate from one tank at a time and the excess fuel returned to this tank. In this case, the fuel return valve of the tank not being used must be closed. In small vessels it is not practical to operate off one tank as the vessel would develop a list, therefore engines receive their fuel from the port and starboard fuel tanks.
Fuel lift pump - Unless there is a day tank where the fuel is fed by gravity to the engine, it will be necessary to have a fuel lift pump to get the fuel from the tanks to the fuel pump. A fuel lift pump can be a gear, diaphragm or plunger type.
Fuel injection pump - It accurately meters the fuel and delivers it under high pressure at a precise moment to the spray nozzle of the fuel injector.
Fuel injector -  It is a spring loaded valve located in the cylinder head and allows the fuel, under pressure from the fuel pump, to enter the combustion space. It enters in an atomised form to allow it to mix completely with the hot compressed air so that ignition can take place with efficient combustion. Excess fuel is returned to the tank.

Fuel transfer pumps

Unless the vessel’s fuel tanks are positioned above the level of the engine or a day tank is installed at sufficient height, fuel cannot be gravity fed to the engine’s fuel injection pump. To assist in drawing fuel from the tank/s a fuel transfer pump is fitted between the tank/s and the fuel injection pump. Fuel transfer pumps are also commonly referred to as transfer, lift and charge pumps.
Diaphragm type transfer pump
The diaphragm type transfer pump is mechanically driven by a special lobe on the camshaft. The lobe pushes against the lever causing the diaphragm to be pulled down against a spring pressure, creating a partial vacuum.
A first check valve opens and draws in fuel, filling the chamber between the diaphragm and check valves. As the lever moves off the lobe of the cam, the diaphragm spring pushes the diaphragm up, closing the first check valve forcing fuel through a second check valve and into the fuel pump. An external lever is provided to permit manual operation of the pump for priming purposes.
The pump will deliver more fuel than is required. The fuel not being used will build up pressure in the line between the fuel pump and the fuel transfer pump causing the second check valve to close. The downward movement of the diaphragm will allow more fuel to enter through the first check valve into the chamber. The first check valve will close and as the return spring cannot overcome the pressure in the line between the fuel pump and the second check valve, the lever will be held off the cam until more fuel is required.
This diaphragm pump could be attached to the side of the fuel pump and actuated by a cam on the camshaft for the fuel pump. Alternatively, it may be attached to the block and actuated by a cam on the main camshaft.
Plunger type transfer pump
The plunger type fuel transfer pump is mechanically driven by a special lobe on the camshaft. The lobe pushes against the plunger in the fuel transfer pump to create the pumping action. Check valves control the direction of fuel flow, and prevent fuel bleed back during engine shut down .
As the high point on the cam lobe rotates away from the fuel transfer pump, the spring forces the piston towards the camshaft. The pressure of the fuel in the piston bore closes the first check valve and opens a second check valve forcing fuel to the low pressure supply line. As the piston moves, a third check valve opens and fuel is drawn into the spring cavity.

As the high point of the cam lobe rotates towards the fuel transfer pump, the plunger and piston are forced towards the inlet. The pressure of the fuel on the spring side of the piston causes the third check valve to close and first check valve to open, allowing the fuel in the spring cavity to flow to the other side of the piston.
A second plunger allows manual priming and bleeding of air from the system. When the plunger is depressed, the first check valve prevents back flow forcing fuel through the second check valve. When the plunger is released, the spring forces the plunger outward. This action creates a suction that causes the second check valve to close and the fuel is drawn through the open first and third check valves.
If the pump supplies more fuel than is required, the fuel will build up the pressure in the line between the plunger pump and the fuel pump. The pressure build up will hold the plunger stationary against the plunger spring an away from the arm, effectively stopping pump operation until more fuel is required.
Gear type transfer pump
This pump consists of two meshed gears in a closely fitted housing. It has inlet and outlet ports opposite one another. One gear is driven by the power source and in turn drives the other. As the gear teeth separate and travel past the inlet port, a partial vacuum is formed. Fuel entering the inlet port is carried to the outlet port in pumping chambers formed between the gear teeth and the housing. As the gear teeth mesh at the outlet there is no place for the fuel to go but out.
Vane type transfer pump
In the vane type pump, a slotted rotor driven by a drive shaft rotates between closely fitted side plates, and inside of an elliptical or circle shaped ring. Polished, hardened vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force. Between succeeding vanes, pumping chambers are formed which carry oil from inlet to the outlet. A partial vacuum is created at the inlet as the space between the vanes open. Fuel is squeezed out of the outlet as the pumping chamber size decreases.
A jerk type fuel injection pump can have a separate unit for each cylinder or multi-elements where a number of pump elements and a camshaft are housed in the one casing.

Single element jerk type fuel injection pump

Multi element or in line fuel injection pump

In general, jerk type fuel injection pumps comprise
·      barrel
·      delivery valve
·      rack and pinion
·      camshaft and
·      spring

Fuel injection pump element
Barrel and delivery valve
Each barrel is locked into the housing in such a way that the upper section, which contains two ports placed at 180 degrees and known as intake port and spill port, is completely immersed in fuel supplied by the fuel lift pump.
The barrel is closed at its upper end by a spring loaded pressure valve known as a delivery valve.  An injector pipe is connected between here and the injector.

The plunger which operates within the barrel is driven on its upward stroke by a roller tappet operating on a camshaft. Contact is kept between the plunger and the tappet by means of a spring which operates in a similar fashion to an inlet or exhaust valve spring.  The plunger has a slot and a helix cut into it near the top.
Rack and Pinion
A rack is fitted to the pump to engage with a pinion on the outside of a sleeve.  The sleeve fits over the plunger and has slots engaging with keys.  This allows the plunger to be rotated by the fuel rack as the plunger moves up and down. The end of the fuel rack is attached to the governor.


Fuel metering principle.
When the top of the plunger is below the inlet and spill ports, low pressure fuel flows through the inlet and spill ports into the barrel. It fills the space above the top of the plunger to the closed delivery valve and also down the slot of the plunger and into the space below the helix.
The cam pushes the plunger up and injection commences when the top of the plunger covers the inlet and spill ports. As the plunger moves up, the trapped fuel is delivered under high pressure through the delivery valve to the injector until the helical grove on the plunger uncovers the spill port.
This allows the fuel pressure above the plunger to fall to the suction pressure through the vertical slot. The plunger will rise further to complete its stroke but no fuel will be pumped. As the lobe of the cam goes past top dead centre, the spring will cause the plunger to return to the bottom of its stroke.
To vary the amount of fuel injected into the cylinder, the plunger is rotated by the fuel rack and this causes the  helical groove to uncover the spill port earlier or later depending on whether less or more fuel is required.

The fuel rack is attached to the governor. If the propeller comes out of the water the engine starts to speed up, the governor reacts by moving the fuel rack, causing the helical groove to uncover the spill port earlier or cuts off the fuel altogether. As the propeller comes back into the water, the engine starts to slow down and the opposite occurs.
To cut off the fuel to stop the engine, the plunger is rotated by the rack until the vertical slot is in line with the spill port so no fuel is delivered as the plunger moves up. 
Calibration and timing of a multi-element fuel injection pump
In a multi element pump, each element is calibrated and timed on a test rig. To calibrate a pump, each element is connected up to a calibrated test tube. The pump is run and then each test tube is checked to ensure that each element has delivered the same amount of fuel. Each element is timed to ensure that injection commences at the precise time in the stroke.
If the injection occurs too early on the compression stroke, it will result in high peak pressures and will subject the engine to unsafe stresses. This is caused by the tendency of the pressure to reverse the rotation of the engine and evidence by excessive detonation which is known as diesel knock.
Fuel Injection Pump Systems
Distributor Pump
The distributor pump incorporates a single pumping element and automatic metering system. This make it unnecessary to calibrate and balance a number of pumping elements which is the case of multi-element pumps.
The main components of the distributor pump are:
·      internal transfer pump
·      metering valve
·      governor
·      rotor and cam ring assembly (pumping element)
·      timing advance mechanism
·      maximum fuel delivery adjustment
Distributor type fuel injection pump

The fuel transfer pump (or fuel lift pump) draws the fuel from the fuel tank through a pre-filter and pumps it to a filter head into a combined filter/water separator where any contaminants and water are removed.
The fuel then is pumped to the distributor pump which pressurises, controls timing, distributes and meters an amount of high pressure fuel to the injectors.
The distributor pump uses an internal transfer pump to increase the fuel pressure in relation to engine speed. Fuel then flows through a solenoid valve to the timing advance and the annular groove surrounding the rotor.
A metering valve determines the amount of fuel made available to the pumping section of the rotor. Fuel flow is either increased or decreased depending on rotation of the metering valve by the governor. A mechanical shut down lever can also be used to move the metering valve to the closed position, stopping fuel flow to the injectors and the remainder of the engine.

The governor is connected to both the engine throttle and the metering valve, controlling the fuel flow in relation to movement in the engine throttle.
Fuel from the metering valve flows through a metering port into the charging port in the rotor. As the rotor revolves these two ports fall out of alignment, trapping fuel in the rotor. As the rotor continues to revolve the fuel is placed under increasing pressure and eventually the rotor’s charging port aligns with delivery ports and the fuel escapes to the injector.
Rotors and ports

The PT (pressure/time) fuel system has been developed and refined by Cummins over a long period.
The PT system uses injectors which meter and inject the fuel. Metering is based on a pressure/time principle.
Pressure - Time principle and is accomplished by a fixed size opening in the injector and the pressure of the fuel supplied to the injector.
Fuel system -
The fuel is drawn from the tank through a filter by the fuel pump, delivered to the injectors with 80% of the fuel being returned to the fuel tank.
Cummins PTG-VS fuel pump
Cummins PTG-VS fuel pump
The main components of this fuel pump are:
·      Gear pump
·      Pulsation damper
·      Magnetic filter
·      A standard governor
·      Throttle shaft for the standard governor
·      A variable speed governor
·      Shut down valve

A gear pump delivers fuel under pressure through a pulsation damper, which dampens out fuel surges created by the gear pump action. A magnetic filter is used to remove any metal debris from the fuel.
The fuel flows from the magnetic filter to a standard governor which controls the engine idle speed and the fuel pressure delivered by the fuel pump. The standard governor is located in series with the variable speed governor, which is positioned in the fuel pump housing. The variable speed governor controls the fuel flow to the injectors in relation to the engine speed.
The amount of fuel which flows to the injectors is dependent upon the fuel pressure from the fuel pump and the time the feed port is allowed to remain open. Hence the “pressure - time” theory to this fuel injection pump system.
Detroit diesel mechanical unit fuel injection system

Detroit Diesel fuel system
Fuel is drawn from the supply tank through the strainer and enters the fuel pump on the inlet side. On leaving the pump under pressure, fuel is forced through the fuel filter into the fuel manifold and from there through fuel pipes to the inlet side of the unit injectors. Surplus fuel returns from the outlet side of the unit injectors through outlet fuel pipes into the return manifold, from where it flows back to the supply source.
The fuel pump is of the gear type with an in built relief valve.
A non-return valve can be installed between the fuel strainer and the source of supply to prevent fuel draining back when the engine is not running.

A restricted elbow is located at the end of the outlet manifold to maintain pressure in the fuel system between the inlet and outlet fuel passages.
Detroit diesel fuel pump
Mechanical unit injector
The unit injector is a single unit which combines all the necessary components to provide complete and independent fuel injection to each cylinder.
The unit injector performs four functions:
·      creates the high fuel pressure needed for efficient fuel injection
·      meters and injects an accurate amount of fuel
·      atomises the fuel to assist mixture with air in the combustion chamber
·      times the injection of fuel into the combustion chamber
Unit injectors have the advantage that there are no high pressure fuel lines and the continuous flow of fuel serves to cool injector components while also preventing vapour pockets from forming.

Detroit Diesel Mechanical Unit Fuel Injector
Fuel enters the injector through a filter cap and element and flows into a supply chamber. Fuel also flows into space below the injector plunger. The plunger is moved by a special cam via a push rod and rocker assembly. As the plunger moves up and down inside a bushing, fuel is fed through two ports in the bushing into the supply chamber.

Fuel injection assemblyDetroit Diesel 2 stroke engine
For engine speed the plunger can be rotated within the bushing using quadrant gear which meshes with a control rack.
Fuel metering is achieved by rotating the plunger which varies the relationship between two helices machined into the lower portion of the plunger and fuel inlet ports in the bushing. The positioning of the helices and ports control the plunger’s stroke and the amount of fuel injected into the cylinder.
On the pumping stroke a portion of the fuel is forced through the lower port into the supply chamber until the lower plunger helix shuts off the port. Fuel trapped below the plunger is then forced through a central hole in the plunger and so through the upper port into the supply chamber until that port is closed by the upper plunger helix. With both ports closed fuel pressure builds up during the remainder of the plunger stroke until it is sufficient to lift the injector valve from its seat, at which point injection commences.
The spray tip incorporates a check-valve whose function is to prevent fuel dribble into the combustion chamber after the injection cut-off point should the injector valve fail to return to its seat.
On the injector plunger’s return stroke, the high pressure area inside the bushing is again filled with fresh fuel through the two inlet ports. This maintains a constant circulation of cool fuel which helps in reducing injector temperatures and effectively removes all traces of air.

Excess fuel is fed back to the fuel return manifold and subsequently, the fuel tank, through the injector outlet opening which contains a filter element similar to the one on the fuel inlet side. When the control rack is pulled back to the cut-off position the upper port is not closed by the helix until after the lower port is uncovered. Consequently, all fuel is passed back to the supply chamber and no injection takes place.
When the control rack is in the full injection position, the upper port is shut off shortly after the lower port has been closed by the position of the helix. This rack position is set to give maximum effective plunger stroke and maximum fuel delivery. As previously mentioned, intermediate throttle positions are provided by the relative position of the helical contours to the inlet ports so that both the effective stroke of the plunger and the commencement timing of the injection are altered.
Electronic unit injector
Major engine manufacturers also supply electronically controlled engines. An example of an electronic control is Detroit Diesel Electronic Controls (DDEC). These engines employ an electronic unit injector .
·      The electronic unit injector (EUI) is built on their patented mechanical unit injector design.
·      The design simplifies the plunger  and bushing. It also replaces the mechanical rack with an electronic solenoid.
·      It allows precise metering and injection timing.
·      The amount of fuel injected and the timing are determined by information fed into the microprocessor (Electronic Control Module) from sensors located on the engine.
Fuel injector assembly
A fuel injector is a spring controlled valve located in the engine cylinder head and allows the fuel, under pressure from the fuel pump, to enter the cylinder. It enters in an atomised form to allow it to mix completely with the hot compressed air so that ignition can take place with efficient combustion.

Fuel injector
Spray nozzle assembly
The fuel injector consists of a nozzle body and valve. The nozzle body incorporates the valve  seat and has holes or orifices in it to atomise the fuel. The valve must seal effectively on the valve seat to allow for a clean cut off of fuel to the cylinder. A leaking valve causes misfiring and irregular speed, particularly on light loads. The valve and nozzle body are lapped to form a mated assembly. Therefore the valve and nozzle body cannot be exchanged individually. A nozzle cap attaches the nozzle to the body of the injector.
Types of spray nozzles
There are different types of spray nozzles. The type of spray nozzle used depends upon the design of the combustion chamber and the angle of the injector. It must spray the fuel so that it mixes completely with the hot compressed air for efficient combustion to take place. Four types of spray nozzles are the single hole, multi-hole, pintle and pintaux.

Types of spray nozzles.
The single and multi-hole spray nozzles are similar in that when the valve opens, the fuel ejected can be directed through one hole in the case of the single spray nozzle or through any number of holes at any angle in the case of the multi-hole spray nozzle.
The pintle and pintaux are also similar. A pintle on the valve projects past the valve seat and slightly past the end of the nozzle. There is a slight but exact clearance between the pintleand the injection hole. The pintle size and shape can be varied so as to meet any spray pattern requirement. The pintle prevents the formation of carbon deposits in the injection hole. Pintlenozzles are used in engines with adequate air turbulence such as pre-combustion chambers or turbulence chambers.
When the fuel pressure opens the valve, the pintle causes a conical spray pattern. It also allows a relatively small proportion of the fuel to be injected as the valve starts to open, followed by the bulk of the fuel thereby slowing down the pressure rise in the cylinder bringing about smoother combustion and engine running. The pintaux differs in that it has a hole at an angle where fuel sprays out separately from the conical pattern for pilot injection.  

Nozzle holder or body - A nozzle holder forms the body of the injector. It is fitted with a flange to secure it to the cylinder head. It has drilled passages for the fuel to flow to the valve and for the leak off from the valve stem.
Spring and adjustment screw - The valve is held on its seat by a spring operating on a steel spindle. The compression of the spring can be adjusted by a screw and a locknut so that the valve opens at the recommended pressure.
Cap nut - A cap nut is screwed onto the top of the nozzle holder to enclose the adjustment screw and to seal the unit.

The fuel injector pump delivers a set amount of fuel under pressure to the fuel injector.
The pressure causes the valve to open against the spring and the fuel to spray into the cylinder.
When the helix on the plunger of the fuel injector pump uncovers the spill port, the pressure of the fuel drops quickly and the spring in the fuel injector causes the valve to shut.
The delivery valve in the fuel pump also closes and fuel is maintained in the injector pipe.
The needle valve is a neat fit in the nozzle and fuel flows through the small clearance to lubricate it. This fuel is called leak off and is returned to the fuel tank.
CAUTION:Injectors deliver extremely high pressures. You should exercise due care when dealing with this equipment as there is a clear risk of serious and permanent injury.

2.5 Rectifying injector faults

Any injector problem at sea can be rectified by replacing the injector with a spare. However the candidate is required to know how the following faults can be identified and rectified.
Incorrect opening pressure
Too low an opening pressure will cause the valve to chatter on its seat. Fuel will be injected into the cylinder earlier. It is caused by insufficient compression on the spring.
Too high an opening pressure will cause the valve to hammer on its seat. Fuel will be injected into the cylinder later. It is caused by too much compression on the spring.
The spring adjusting screw has a lock nut which may have slackened off causing insufficient compression on the spring.
The spring may break. Replace the spring.
The correct opening pressure can only be obtained by placing the injector in a test rig and adjusting the tension on the spring until the correct opening pressure is obtained. Whilst on the test rig, the spray pattern of the fuel leaving the nozzle can also be checked.
Distorted spray form
Spray nozzle orifices are partially clogged. Spray nozzles should be cleaned by first soaking them in either kerosene or clean fuel to soften the dirt. The spray holes or orifices can be cleaned with a pointed piece of wood. Do not use a piece of wire. 
Dripping injector
The valve is not sealing on its seat. Grind it in with the finest grade of grinding compound. Excessive grinding causes the valve to seat too deeply in its seat causing a lagging of the fuel admission which results in late combustion and therefore loss of power.
In addition, the valve stem may be bent and this will cause the valve not to seal on its seat and the valve stem will be tight in the nozzle body. The valve and nozzle body are lapped to form a mated assembly. Therefore the valve and nozzle body cannot be exchanged individually. Replace with a new valve and nozzle body. The opening pressure will then have to be adjusted.

Dirt between the valve and its seating
Spray nozzles should be cleaned by first soaking them in either kerosene or clean fuel to soften the dirt. Do not use anything metallic or abrasive to clean them. Grind it in with the finest grade of grinding compound. Excessive grinding causes the valve to seat too deeply in its seat causing a lagging of the fuel admission which results in late combustion and therefore loss of power.
Injector valve sticking in the nozzle body
The valve stem may be bent and this will cause the valve stem to be tight in the nozzle body and the valve not to seal on its seat. The valve and nozzle body are lapped to form a mated assembly. Therefore the valve and nozzle body cannot be exchanged individually. Replace with a new valve and nozzle body. The opening pressure will then have to be adjusted.
Alternately, there may be dirt between the valve stem and the nozzle body. It may be possible to clean the dirt away and reuse the assembly. If however, there has been grit passing through the fuel injector, it is most likely that there is pick up on the valve stem and body thereby scoring them. Pick up is when metal from one part is transferred to its mating part and scores or grooves it. Further operation in this condition could cause the valve stem to seize in the nozzle body. Any pick up on the valve stem and nozzle body will require the assembly to be replaced.
Too much fuel escaping at the leak off pipe
Caused by excessive clearance between the valve stem and the nozzle body resulting from wear or pick up from dirty fuel or corrosion by water contaminated fuel. A fine clearance is required to maintain the fuel pressure and allow some fuel to pass by to lubricate. Replace the valve and nozzle assembly.  

Exhaust emissions can be related to the operation and condition of the engine.
Black smoke
indicates a fuel or air problem. For efficient combustion, the ratio of fuel to air must be maintained otherwise incomplete combustion will take place resulting in black smoke.
·      Blocked or partially blocked air cleaner
·      Turbo charger not attaining sufficient speed
·      Poor compression
·      Incorrect fuel pump timing, Faulty fuel pump
·      Incorrect valve timing
·      Faulty fuel injectors - dirty nozzle, incorrect opening pressure, excessive leak off, valve not seating in body   
·      Engine overloaded
Blue smoke
indicates that lubricating oil is being burnt. Caused by:
·      Worn, broken or sticking piston rings and/or worn cylinder liner bores
·      Worn valve guides
·      Valve stem seals leaking
·      Turbo charger seals leaking
·      Oil bath type air cleaner overfull
White smoke
or white exhaust vapour indicates water or moisture.
·      Water in the fuel
·      Moisture in the air
·      Cold cylinder liner bores and combustion space when first starting engine
·      Leaking cylinder head gasket between cylinder and cooling water passage.

The power requirements of an engine may vary continually due to fluctuating loads, therefore some means must be provided to control the amount of fuel required to hold the engine speed reasonably constant during such load fluctuations. To accomplish this control, a governor is fitted to the engine.
There are a number of different types of governors, but only two will be mentioned.
Constant speed governor
It is used to maintain the engine at the same speed. For example an auxiliary engine driving a generator may have a fixed speed of 1800 rpm. The electrical load will vary. If the load is increased, more fuel is required otherwise the speed will drop. The drop in speed will cause the governor to alter the fuel pump to supply more fuel so the 1800 rpm is maintained.
Variable speed governor
It is used to maintain a set idling speed, a maximum speed and any desired speed between these limits regardless of any load change. The desired speed is set by a speed control lever or wheel. This type of governor is used on propulsion engines and a simple mechanical and hydraulic type are described herein.
Mechanical and hydraulic governors
Mechanical governors are limited as to their sensitivity due to the fact that the governor flyweights must not only limit the speed, but also perform the physical work of moving the fuel control mechanism.
In addition, speed droop is inherent in them, so they are incapable of maintaining constant speed with varying load without manual adjustment.
The steady decrease in the speed of an engine caused by an increase in load from no load condition to full load, without change in the adjustment of the governor is known as “speed droop”.
A governor which regulates the engine speed so that there is zero speed droop is termed “isochronism”.

A hydraulic governor of the proper design is not only isochronous but is extremely sensitive because the governor flyweights are used to limit speed only, the work of moving the fuel control mechanism being performed hydraulically.
A mechanical governor cannot make any adjustment to the fuel supply until the engine speed has changed iethey cannot anticipate, they can only correct.
Mechanical variable speed governor
Mechanical variable speed governor

The governor is engine driven which causes the flyweights to rotate.
When the engine is operating at normal speed, the centrifugal force acts on the rotating flyweights and is balanced by the vertical speeder spring force.  The control sleeve remains stationary.
If the engine load decreases, the engine speed increases (ie. the propeller coming out of the water).  The centrifugal force acting on the flyweights also increases causing the flyweights to move outwards and the control sleeve upwards. 
This moves the fuel rack so less fuel is delivered. The upward movement in the control sleeve increases the compression in the speeder spring and hence the speeder spring force. This increased spring force and the control sleeve remains stationary in the new position.
If the engine speed decreases (ie. the propeller going back into the water), the opposite to the above occurs. Thus the control sleeve moves up and down as the engine speed fluctuates because of load variations.
The normal operating speed of the engine can be manually adjusted by increasing or decreasing the speeder spring compression and hence the speeder spring force by the speed control lever.
Hydraulic variable speed governor
Hydraulic variable speed governor.

The governor is engine driven which causes the flyweights to rotate.

When the engine is operating at normal speed, the centrifugal force acts on the rotating flyweights and is balanced by the vertical ballhead spring force and the piston valve remains stationary.

If the engine load increases, the engine speed decreases, the centrifugal force acting on the flyweights also decreases causing the flyweights to move inwards and the ballheadspring to move the piston valve downwards. The piston valve, on moving downwards, will admit oil under the power piston. This pushes the power piston upwards, compressing the return spring and moving the fuel control towards more fuel. The movement of the compensating lever slightly decreases the force on the ballhead spring and returns the piston valve to its neutral position.

The normal operating speed of the engine can be manually adjusted on the speed adjustment wheel. To increase the engine speed, the force on the ballhead spring is increased causing the piston valve to admit more oil under the power piston which in turn increases the supply of fuel.

The compensating lever is fitted to stop the governor from hunting. Hunting is when the speed is below or above the control speed, the governor will continue to adjust the fuel control. To avoid hunting, a governor mechanism must anticipate the return to normal speed and must stop changing the fuel control setting slightly before the new setting, required for sustaining the control speed, has been reached.

Pneumatic Governor
Pneumatic governor
The pneumatic governor operates on the well known fact that air passing through a pipe tends to create a vacuum in a part of smaller diameter. The engine suction through a venturi(a tube with a narrowing throat or constriction designed to increase the velocity of the gas or fluid passing through it) provides the necessary suction and in turn operates a diaphragm control connected directly to the control rack of the fuel injection pump.

The pneumatic governor consists of two main parts:
1. The venturi air flow control unit mounted between the air induction manifold of the engine and the air intake filter; and
2. The diaphragm unit mounted on the end of the fuel pump housing.

The venturi unit - has a butterfly valve fitted at its throat and is actuated by the throttle. The butterfly valve is limited by stops for both the idle and maximum speeds. A vacuum pipe is taken from the same center line as the butterfly valve to the diaphragm unit.
The diaphragm unit - has a diaphragm, a light spring dampening out any oscillations and keeping the fuel pump control rack in the full open position. A manually operated lever is fitted to the control rack and is used to stop the engine by drawing the control rack into the stop or no fuel position.
The operation of the governor is as follows:
When the engine is at rest, the lever is released and the spring forces the control rack into the full load position. By setting the excess fuel device rack allowed to move automatically to the extent of its travel, placing the fuel pump plungers into starting position (maximum fuel delivery). The engine is ready to start.
(The excess fuel device is located at the opposite end of the fuel control rack and consists of a plunger, or latch, which when released, allows the rack to move as previously stated. As soon as the engine starts and the governor takes charge, this device resumes its normal load position automatically, preventing the control rack from again going to the starting position).
The manual control is set to running position and the engine started.
When the engine starts and is idling, the butterfly valve is almost closed. A high vacuum is immediately created in the corresponding pipe and airtight compartment in the diaphragm unit.
The air in the adjoining compartment is at atmospheric pressure, therefore the pressure on this side of the diaphragm is higher than that existing in vacuum compartment. This causes the diaphragm to move the control rack towards the stop position until the engine is running at the predetermined idling speed.
When the throttle is operated to increase the engine speed the butterfly valve is opened wider, this decreases the velocity of the air passing the mouth of the connecting tube.

The result is an increase of pressure in the vacuum compartment of the diaphragm unit with the spring forcing the diaphragm and the control rack towards the maximum speed position, so increasing the amount of fuel delivered to the engine.
Fluctuations can exist in the induction manifold. To dampen these fluctuations, an additional adjustable spring controlling the diaphragm is fitted. This auxiliary spring comes into operation at predetermined speeds and can be adjusted by a screw to suit the engine requirements.
This governor is extremely sensitive and efficient but has the bad fault that should any leakage occur that will tend to destroy the vacuum, the governing effect will be lost and the engine may race.
Governors are driven by some part of the engine that rotates. It may be off the camshaft, or be mounted on the scavenge blower and driven by the upper blower rotor or be attached to the end of the fuel pump, or enclosed in the fuel pump housing, or by some other method.
Mechanical governors are lubricated by oil splash. Oil entering the governor is directed by the revolving flyweights to the various moving parts requiring lubricating.
Governor difficulties are usually indicated by speed variations of the engine. However, speed fluctuations are not necessarily caused by the governor and, therefore, when improper speed variations become evident, the unit should be checked for excessive load, misfiring or bind in the governor operating linkage.
Dirty oil is a cause of hydraulic governor troubles.

Remote control of governors
Some hydraulic governors are equipped with a reversible synchronising motor which is mounted on the governor cover. This motor makes a close adjustment of the engine speed possible by remote control and is especially valuable for synchronising two generators from a central control panel or bridge control.
Electrical / hydraulic governors
The Electric Fuel Control (EFC) governor is an electrical sensing system that can be adjusted for isochronous engine speed droop. This governor will provide rapid fuel rate changes to improve the transient response to the load change.
It consists of:
Magnetic pick up - This is an electromagnetic device that is mounted in the flywheel housing. As the flywheel gear teeth pass the pick-up, an alternating current (AC) voltage is induced, one cycle for each gear tooth. This electrical signal is directly proportional to the engine speed and is fed to the governor control.
Governor control - The governor control is an all electric solid state module which compares the pulses (electric signal) from the magnetic pick-up with a speed control reference point. A current output is supplied to the actuator which rotates the actuator shaft to control the fuel flow to the engine.
Actuator - The actuator is an electromagnetic rotary solenoid valve, the turning action of the shaft regulates the fuel pressure and therefore determines the engine speed and power. (In other governors, there are variations in that they still have an electromagnetic solenoid valve but it is not a rotary type, and it still controls the fuel pressure).

2.8 Testing and setting a mechanical variable speed governor
All governors are properly adjusted before leaving the factory. However, if the governor has been recondition or replaced, minor adjustment might be required.
As the procedure for adjustment vary between makes and models of governors, the Owners Manual should be followed.
Caution - To prevent maladjustment, it is the practice of some manufacturers to seal the governor mechanism after it has been adjusted on the test bed, and, if the seal is broken, to decline responsibility for failure in performance.
Interference with the tension of the governor springs may cause the speed of the engine to rise beyond the safety limit.  Interference with the maximum fuel stop may result in the injection of too much fuel, thus causing excessive exhaust smoke and overheating.
The usual adjustments are for the maximum no-load speed and the idling speed, although there maybe a number of steps to affect a setting.
Adjustments should only be made after the engine has reached normal operating temperature. An accurate tachometer should be used for the engine speed.
Maximum no-load speed
A stop is used to limit the compression of the governor spring which determines the maximum speed of the engine.
This adjustment will only affect the maximum speed and have no effect on intermediate speed control positions. Set the throttle at full speed and when it is running at this speed, turn the adjusting screw so that the maximum speed, as recommended by the manufacturer, is obtained. Tighten the lock nut on the adjustment screw.
Idling speed
A stop is used to limit the travel of the fuel pump rack so that the slot in the plunger does not line up with the spill port and stop the engine.
With the throttle in the idle position, loosen the lock nut and turn the adjusting screw until the engine is running at the manufacturers recommended idling speed. Tighten the lock nut.


No comments:

Post a Comment

Have something to say? Please write here...