Saturday, July 6, 2013

MARINE ENGINE Series: Principles of Diesel Engines


MARINE ENGINE & PROPULSION SYSTEMS

Principles of Diesel Engines

Before beginning this module, let’s look at some terms relating to internal combustion:
Force
is the influence which tends to change the motion or direction of a body at rest or in motion. A simple explanation is pushing or pulling.
From the above, applying a force would either:
·      Start moving a body from rest or bring a moving body to rest.
·      Increase or decrease the speed of a moving body.
·      Change the direction of motion of a moving body.
Force is measured in newtons (N).

Work
is the use of energy to overcome resistance. The amount of work done is from moving an applied force through a distance. The unit of measurement of doing work is the joule.
The force is measured in newtons (N) and the distance is measured in metres (m). From the formula Work = Force x Distance, work would be in newton metres (Nm). To prevent confusion between ‘work’ and ‘torque’, the unit given to the formula for work is the joule (j).
One newton metre = one joule.
Torque
is when a force tends to cause a movement about a point. Torque is also called a turning or twisting effort. Torque = Force x Distance. Torque is the force exerted, but notmoved, over a distance.
Force is measured in newtons (N) and distance is measured in metres (m). Torque is therefore measured in newtonmetres (Nm).
As an example, the force on the piston of an engine exerts a turning moment on the crankshaft.

Power
is the amount of work done or energy expanded in a given time. Also expressed as the capacity to do work. Watt (W) is the unit measurement of power. A watt is the power used when energy is expended or work done at the rate of one joule per second.
Power = Force x Distance
              Time in seconds
As force is in newtons (N), distance in metres (m), and time in seconds (s), the answer will be in newton metres per second or joules per second. (1 newton metre = 1 joule). However, as one joule per second = one watt, the final answer will be in watts.
Power of an engine is measured in kilowatts (kW) rather than watts (W). 1000 W = 1 kW.
Thermal efficiency
Thermal efficiency is the ratio of work done at the flywheel to the amount of energy contained in the fuel. Thermal efficiency is expressed as a percentage.
Calorific value
Fuel contains a specific amount of heat energy or heat value which is released when the fuel is burnt. This is the calorific value of the fuel. It is measured in joules per kilogram of fuel.
Volumetric efficiency
is the ratio between the  swept volume of a cylinder and the actual volume of air drawn in during the induction stroke. The efficiency varies considerably, depending on the design and operating conditions but especially with engine speed. A turbo charged engine will have a higher volumetric efficiency (in excess of 100%) than that of a normally aspirated engine (less than 100%). Swept volume is the volume in the cylinder between TDC and BDC of the piston.
Turbulence
also called swirl, is the circular movement of the air as it enters the combustion chamber. The swirling motion or turbulence is encouraged by design considerations as it enhances flame propagation and is especially important at light engine loads. It is a desirable characteristic in the flow of air into the cylinder. In most engines, a rapidly swirling motion is deliberately induced and the violent movement helps ensure even mixing of the fuel and air. It also speeds up the combustion process once the fuel has ignited.
Scavenging
is the term used for eliminating the burned exhaust gases from a cylinder. The incoming air removes, or scavenges, as much of the burnt gases as possible. Valve overlap assists in the scavenging process.
Compression ratio
is the ratio between the volume of the air before and after it has been subject to compression. A compression ratio of 12:1 means that during the pistons travel from the lowest to the highest point in the cylinder, the air has been compressed to one-twelfth its original volume. A diesel engine needs a high compression ratio to get sufficient heat in the compressed air to ignite the fuel.
Compression
  ratio   = piston displacement + clearance volume
                           clearance volume
Valve overlap
is the period which both the inlet valve and exhaust valve are open at the same time. The inlet valve opens before top dead centre (TDC), say at 10° and the exhaust valve closes after TDC, say at 35°. The opening of the inlet valve overlaps the closing of the exhaust valve. The overlap in this case would be 35° .
The purpose of valve overlap is to ensure that are exhaust gases are discharged from the cylinder and the cylinder receives a fresh charge of air to make it more efficient when combustion next takes place. It also has a cooling effect.
Valve rotators
are devices which cause a valve to rotate each time it opens. It can be fitted to either end of the valve spring. Its purpose is to ensure even wear and prevent exhaust valves from burn out.
Dwell
is the angle that the valve remains in the fully open position. The profile of the lobe of the cam causes the valve to open until the lobe flattens out. The valve stays in this fully open position which is the angle of dwell until the other side of the lobe is reached when the valve starts to close.


Cam lift
is the distance from the peak of the lobe of a cam to its axis minus the distance from the back of the cam to its axis. Another description would be the distance the valve opens plus the valve lash or tappet clearance measurement.

Cam profile

There are two types of diesel engines, a four stroke cycle and a two stroke cycle.
Four stroke cycle diesel engine
In a four stroke cycle engine, four strokes of the piston are required to complete one cycle. The four strokes are induction, compression, power and exhaust.
The actual opening and closing of the inlet and exhaust valves and the period of injection of the fuel can be taken from the timing diagram. Timing diagrams will vary between engine models and manufacturers.









Four stroke timing diagram.

The above diagram is for a Caterpillar series 3600 turbo charged after cooled engine. As can be seen from the timing diagram, the induction stroke commences when the inlet valve opens 10° before TDC when air is drawn into the cylinder as the piston moves down. The inlet valve closes 1° before BDC.
The air is now trapped in the cylinder and as the piston rises on the compression stroke, the air is compressed. As the air is compressed, it rises in temperature. When the piston reaches 19° before TDC, the injection of fuel commences and continues until 73° after TDC.
The heat in the compressed air ignites the fuel and combustion takes place. The gases expand forcing the piston down on the power stroke.
The exhaust valves opens at 26° before BDC and the exhaust gases commence and are discharged as the piston rises on the exhaust stroke. Most of the exhaust gases have been discharged as the piston nears TDC. However, at 10° before TDC, the inlet valve opens and air enters the cylinder and helps discharge any remaining exhaust gases until the exhaust valve closes at 3° after TDC.
The whole cycle is then repeated.
Both the exhaust valve and inlet valve are open from 10° before TDC to 3° after TDC, an overlap of 13°. This is referred to as “valve overlap” and ensures that all the exhaust gases are discharged from the cylinder and the cylinder receives a fresh  charge of air to make it more efficient when combustion next takes place.
Therefore there is one power stroke for every cycle or two revolutions of the crankshaft.
Two stroke cycle diesel engine

In a two stroke cycle engine, two strokes of the piston are required to complete one cycle.
The two strokes are compression and power. The events of compression, injection of the fuel, combustion and expansion of the gases take place in the same order as the four stroke engine, but the exhaust of the burnt gases and the induction of air take place at the bottom of its stroke. This is the chief difference between the two stroke cycle and the four stroke cycle.
There are variations in two stroke cycle engines. The type described here is the most common to be found in marine engines. It has inlet ports and exhaust valves.


In this two stroke cycle engine, all the valves are exhaust. The inlet holes or ports are in the lower section of the cylinder liner wall.
The piston uncovers the inlet ports as it moves down the cylinder. The piston covers the inlet ports as it moves up the cylinder. This action has the same effect as a valve opening and closing.
An engine driven scavenge blower is fitted and the incoming air is blown into the cylinder through the inlet ports when they are uncovered by the piston.
Two stroke timing diagram.
The above timing diagram is for a Detroit Diesel model 16V-149 turbo charged inter cooled engine. As can be seen from the timing diagram above,  induction commences at 49° before BDC when the piston has uncovered the inlet ports. Air is forced into the cylinder by the scavenge blower as the piston moves down to BDC and back up again until it covers the inlet ports at 49° after BDC.
As the piston rises, the exhaust valve closes at 62° after BDC. The air is now trapped in the cylinder and the piston rises on the compression stroke. As the air is compressed, it rises in temperature.

Fuel is injected before TDC and continues after TDC. Detroit Diesel do not give the period of injection as this will vary depending upon the engine speed, the load and the size of the injectors. The camshaft contains the exhaust valve cams as well as the unit injector cams. Therefore, if the exhaust valve timing is correct, the unit injector timing will be correct providing the injector follower is adjusted to a definite height in relation to the unit injector. A special gauge is supplied to set this height.
The heat in the compressed air ignites the fuel and combustion takes place. The gases expand forcing the piston down on the power stroke.
The exhaust valve opens at 83° before BDC allowing the burned gases to escape into the exhaust manifold. However, at 49° before BDC, the inlet ports are uncovered by the piston and air enters the cylinder and helps discharge any remaining exhaust gases until the exhaust valve closes at 62° after BDC.
The whole cycle is then repeated.
There is one power stroke for every one revolution of the crankshaft.
Combustion chamber design, which includes the shape of the cylinder head, the shape of the top of the piston and the air flow through the inlet ports, is one of the most important factors in efficient operation of the diesel engine. Because of the very short space of time available in a diesel engine in which the fuel and air can mix, various methods have been devised in an attempt to give improved mixing and combustion.



Combustion chambers can be of several designs but all are concerned in creating turbulence to the air during the compression stroke. In the diesel engine, the fuel is in the form of fine particles sprayed into the cylinder after the air has been compressed. To secure complete combustion, each particle of fuel must be surrounded by sufficient air. The mixing of the air and fuel is greatly assisted by the combustion chamber air turbulence.
Some engines have helical inlet ports to provide additional swirl.
Generally, combustion systems can be classified as direct and indirect injection types.


·      Direct injection.
·      Indirect injection, the two most common types being:
·      Turbulence chamber and
·      Pre-combustion chamber.
·       
The larger, slow speed engines and medium speed engines do not have the same difficulty in achieving good combustion as small high speed engines.
Direct injection

Direct injection combustion chamber
With direct injection, the fuel is injected directly into the combustion chamber which is usually formed by a cavity in the piston crown. 
This cavity is carefully shaped to promote air swirl and the direction of the injector nozzle ensures that rapid mixing of the fuel and air assists complete combustion.
Advantages - It is claimed that direct injection gives higher thermal efficiency with lower fuel consumption. This is bought about by the fact that no heat is lost or power wasted in pumping air through a restricted opening into the separate chamber or in discharging the gases from the chamber. This gives easier starting and generally this type of engine does not require a starting aid device, such as glow plugs.
Disadvantages - This kind of injection is prone to “diesel knock”.

Indirect injection

Pre-combustion piston
The indirect injection or separate chamber system is where a separate small chamber is connected to the main chamber by a narrow passage or orifice.
The pre-combustion chamber and the turbulence chamber (also called a compression swirl chamber) work on the same principle. The main physical difference is the location and size of the connecting passage.
Swirl Chamber


With pre-combustion chambers only about 30% of the combustion air is forced into the chamber, fuel is injected and primary burning takes place in the chamber. This prevents too sudden a rise in pressure which can contribute to the so called ‘diesel knock’. The burning mixture of fuel and air is vigorously expelled through the connecting passage into the main combustion chamber or cylinder where an excess of air permits combustion to be completed.
Advantages - lower injection pressures can be used, resulting in less wear of injector nozzles; simpler design of nozzle equipment, which are easier to maintain, and smoother idling of the engine.
Engine manufacturers may in some instances use either design in their range, depending on operating requirements.
Disadvantages - not as efficient as direct injection. It can also be prone to pre-combustion burn-out.
Valve timing is the critical relationship between the position of the crankshaft and the opening and closing of the inlet valves and exhaust valves. The valve train is geared or has a chain drive with sprockets on the camshaft and crankshaft.
Any slight variation from the correct timing setting will result in loss of power and overheating. Any large variation and the engine will not start.
To accurately check the valve timing, it will be necessary to remove the timing cover to gain access to the timing gears.
The gears or sprockets are fitted to the crankshaft and camshaft by keys so they can only be fitted in one position. However, they can be incorrectly lined up to each other.
Th operators manual will indicate what the timing marks look like and in the case of chains, what the sprockets should line up with. Typical lining up marks for gears are shown below:



Gear lining up marks
When timing has been found to be correct, the tappet clearances (also referred to as valve lash) should be checked. Whenever the cylinder head is overhauled, the valves are reconditioned or replaced, or the valve operating mechanism is replaced or disturbed in any way, the tappet clearance must be adjusted. Also when the cylinder head has been re-tightened after the initial run in period.
When the valve and valve operating gear heats up in service, the clearance between the rocker arm and the valve stem decreases. If insufficient clearance is allowed, the valve will be prevented from seating. The correct clearance will be specified by the engine manufacturer. In the Operators Manual, some manufacturers state clearances for when the engine is at its normal operating temperature, others when the engine is cold, while some give both.
Clearances will vary as much as 0.128 mm (0.005”) between a cold and the normal operating temperature of an engine. Usually, an exhaust valve will have a greater clearance than an inlet valve because of their different operating temperatures. Too much clearance will cause excessive wear, noisy operation and altered valve timing, that is, late opening and early closing.
If the clearance is insufficient and the valve does not seat properly, it will result in:
·      loss of compression through valve leakage
·      burning and eroding of the valve and seat, and
·      general overheating.



In the extreme, it is possible that the piston could strike the valve resulting in a bent valve stem, damaged piston or worse if the valve or piston should break.
When the valve operating mechanism is disturbed in any way, the engine is cold, but only a hot tappet clearance is given, the tappet clearance must be checked. If required, a further adjustment when the engine is at its normal operating temperature.
The most common form of adjustment for tappet clearance is by means of a screw and lock nut located in one end of the rocker arm. The clearance is measured by means of a feeler gauge between the valve stem and rocker arm when the valve is in the fully closed position. This is usually done when the piston, under the valve being adjusted, is on top dead centre at the end of the compression stroke.
An easy way to identify the above is as follows:
On a six cylinder engine with a firing order of 1 5 3 6 2 4, turn the engine over in the direction of rotation. When the inlet valve and exhaust valves are rocking on number 6 cylinder (ie. the piston finishing its exhaust stroke and starting its induction stroke) adjust the inlet and exhaust valve clearances on number 1 cylinder which will just be completing its compression stroke and commencing its power stroke.
On the crankshaft, the bottom end journals on numbers 1 and 6 are 180° to each other, 2 and 5 are 180° to each other, and 3 and 4 are 180° to each other.
What you are doing is adjusting number 1 tappets while number 6 is rocking, then adjust number 5 because it is the next one in the firing order to be on top dead centre while number 2 is rocking, adjust number 3 while number 4 is rocking, adjust number 6 while number 1 is rocking, adjust number 2 while number 5 is rocking, and adjust number 4 while number 3 is rocking.
On a Detroit Diesel, the exhaust valve/s can be adjusted on the cylinder on which the unit injector follower is fully depressed. This means that fuel injection is taking place so it is at the end of the compression stroke and the beginning of the power stroke.


Early injection
If the injection occurs too early on the compression stroke, it will result in high peak pressures.  This will subject the engine to unsafe stresses 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.
Late injection
Retarded injection or late burning gives incomplete combustion causing too low a power output and overheating.
Timing instructions
It will be necessary to follow the manufacturers instructions in the Owners Manual to time the fuel pump to the engine as different methods are employed.
Timing principle
The principle is that fuel injection commences on the compression stroke just before top dead center. With a four stroke, the piston also comes up to top dead center on the exhaust stroke. Make sure it is on the compression stroke.
As with timing inlet and exhaust valves, the fuel injection pump must be timed to inject fuel at the correct angle on the compression stroke. This means that the gear driven shaft to the pump must also be lined up in the gear wheel train. Otherwise, difficulty might be experienced in lining up the holes in the drive coupling.
Timing engine to pump
The flywheel is usually marked with a TDC and with an injection mark that is before the TDC mark when turning the engine over in the direction of rotation. Turn the engine over in the direction of rotation until its number 1 cylinder is on the compression stroke and the injection mark is lined up.
The fuel injection pump must also be lined up on number 1 element or port at the commencement of injection. The Owners Manual will identify the position of the lining up marks as brands of pumps differ. When the lining up marks on the pump correspond, the drive couplings can be bolted together.


Alternative method of timing
To make it easier still, some manufacturers make provision for locking the fuel injector pump shaft at a position corresponding to top dead center for number 1 cylinder. A further pin is then located in a hole in the camshaft timing gear that is top dead center for number 1 cylinder. The drive couplings can then be bolted together and the pins removed.
As the pin is located in a hole in the camshaft, it can only be on the compression stroke on a four stroke engine.
Checking the timing of a fuel pump
The timing may be checked as follows:
1.         Remove the delivery valve and spring from number 1 element in the fuel injection pump.
2.         Open the throttle to the full position. (If the throttle is left at the stop position, the slot in the plunger will be in line with the spill port and no fuel will be delivered.)
3.         Rotate the engine in its operating direction until number 1 cylinder is on the compression stroke. Keep rotating the engine slowly and when the mark on the flywheel, indicating the start of injection is lined up with the timing indicator mark, fuel will immediately start to rise from where the delivery valve was removed. (This will mean the top of the plunger has just covered the inlet and spill ports and injection is starting).
4.         If fuel starts to rise before or after the timing marks are in line, the fuel pump timing is out and will have to be adjusted.
Detroit Diesel unit injector
 On a Detroit Diesel, the cam that actuates the unit injector is on the same shaft as the cams for the exhaust valves. If the exhaust valves are correctly timed, that is they open and close at the correct angles, then the unit injector timing must be correct. It is then only a matter of adjusting the unit injector follower to get the correct height in relation to the unit injector body. A special gauge is supplied for this purpose.
Cummins PT injector
On the Cummins PT system, it is only a matter of setting the clearance between the rocker arm and the injector.

A turbo charger (sometimes called a turbo blower) can be fitted to both two and four stroke engines to increase the volumetric efficiency and thus their power output.
Advantages
The advantage of a turbo charger is that fuel consumption is lower than that of a normally aspirated engine of the same power output.
In addition, the turbo charger utilises the exhaust gases of the engine so no power from the engine is required to drive it.
The turbo charger causes a larger mass of air into the cylinder to that of a same cubic capacity normally aspirated engine. This allows for a proportional increase in the amount of fuel that can be injected and burnt in the cylinder thereby providing an increase in the power output of the engine. 
Components of a turbo charger
The components of a turbo charger are shown below.
Turbo charger


Rotor assembly
It has a rotor shaft which has exhaust gas turbine blades on one end and air compressor blades on the other end.

Casings
The exhaust gas turbine blades are housed in a casing which is attached to the exhaust manifold and to the exhaust pipe. Some casings are fresh water cooled to minimise the heat radiated out into the engine space. This allows for a cooler engine space, cooler air entering the engine air intake and therefore more power again. A nozzle ring is fitted inside the casing to direct the flow of exhaust gases to the turbine blades.
The air compressor blades are also housed in a casing which has an air cleaner on the intake side and is connected to the intake manifold on the discharge side. Where an engine is after cooled, the discharge side is connected to the after cooler which is then connected to the intake manifold.
Both the above casings are attached to a centre casing which contains the bearings, seals and method of lubrication.

Bearings and lubrication
The shaft may rotate in white metal bearings which can be lubricated from the engine driven oil pump. This method of lubrication also allows the oil to remove some of the heat in the turbo charger. One bearing locates the shaft and takes the small residual thrust, the other bearing allows the shaft to move longitudinally to accommodate the differential thermal expansion of casings and shafting.
Alternatively, the smaller turbo chargers usually incorporate a ball bearing for positioning at the compressor end  and a roller bearing to accommodate axial expansion at the turbine end of the rotor shaft. The bearings may have their own reservoir which forms part of the turbo charger. These reservoirs usually have round oil level sight glasses with two horizontal lines marked to indicate the high and low levels. Seals are fitted to retain the oil.


Operation of the turbo charger on a diesel engine

In a four stroke engine, exhaust gases flow from each cylinder into the exhaust manifold and then past the turbine blades of the turbo charger. With the engine running at full speed, the turbo charger can obtain speeds up to 100,000 revolutions per minute (rpm).
The air compressor blades will revolve at the same speed. Air is drawn through the air cleaner and forced under pressure into the intake manifold. When the inlet valve opens on the induction stroke, with the piston descending in its cylinder, air is forced into the cylinder.
It is necessary to reduce the turbo charger speed in stages or slowly for two reasons:
1.         If the engine speed is reduced from full engine speed to stop quickly and the bearings of the turbo charger are lubricated by the main engine driven lubricating oil pump, the engine, on stopping, will cease to supply the lubricating oil to the turbo charger bearings. Because of its high speed, it will take some time for the turbo charger to come to rest and the bearings could be damaged.
2.         The exhaust gas side of the turbo charger operates at a very high temperature. It is preferable to reduce the temperature gradually rather than quickly to prevent unequal contraction of the turbo charger parts as it slows down.
Monitoring the performance
Normally, as part of the purchase of a new engine, the engine distributor or dealer will do an installation and pre-run check. The following will be recorded:
·      The speed of the turbo charger at a nominated engine speed.
·      Air flow in.
·      Air flow out.
·      Air pressure after the compressor blades.
·      Exhaust gas flow.
The flow of air going into the turbo charger is important. The air is taken from the engine room so sufficient ventilation to the engine room is required to ensure there is enough for the engine as well as cooling the engine room.

The exhaust gas flow is also important. It ensures the installation of the exhaust piping is within limits and not restricting the performance of the engine.
As the above is recorded, checks can always be carried out and readings compared with the initial ones.
1.7 After coolers (Charge air coolers)
An after cooler is also called an inter cooler or a charge air cooler.
An after cooler is fitted where an engine is turbo charged, however it is not necessary to fit one. Therefore an engine can be turbo charged or can be turbo charged and after cooled.
The reduction in air temperature will increase the density of the inlet air resulting in more air entering the cylinder. More fuel can then be injected and burnt, giving increased power.
The after cooler is fitted between the air compressor side of the turbo charger and the air intake manifold on the engine.
In the after cooler, air passes over the outside of the tubes while the engine cooling water or sea water passes through the tubes usually in the opposite direction (contra flow). Fin plates are attached to the outside of the tubes to increase the surface area for the air, thereby giving a better transfer of heat.
Maintenance
·      Sea water flowing through the tubes will tend to leave deposits in less time that if fresh water was used. The end covers can be removed and a wire brush pushed and pulled through the tubes. If the scale is not removed by the brush, the tube nest will have to be chemically cleaned.
·      On the air side, usually no maintenance is required if the air cleaner is doing its job and the filter is changed regularly.
·      A leaking tube will cause the cooling water to pass into the air side. Depending on the design, the air may enter at the bottom and leave at the top to prevent water carrying over with the air. A drain cock is fitted at the bottom.
·      As the air passes through the after cooler, its temperature may be reduced until it is below the saturation temperature. Heavy condensation of water vapour may then follow, this water being carried into the engine. If this is a problem, a water separator can be mounted between the after cooler and the air inlet manifold.

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