Cylinder Combustion in Marine Diesel Engines
Hydrogen has a higher calorific value than carbon, therefore, more heat may be obtained from fuels containing higher Hydrogen/Carbon ratios.
The lower specific gravity of hydrogen than carbon allows a rough rule of thumb to be; the higher the Specific Gravity, the lower the Calorific Value (and quality) of the fuel. The presence of impurities clouds the issue slightly
For efficient combustion an ignition source and sufficient oxygen need be present to completely oxidise the Hydrogen to water vapour and the carbon to carbon-dioxide.
The combustion is required to occur in a short period of time in an internal combustion engine, there are five essential requirements to ensure this:
1. Correct Air/fuel ratio -There must be sufficient oxygen to burn not only the hydrogen and oxygen present but also any other combustibles, such as sulphur. To be effective and efficient all the fuel must be burnt in the cylinder i.e. all the hydrogen must be burnt to water and all the carbon must be burnt to carbon dioxide. As the time for combustion is short excess air must be supplied to increase the possibility of the fuel being in close proximity to the oxygen molecules. The correct maintenance of the scavenge system including turbocharger suction filters is therefore essential.
2. Atomization -To ensure that the fuel breaks down into its constituent elements as quickly as possible it is atomised, which means it is injected into the cylinder under pressure through a small orifice (high surface area/volume ratio allowing rapid oxidation ).
3. Mixing - Atomised fuel made up of fine droplets does not penetrate well into the cylinder combustion space , mixing with the air is promoted by giving the a swirling motion.
4. Injection Timing - As the fuel burns it creates a pressure wave which acts against the piston.
If the injection is too late, the piston is travelling down the liner. The pressure wave created by ignition moves rapidly down to meet the piston causes excessive shock loading on the top of the crown (this is the characteristic 'Diesel knock' of engines when started from cold).Less power is derived as the correct pressure does not act on the piston during the early stages of the stroke.
If the injection is too early then very high temperatures and high peak pressures can be generated caused by the rapid combustion period occurring when the space available is very small. This can lead to increased engine efficiency but also to overloading of the bearings, particularly the top end bearings.
5. Compression temperature - The diesel engine is a compression ignition engine , this means that the ignition of the fuel is reliant on the temperatures generated by the compression of the combustion air.
The compression ratio is set at the design stage to give the correct temperature. However, loss of compression, say by a leaky exhaust valve or piston rings can lead to a late timing of ignition. A similar effect can occur if the cylinder parts are not kept at the correct temperature
Cylinder mixing:
Combustion chamber pressure curve:
Phase one Ignition delay - Fuel injection does not start immediately the pump plunger begins to lift, there is a delay due to compression of the fuel and expansion of the pipework. Although liquids are often classed as being incompressible, they can be compressed to some extent at the pressures involved. Pipework will expand at these pressures and a certain amount of oil must be delivered in order to take account of these factors. Pump timing can be adjusted to take account of this because the amount remains the same at all engine speeds. When oil pressure reaches a high enough value the injector needle will lift and injection commences.
Ignition lag - The duration of this period is set as a definite period of time, irrespective as to how fast the engine turns, and that period depends upon the chemical structure of the fuel. Basically, the lag period depends upon the number upon the number of molecular bonds which must be broken in order to release atoms of hydrogen and carbon from the fuel molecule. The longer and more complex the molecular chain, the greater will the amount of heat energy required to release the atoms and the longer will be the amount of heat energy required to release the atoms and the longer will be the ignition lag period. Because modern residual fuels result from complex blends of crude oil of many different types, they are complex structures and the ignition quality may be very variable between nominally the same grade of fuel. Formerly the cetane number was used to define ignition quality but cetane is a single element fuel and relating this to the complex nature of residual fuels is not realistic. The general term ignition quality is now used.
Ignition lag is the preparation period of the fuel within the cylinder for spontaneous ignition and beginning of combustion. The physical and chemical processes occurring during this period are characterised by weak absorption and liberation of heat. Thus there is little if any deviation from the compression curve. The length of the lag period depends on the fuels ignition quality and nothing else. The higher the ignition quality, the shorter will be the lag period, and the lower the ignition quality, the longer the lag period.
The constant nature of the lag period has little effect in the marine slow speed engine. For an automobile engine operating at much higher speeds this period is a significant proportion of crank angle. As the revs of the engine increase ignition of the fuel will occur later leading to a possibility of 'pinking', a timing retard is therefore required.
Phase two - Uncontrolled or rapid combustion period over a short period (5 to 10 degrees). Initially considerable heat is given off. This causes violent chemical reactions in the air vapour mix which has built up during the first phase. Between 40 to 70% of available energy is released during this phase
Phase three-Controlled burning period. Characterised by a slower pressure rise at the end of the injection period. The physical and chemical processes occurring during this phase are identical to those in the previous phase. The rate of pressure rise reduces as the piston sweeps down the liner.
The time available for combustion is relatively small with higher soeed short stroke engines, but is greater for slow speed long stroke engines. These can ten burn lower quality fuels with higher carbon content.
Heating of residual fuel:
When burning residual fuel, heating is required in order to reduce the viscosity at the injectors to approximately that of diesel oil. This ensures good atomization and brings the temperature of the.fuel closer to the ignition point.
Heating the fuel helps separate solid and liquid contaminants in tanks and in centrifuges, and allows it to flow readily from the tanks to fuel manifold where the final heating for injection takes place. Fuel lines are provided with booster or surcharge pumps on order to force fuel from the tanks through final heaters to the fuel injection pumps, thus ensuring that oil is always available at the pumps. If oil is heated to high temperature it is essential that it is kept under pressure to prevent gassing up of the HP pumps. Heating requires the fuel pump and injector clearances to be increased.
Atomization:
For good combustion the oil droplet size in the combustion space should be at a minimum, and so have a maximum surface area to volume ratio. This ensures rapid heating and an increase in the percentage of fuel molecules in contact with the combustion air. Droplet size should be about 10mm diameter. However, as the droplet size reduces so it ability to penetrate into the combustion space reduces. This is because the droplet has little mass so has little momentum and will be quickly slowed by friction of the dense combustion air. This will produce poor combustion due to the inefficient mixing with the air.
This size must be balanced with the problems of oversized droplets. This is not only with the surface area to volume ratio, also, large droplets can have too great a penetration, still burning fuel can contact with the liners and cylinder wall causing erosion and burnaway. Unburnt fuel can pass down the liner walls where it can mix with the unburnt cylinder liner oil and accumulate in the scavenge risking a potential fire. On trunk piston engine fuel dilution of the crankcase oil can result.
Effect on oil droplet after injection
High pressure fuel is forced through small holes in the injector tip and this produces a high velocity jet of fuel. Friction between the fuel jet and the compressed air causes the fuel jet to break down into droplets, the size of which depend upon the density od the compressed air and the velocity of the jet. In order to achieve the optimum jet, fuel pressure and hole diameter must be within well defined limits. In general the length/hole ratio should be about 4:1.
Larger droplets may be produced by enlarging the hole or reducing the fuel pressure whilst smaller droplets may be formed by using smaller diameter holes or higher fuel pressure. Slow running results in larger droplets because fuel rail pressure falls as there is a longer period of time for injection to take place. Slow running for short periods is not a problem, for longer period 'slow steaming' nozzles with reduced diameter holes are used. Over a period of time injector nozzles will wear increasing hole diameter and require their replacement.
Power Cards
A power card is a graph of cylinder pressure against time, it was originally drawn using a mechanically driven pen onto graph paper mounted on a drum. The drum was rotated by string, via a cam on the camshaft and pushrod. As the drum rotated the pen mounted on the linkages was pressed up to the paper. For clarity the pen is released once a single cycle has passed otherwise slight fluctuations in power demand could lead to several cycles being superimposed on one another blurring the image.
The indicator is a sensitive piece of equipment which can malfunction and so it must be treated with care. It can only be used effectively on an engine operating below 200 rpm due to the difficulty involved in getting only a single line on the card. In addition the inertia in the drum can lead to delays distorting the shape. For higher speed diesels either peak pressure indicators are used, or sophisticated electronic monitoring equipment is required with oscilloscope type displays. The time base for these is off transducers mounted on the flywheel.
It is important that the indicator is kept well lubricated with a light high quality oil . Prior to mounting the indicator the indicator cock is blown through to ensure it is clear. Compression cards are then first taken to check for errors caused by wear or friction in the instrument.
Compression curves:
Two stroke cycle power card:
Bottom dead centre
1. scavenge port closed
2. exhaust port shut-commence of compression
3. fuel injection
4. top dead centre
5. 7post combustion expansion
6. exhaust port opens
Four stroke cycle:
• 3-4-5 fuel injection and combustion
• 5-6 expansion
• 6-7-8-Exhaust valve open
• 8-9-10 overlap, exhaust remains open whilst air enters
• 10-1 aspiration and exhaust valve closes
Power calculation:
The area swept out by the power stroke will give the power developed by the engine. It should be noted on a four stroke most of the non-power stroke occurs below atmospheric on a naturally aspirated engine and so gives a net loss of power.
Power = p.A.L.n
p - mean average pressure in the cylinder
A-area of piston[m3]
L-stroke [m]
n-revolutions per second
From a power card this is altered to:
Power = area of diagram/length of diagram x Indicator spring constant
By use of an instrument called a Planimeter the area scribed out by the pen could be measured giving the power generated by the cylinder. In addition, through experience, certain problems could be diagnosed by looking at the shape drawn.
Fault diagnosis:
As indicated there are practical difficulties with use of the power indicator instrument on a high speed four stroke engine. Therefore the following is based around the two stroke cycle.
The light spring diagram For this, the spring is replaced with one of much lower spring constant. In this way the operation at the lower pressures, i.e. around bottom dead, may be examined. In particular this gives indication of blocked or restricted scavenge and exhausts. To further clarify, the motive effort for rotating the drum is often by hand so only a small part at the end of the stroke is covered.
Draw card (90 deg. out of phase):
Scavenge port opens at 140 degrees after top dead and closes 140 degrees before top dead.
Early injection:
Early injection can be caused by incorrect fuel timing, broken or wrongly set up fuel injector, incorrect fuel condition, overheating of parts around the combustion space.
Its effect is to increase the maximum cylinder pressure. There will be an increase in combustion efficiency but the increased peak pressure leads to overload of the bearings and shock to pressure parts.
Late injection:
Late injection can be caused by loss of compression, insufficient scavenging, delayed timing, incorrect fuel condition and atomisation, undercooled parts around the combustion space. It results in a condition called diesel knock where the flame front travels rapidly down the liner to strike the receding piston. In addition, leads to afterburning and high exhausts
Afterburning:
Causes loss of power, smoke and high exhaust temperatures. Can lead to damage to exhaust valves and seats as well as piston crowns. Fouled turbocharger and waste heat recovery units. High cylinder temperatures causes problems with lubrication
Leaking fuel injector:
Detected by loss of power, smoky exhaust and high temperatures. A knock can be heard on the fuel supply system. Can lead to after burning
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