Propulsion

Diesel Engine Combustion Process

Cylinder Combustion

Fuel oil is a hydrocarbon consisting of hydrogen and carbon, together with other elements most of which are unwanted.


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;

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.

Atomisation-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 ).

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.

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.

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 atomisation 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.

Atomisation

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 dia.


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 of 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/stiction in the instrument.




Compression curves


Two stroke cycle power card


1- Bottom dead centre

2,8 - scavenge port closed

3 - exhaust port shut-commence of compression

4 - fuel injection

5 - top dead centre

6 - post combustion expansion

7 - exhaust port opens




Four stroke cycle





Shown above are typical power cards for 4 stroke engine. The lower one shows the effect of improving turbocharger efficiency. That is some mechanical effort is made by the charge air pressure lowering fuel consumption. Poor timing can negate this effect



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

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 (90o 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


Fuel injection for future high-speed engines

Diesel engine manufacturers in both propulsion and genset applications are concerned with the development aim of low fuel consumption, reliability, and long service life. Other important issues are low soot, NOx, CO, particle emissions, and good dynamic characteristics; noise levels are also becoming increasingly important.


To achieve these requirements more accurate control is required of the timing, quantity and shape of the fuel injection is required.


Modern design has moved towards the use of electronics to achieve this


Conventional injection systems with mechanical action include inline pumps, unit pumps with long HP fuel lines and injectors. A cam controls the injection pressure and timing, while the fuel volume is determined by the fuel rack position. The need for increased injection pressures in more modern designs means that the variable time lag introduced by distortion of the pipework and compressibility of the fuel cannot be accounted for. Therefore this type of design is losing favour




Unit Injector


A comparison between unit pump and unit injector systems has been made assuming the unit injector drive adopts the typical camshaft/pushrod/rocker arm principle. With the aid of simulation calculations the relative behaviour of the two systems was investigated for a specified mean injection pressure of 1150bar in the injector sac. The time-averaged sac pressure is a determining factor in fuel mixture preparation, whereas the frequently used maximum injection pressure is less meaningful.


The pressure in a unit pump has been found to be lower than in a unit injector, but because of the dynamic pressure increase in the HP fuel line, the same mean injection pressure of 1150 bar is achieved with less stress in the unit pump.


With the unit injector, the maximum sac pressure was 1670 bar-some 60 bar higher than the unit pump. To generate 1150 bar the unit injector needed 3.5kW-some 6% more power. During the ignition delay period, 12.5% of the cycle related amount of fuel was injected by the unit pump as against 9.8% by the unit injector. The former is, therefore, the overall more stiffer system.


Translating the pressure differential at the nozzle orifice and the volume flow into mechanical energy absorbed, the result was a higher efficiency of 28% for the unit pump, compared to 26% for the unit injector.


From the hydraulic aspect, the unit pump offers benefits in that there is no transfer of mechanical force between the pushrod drive to the cylinder head and less space is needed for the fuel injector which gives better design possibilities for inlet and exhaust systems


With conventional systems, the volume of fuel injected is controlled by the fuel rack, and matching the individual cylinders requires the appropriate engineering effort. The effort increases considerably if the injection timing is done mechanically.




Unit pump with solenoid valve control.


The engineering complexity involved in being able to freely select fuel injection and timing can be considerably reduced by using a solenoid valve to effect time-oriented control of fuel quantity. To produce minimum fuel injection quantity, extremely short shift periods must be possible to ensure good engine speed control. Activation of the individual solenoid valves and other prime functions, such as engine speed control and fuel injection limitation , are effected by a microprocessor controlled engine control unit (ECU). Optional adjustment of individual fuel injection calibration and injection timing is thus possible with the injection period being newly specified and realised for each injection phase; individual cylinder cut out control is only a question of the software incorporated in the ECU.


With cam-controlled injection systems, the injection pressure is dependant on the pump speed and the amount of fuel injected. For engines with high meps in the lower speed and low-load ranges, this characteristic is disadvantageous to the atomisation process as the injection pressure drops rapidly. Adjusting the injection timing also influences the in-system pressure build-up, e.g. if timing is advanced, the solenoid valve closes earlier, fuel compression starts at lower injection pressures which, in turn, is detrimental to mixture preparation.


To achieve higher injection pressures extremely steep cam configurations are required. As a result, high torque peaks are induced into the camshaft which involves a compensating amount of engineering effort regarding the dimensioning of the camshaft and the gear train, and may even require a vibration damper.

So while the solenoid valve controlled system has a number of advantages it retains the disadvantages of the conventional systems. In the search for a flexible injection system this system only represents a half step.




Common rail injection system (CRIS)


With the CRIS the HP pump delivers fuel to the rail which is common to all cylinders. Each injector is actuated in sequence by the ECU as a function of the crankshaft angle. The injector opens when energised and closes when deenergised. The amount of fuel per cycle is determined by the time differential and the in-system pressure. The actual in-system pressure is transmitted to the control unit via a pressure sensor and the rail pressure is regulated by the ECU via the actuator in the fuel supply to the HP pump. For rapid load shedding the pressure regulator restricts the HP pump to a maximum of 1330bar, compared to the specified 1200bar


With this system the injector incorporates several functions. The nozzle needle is relieved by a solenoid valve and thus opened by the fuel pressure. The amount of fuel injected during the ignition delay period is regulated by the nozzle opening speed. After the control valve is deenergised as additional hydraulic valve is activated which ensures rapid closure of the needle valve and. Therefore minimum smoke index. With this servo-assisted injector the opening and closing characteristics can be adjusted individually and effected extremely precisely. It is capable of extremely high reaction speeds for controlling minimum fuel quantities during idle operation or pilot injection.


Compared to a conventional system the pumping force is considerably lower, with pressure generation accomplished by a multi-cylinder, radial-piston pump driven by an eccentric cam. Pressure control is realised by restricting the supply flow. Locating the high-pressure pump on the crankcase presents no problems while the fuel injection control cams are deleted from the camshaft which can therefore be dimensioned accordingly. Fuel quantity injected is determined by the ECU and is a function of desired and measured engine speed. The CRIS allows very fast response times in the region of 10ms


In the event of single injector failure the injector is shut off via the shut off valve. This allows the remaining cylinders to be operated in a get you home mode.


The CRIS system offers the best characteristics and costing for future High speed engine injection systems


Low Sulphur Fuels

Sulphur contained in the fuel forms metallic sulphides that coat the internal surfaces of the fuel injection equipment including the fuel pumps and the fuel injectors. These sulphides have low shear resistance and act as EP additives similar to that found in lubrication oils. Extremely low sulphur fuels in use on the automotive transport industry have led to the use of lubricity additives. In the marine environment the reduction in sulphur content has been less dramatic.


Marpol Annex VI(regulation 14) and the creation of Sulphur Emission Control Area means it wil be a requirement to use only fuels with a certain maximum sulphur content. In the addition to the increased cost of these low sulphur fuels it is necessary to factor in the possibility of increased wear and tear on the engine components.


Low sulphur fuels are normally low viscosity oils such as gas oil. Carefull planning has to be done both at the design level ( to ensure sufficient storage capacity) and at the operational and maintenance levels due to the known difficulties in changing over from a heated fuel to a non heated or one with reduced heating capacity.


Fouling

A not uncommon failure of Marine diesel engines is a fouling of turbocharger nozzle rings and blades with compounds containing sodium and vanadium as well as more traditional carbon deposits.
This leads to increased exhaust gas temperatures, loss of plant efficiency and reduced reliability. The degree and type of fouling is dependent on the constituents and amounts of contaminants in the fuel oil.

Corrosion

More specifically high temperature corrosion directly caused by the presence of compounds of sodium and Vanadium at temperatures over 500'C.

Sodium and Vanadium are found in heavy fuels up to 200ppm and 600ppm respectively forming Vanadium Oxides ( chiefly V2O5), Sodium oxidises to Na2O and with sulphur also contained, sulphates NaSO4 which are able to react further with vanadium oxides.

The various compounds that may be formed from these have a wide variety of properties one of the most significant of which is the melting points

At the moment of solidification certain compounds can release molecular oxygen which can attack the metal surface. Oxygen may be re absorbed into the deposition thus forming an oxygen pump which aggressively attacks the surface of the metal during melting/solidification processes at around 530 to 600'C. The iron oxide (or Nickel oxide for Cr-Ni Steels) diffuses into the melting cake. A typical reaction is

Na2O.6V2O5 <-> Na2O.V2O4.5V2O5 + 1/2 O2

The parts of most concern in marine diesels are Exhaust valves, piston crowns as well as components of the turbocharger such as the nozzle ring and blades.

Effects of ratio of Sodium and vanadium in fuel

Composition  Melting Point oC  

V2O5  670  

Na2O.V2O5  682  

2Na2O.V2O5  643  

Na2O.V2O4.5V2O5  535  

5Na2O.V2O4.11V2O5  535  

Na3Fe(SO4)3  543  

Na2SO4  887  

Fe2SO4)3  720 (decomposition)  

From the table above it can be seen that the ratio of Sodium to Vanadium in the compounds greatly influences the melting point and thereby the corrosive and slagging effect.
The danger zone is taken to be
Na/V ratio of 0.08 to 0.45 of which 0.15 to 0.30 is particularly destructive

Effects of Temperature and SO2

The temperature of the components in the diesel engine will decisively influence the temperature at which corrosion takes place. In addition the presence of SO2 causes the formation of sulphates in the melt

SO2 + V2O5 -> SO3 + V2O4

SO3 + Na2O -> Na2SO4

The sodium sulphate cannot exist in the melted Sodium Vanadates and is released to further attack the metal surfaces. The SO3 may combine to form sulphurous deposits stripping protective oxide layers from the metal surfaces.

Bunker quality and the effects of Fuel Conditioning

A look at a general cross section of the fuel oils being supplied around the world reveals that a significant portion contain sodium and vanadium in ratios around that considered to be the most destructive.

Passing the fuel through a purifier was the effect of reducing the Sodium content significantly although there is little effect on the Vanadium content.

I have recently been in correspondence with an engineering manager of a large power generation plant. His concern was that the fuel being supplied to the engines had a water content greater or equal to 0.25%.
Water was being introduced into the fuel at an early stage of its conditioning as a method of washing the sodium from the bunkers being supplied in an attempt to reduce the effects of sodium vanadium corrosion.

Vanadium may be found in lubricating oil.

Alteration of the Na/V ratio

As mentioned careful purification can have a significant effect on the amount of Sodium in the Fuel. However Sodium can be re-introduced into the combustion process in the form of salt water spray laden air of due to leakage of sea water cooled air coolers. It should be noted that where ratios are equivalent corrosion processes were the sodium was already contained in the fuel are significantly higher.

Even when Na/V ratios are out of the danger zone it is possible for pockets or 'banks' of products to build up and be released to form these damaging ratios. Typically Sodium deposits may be found in the scavenge areas and Sodium and Vanadium deposits in the exhaust areas.

Fuel additives

Magnesium salt based additives are available on the market. The effect of these is to increase the melting point of the compounds formed. deposits tend to be loose and easily removed and little corrosion may be evident

Recommendations

An exhaust gas temperature of 530 to 560'C and Na/V ratios of 0.15 to 0.30 are the danger zones. For reasons described it is very difficult to avoid these ratios, however the following recommendations are given which should significantly reduce corrosion and could possibly influence the degree of slagging.

Further Reading:
Mechanisms of High Temperature Corrosion in Turbochargers of Modern Four Stroke Marine Engines:Motoren und Energietechnik GMbh Rostock


Uptake Emission control

Exhaust emissions from marine diesel engines largely comprise nitrogen, oxygen, carbon dioxide and water vapour, with smaller quantities of carbon monoxide, oxides of sulphur and nitrogen, partially reacted and non-combusted hydrocarbons and particulate material. SOx and NOx emissions, together with carbon dioxide, are of special concern as threats to human health and the environment.

Dominating influences in the formation of NOx in the combustion chamber are temperature and the longer the residence time in the high temperature, the more thermal NOx will be created.

Typical emissions from a MAN B&W low speed engine


SOx generation is a function only of the fuel oil sulphur level and is therefore best addressed by burning lower sulphur fuels. Emissions are considered low for a given sulphur level thanks to the high efficiency of large diesel engines.


Emissions of carbon monoxide (CO) , also low for large diesel engines are a function of the air excess ratio, combustion temperature and air/fuel mixture.

During the combustion process a very small part of the hydrocarbons (HC) in the fuel is left unburned: up to 300ppm in large two-stroke engines, depending on the fuel type.


Particulate emissions (typically 0.8 to 1 g/kWh) originate from partly burned fuel, ash content in the fuel and cylinder lubricated oil/dosage; and deposits peeling off in the combustion chamber and exhaust gas system

Emission factors (g/kWh) for marine engines under steady state.


   

Maximum allowable NOx emissions for marine diesel engines



Low speed engines

Medium speed engines  

NOx

18.7

13.8

CO

2.1

1.8

HC

0.5

0.6

S02

>21.0 x Sulphur content of fuel




There are two main approaches to reducing NOx


Primary methods include: reducing the maximum combustion pressure by delayed fuel injection, recirculating the exhaust gas, reducing the amount of scavenge air, injecting water into the combustion chamber or emulsified fuel. And the use of special fuel nozzles.


Reducing the firing pressure via fuel injection retardation readily lowers the peak temperatures and yields lower NOx but also invariably reduces the maximum temperature and leads to higher fuel consumption.


Different fuel valve and nozzle types have a significant impact on NOx generation, as well as on smoke and hydrocarbon emissions, and the intensity of the fuel injection is also influential. The influence on NOx is due to the control by the fuel injection system of the formation and combustion of the fuel/air mixture, the local temperature level and the oxygen concentration in the fuel area.


MAN B&W cites tests with a K90MC engine at 90% load which yielded the following results (NOx/ 15% oxygen):
Standard fuel nozzle 1594ppm
Six hole fuel nozzle 1494ppm
Slide type fuel nozzle 1232ppm


it was verified years ago that water emulsification of the fuel can achieve a significant reduction in NOx emissions with no detrimental effect on engine maintenance costs, MAN B&W Diesel citing long experience with low speed engines in power stations. The influence of water emulsification varies with low speed engine type but generally 1% of water will reduce NOx by 15


A standard engine design allows the addition of some 15% water at full load, says MAN B&W, thanks to the volumetric efficiency of the fuel injection pumps-but does not represent a limit from the combustion point of view. Larger ratios have been tested - up to 50/50 fuel and water- with the same or similar impact on NOx reduction but this would call for engine modifications.


Emulsification is performed before the circulating loop of the fuel system, in a position in the fuel flow to the engine from which there is no return flow. Thus it is the fuel flow that controls the water flow. The water flow could also be controlled by measuring the NOx in the exhaust, should continuous NOx monitoring be required.


Water can also be added to the combustion space through separate nozzles or by stratified segregated injection of water and fuel from the same nozzle (see SWFI). The results are similar but retrofitting emulsifiers is simpler.


Humidifying the scavenge space id another way of introducing water into the combustion zone though not as appealing since too much water can cause damage to the cylinder conditions.


Schematic of exhaust gas recirculation system and water emulsified fuel system


Exhaust gas recirculation (EGR) can be applied to modify the inlet air and reduce NOx emissions, a technique widely used in automotive practice. Some of the exhaust gas after the turboblower is led to the blower inlet via a gas cooler, filter and water catcher.


The effect of EGR on NOx formation is partly due to a reduction in the combustion zone and partly due to the content of water and carbon dioxide in the exhaust gas. These constituents have high specific heats, so reducing the peak combustion temperature which, in turn reduces the generation of NOx.


Kawasaki Heavy Industries on a MAN 5S70MC


Stratified fuel-water injection (SWFI)

The effects of water addition on diesel spray combustion include a thermal effect due to the large latent heat of evaporation and the specific heat of water and a chemical effect due to water gas reaction with free carbon. It is believed that the lowering of the combustion temperature by these effects in the region of combustion contributes to the suppression of NOx generation.


The aim of SFWI is to add a large quantity of water to the fuel spray after ignitability has been ensured by injecting completely pure fuel oil at the start of injection. Water and fuel are injected separately through the same valve.


The hydraulically actuated piston delivers water via the solenoid at a time when fuel oil injection is not taking place.


Delivered water is at a greater pressure then the oil delivery, it pushes back fuel in the passage between the injection pump and the injection valve. By this process, fuel and water are injected into the cylinder during the next injection cycle while retaining stratification in the sequence: Fuel - water- fuel.


During this cycle the rack becomes higher by an amount corresponding to the amount of water injected.


The following points of note came from an in service trial


Selective catalytic reduction (SCR)

SCR can reduce NOx levels by at least 95%. Exhaust gas is mixed with ammonia before passing through a layer of a special catalyst at a temperature between 300 to 400oC. The lower limit is mainly determined by the sulphur content of the fuel: at temperatures below 270oC ammonia and SOx will react and deposit as ammonium sulphate; and at excessively high temperatures the catalyst will be degraded (the limit is around 400-450oC).


NOx is reduced to harmless waste products nitrogen and water vapour. In addition some soot and hydrocarbons in the exhaust are removed by oxidation in the SCR reactor.


Ammonia is stored as a liquid gas under pressure of 5-10bar in a deck mounted storage tank protected to prevent overheating. A computer controlled quantity of evaporated gas is led to the engineroom via a double skinned pipe. A bypass arrangement allows the SCR to be redundant when away from controlled areas.



A flow of air is taken from the scavenge and used to dilute the ammonia in a static mixer. The ammonia concentration is thus below the L.E.L. before it enters the exhaust pipe. The minimum engine load for NOx control with SCR is 20-30% unless more comprehensive temperature control systems are installed. At lower loads the catalyst is by passed.


Ammonia fed to the SCR reactor can be liquid, water free ammonia under pressure, an aqueous ammonia solution at atmospheric pressures or in the form of urea carried as a dry product and dissolved in water before use.


Fuel Pumps

Bosch Scroll pump

The Bosch scroll pump consists of a plunger running in a barrel. The plunger is shaped as per the diagram and is rotated in the barrel by the fuel rack.


Position one-The plunger is travelling down the barrel and the suction and spill ports are uncovered. A charge of oil enters the chamber

Position two-The suction and spill ports are covered and the barrel is travelling up the barrel. Pressure builds up until the fuel valve opens and injection commences

Position three-the spill port is uncovered, pressure above the plunger rapidly drops as the oil spills out. End of injection


It can be seen that by rotating the plunger the bottom edge uncovering the spill port can be moved. In this way the amount of fuel delivered is varied. On this only the end of injection timing is varied. Start of injection is constant. Some adjustm =481>


A standard bosch fuel pump can be fitted with a profiled plunger. The advantage of this is that the combustion process can be controlled to suit load conditions thereby improving efficiency.

Variable beginning and end-Variable Injection Timing (VIT) control


This allows for ideal matching of load to injection timing for various qualities of fuel. The Barrel insert can be moved up and down by action of the Nut. This has the effect of altering the position of the spill port relative to the plunger stroke. Therefore the beginning of injection is altered. The end of injection is varied by its normal way of rotating the plunger.


The Nut, which moves linearly, is controlled by the VIT rack, this is altered- continuously by the engine management.



Pump adjustment-Individual pumps may be adjusted in order to account for wear in the pump itself or the entire range of pumps can be adjusted to suit particular loads or fuel ignition quality. Individual pumps are adjusted by means of the screwed links from the auxiliary rack to the nut, just as the main rack adjustment is carried out. Adjustment of all pumps is simply by movement of the auxiliary fuel rack.


Problems associated with jerk pumps-the main problem with pumps of this type is that sharp edges on the plunger and around the spill port become rounded. As injection commences when the spill port is covered by the plunger, this means that later injection takes place. With the variable injection pump this can be accounted for by lowering the barrel insert and hence the spill port, so that it is covered at the required point. In older type pumps, adjustment required washes and shims to be placed between the plunger foot and cam follower or shims removed from below the pump body in order to lower it and the spill port relative to the plunger. Wear also causes leakage between the plunger and barrel but the only solution is replacement. Original timings must be restored. The period of fuel injection




Typical fuel pressure curve at outlet from pump


A-Pump spill closes (approx. 8o)
B-Fuel injector opens (approx. -4
o)
C-Spill opens (approx. 12
o)
D-Fuel injector closes (approx. 16
o)
E-Reflected pressure wave
F-Period of partial equilibrium i.e. the rate of delivery from the rising plunger in the barrel equals the flow out of the injector, therefore no pressure rise. Instability of the wave form can indicate too low viscosity fuel supplied.
G- Injection period (approx. 20
o)

It can be seen that the maximum pressure generated by the pump is far higher than the opening pressure by the injector ( 650 against 350 Kg/cm2). Engine monitoring equipment can be used to generate this graph allowing diagnosis of the fuel supply equipment. For example, the rate of rise of pressure before the fuel injector first opens indicates wear in the fuel pump.

Period of equilibrium


This is the period between the beginning and end of stroke and can be divided into three periods.

Delivery with no injection- being subject to high pressure the fuel reduces in volume, about 1 %. This causes a loss of effective plunger stroke and hence delays the start of injection. The main factor in this is the length of fuel pipe. The effect must be considered when advancing the fuel cam in relation to engine speed.

Main injection period-This is directly related to the effective stroke of the fuel pump plunger and consequent engine load. The engine speed can alter the resilient pressure fluctuations in the fuel pipe and so alter the fuel delivery curve and cause irregular discharge from the injector.

Secondary injection period-This is referred to as 'dribbling' and is due entirely to the resilient pressure fluctuations in the fuel piping and related to engine speed. The fuel oil passing to the injector has kinetic energy. At end of injection a low pressure wave passes through the fuel closing the needle valve in the injector. The kinetic energy in the fuel is converted to pressure energy and a pressure wave is formed. This can be seen below as the 'reflected pressure wave'. Avoided by fitting short, large diameter rigid fuel lines and having a sharp cut off at the fuel pump or an anti dribble device.


Effects of high speed;

a, start of injection can be delayed 3 to 10o - counteracted by advancing fuel cam by appropriate amount.

b, fuel pressure can be reduced by half maximum desired


Anti-dribble Non-return valve-fitted to fuel pump discharge


Variable rate injection

In an effort to improve the combustion characteristics of the burn period profiled cams have been used which reduce the initial rate /*of delivery smoothing out the process.


Sulzer Type Fuel pump


The Sulzer differs from the Bosch scroll pump in that it operates with a plain plunger, timing being effected by operation of valves.


The cam, which is driven via gears by the crankshaft forces the plunger up the barrel thereby delivering fuel to the injectors during the period that both suction valve and discharge valve is shut.


The eccentric cam which alters the timing of spill is rotated via the fuel rack driven from the governor. The eccentric cam altering the opening and closing of the suction port, may be altered manually or driven off an engine management system to change the beginning of injection.



Common Rail System

When compared to the jerk system the common rail system is said */to be quieter, gives more accurate control of fuel pressure, has no high torque's or sudden loads transmitted to the camshaft.


High pressure fuel (300 bar) I delivered from a crank driven constant output pump to the fuel main, which supplies all the cylinders. The pump drive is chain driven from the crankshaft. The cam operated timing valves control the start and the duration of fuel injection to each cylinder. The pressure can be controlled by air operated relief or spill valves. The air pressure is controlled by a cam operated reducing valve. The excess fuel is spilled from the HP main and passes to the buffer. An overspeed trip collapses fuel pressure to a drain tank.


Modern common rail system. Modern requirements for very precise fuel injection timing and delivery, varying fuel quality and load/speed variations has led large slow speed engine designers to the common rail system.


An electric driven high capacity pump supplies fuel to electric operated solenoid valves. One solenoid is fitted for each fuel valve. By computerised control the requirements can be met




Modern Hydraulically driven pump

Development of the slow speed engine has lead to the 'camshaftless' design. Here the motive force for the fuel pump has changed from mechanical cam and follower to hydraulic.


Hydraulic oil is supplied via either a dedicated supply or more 8*normally common rail system. Accumulators are fitted on the pumps to smooth the motive oil pressure at the pump.

Hydraulic oil is diverted from the system to the pump actuation piston via an electrically controlled solenoid valve. This valve has three positions the middle being neutral.


The Control of the solenoid valve is carried out by the engine management system and is affected by such parameters as engine loading, engine revs, fuel quality and exhaust gas condition fuel oil flows under 8 bar boost pressure through a non-return suction valve and the piston falls to start of stroke position


The solenoid valve may be proportioning in that it may control the flow rate to the power piston thereby changing the rate of fuel injection flow. For example, at lower loads a higher rate on injection may be allowed for. This has the effect of increasing Pmax, gives better heat release and thereby improving fuel economy


Puncture Valve

The puncture valve consists of a piston which communicates with the control air system of the engine. In the event of actuation of the shut-down system, and when 'STOP' is activated, compressed air causes the piston with pin to be pressed downward and 'puncture' the oil flow to the fuel valve. As long as the puncture valve is activated, the fuel oil is returned through a pipe to the pump housing, and no injection takes place. I have also added a few more comments and attached a file to further explain the function(s) of the puncture valve.


MAN B&W reversing: Reversal of the fuel pump follower only takes place while the engine is rotating. If the engine has been stopped from running ahead and started astern, the fuel pump follower will move across as the engine starts to rotate and before fuel is admitted by venting the fuel pump via the "puncture" valve.