diesel engine and steam boiler

7/29/2019 Diesel Engine and Steam Boiler

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Diesel Engine and Steam Boiler, Fuel Atomization and

Combustion

Diesel Engine Fuel AtomizationThe essence of a diesel engine is the introduction of finely atomized fuel intothe air compressed in the cylinder during the pistons inward stroke in such a

condition that it burns quickly and cleanly. The heat generated by the

compression (425 550oC), which is normally nearly adiabatic, is crucial in

achieving ignition where the vapour of the fuel that burns not the fuel itself, so

the injection system's primary function is to vapourise the fuel in the cylinder,

and thus maximize the fuel/air mixing. To achieve good combustion inside the

cylinder there are essential factors that influences and they are follows:

AtomizationBy breaking up the fuel into fine particles, the fuel will heat up and mix more

readily with the air, as the surface area interface has increased. This is achieved

by a fuel injector where by increasing the fuel pressure, and passing it through

small holes the fuels 'tears' or shears to form small spheres within the

combustion space (fig.1). Smaller diameter holes and/or increased fuel pressure

increases atomization.

Figure 1. Atomization of fuel.


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Penetration

In order that the fuel doesn't burn in one area of the combustion chamber, and

hence cause local oxygen depletion, it must be spread out into the chamber

evenly. The fuel must spread to all parts and not impinge as liquid onto the

cylinder walls. The centrally placed injector gives a more uniform pattern.Increasing the nozzle hole length or decreasing the cylinder air density gives

greater penetration.

Figure 2. Penetration and Turbulence

Turbulence

To achieve good air/fuel mixing, the air in the cylinder should be turbulent. This

is to allow the mobile air to reach all the fuel particles. Turbulence is achieved

by swirl (generated as the air enters the cylinder), and squish (generated as thepiston compresses the air charge). Swirl occurs in the 4 stroke engine by the air

flowing into the cylinder tripping over the inlet valves; and the 2 stroke by thetangential entry slots cut into the cylinder liner (fig.3). Turbulence is normally

fixed by engine design rather than operational variables, although changes in

scavenge pressure will affect turbulence.


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Figure 3. Tangential entry of scavenge air into the cylinder liner through ports

Ignition Delay

This is the time between injection of the fuel and its spontaneous ignition. This

delay can be around 15, so that fuel which starts injection at 18BTDC will not

start to bum until 3BTDC. The delay is mainly caused by the time required for

the fuel to heat up to its auto-ignition temperature (around 350oC); and as such

cylinder conditions and fuel structure are the main variables in determining the

actual delay time. As cylinder conditions are cooler at low loads, delay periods

are greater; also injecting the fuel earlier in the cycle also means cooler cylinder

conditions. A greater proportion of aromatics in the fuel make-up will increase

delay times. If the delay period is too great then the initial combustion of fuel

will be too severe, leading to high rates of pressure rise, and subsequent pistonring and bearing damage. To reduce these effects we should always ensure

scavenge temperatures are sufficient, and avoid using fuels with high CCAI

ratings (Calculated Carbon Aromaticity (aromatics-benzene ring) Index-800

870). For measure of the ignition quality (delay) in distillate fuels are given as

Cetane number (Cetane very easily under compression, and assigned a number

of 100, while alpha-methyl naphthalene was assigned a cetane number of 15)

High number the better ignition quality; i.e. shorter time between injection and

ignition.
http://en.wikipedia.org/wiki/Cetanehttp://en.wikipedia.org/wiki/Cetane


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Figure 4. Typical Draw card diagram (crank angle based).

Further there are other factor that influences the combustion is the:

Viscosity is the resistance of a fluid to shear. To reduce pumping effort and

achieve correct injection, the fuel must be at the correct viscosity, and asviscosity is dependent upon temperature, and is often used as a controlling

factor. Usual injection viscosity 10 - 15 cSt.

Density is the measure of the oils mass per unit volume and as a rough guide

the higher the density the lower the quality of oil supplied. This indirectly

influences the performance of the combustion as it is a vital component in thecentrifuge process. An upper limit of 0.991 g/ml is still valid for the operation

of many centrifuges; hence oils above this level will not be cleaned sufficiently

for motor powered vessels.

Conradson Carbon is a measure of the carbon residue from a fuel when burnt

without air. A high value indicates an increased fouling tendency of a fuel,

especially following ineffective combustion.

Asphaltenes are high mass hydrocarbon structures which contain a high carbon

to hydrogen ratio, and entrap water, ashes and other impurities. Due to their

complex chemical structure, asphaltenes are slow burning and hence need to be

in the combustion process for longer. High asphaltenes contribute towardsincompatibility of fuels.


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Designing diesel engines to produce their maximum power requires careful

matching of the combustion chamber geometry. A large number of successful

designs have been developed that involve very different approaches. Diesel

engines can be divided into direct injection (DI) engines and indirect injection(IDI) engines. Direct injection engines have the fuel injected directly into the

main chamber above the piston (fig.5).

Figure 5. Direct injection Combustion system

The wide flat chambers are associated with high pressure injection systems and

the deeper bowls are used with high swirl. Direct injection engines dependprimarily on the kinetic energy of the fuel spray to mix the air and fuel. This

dependence increases the importance of the fuel injection system for optimizing

the combustion system in DI engines. Increased air swirl can enhance the fuel-

air mixing. Large bore engines use higher injection pressure and low swirl or

quiescent combustion chambers. Successful design of DI combustion systems

requires optimum matching of the number of holes, the air swirl level, and the

fuel injection pressure.

Indirect injection engines have the fuel injected into a separate chamber (fig.6)that is connected to the main chamber by one or more small passageways.

Many different types of combustion chambers have been developed for IDI

engines but the most successful recent designs have used some variation of the

Ricardo Comet swirl chamber design (fig.6 (a)). During compression, air is

forced at high velocity, from the main chamber, through the narrow connecting

passage, and into the swirl chamber, orprechamber. As this air enters the

prechamber, the chamber shape turns the flow and induces a strong swirl in the

chamber. Fuel is injected into the swirling flow and ignited after a brief ignition

delay. The fuel-air ratio in the swirl chamber is relatively rich, because only

about half of the trapped air is present in the swirl chamber.


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Figure 6. Cross-sections of IDI combustion system and a pintle nozzle

As the pressure in the swirl chamber rises because of combustion and the

pressure in the main chamber falls because of piston motion, the burning gases

expand into the main chamber where the CO and unburned hydrocarbons burn

with the remaining air. Indirect injection engines have been popular for light-

duty diesel applications because of their lower NOx emissions, wider speedrange, and quieter operation. However, the high flow velocities in the swirl

chamber and connecting passage produce greater heat transfer losses for IDI

engines than for DI engines. These engines generally require compression ratios

greater than 20:1 for reliable starting and acceptable fuel economy. Most IDI

engines still require some form of starting aid such as a glow plug located in theswirl chamber. Because of the high swirl rates in the prechamber, IDI engines

can achieve the fuel/air mixing rates required for high-quality combustion with

low pressure fuel injection. Most IDI engines use relatively inexpensive

distributor-type fuel injection pumps and either single hole or pintle-typeinjectors (Fig 6(c).

The injection pump, injectors, and the air induction system must be properly

matched to the chamber design.


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Fuel Injectors.

The injectors function is to atomize the high pressure fuel oil, and act as a non-

return valve. In order to act as a non-return valve, a spring is incorporated to

close the injector needle when the fuel pump spills the high pressure fuel. But a

quick needle opening is required to avoid low fuel pressure at the injectornozzle at the start of injection. The most common way of achieving these

conflicting requirements is the differential needle valve principle where the high

fuel pressure gives a quick snap opening to the injector.

/4 (D2-d

2) Effective area just before opening.

/4 D2

Effective area just after opening.


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Figure 7. Fuel injector diagram

The hydraulically operated fuel injector (fig.7) is fitted to inject the fuel in small

droplets in a diesel engine. The general design is similar for most engines and

consists of a spring loaded non-return needle valve operated hydraulically by a

fuel pressure wave from the fuel pump to discharge fuel at high pressure

through an atomizer nozzle. It consists of a valve body or nozzle holder to

which the nozzle or atomizer is secured by a retaining nut. The valve bodycontains the spring and its compression nut, with an intermediate spindle if


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required. Surfaces between the body and atomizer are ground and lapped to

form an oil pressure-tight seal. A dowel ensures alignment of the oil passages.

The needle valve is lapped into the bore of the atomizer and these must be kept

as a matched unit. There are two chambers in the nozzle, the upper one being

charged with fuel oil from the fuel pump and sealed by the needle valve whenclosed. The lower chamber, or sac, is sealed by the mitre seat of the needle

valve and of small atomizer holes. Injector spring compression is adjusted under

test and a compression ring fitted. It is set to allow the needle valve to lift or

open at a predetermined fuel pressure. The intermediate spindle conveys the

spring compression to the needle valve and may be arranged to limit its lift. The

valve will open when the pressure from the fuel pump acting on the shoulder of

the needle valve overcomes the spring compression. As the needle valve lifts,

oil flows to the lower chamber allowing fuel at high pressure to pass through the

atomizer holes into the combustion chamber. When the fuel pump cuts off

pressure, the valve will close under spring compression. Since the full area of

the needle is now exposed to pressure, closing will occur at a pressure lower

than that at which it opened. The action of the needle valve must be rapid and

positive with no oil leakage. Valves should be primed if the engine has been out

of service or during preparation for commencement of a voyage.

Present two-stroke engine injectors are of uncooled type unlike the previous

type of injectors where it was cooled by water. This eliminates the problem ofoil contaminating the cooling water. Each injector is also fitted with a spring-

loaded circulating valve (fig.8) which permits hot fuel at the circulating pumppressure to pass through passages in the valve body before returning to a buffertank in the oil system. This maintains the valve at the correct temperature at all

times, allowing the engine to be manoeuvred on heavy fuel. The high pressure

(around 15 bar) wave of the engine fuel pump immediately depresses the spring,ensuring that the circulating passages are sealed off before the high pressure

lifts the needle valve and injects fuel into the cylinder. The ideal position for a

fuel injector is in the centre of the cylinder cover, allowing a symmetrical,conical spray pattern in the combustion chamber. This is achieved, in most four-

stroke engines. In large engines with a centrally placed exhaust valve, theinjectors are placed symmetrically around the cover.

With the injector's position in a high temperature region of the cylinder cover, it

can be prone to fault. Once the injector becomes faulty, this quickly reduces

engine efficiency as the fuel is not achieving the optimum atomization and

penetration.


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Fuel Pumps.

The engine driven fuel pumps have three functions:-

1) Produce the high pressure required for injection, normally over 700

bar, with higher pressures for lower grade fuels.

2) Inject the fuel at, the correct cycle position, the most commonsequence is for fixed start and variable finish, but variable start is

more commonly in use for greater efficiency.

3) Control the quantity of fuel delivered, and hence vary the engine

power. The pumps must also be able to cease delivery whilst the

engine is running i.e. for emergency stop requirements.

The most common type of pump is the jerk pump and the main variants are port

controlled or valve controlled.

Port Controlled:-

Commonly used by medium speed engines and by some slow speed engines.

Helical springs are fitted to return the plunger on its down stroke and to

maintain contact of follower on the cam. The plunger has a characteristic helix

machined on it (Fig.9).Normally the start of injection is fixed, and occurs when

the top of the plunger covers the spill ports. At this point is moving at the

correct velocity to give a rapid raise in fuel pressure. The variable end of

injection is dependent upon the effective length of the helix (effective stroke)

which is altered by rotating the plunger and when the curved surface of the helix

uncovers the suction port and allows fuel pressure above the plunger to fall tothe suction pressure through a vertical slot or hole. A rack is fitted to the pump

to engage with a pinion machined on the outside of a sleeve. The sleeve fits

over the plunger and has slots engaging with keys. In this way the plunger maybe rotated by movements of the rack. Oil supply to the pump suction is by

means of a continuously operating supply or surcharge pump which causes

flooding of the fuel pump chamber as soon as the suction port is uncovered bythe plunger. In some pumps, a non-return spring-loaded discharge valve is

fitted. This is arranged to reduce pressure on its discharge side as it closes,

ensuring positive seating of the fuel injector needle and reducing cavitationwithin the pump. These surfaces are a close fit, the small clearance allowing

some leakage to lubricate the plunger. Larger clearances are necessary for the

higher temperatures when burning heavy fuel.


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Figure 9. Port Controlled fuel pump and timing control


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Valve Controlled:-

This pump is commonly used on the Sulzer engines, and consists of blind

plunger with fuel delivery controlled by means of two valves. The valves are

operated by push rods and controlled by rotating the eccentrics (fig.9). The

suction valve controls the start of delivery in that upward movement of theplunger closes the so that the pressure starts to rise and the injection begins. The

eccentric on suction valve is operated by the variable injection timing control.

The spill valve controls the point at which delivery end, and this is activated by

the upward motion of the plunger. The eccentric of the spill valve is operated by

the governor, and by which fuel regulated.


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Figure 10. Valve Controlled fuel pump

VARIABLE INJECTION TIMING (VIT):-

Changing the start of fuel injection to an earlier or advanced angle will reduce

fuel consumption. This is because earlier injection will raise the cylinder

pressures and temperatures which increase cycle frequency. This advancing of

the fuel timing would be carried out on the valve controlled units by rotating the

eccentric controlling the suction valve on the Sulzer series, and by vertically

moving the barrel (fig.11) on the M.A.N B&W MC series engine or byconfiguration of the plunger control helix (fig.12) enables ignition pressure to

be maintained over a fairly wide load range, thus achieving a favourable fuel

consumption, not only on full load but also in the range of maximum use, i.e., at85 per cent output. The amount that the fuel timing can be advanced is limited

by:

1) Max. pressure in the cylinder. At the high loads putting the fuel into

early would mean excessive max.pressures.

2) Combustion pressure rise. Similar to above and high pressure rises aredetrimental to piston rings and bearing loading.


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3) Uneven running. This would occur at low loads if the fuel is injected too

early, combustion becomes explosive rather than gradual, and the engine

becomes noisy and hard to start/idle.

Figure 11. MAN B&W VIT fuel injection pump.

Figure 12. Plunger configured for variable start of injection.


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Injector Faults.

1) Mechanical defects such as broken spring (fatigue corrosion) will allow

injector to open early, but delivered fuel is not correctly atomized. The fuel

will also dribble into cylinder late in the cycle increasing smoke and

afterburning.

2) Leaking needle valve, caused by poor fuel filtration repeated shock loading

of needle onto seat. This also gives fuel dribble which leads to carbon

trumpets and distorted spray pattern. Again smoke will be seen in the exhaust

plume.

3) Poor fuel valve cooling also produces carbon trumpets. Check cooling system

on that cylinder for defects, or fuel injector/ cylinder head seating faces; if

separate fuel valve cooling then this would be the main cause.

4) Leaking at internal mating faces. This would produce leakage from the spill

port and reduce the quantity of fuel delivered.

So how do we know an injector is faulty in service. Assuming we have an

engine with the facility to measure all parameters.

A) Cylinder exhaust temperatures varies from average.

B) Check on cylinder pressure shows it is lower than average.

C) Exhaust gases become more smoky, indicating unburnt fuel.

Pump Overhaul would be Carried out:-

1) Renewal of pump seals, thus external leakage occurs, or camshaft crankcase

oil becomes contaminated.2) Sticking plunger rotation. This must be free to operate otherwise the fuel

cut-out function may not stop the engine.

3) High internal plunger leakage. This would affect the rate of pressure rise,limit the maximum pressure delivered, and reduce the quantity of fuel

delivered. Thus fuel would be slower burning and injection later.

4)Delivery valves defects. Broken spring or worn valve will affect the quantityof fuel delivered, and thus would influence cylinder imbalance.


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Steam Boiler Fuel Atomization and CombustionCombustion in a boiler is the controlled generation of useful heat by the rapid

chemical combination of oxygen with the combustible elements of a fuel. In aboiler, ignition is provided as the fuel and air are brought together so that there

is a progressive burning of fuel and a flow of combustion products to thechimney. The mixing of air & fuel is carried out by the burner unit in a boiler.Furnace Explosions result from the ignition of unburnt fuel and oxygen after

they have accumulated in the boiler and not in the burner itself as could be

expected. Before the accumulation can explode, air & fuel must be present incertain ratios. When the ratio of the explosion has been reached an ignition

source is necessary, e.g. hot metal, hot refractory, hot gases trying to fire the

boiler without proper purging. Explosion is the uncontrolled, extremely rapid,chemical combination of a fuel with oxygen. It results in a rapid expansion of

combustion gases which leads to a rapid increase of furnace pressure causingblow back of combustion outside the air register or bursting the air register or

the weakest section in the boiler furnace.

The burner unit or air register (fig.13) consists of an atomizer assembly, ignition

rods for initiating a fire by providing an electric arc between the rods, baffles

and swirler or diffuser vanes to swirl the air to give good mixing of the air and

fuel with a damper to control the air flow. An inspection port to observe the

flame visually and a flame detecting device (flame eye) to prevent fuel being

sprayed into the furnace by cutting off the fuel supply to the burner unit in the

event of failure of the fuel not burning in the furnace.


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Figure 13. Air register.

The burner unit operates automatically, and a simple automatic combustion

system (Fig.14) suitable for many auxiliary boilers. The burner has a spring

loaded valve which closes off the fuel to the atomizer when the fuel pressure islow, and when the pressure is high the valve opens and combustion is re-

established. The solenoid valves are two way so that fuel can be deliveredthrough either of the two outlets. The spill valves are spring loaded and when

one is in operation it provides the only path for the fuel to return to the suction

side of the pump. The pressure in the system will be forced to build up to the

spill valve setting. Combustion air is supplied by a constant speed fan and a

damper arrangement controls the air flow. The system operates as follows:

First the fuel is heated by an electrical heater where its thermostatically. At

this point the fuel pump and forced draught fan automatically start up and air

from the fan purges the furnace free from any oil vapour which may be present

while the oil circulates through the system through the circulating solenoid

valve until the oil temperature in the system attains the desired working

temperature. When the oil in the system reaches the wording temperature, Theoil circulating solenoid valve changes position and the oil now flows through


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the low flame spill valve the ignition arc comes on and the oil pressure in burner

builds up and opens the piston valve. The atomized fuel is ignited and once the

flame is established, control of the oil valve and fan damper depends upon the

steam pressure. With low steam pressure the oil valve is actuated to deliver oil

through the high flame spill valve and when the steam pressure rises, the fuel isswitched back to the low flame spill valve. The fan damper is operated

simultaneously to adjust the air flow to suit the high or low flame conditions.

The solenoids are controlled by a pressure switch operated by the steam

pressure.

One type of burner atomizer nozzle is the pressure jet nozzle (fig.14) which is

extensively used on most auxiliary boilers. The fuel is supplied to the burner at

high pressure (10 bar), where it emerges through tangential slots, which gives a

swirl to the oil before exiting the burner tip via the nozzle hole, here the heated

oil is passed through the atomizer orifice where it is atomized; this process

breaks it up into a fine spray of droplets, so presenting a very large surface area

of oil to the combustion processes and mixes with the swirling air. To adjust the

quantity of fuel admitted, the fuel pressure is altered. But low fuel pressure also

causes poorer atomization, and this limits fuel pressure variation obtainable.

The nozzle has to be maintained by opening and cleaning the filters and

checking the inner passages and the orifice for on restrictions. The orifice

become oversize after prolonged operation , and if this is not changed will cause

the oil droplet form to be bigger thereby give rise to poor combustion and

impinging and burning on the refractory causing failure of the material.

The combustion stage itself commence, and in a boiler furnace a type of

combustion often referred to as a suspended flame' is used. For this a stream of

oil particles and air enters the combustion zone at the same rate at which the

products of combustion leave it. The actual flame front therefore remains

stationary, while the particles pass through it, undergoing the combustion

process as they do so. The combustion zone itself can be subdivided into two

main stages; these are referred to as the primary and secondary flames (fig.15).


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Figure 13. Elementary automatic combustion system


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Figure 14. Pressure Jet Nozzle.

Primary Flame

For the oil to bum, it must be raised lo its ignition temperature, wherecontinuous vaporization of the oil required for its combustion takes place. Note

this temperature should not be confused with the flash point temperature of the

oil, where only the vapour formed above the oil in storage tanks, etc. will burn.

The ignition or burning temperature should normally be at least some 20Cabove this value. This ignition temperature cannot be obtained in the fuel oil

heaters, and therefore the heat radiated from the flame itself is utilized so that,as the cone of atomized oil leaves the burner, the lighter hydrocarbons are

rapidly raised to the required temperature by the heat from the furnace flame;they then vaporize and burn to form the primary flame. The heat from this

primary flame is now used to heat the heavier constituents of the fuel to their

ignition temperature as they, together with the incoming secondary combustionair, pass through the flame. The stability of the combustion process in the

furnace largely depends upon maintaining a stable primary flame and, to ensureit is not overcooled, a refractoryquarl is usually placed around it so as to radiateheat back to the flame. The primary flame should just fill the quarl. If there is

too much clearance excessive amounts of relatively cool secondary air enter the

furnace too little and the heavier oil droplets impinge on the quarl and form

carbon deposit s. Another important factor for the formation of the primary

flame is that it must be supplied with primary air in the correct proportion and at

the right velocity. In the case of air registers using high velocity air streams this

is done by fitting a tip plate which spills the primary air over into a series of

vortices. This ensures good mixing of the air and fuel and, by reducing the


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forward speeds involved, helps to maintain the primary flame within the

refractory quarl.

Figure 15. Combustion in a boiler furnace

Secondary Flame

The larger oil droplets, heated in their passage through the primary flame zone,

then vaporize and begin to burn. This, although a rapid process, is not

instantaneous, and so it is essential that oxygen is supplied steadily and

arranged to mix thoroughly with the burning particles of oil. An essentialfeature for the stability of this suspended secondary flame is that the forward

velocity of the air and oil particles must not exceed the speed of flame

propagation. If it does the flame front moves further out into the furnace and the

primary flame will now bum outside the quarl with resulting instability due to

overcooling. The careful design of the swirl vanes in the air register can be used

to create the required flow patterns in the secondary air stream. The secondary

flame gives heat to the surrounding furnace for the generation of steam.

diesel engine and steam boiler

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