scavenge fires

8/4/2019 Scavenge Fires

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Santillan, JessielorenSantillan, Rexie

Santos, John DanielSayson, Gerardo Jr.


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A basic part of thecycle of an internalcombustion engineis the supply of fresh

air and removal ofexhaust gases. Thisis the gas exchangeprocess. Scavengingis the removal ofexhaust gases byblowing in fresh air.

Fig. Uni-flowScavenging


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Charging is the filling of the engine cylinderwith a supply or charge of fresh air ready forcompression. With supercharging a largemass of air is supplied to the cylinder by

blowing it in under pressure. Efficientscavenging is essential to ensure a sufficientsupply of fresh air for combustion.


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A build-up of this flammablemixture presents a dangeras a blow past of hot gasesfrom the cylinder mayignite the mixture, andcause a scavenge fire. Aloss of engine power willresult, with high exhausttemperatures at the

affected cylinders. Theaffected turbo-chargersmay surge and sparks willbe seen at the scavengedrains.


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Cylinder oil can collect in the scavenge spaceof an engine. Unburned fuel and carbon mayalso be blown into the scavenge space as aresult of defective piston rings, faulty timing,

a defective injector, etc.


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Indications of a scavenge fire are: loss in power and

irregular running of the engine,

high exhaust temperatures of correspondingunits,

high local temperature in scavenge trunk,

surging of turbocharger, and

sparks and smoke emitted from scavengedrains.


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External indications will be given by:

a smoky exhaust and

the discharge of sooty smuts or carbonparticles.


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If a scavenge fire starts two immediateobjectives arise-

they are to contain the fire within thescavenge space of the engine and

to prevent or minimize damage to theengine.


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1. Once a fire is detected the engine should be: slowed down,

fuel shut off from the affected cylinders and

cylinder lubrication increased.


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2. All the scavenge drains should be closed. Asmall fire will quickly burn out, but wherethe fire persists the engine must be stopped.

3. A fire extinguishing medium should then beinjected through the fittings provided in thescavenge trunking. On no account shouldthe trunking be opened up.


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To avoid scavenge fires occurring: the engine timing and equipment

maintenance should be correctly carried out.

scavenge drains should be regularly blownand any oil discharges investigated at thefirst opportunity..


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The scavenge trunking should be regularlyinspected and cleaned if necessary. Wherecarbon or oil build up is found in thescavenge, its source should be detected and

the fault remedied.


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It is one of the most important parts ofa marine diesel engine.

A fuel injection system does the work ofproviding the right amount of fuel to theengine cylinder at the right moment.

It is also extremely important that the fuelinjected inside the engine enters the cylinder

at the right combustion situation for thehighest combustion efficiency.


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Fuel injection is donewith the help ofcams andcamshaft. The speed

of the cam shaft issame as the enginespeed in a twostroke engine and

half the enginespeed in a fourstroke engine.


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As you know the function of a fuelinjection system is to deliver fuel to theengine cylinders under specific conditions:

at a high pressure, at the proper time,

in the proper quantities,

and properly atomized.


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Fuel Injection System

Solid Injection Type

Jerk Pump System

Helix Type

Injection Pump

Valve

Controlled

Injection Pump

Common

Rail System

Air Injection

Type


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The operating principle of most air injectionsystems has been similar to the one first usedby Rudolph Diesel. Fuel oil was metered anddelivered by a pump to the atomizer, which

was in communication with a high pressureair storage tank supplied by an aircompressor, and injection occurred when theinjector valve was opened by a cam actuatedmechanism. High pressure air then flowedinto the engine cylinder carrying along withthe metered fuel as a finely atomized spray.


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One of the first satisfactory airless injectionsystem was applied to an engine in 1920 byJames McKechnie of Vickers. Ltd. Of England.As shown in the figure, oil was delivered by a

metering pump to a spring loaded plunger,which was raised by a cam. Tripping of thecam allowed the fuel to be injected into theengine cylinder as the spring returned the

plunger to its bottom position.


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The Mckechnie Solid InjectionSystem


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The Jerk pump system consists of individualfuel injectors for each cylinder. This meansthat the injection of each cylinder is exclusiveof each other. The injector pump is operated

once every cycle using the cam and camshaft.


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In order to ensure that the camshaft and theinjection runs simultaneously to deliverperfect timing of fuel injection, the barrel andplunger of the injector pumps are selected of

appropriated size to suit the engine fuelrequirements. The fuel delivery is facilitatedwith the help of ports in the barrel and slotsin the plunger or adjustable spill valves.


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All the injector valves are preset to a specificpressure. The needle of the valve lifts exactlyto this pre-set pressure, ensuring that thefuel completely atomizes once it enters the

cylinder.


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Valve Control DischargeType

Helix Or Helical EdgePump

The valve controldischarge type pump is

generally found onslow speed two-strokeengines.

The helix type arefound on high or

medium speed fourstroke engine.


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The injector pump is operated by a camwhich drives the plunger up and down. Thetiming of the injection can be altered byraising or lowering the pump plunger in

relation to the cam. The pump has a constantstroke and the amount of fuel delivered isregulated by rotating the pump plungerwhich has a specially arranged helical groove

cut into it.


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The fuel is supplied to the pump throughports or openings. As the plunger movesdown, fuel enters the cylinder. As the plungermoves up, the ports are closed and the fuel is

pressurized and delivered to the injectornozzle at very high pressure. When the edgeof the helix uncovers the spill port pressure islost and fuel delivery to the injector stops.


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A non-return valve on the delivery side of thepump closes to stop fuel oil returning fromthe injector. Fuel will again be drawn in onthe plunger down stroke and the process will

be repeated. The plunger may be rotated inthe cylinder by a rack and pinionarrangement on a sleeve which is keyed tothe plunger.


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This will move the edge up or down to reduceor increase the amount of fuel pumped intothe cylinder. The rack is connected to thethrottle control or governor of the engine.

This type of pump, with minor variations, isused on many four diesel engines.


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In the variable injection timing pump used inMAN B&W engines the governor output shaftis the controlling parameter. Two linkages areactuated by the regulating shaft of the

governor.


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In the common rail system, each cylinderdoesnt have an individual fuel pump but onlyone high-pressure multiple fuel plungerpump for all the cylinders. A manifold or rail

is where the fuel accumulates at highpressure before entering the cylinders. Thiscommon rail supply fuel to all the cylinders.


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A timing valve in provided between the railand the injector to control the timing andextent of fuel delivery. The common rail isalso provided with spill valves to release

excess pressure. The injectors in a commonrail system are often known as fuel valves.


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In 1913 Vickers, Ltd. developed the commonrail system which became very popular . Amulti-plunger pump delivered fuel to anaccumulator and header of large capacity withthe fuel pressure maintained at about 5000 psi

by a relief valve, and the fuel was sprayed intothe engine cylinders through mechanicallyoperated injection nozzles. Fuel metering wascontrolled by varying the period of opening ofthe injection valves. The first American engine

with common rail injection system was built bythe Atlas Imperial Diesel Company of Oakland,California in 1919.


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scavenge fires

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