Fuel Injection in the CI Engine

7/30/2019 Fuel Injection in the CI Engine

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Fuel Injection in the CI

EngineFor the compression ignition engine, it is

very important to promote a means ofinjecting fuel into the cylinder at theproper time in the cycle. This is so

because the injection system starts andcontrols the combustion process.


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Objectives of the Injection SystemThe injection system of the compression ignition

engine should fulfil the following objectivesconsistently and precisely:

1. Meter the appropriate quantity of fuel, as demandedby the speed of, and the load on, the engine at thegiven time.

2. Distribute the metered fuel equally among cylindersin a multi-cylinder engine.

3. Inject the fuel at the correct time (with respect tocrank angle) in the cycle.

4. Inject the fuel at the correct rate (per unit time orcrank angle degree).

5. Inject the fuel with the correct spray pattern andsufficient atomization as demanded by the designof the combustion chamber, to provide properpenetration also.

6. Begin and end injection sharply without dribblingor after injection.


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To accomplish these objectives, a number of functionalelements are required. These constitute together, thefuel injection system of the engine. These elements

are as follows.1. Pumping elements to transfer the fuel from the tank to

the cylinder, along with the associate piping andhardware.

2. Metering elements to measure and supply the fuel at

the rate as desired by the speed and load conditionsprevailing.

3. Metering controls to adjust the rate of the meteringelements for changes in load and speed of the engine.

4. Distributing elements to divide the metered fuel

equally among the cylinders in a multi cylinderengine.

5. Timing controls to adjust the start and stop ofinjection.

6. Mixing elements to atomize and distribute the fuel

within the combustion chamber


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Function of fuel injection equipmentThe function of fuel injection equipment is to supply the

engine with fuel in qualities exactly metered inproportion to the power required and timed withutmost accuracy, so that the engine will deliver thatpower within the limits prescribed forfuelconsumption, exhaust smoke, noise and exhaust

emissions.The fuel must be injected through suitable nozzles at

pressures high enough to cause the required degreeof atomization in the combustion chamber and toensure that it mixes with sufficient air for complete

combustion in the cycle time available.In multi cylinder engines the periods of injection, the

timing and the delivered quantity must be accuratelymetered to ensure an even balance between thecylinders.


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For an engine developing 3kW at 60rev/s, ofcylinder capacity 0.2 liter the fuel delivery at fullload would be approximately 10mm3 in 1.2ms,repeating this 30 times every second. At noload the quantity will be reduced toapproximately to 3mm3.

In general terms the injection period and thepressure increase with engine size: small directinjection (DI) engines will have a period about25 degrees crank travel and an injectionpressure exceeding 400barwhilst large enginesmay have periods approximating 40degrees

with pressures in excess of 1000 bar. Enginesrequired to meet future limits ofexhaust NOxemissions will need shorter injection periodswith corresponding higher injection pressures.


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The equipment for a six cylinder medium-sized

high speed turbo charged vehicle engine

developing 110kW at 43.3rev/s will have a fullload delivery of 65mm3 with an injection

period of approximately 26degrees crank

travel. The nozzle will have a total orifice area

of approximately 0.247mm2

(equivalent to fourholes of 0.28mm diameter) and the peak

injection pressure will be about 450 bar. To

meet a NOx emission standard of 10g/kWh the

injection period will have to be reduced toabout 23 degrees crank angle for the same

hole diameter. This will increase the probable

peak line pressure to 650 bar.


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Fuel Injection Systems

There are two main classifications for

fuel-injection systems, namely

1. air injection which had become

obsolete but now some interest has

been shown by researchers (however

very high pressure is required for air)

and

2. solid (or airless) injection systems.


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The airless, mechanical, or solid injection

systems consist of three types.

1. Individual pump system: This consists of aseparate metering and compression pumpfor each cylinder.

2. Distribution system: This consists of asingle pump for compressing the fuel(which may also meter), plus a deliverydevice for distributing the fuel to thecylinders (which may also meter).

3. Common rail system: A single pump forcompressing the fuel, plus a meteringelement for each cylinder.


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pACm fnDf

2

NpACm fnDf

3602


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Spray Structure


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The Sauter Mean Diameter

If Ni is the fraction of droplets counted in

size interval di, then the Sauter Mean

Diameter SMD is given by

1

2

1

3

i

ii

i

ii

dN

dN

SMD


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F l Filt


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Fuel FiltersA low pressure (2.5 bar) transfer pump or fuel feed

pump is required to lift the fuel from the tank, toovercome the pressure drop in the filters, and tocharge the metering or pressuring unit. Three filtersare recommended, namely,

1. A primary stage (a metal- edge filter to removecoarse particles, larger than 25 microns).

2. A secondary stage (a replaceable cloth, paper or lint

element to remove fine particles from about 4 to 25microns) and

3. Final stage (a sealed, non-replaceable element) toremove fine particles that escaped the secondary

stage.


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Quantity of Fuel and the Size of

Nozzle Orifice

The quantity of fuel injected per cycle is dependent

on the power output of the engine. The size of

droplets depend on the velocity which should be

of the order of 400 m/s. As mentioned earlier,this velocity is given by

where h is the pressure difference between

injection and cylinder pressures, measured in

meters of fuel column.

ghCV df 2


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The volume of fuel injected per second, Q, is given by

60

60

3604

2.

if

N

N

VdQ

where d is the diameter of one orifice in m,

Ni is the number of injections per minute, = N/2

for a 4-stroke engine,N is the engine speed in rev/min,

is the duration of injection in crank angle

degrees,

Q is expressed usually in mm3/degree crank

angle/liter cylinder displacement volume

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Fuel Injection in the CI Engine