fuel metering
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© RubaiyatMilitary Institute of Science & Technology, ME Dept.
Fuel Metering CI Engine
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OBJECTIVESOBJECTIVESThe injection system of CI engine should fulfill
following objectives consistently & precisely:• Meter the quantity of fuel demanded by the speed
of, & the load on the engine• Distribute metered fuel equally among the cylinders• Inject fuel at the correct time in the cycle• Inject the fuel at correct rate• Inject fuel with the spray pattern & atomization
demanded by the design of combustion chamber• Begin & end the injection sharply without dribbling
or after-injection
Military Institute of Science & Technology, ME Dept.
© Rubaiyat
With diesel engines, fuel is sprayed directly into the cylinders at the time combustion is intended to occur.
Diesel fuel injection system operates at high pressure- on the order of 100 MPa, for the following reasons:
1. the fuel pressure must be greater than the compression pressure in order to inject the fuel into the cylinder at the time combustion is to commence.
2. the fuel velocity relative to air, needs to be large so the atomized droplets will be small enough for rapid evaporation and ignition.
Military Institute of Science & Technology, ME Dept.
CI INJECTION SYSTEMS
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CI INJECTION SYSTEMS
A. AIR INJECTION: Obsolete
B. MODERN SYSTEMS:
1. Individual Pomp System: A separate metering &
compression pump for each cylinder
2. Distributor System: A single pump for compressing the fuel & a dividing device for distributing the fuel to the cylinder
3. Common Rail System: A single pump for compressing the fuel & a metering element for each cylinder
Military Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
CI INJECTION SYSTEMS
filter filter filter40 psi 40 psi 40 psi
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Common Rail System
Military Institute of Science & Technology, ME Dept.
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Common rail injector
Military Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
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Pumping element
Military Institute of Science & Technology, ME Dept.
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Pumping element
Give a constant start of injection for various loads at constant speed.
Injection will start at the same position of the crankshaft
Duration of injection will be maximum at full load
Hence it has a constant beginning and variable ending of delivery
Military Institute of Science & Technology, ME Dept.
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Pumping element
Start of injection is advanced as the load is increased, while delivery ends at a constant crank angle.
So it has variable beginning and constant ending
Sometimes found at low-compression SI oil engine
Military Institute of Science & Technology, ME Dept.
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Pumping element
it has variable beginning and variable ending
Sometimes specified for small CI automotive engine.
Military Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
Spray Development
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Spray Developmento At low pressure difference single drops are formedo These merge into a stream as the up-stream pressure
is increased o Further increase in pressure causes the stream to
break into sprayo ‘Breakup distance’ decreases- cone angle increases
as pressure difference increases until apex of the cone practically coincides with the orifice
o Heavy fuels required a greater pressure difference to reach this stage than do light fuels
Military Institute of Science & Technology, ME Dept.
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o Two velocities of the spray can be recognized:• The spray-tip velocity• The average individual-particle velocity
o Two type of penetration can be recognized: • Penetration of the spray tip • The average penetration of the individual-
particleso Rate of penetration of the spray tip increases as the
penetration pressure increases although the maximum penetration is virtually independent of the injection pressure
Military Institute of Science & Technology, ME Dept.
Spray Development
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o High injection pressure cause fine atomization o The dispersion of droplets, in any one cross section
of the spray, become more even:
1. As the distance increased from orifice to cross section
2. As the air density is increased
3. As the oil viscosity is decreased
4. As the injection pressure is increased
Military Institute of Science & Technology, ME Dept.
Spray Development
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CI Engine Nozzles Classification:
Military Institute of Science & Technology, ME Dept.
Closed Nozzle
CI Engine Nozzle
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Open Nozzle o The simplest type of nozzleo Disadvantages:
i. High injection pressure encountered at high speeds
ii. Tendency of the nozzle to dribble between injections
o If the pump is driven by the engine & the speed is doubled, the velocity of the incompressible fluid through the orifice will also double since the pump is displacing fuel at double the original rate.
o Velocity of a fluid is proportional to √△pMilitary Institute of Science & Technology, ME Dept.
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Closed Nozzleo The oil is led to a pressure gallery that surrounds an
inclined surface of the plungero When the oil pressure is sufficient to lift the plunger
or needle against resistance of the spring, the full injection pressure is exerted against the orifice
o Two styles of closed nozzles are available:– Single-hole nozzle– Multi-hole nozzle
o Valve Opening Pressure(VOP) & Valve Closing Pressure(VCP) is different
o Values of VCP/VOP varies from 0.6 to 0.9 in commercial nozzle
Military Institute of Science & Technology, ME Dept.
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Closed Nozzle
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Pintle nozzle tends to give:• Hollow conical spray• Cone angle varies from 0-60o depending
upon the taper of the pintle• Maintenance is reduced from carbon
formation on the nozzle face since injection pressure is minimum
• Pintle also promote atomization at the expense of penetration
Military Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
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FUEL METERING
SI ENGINE
Military Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
© RubaiyatMilitary Institute of Science & Technology, ME Dept.
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