© RubaiyatMilitary Institute of Science & Technology, ME Dept.

Fuel Metering CI Engine

© Rubaiyat

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

© Rubaiyat

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

© Rubaiyat

Common Rail System

Military Institute of Science & Technology, ME Dept.

© Rubaiyat

Common rail injector

Military Institute of Science & Technology, ME Dept.

© RubaiyatMilitary Institute of Science & Technology, ME Dept.

© RubaiyatMilitary Institute of Science & Technology, ME Dept.

© Rubaiyat

Pumping element

Military Institute of Science & Technology, ME Dept.

© Rubaiyat

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.

© Rubaiyat

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.

© Rubaiyat

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

© Rubaiyat

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.

© Rubaiyat

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

© Rubaiyat

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

© Rubaiyat

CI Engine Nozzles Classification:

Military Institute of Science & Technology, ME Dept.

Closed Nozzle

CI Engine Nozzle

© Rubaiyat

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.

© Rubaiyat

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.

© Rubaiyat

Closed Nozzle

Military Institute of Science & Technology, ME Dept.

© Rubaiyat

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.

© Rubaiyat

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.