Fuel Injection in the CI
For the compression ignition engine, it is
very important to promote a means of
injecting fuel into the cylinder at the
proper time in the cycle. This is so
because the injection system starts and
controls the combustion process.
Objectives of the Injection System
The injection system of the compression ignition
engine should fulfil the following objectives
consistently and precisely:
1. Meter the appropriate quantity of fuel, as demanded
by the speed of, and the load on, the engine at the
2. Distribute the metered fuel equally among cylinders
in a multi-cylinder engine.
3. Inject the fuel at the correct time (with respect to
crank angle) in the cycle.
4. Inject the fuel at the correct rate (per unit time or
crank angle degree).
5. Inject the fuel with the correct spray pattern and
sufficient atomization as demanded by the design
of the combustion chamber, to provide proper
6. Begin and end injection sharply without dribbling
or after injection.
To accomplish these objectives, a number of functional
elements are required. These constitute together, the
fuel 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 and
2. Metering elements to measure and supply the fuel at
the rate as desired by the speed and load conditions
3. Metering controls to adjust the rate of the metering
elements for changes in load and speed of the engine.
4. Distributing elements to divide the metered fuel
equally among the cylinders in a multi cylinder
5. Timing controls to adjust the start and stop of
6. Mixing elements to atomize and distribute the fuel
within the combustion chamber
Function of fuel injection equipment
The function of fuel injection equipment is to supply the
engine with fuel in qualities exactly metered in
proportion to the power required and timed with
utmost accuracy, so that the engine will deliver that
power within the limits prescribed for fuel
consumption, exhaust smoke, noise and exhaust
The fuel must be injected through suitable nozzles at
pressures high enough to cause the required degree
of atomization in the combustion chamber and to
ensure 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 accurately
metered to ensure an even balance between the
For an engine developing 3kW at 60rev/s, of
cylinder capacity 0.2 liter the fuel delivery at full
load would be approximately 10mm3 in 1.2ms,
repeating this 30 times every second. At no
load the quantity will be reduced to
approximately to 3mm3.
In general terms the injection period and the
pressure increase with engine size: small direct
injection (DI) engines will have a period about
25 degrees crank travel and an injection
pressure exceeding 400bar whilst large engines
may have periods approximating 40degrees
with pressures in excess of 1000 bar. Engines
required to meet future limits of exhaust NOx
emissions will need shorter injection periods
with corresponding higher injection pressures.
The equipment for a six cylinder medium-sized
high speed turbo charged vehicle engine
developing 110kW at 43.3rev/s will have a full
load 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 four
holes 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 to
about 23 degrees crank angle for the same
hole diameter. This will increase the probable
peak line pressure to 650 bar.
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)
2. solid (or airless) injection systems.
The airless, mechanical, or solid injection
systems consist of three types.
1. Individual pump system: This consists of a
separate metering and compression pump
for each cylinder.
2. Distribution system: This consists of a
single pump for compressing the fuel
(which may also meter), plus a delivery
device for distributing the fuel to the
cylinders (which may also meter).
3. Common rail system: A single pump for
compressing the fuel, plus a metering
element for each cylinder.
m f CD An 2 f p
m f CD An 2 f p
The Sauter Mean Diameter
• If ΔNi is the fraction of droplets counted in
size interval Δdi, then the Sauter Mean
Diameter SMD is given by
N d i i
SMD i 1
A low pressure (2.5 bar) transfer pump or fuel feed
pump is required to lift the fuel from the tank, to
overcome the pressure drop in the filters, and to
charge the metering or pressuring unit. Three filters
are recommended, namely,
1. A primary stage (a metal- edge filter to remove
coarse particles, larger than 25 microns).
2. A secondary stage (a replaceable cloth, paper or lint
element to remove fine particles from about 4 to 25
3. Final stage (a sealed, non-replaceable element) to
remove fine particles that escaped the secondary
Quantity of Fuel and the Size of
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
V f Cd 2 gh
where h is the pressure difference between
injection and cylinder pressures, measured in
meters of fuel column.
The volume of fuel injected per second, Q, is given by
2 60 N i
Q d V f
4 360 N 60
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,
θ is the duration of injection in crank angle
Q is expressed usually in mm3/degree crank
angle/liter cylinder displacement volume