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Fuel Injection in the CI Engine
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:
Meter the appropriate quantity of fuel, as demanded by the speed of, and the load on, the engine at the given time.
Distribute the metered fuel equally among cylinders in a multi-cylinder engine.
Inject the fuel at the correct time (with respect to crank angle) in the cycle.
Inject the fuel at the correct rate (per unit time or crank angle degree).
Inject the fuel with the correct spray pattern and sufficient atomization as demanded by the design of the combustion chamber, to provide proper penetration also.
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.
Pumping elements to transfer the fuel from the tank to the cylinder, along with the associate piping and hardware.
Metering elements to measure and supply the fuel at the rate as desired by the speed and load conditions prevailing.
Metering controls to adjust the rate of the metering elements for changes in load and speed of the engine.
Distributing elements to divide the metered fuel equally among the cylinders in a multi cylinder engine.
Timing controls to adjust the start and stop of injection.
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 emissions.
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 cylinders.
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
air injection which had become obsolete but now some interest has been shown by researchers (however very high pressure is required for air) and
solid (or airless) injection systems.
The airless, mechanical, or solid injection systems consist of three types.
Individual pump system: This consists of a separate metering and compression pump for each cylinder.
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).
Common rail system: A single pump for compressing the fuel, plus a metering element for each cylinder.
Spray Structure
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
Fuel Filters
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,
primary stage (a metal- edge filter to remove coarse particles, larger than 25 microns).
A secondary stage (a replaceable cloth, paper or lint element to remove fine particles from about 4 to 25 microns) and
3. Final stage (a sealed, non-replaceable element) to remove fine particles that escaped the secondary stage.
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.