13-07-2012, 03:04 PM
Internal Combustion Engines
Internal Combustion.ppt (Size: 1.51 MB / Downloads: 139)
Injection System
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.
Difference between the Carburation and Injection
Basically, the purpose of carburetion and fuel-injection is the same viz., preparation of the combustible charge. But in case of carburetion fuel is atomized by processes relying on the air speed greater than fuel speed at the fuel nozzle, whereas, in fuel-injection the fuel speed at the point of delivery is greater than the air speed to atomize the fuel.
In carburetors, air flowing through a venturi picks up fuel from a nozzle located there. The amount of fuel drawn into the engine depends upon the air velocity in the venturi. In a fuel-injection system, the amount of fuel delivered into the air stream going to the engine is controlled by a pump which forces the fuel under pressure.
Objectives of an Injection System
The injection system of the compression ignition engine should fulfill 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.
Fuel Injection Systems
In this system, fuel is forced into the cylinder by means of compressed air. This system is little used nowadays, because it requires a bulky multi-stage air compressor. This causes an increase in engine weight and reduces the brake power output further.
One advantage that is claimed for the air injection system is good mixing of fuel with the air with resultant higher mean effective pressure.
Another is the ability to utilize fuels of high viscosity which are less expensive than those used by the engines with solid injection systems.
These advantages are off-set by the requirement of a multistage compressor thereby making the air-injection system obsolete.
Individual Pump and Nozzle System
In this system, each cylinder is provided with one pump and on injector.
In this arrangement a separate metering and compression pump is provided for each cylinder.
The pump may be placed close to the cylinder as shown in Fig. (a) or they may be arranged in a cluster as shown in Fig. (b).
The high pressure pump plunger is actuated by a cam, and produces the fuel pressure necessary to open the injector valve at the correct time.
The amount of fuel injected depends on the effective stroke of the plunger.