Saturday, 14 July 2012

Overview of primitive and existing systems

           

Overview of primitive and existing systems

       Achieving the highest efficiency from a modern diesel engine depends on attaining a perfect combustion process in the cylinder each time injector fires. Various concepts have emerged with time to overcome the deficiencies of older systems and to improve upon them. Many of them have been successfully put to use. 

1.  Air- injection system:
 

The relevant fuel valve is mechanically operated. These valves consist essentially of three parts, namely a casing, a needle or mushroom valve, and an atomizer. The casing houses the needle the valve and atomizer, and forms a receptacle for the fuel charge and injection air; the valve, which is operated by a cam through a rocking lever, admits the fuel into the cylinder at a correct time; while the atomizer restricts the flow of fuel and breaks it up into small particles. The casing has two passage opening into it, the smaller from the fuel pump leading to the lower part of the casing immediately above the atomizer, and the other from the injection air bottle admits the injection air at the upper part of the casing. At the points where the fuel and injection air enter the casing small non return valves are usually provided, their object being to prevent injection air which has already passed into the fuel valve from entering the fuel pipe connecting the fuel pump and the fuel valve and causing air –lock, and also to prevent fuel from entering the injection air-pipe.
   
As a safeguard against pressures sufficient to cause damage occurring in the fuel valves, a bursting disc is provided on each valve. The functions of the atomizer are: ·         To break up the fuel into small particles

·         To cause the small particles of the fuel and injection air to mix together.

·         To regulate the rate at which the fuel enters the cylinder when the valve opens.
 

                  The fuel pump delivers exact amount of fuel, necessary to develop the required power, into the fuel valve casing every cycle, where it awaits the opening of the valve. When the valve opens, the whole of the fuel in the casing is forced into the cylinder by the injection air, the pressure of which is greater than the prevailing pressure in the cylinder. Therefore when the valve closes the casing contains only the injection air at 1000 lbs/sq in, and when the next charge of fuel is delivered into the casing it will lie on top of the uppermost atomizing ring with compressed air above and below it. Thus when the valve opens, air below the fuel charge escapes into the cylinder, and the resulting sudden reduction in pressure will cause the air above the fuel charge to begin moving towards the cylinder at a high velocity.
                                             
                  As the only way for the air to reach the cylinder is through the slots or holes, as the case may be, in the atomizing rings, it chops the fuel up in small pieces, as it were, and carries the “pieces” with it into the cylinder.
 
                       The operation of atomizing and injecting the fuel charge is comparatively gradual. The arrangement of the injector valve housing containing the fuel atomizer can clearly be seen.   
                    The fuel valve begins to open and admission of the mixture of fuel and injection air commences, irrespective of whether the engine is two stroke or four stroke type, when the crank is from 1-5 degree before the end of compression stroke, the actual point depending upon the rotational speed of the engine and the shape of the fuel cam toe-piece employed to open the valve. The object of starting injection slightly before the piston reaches the end of the combustion stroke is to ensure that the combustion process has begun immediately the piston begins its working stroke. If the combustion process is delayed until after the piston begins to move on its working stroke the thermal efficiency will be reduced. The fuel valve remains open only long enough to ensure the whole of the fuel charge being injected. If the valve remains open longer than this no harm will result, but the consumption of injection air will be much greater than it need be, and overall efficiency of the plant will be reduced, since it is not possible to regain the whole of the work expanded in compressing the air. If, on the other hand, the period the fuel valve remains open is insufficient, a little of the fuel charge will be left in the valve, each cycle, and eventually the engine will stop owing to unatomised fuel being injected into the cylinder. Such a state of affairs will result in any engine if the lift of the fuel valve is very much less than is ought to be, or if the obstruction to the flow of fuel through the atomizer is excessive.

                                     If the injection air pressure is lower than the cylinder pressure, fuel will not be injected into the cylinder and some of the hot compressed air into the cylinder will flow into the fuel valve, when the valve opens and may ignite the fuel therein, with grave consequences. Should the fuel ignite dangerous pressures may result in the fuel valve, since by the time the fuel has started to burn, the fuel valve will have closed and the burning of fuel will take place in a confined space. The injection air pressure must, therefore, never be less than about 200 lbs/sq greater than the compression pressure in the cylinder. 

                                      In all air injection engines the fuel valve closes when the crank is around 40 degree beyond the TDC, which gives the period of opening in the neighborhood of 45 degree, but the maximum lift of the valve varies according to the size of cylinder.

 1.  Airless injection system:
    
                        

                                  Small land-type engines have operated on this system of fuel injection for great many years, but it was not until about 1930 that it was universally adopted for large marine engines.
  
                                                                                                                                   
                                                                                                                              Prior to this the air injection engine could burn a wide range of distilled fuel and achieve a higher break mean pressure in the cylinders, even though its mechanical efficiency was lower by some 8%, due to the power required to drive the air compressor. This superiority over the airless injection system resulted from two factors, the first being the supercharging effect of the injection air which increased the available oxygen by about 5%, and secondly the action of the air in promoting turbulence, and consequently better mixing, of the atomized fuel and air.
     
Technical improvements in the fuel pumps and fuel valves of airless injection systems have eliminated the advantages possessed by the air-injection system, and has shown the former system to be no less superior on all grades of fuel commonly used.

Now that it is usual to employ the heaviest grade of boiler fuel, which must be heated to obtain the desired degree of atomization, the absence of injection air is an advantage, because in expanding down from 1000-5000 psi the resulting refrigerant effect would tend to cool the fuel as it entered the cylinder.

Airless injection system consists of two broad categories:

a)    Common rail injection system

b)   Jerk type injection system


A. Common rail injection system:

It has one or more high pressure multiple plunger fuel pumps. Fuel is discharged in to a manifold or rail, which is maintained at a very high pressure. Sufficient volume capacity is provided in the high-pressure pipeline by pump and accumulator bottle, which serves to decrease the pressure fluctuation at quick load changes. The metering and timing of fuel injection is controlled by mechanically operated fuel valves in the earlier types of engine and by the timing valves in the later type of engines like P&J type of Doxford engine. This timing valve times and meters the fuel injection in the combustion chamber. The valve is operated by camshaft the duration of opening of timing valve and hence the period of injection of fuel in the cylinder regulates the quantity of fuel admission.

B. Jerk type injection system:

It can be broadly classified into two categories

I.      Port controlled

II.    Valve controlled
                                                                                                                       
             
·        Port controlled fuel pump ( BOSCH  type)

Undoubtedly the most popular system used on board today consists of the BOSCH type Reciprocating pump of the kind shown in the Fig. feeding fuel  into high-pressure pipeline, which leads directly to the fuel injector v/v.

                             Essentially the pumping element is a robust sleeve or barrel, which envelops a close fitting plunger. In its implicit form the barrel has a supply part at one side and a spill port at the other side. The fuel cam through a roller follower actuates the plunger. As soon as the rising plunger covers the supply and spill ports, the fuel is pressurized and displaced through the delivery valves towards the injectors. A sharp pressure wave is generated which runs through the high pressure piping to the injector causing the injector valve to open and inject. As soon as a relieved area on the plunger uncovers the spill port, the injection ceases. The relief on the plunger has a helical control edge so that rotation of the plunger by means of the control rod varies the lift of the plunger during which the spill port is closed and therefore the fuel quantity injected and the load carried by the engine.
                              
                                                                                                                       
In the figure shown above it can be seen how some special modifications to the conventional BOSCH type pump makes the unit more reliable.

This injection pump contains the following features:
·         The mono element design is a rigid and distortion-free solution even at high injection pressures.

·         A constant pressure relief valve eliminates the risk of cavitation erosion by maintaining a residual pressure, which is on a safe level over the whole operating field.

·         A drained and sealed-off compartment between the pump and the tappet prevents leakage fuel from mixing with lubricating oil.

·         Precaliberated pumps are interchangeable.

This design however suffers from a number of deficiencies:

§  The pressure pulses can easily runback and forth between the pump and injector several times before the injector is actually forced to open and inject. Thus, it becomes very much essential for the operator to ensure that the system is in excellent condition and to ensure that the fuel is properly treated and free of dirt. If, for example, nozzle spray holes are partially blocked by extraneous elements or carbon particles, the pressure wave may not be sufficiently reduced within the system. This eventually results in destruction of fuel pump cam or other vital parts of the injection system upon the next stroke.

§  When the spill port opens and a pressure upto 1600 bar is released, cavitation and/or erosion is likely to occur, affecting both the housing directly opposite the port, and the plunger land which is still exposed to the port at the instant of release.

§  There is also the effect of dilution due to pressure, the need for lubrication, and the need to prevent fuel from migrating into spaces where it could mingle with crankcase oil.




·        Valve controlled fuel pump:
                                                                                               
     Fig. (1.4) Shows a simple schematic diagram of operational and the fuel metering device of a valve control type fuel pump which has both end control of fuel injection. Normally these pumps are built together in pair. The pump barrel is completely filled with fuel under positive pressure during the downward stroke of the plunger. The pressure of the supply fuel oil opening the suction valve as the plunger performs its return stroke and later part of stroke commences filling of the pump barrel by push rod lifting the suction valve further. The injection of fuel begins only after the plunger has advanced further in its stroke when the suction valve is seated. The injection of fuel begins only after the plunger has advanced further in its stroke when the suction is seated. The delivery is continued till it is terminated by the spill valve lifted mechanically near the end of the plunger stroke. The time of opening of spill valve or closing of suction valve depends on the engine load and is variable by regulating mechanism. Delivery takes place through the spring loaded discharge valve. Fine adjustment of opening or closing of spill or suction valve is done by adjusting screw fitted on push rod. Therefor both end control of fuel injection is achieved with this type of pump very easily. Fuel pumps are provided with pneumatic safety mechanism that shuts of the fuel to the engine if the speed of the crankshaft exceeds the permitted maximum. To protect the fuel pump housing against excess pressure there is a safety valve provided which opens at a preset pressure of 900 bar.
                                                                                                                                      

     The safety valve is always enclosed in housing so that in case of the valve opening the fuel is drained to fuel oil leakage tank. When HFO is being used, the pre-heated oil must be circulated before the engine is started.  When the engine is being reversed the camshaft and thus all the fuel cams are turned relative to the crankshaft in such a way that fuel injection takes place at the correct timing for both ahead and astern operation.


 Comparison of the two systems


#

 

Common rail injection

 

Jerk type injection

1
All the pumps deliver to a common pipe called common rail or manifold. The system pressure at around 400-550 bar.
Pumps deliver to individual cylinder, fuel injector or injectors. Pump discharge pressure can go upto 1300 bar even.
2
Since it involves only common manifold, so any leakage or damage to this pipe will lead to engine stoppage till the defect is rectified.
Its respective pump drives each cylinder unit so any defect or failure only leads to particular unit suspension and other units would be working normally.
3
Required power for driving pumps is normal and its around 2% of the total power developed in the engine.
Required power for driving pumps is around 6% of power developed in the engine.
4
Eccentric runs drive pump assembly and hence it is very smooth and noiseless.
This is very noisy due to sudden pressure rise caused by the stiff cam profile on the pump plunger.
5
If a single pump in the system malfunctions, then that can be cut down from the system and the capacity of the other pump can be increased to adjust the defective pump output so that the overall output of the engine remains same.
Since pumps deliver to individual units, any malfunctioning of a pump cuts off supply to that particular unit resulting in engine power loss.
6
Cams on the timing valve assembly(as in Doxford Engine) are symmetrical in relation to the overall engine output remains same.
Ahead and astern running are timed differently. The firing order is different.
7
Pressure in the common rail is relatively lower; hence is less efficient.
High pressure ensures greater penetration and better atomization.
8
Runs with least stress in the gearing system and consumes less power from the engine.
Imparts more stresses on the gearing system; hence consumes more power from engine.
9
Has valve related problem (timing valve); hence more maintenance is required.
Fuel pump has only are discharged valve; hence maintenance requirement and related problems is less.