La violencia y sus formas, en el caso del relleno Doña Juana

The DPA fuel injection pump (Fig. 3.37) serves the same purpose as the multi-element type and offers the following advantages:

.

it is smaller, more compact and can be fitted in any position not just horizontal;

.

it is an oil-tight unit, lubricated throughout by fuel oil;

.

only one pumping element is used, regardless of the number of cylinders to be supplied;

.

no ball or roller bearings are required and no highly stressed springs are used;

.

no phasing is required; calibration once set is equal for all cylinders;

.

an automatic advance device can be fitted. In this pump the fuel at lift pump pressure passes through a nylon filter, situated below the inlet union, to the transfer pump. Fuel pressure is increased by the transfer pump, depending on the speed of rotation of the pump and controlled by the regulating valve. The regulating valve maintains a relationship between pump speed and transfer pressure, which at low revolutions is between 0.8 and 1.4 kg/cm2 (11–20 lbs/inch2) increasing to between 4.2 and 7.0 kg/cm2 (60–100 lbs/inch2) at high revolutions. From the transfer pump, fuel flows through a gallery to the metering valve. The metering valve, which is controlled by the governor, meters the fuel passing to the rotor depending on engine requirements. The fuel is now at metering pressure, this being lower than transfer pressure. As the rotor rotates, the inlet ports come into alignment and fuel enters the rotor displacing the plun- gers of the pumping elements outwards until the ports move out of alignment. Further rota- tion brings the outlet ports of the rotor into alignment with one of the outlet ports which are spaced equally around the hydraulic head. At the same time, contact between the plunger rollers and the cam ring lobes forces the

pumping elements inwards. Fuel pressure

between the plungers increases to injection level and fuel is forced along the control gal- lery, through the outlet port to the injector pipe and injector. As the next charge port in the rotor aligns with the metering valve port, the cycle begins again. Figure 3.38 illus- trates this.

The inside of the cam ring has as many equally spaced lobes as there are cylinders in the engine. Each lobe consists of two peaks, the recess between being known as the retraction curve. As the pumping element rollers strike the first peak, injection takes place. On reaching the retraction curve, a sudden drop in pressure occurs and injection stops without fuel dribbling from the injector. Further movement of the rotor brings the rollers into contact with the second peak which maintains residual line pres- sure until the outlet port moves out of

alignment. The cam ring rotates within the pump housing varying the commencement of injection. Movement is controlled by the advance/retard device.

It should be remembered that this type of pump is lubricated by the fuel oil flowing through the pump and if it runs out of fuel at any time, due to a new pump being fitted, any parts of the fuel system being disconnected or the filter elements having been changed, then to prevent damage to the pump occurring the fol- lowing method should be used to bleed the sys- tem of air.

1. Slacken the vent screw on the fuel filter. This may be a banjo union of the ‘leak- off’ pipe. Operate the lift pump until fuel free from air flows from the filter. Tighten the vent screw.

2. Slacken the main feed pipe union nut at the pump end and operate the lift pump until

fuel free from air flows from the union. Tighten the union.

3. Slacken the vent screw of the governor con- trol housing and the vent screw in the hydraulic head. Operate the lift pump until fuel free from air flows from the vent screws. Tighten the hydraulic head vent screw and governor control housing vent screw.

4. Crank the engine by hand one revolution and repeat the operations listed at 3.

5. Slacken all high-pressure injection pipes at the injector end and turn the engine over with the starter until fuel free from air flows from unions. Tighten unions and start engine.

Note: Always ensure that the stop control is in the start position and the throttle is wide open when bleeding the pump.

Figure 3.39 shows one type of injector pump. This is fitted with a mechanical means of gov- erning the maximum and minimum revs of the

engine. The automatic advance operates

when the engine is stationary, enabling injec- tion to take place earlier than normal when starting the engine. This gives easier starting and reduces the amount of smoke being passed through to the atmosphere.

Injectors

The injector can be considered as an automatic valve which performs a number of tasks. It may vary in design but all conform to the following requirements:

.

it ensures that injection occurs at the correct pressure;

.

it breaks up the fuel into very fine droplets in the form of a spray which is of the correct pattern to give thorough mixing of the fuel with the air;

.

it stops injecting immediately the injection pump pressure drops.

Fig. 3.38 Cycle of operation (a) charge phase, (b) injection phase

The nozzle is the main functional part of the injector. It consists of a needle valve and nozzle body machined to fine tolerances and lapped together to form a mated pair. Extreme care must be exercised when handling this component.

Lubrication is achieved by allowing a con- trolled amount of fuel to leak past the needle valve. Provision is made for this back-leakage to return either to the filter or fuel tank, it also circulates heated fuel to help overcome the problem of the fuel freezing in very cold weather.

Adequate cooling of the injector is most important and is catered for by careful design and positioning in the cylinder head. To pre- vent over-heating, it is essential that the cor- rect injector is fitted and that any joint washers are replaced each time the injector is removed (do make sure the old joint washer is removed before fitting a new one). No joint washer is required on those engines fitted with copper sleeves which form part of the injector housing. In addition to the normal copper sealing washer, a corrugated type steel washer is sometimes located between the nozzle and the heat shield. It is essential that the outside edge faces away from the nozzle, otherwise serious overheating of the injector will occur. To prevent the washer turning over whilst fit- ting it should be fed down a long bladed instrument such as a screwdriver or a suitable length of welding rod.

In document CORPORACIÓN UNIVERSITARIA MINUTO DE DIOS PROGRAMA DE TRABAJO SOCIAL (página 49-53)