Fuel pumps and injectors
Over the years there have been many forms of fuel pump and fuel injection systems. Present trends have settled, almost without exception, one the ‘jerk pump’ method, and by far the greatest proportion use pumps with the well known helix form of fuel regulation.
The previous section described why the fuel must be raised to the correct pressure and then accurately injected into the cylinder at a precise point. To achieve this, the oil could be pressurised into an accumulator and then released by controlling the opening of the injector (perhaps electronically, or through the timing valves of the common rail method). Alternatively, the oil can be rapidly raised in pressure over a very few degrees of crank angle, and the pressure thus developed can be used to operate the fuel injector. This, in essence, is the jerk system. Such a method creates enormous, and almost instantaneous, forces throughout the system. Consider the cam that generates the pumping effect. For such pressures to be developed quickly, the pumping face of the cam must rise rapidly from the base dwell surface. Then, as the pressure builds up, there will be a substantial back pressure onto this cam face. To carry these high loads without damage to either roller or cam, the cam face must be of adequate area and well lubricated.
The pump itself, in simplistic terms, consists of a ram carried in a matched barrel. There are return springs and racks etc. but the principle of the pump can be understood from these two components. The ram carriers an accurately machined helical groove cut into its flank. The top of the ram is flat (except in some of the more refined units), and as the ram rises up inside the barrel this top face will shut off the oil inlet ports that are machined through the barrel. Any fuel above the ram at this point will be subjected to rapid compression as the roller driving the ram is forced rapidly upwards by the cam. The oil is directed by high pressure piping to the fuel injector. The latter, set to some pre-determined lift value, will open and oil will flow into the combustion space. The end of injection is controlled by the helical groove. As the arm continues to rise, the helix will uncover the ‘spill’ port in the barrel and the pressure will rapidly drop, with the injector automatically closing as the pressure falls off. The quantity of oil delivered will depend upon the rotational position of the helix with respect to the spill port. If the top of the helix is in line with the port, no delivery will take place. Then, as the ram is rotated, the depth to the edge of the helix increases, as does the duration of injection. The start of injection, controlled by the top face of the ram, is therefore always at the same point, unless some form of VIT is fitted to the unit which allows vertical displacement of the barrel. Raising the barrel delays the start of injection; lowering the barrel advances the start of injection. Such fine tuning can be adopted to deal with, and compensate for, changes in the quality of the fuel, and, in particular, variances in the ID characteristics of the fuel.
When the helix uncovers the spill port, pressure release occurs and there is a rapid back flow of fuel. To prevent this flow from evacuating, even partially, the fuel injector and its supply pipe, a non-return valve is fitted at the discharge of the fuel pump. This valve also serves to prevent oil from being drawn back into the pump as the ram returns, under spring pressure, for there will be a period during the return stroke, from the point where the upper face of the ram uncovers it again, when a suction will be developed above the ram (i.e. the reverse of the delivery period). This reduced pressure in the pump body could cause problems, were it not for the non-return valve, for any evacuation of the delivery pipe would mean that it would have to be ‘refilled’ on the next pumping stroke before delivery pressures could be realised again. This would interrupt the timing of fuel injection and is wholly undesirable. However, the closing of the non-return valve, although protecting the pipeline from partial evacuation, could in itself generate another problem. The valve, closing smartly, will halt back flow of oil so suddenly that a reverse wave of oil may develop and, reflecting off the valve, travel back down the pipe to reopen the fuel injector. These shock waves are capable of reopening the injectors, not just once but several times. To prevent this and the problems that would ensue in the power stroke, and combustion in general, the non-return valve is fitted with a small collar just below the mitre seat. This collar, acting like a piston as the valve closes draws a small volume of oil from the pipe line, which is sufficient to reduce the pressure therein so that any reverberating shock waves are of such a low magnitude as to be harmless.
There are many ways of setting the timing of these jerk pumps, with some manufacturers providing special measuring equipment to enable more accurate settings to be achieved. Without these, or any other knowledge of the pump, timing can be monitored by using the ‘spill’ timing procedure. This is achieved by turning the cam shaft until the roller of the pump in question is on the lower dwell of the cam circumference. The HP discharge pipe should then be disconnected (providing the oil supply has been shut off), and the discharge, non-return valve and its spring should be removed, cleaned and stored away carefully. Then a ‘groove’ necked pipe should be fitted to the discharge of the pump. Fuel carefully released to the pump will run through the pump body and out through this goose neck into some receptacle. If the cam is now turned, slowly and carefully, the roller rising up to the pumping flank of the cam will gradually lift the ram inside the barrel until it shuts off the ports and the oil flow will stop. Turning the shaft too quickly would cause the ram to rise and discharge the oil so that it may not be easy to distinguish the point at which the ports are covered. If the procedure is followed carefully, the cessation of oil flow from the goose neck, accompanied by a bubble forming at the pipe end, is clearly discernible. This represents what would be the start of injection under normal running. The angle at which this occurs can be read from the fly wheel or perhaps from the cam shaft protractor. Any adjustments to the timing of the individual pump can be made by raising or lowering the body (respectively retarding the cam shaft will simultaneously adjust all the pumps.
Jerk pumps do not necessarily have to be regulated by helical grooves. Some manufacturers use a plain ram, still operating in the jerk fashion (that is, pumping over a very short period of time), but regulate the start and end of injection be valves built into the suction and delivery sides of the pump. The opening period of these valves is regulated from the engine controls in a similar way to the rotation of the ram (by rack and pinion) in the helix type of pump. As with the helix type of pump the start of injection is always constant with the end of injection controlled by the spill valve, being variable according the load on engine and speed required. As before, differing fuel qualities may require earlier or later injection according to the relevant ID. This can be achieved by the use of an external linkage to the suction valve, the seating of which initiates the start of fuel compression and injection. Thus, VIT is easily achieved by causing this suction valve to seat earlier (for long IDs) or later (for short IDs).
Most of the problems associated with either kind of injection pump are generated by the use of low quality fuels which are both abrasive and corrosive. Wear and tear on the finely machined rams and barrels is the most common area of failure, through scoring and/or seizure, although some valves on the suction sides of pumps have been known to suffer from cavitation damage.
The viscous fuels are usually heated to enable them to be pumped around the system more easily. This hot fuel causes the components of the fuel pump to expand so that they would seize if allowances were not made. When the pumps are cold, therefore, their working clearances are quite large, so the relatively cool diesel oil which may be used for manoeuvring will leak past the rams. Relevant protection should be taken to ensure that this leakage does not contaminate the lubricating oil serving the cam shaft. As the pump is changed over to heavy fuel the temperature should be increased slowly so that the correct working clearances achieved when the oil is at the desire temperature.