10.7.
Injection Pumps
For automobile applications, the three most commonly used fuel injection pumps are ;
(i) The in-line pump system, which comprises of a row of cam-actuated pumping elements, one for each cylinder of the engine, above a camshaft. This type of pump is most commonly used in diesel engines for heavy commercial vehicles.
(«) The distributor type pump system serves several of the engine cylinders by one, two or more plungers from which the fuel is directed to each cylinder by a distributor system. This type of pump is used maximum on cars and light commercial vehicles.
(iii) The unit injector is a combined pump and injection unit, and each unit serves one cylinder. The engine camshaft actuates all these. The main advantage of this design is that fuel at the very high pressure is delivered for injection through a very short duct in the injector itself, instead through a long and relatively vulnerable pipe. The unit injection seems to become most widely used system in the future to combat exhaust emissions. It is less costly in terms of hardware than the other systems, but has the disadvantage that it cannot be incorporated in existing engines without major redesign and changes, which require installation of new production equipment.
Another system, currently under development, uses a hydraulic accumulator to serve a common rail of high-pressure supply system to the injectors. To achieve very high injection pressures, a hydraulic pressure multiplier is installed in between the pump and injectors. The time lag between the delivery of fuel from the pump and the supply to the injector, through various devices is expected to be one of the problems in the system.
The functions of the injection pump are threefold as follows, (i) It supplies fuel precisely at the right point in the diesel cycle for the pressure build-up resulting from ignition that occurs at and after TDC. With current emissions regulations, the injection is required to be varied with engine speed and load; otherwise the injection starting time can be fixed.
(ii) Since the chemical energy content of the fuel injected primarily determines the power output, the pump meters the quantity delivered per cycle to match the torque-speed relationship demanded by the engine. Moreover, equal quantity of fuel should be supplied per injection to each cylinder.
(iii) The fuel must have very high pressure energy so that the injector can effectively atomise, mix and evaporate it in the air in the extremely short time interval available in the cycle. However, to achieve a high rate of delivery in the shortest possible time in a high-speed engine, in principle the delivery characteristic should be a rectangular pulse, having an extremely steep rise and termination of pressure with a flat top in between. In practice, however, it is required to be shaped both to meet emissions requirements and to keep noise down to acceptable levels.
