From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The complexities of fuel injection systems, including the effects of pump speed on fuel line pressure.
| Identifier | ExFiles\Box 179b\3\ img107 | |
| Date | 1st February 1933 | |
| -17- In practice, the case is not so simple as this. There is a lag between the arrival of the pressure wave at the injector and the lifting of the needle valve. The pump chamber may not fill equally well at all speeds, the unloading valve (or other delivery valve) may cause the initial fuel line pressure to vary with speed and may actually produce a partial vacuum in the line. Some or all of these effects may be operating in the R.R. pump but nevertheless the pump lag curves show an increase with decrease of speed and to some extent a stepped form. Some of the effects of pump speed have been considered above. Another of considerable importance is the increase in fuel line pressure for a given control position with speed. As already shown, at speeds below 400 R.P.M. the pressure in the fuel line is not sufficient to ensure continuous injection in the R.R. pump i.e. it cannot exceed 3000 lbs/sq.in. If the speed is raised to 2000 R.P.M. calculation suggests that the line pressure will be of the order of 16,000 lbs/sq.in. Apart from the mechanical strain imposed upon the injection system by these high pressures, the delivery is reduced due to the compressibility of fuel effect and the portion of injection taking place after the spill ports open is prolonged or in other words the injection period is increased. The fuel line pressure for a given speed is also affected to a considerable extent by the volume of oil under pressure. Thus the addition of a fuel line 50 cms. long, 2 mms. bore, to the R.R. pump will (again by calculation) reduce the fuel line pressure at 2000 R.P.M. from 16,000 lbs/sq.in. to 12,000 lbs/sq.in. Reduction of the volume of oil under compression is therefore not necessarily an unalloyed blessing. Sufficient has been said to indicate the general nature of the problem and the difficulties that have to be faced in designing an injection system to operate over a wide speed range. With the advent of two stroke engines this speed range will be further increased and a satisfactory injection system can be developed only by careful design based on experience on the pump testing rig. | ||