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Aviation History
1941
1941 - 0343.PDF
FEBRUARY 6TH, 194*. 117 PETROL INJECTION IN MINIATURE A Continental Conversion SINCE the Germans went over completely to petrolinjection in the engines of their operational militaryaircraft, much thought has been directed towards this method, and its freedom from icing has been held to be a very great advantage over the carburettor. The Con- tinental Series A is a small four-cylinder horizontally opposed air-cooled engine which has adopted petrol injec- tion, and, by courtesy of The Intava World, some particu- lars are given of it hereunder. t Several changes have been made in the 50 h.p. model ;to increase its output to 80 h.p. The compression ratio :has been raised from 5.4 to 7.5 (increasing the b.m.e.p. from 122 to 137 lb./sq. in.) and the r.p.m. raised from€ 00 to 2,700. The 50 h.p. model had fuel of 65 octane cified, but the higher-output engine calls for 80 octane. e bronze valve seats and austenitic steel valves have been -ireplaced by austenitic seats and stellite-faced valves.; It also has been necessary to modify the design of pistons [and rings. "Waffle-iron" finning on the under side of the piston crown increases cooling, and an auxiliary oil throw-off from the connecting-rods improves resistance to Iring- stick ing. • V ' Plunger Motion - Very satisfactory performance is obtained from hydraulic tappets, and they will probably be adopted more exten- sively in the future. While magneto ignition is standard equipment, battery ignition may offer advantages when electric starters are used more generally. The fuel-injection unit was developed in co-operation with the Fuel Injection Corporation, of Muskegon, Mich. Fuel injection prevents ice formation in the induction system without requiring installation of an air heater or manifold jacket. Further, it is claimed that power at height is greater and that there is greater assurance of uniform mixture distribution.under all operating conditions. The device has two constant-stroke plungers having combined reciprocating and turning motion. The latter motion permits each plunger to serve two cylinders. Injec- tion is Into the intake manifold directly ahead of the cylinder. A check valve on the nozzle regulates the dis- charge pressure and also prevents evacuation of the fuel lines at part throttle owing to intake manifold depression. Engine oil directed thro'ugh small passages to each plunger serves not only to lubricate the parts, but also .jeals the plungers against fuel leakage. Any fuel which Jraks past the plunger during the delivery stroke is con- Y ducted to the intake manifold through an annulus and tube. The latter also acts as a return for any excess oil that is trapped in the annulus. Only a very small quantity of fuel and oil is said to leak past the plungers. v " Fuel Control by Restriction Control of fuel delivered at part throttle is accomplished by restricting the inlet to the pump plungers' with a valve that is actuated by the air throttle through a simple linkage. Under part-throttle operating conditions, less than a full- plunger displacement of fuel is required for the proper mix- ture, and restricting the plunger inlet reduces the quantity of fuel to a percentage of the full-stroke volume, depending on the degree of restriction. Throttling of fuel and air is accomplished by a similar action, except that part of the fuel is in the gaseous state after it passes the throttle restriction. Although it might be expected that non-uniformity in fuel distillation range or vapour pressure would produce changes in flow for a given throttle position, experience is reported to have shown that, within the range of fuels used in aircraft engines, the variation from this cause is negligible. Automatic compensation of mixture-strength at height is provided by selection of restriction conditions at the fuel throttle. Flow through the restriction of the fuel throttle depends on the pressure differential, as in any fluid system. There is, however, a reduction in this differential at height because the injector pump does not have the ability to produce the necessary lower absolute pressure on the low- pressure side of the fuel throttle. Compensation at full throttle, as well as at part throttle, is obtained by provid- ing restriction at full throttle and suitably ipcreasing the pump displacement to compensate for this restriction in order to obtain the required full-throttle flow. Future Developments The injector pump is installed at the front of the engine, partly because of the ease of cooling. Additional cooling is obtained by leading the engine air past the injector through a sheet-metal duct, thus helping to prevent forma- tion of vapour ahead of the fuel throttle. A primer is necessary for good starting, because at cranking speeds the injector does not build up sufficient pressure to open the check valve in the nozzle. Spray from the nozzles is exceptionally fine and evenly dispersed due to a whirling action which is imparted to the fuel just before it is dis- charged by-means of offset passages tangential to the final discharge hole. Injection pressure is about 75 lb./sq. in. Some engineers suggest that the use of safety fuels will dictate the adoption of fuel injection, because fuel can be atomised more readily with this system than by conven- tional carburation. Cost will not permit, at least for some time, the wholesale replacement of carburation systems by fuel injection. It has been pointed out, however, that a resulting increase of approximately 10 per cent, in engine selling price would be offset by an accompanying reduction in installation cost owing to the elimination of the intake- air-heater system. Newer refinements in fuel injection may bring appreciable increase in power rating and reduced fuel consumption. Die-cast cylinder heads are held to be a definite possibility for future development. It is quite likely that during the next few years engines with reduced weight-power ratios will be offered, and that the reduction in this ratio will be due to an increase in b.m.e.p. Continental will probably continue to build its direct-drive engines in the present rated speed range of 2,300 to 2,700 r.p.m., as the prospects for obtaining satisfactory airscrew efficiencies at higher speeds without reduction gearing are exceedingly poor. Scottish Airways Time-table 'THERE is a slight alteration to the time-table of Scottish•*• Airways, and Services 4 and 5 have been combined to cover the route Glasgow-Tiree-Benbecula-North Uist-Storno-way, which is now operated on Mondays, Wednesdays, Fridays and Saturdays in each direction. The northbound serviceleaves from Grosvenor Restaurant at 8.45 a.m. and Renfrew Airport at 9.40 a.m., arriving at Stornoway Airport atlz 55 p.m. and the Maritime Buildings at 1.10 p.m. The return journey starts at 1.25 p.m. from the Maritime Buildingsand arrives at the Grosvenor Restaurant at 5.35 p.m. A call can be made at Barra on request if 12 hours' notice is given*nd tide and weather permit. Retreat of Nature IT is reported that three new American factories, atGrumman, North American and General Motors (for Allison engine production), will be built without windows.Artificial lighting and air conditioning will be provided con- stantly so that temperature and humidity will not vary. Thiswill add much to the comfort of workers and will have its effect by increasing the output, but it seems that all suchadvances must be paid for by getting farther and farther away from the natural things of life. If U.S.A. ever has to black-out it will make things much easier as well as saving a lot of money, and will eliminate entirely the danger from shattciedglass.
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