FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1941
1941 - 0569.PDF
MARCH 6TH, 1941. THE CASE FOR THE CARBURETTOR (Continued) injection pump units by an engine-driven supply pump. The delivery pressure of this supply pump, on the German engines, is three or four times as high as the normal de- livery pressure required by the carburettor. Maintaining this high-delivery pressure at great altitudes must involve its own problems. The fuel from the supply pump has to pass through an efficient air separator before it can be fed to the injection pump. No air separator is required by the carburettor. The total weight of the Junkers engine fuel system, excluding the engine-driven supply pump, is approximately 60 lb. The Merlin X carburettor, including Amal valve and fuel pipes, weighs* only 25 lb. Direct- ylfajection equipment for a 24-cylinder engine would be double the weight of the 12-cylinder equipment, but there is no reason why a carburettor for a 24-cylinder engine should be double the weight of a similar 12-cylinde.r type. The Question of Cost It is difficult to estimate the probable cost of a 12-cylinder injection pump, since the standard of work- manship and finish of many of the parts is of necessity so high. The fuel-metering accuracy of the injection pump depends on the accuracy of finish of such parts as the pump barrels and plungers, which are lapped together and are not interchangeable. It is considered that a conserva- tive estimate of cost, both in cash and «nan-hours, would be six to eight times as great as the equivalent carburettor, and this again would be doubled for a 24-cylinder engine. As a matter of interest, it has been found that the Jumo injection equipment comprises some 1,576 parts, of which 327 are different. The Merlin X carburettor, including the Amal valve and fuel pipes, consists of only 433 parts, of which 141 are different. Once more the 24-cylinder engine would double the figure of 1,576 for the injection pump, whereas the corresponding carburettor need not necessarily involve any more parts. Apart from the fact that fuel injection pumps are com- plicated mechanical accessories, the very fact that they are mechanical introduces a potential source of failure, which is not present with a carburettor of the type used on the Merlin engine. Fuel filtration is also of very great importance on petrol injection engines, since the finest particles of grit could easily result in a seized pump plunger. Safety devices are built into the injection pumps to guard against wrecking the complete unit in the event of a plunger seizing, but the result is that the corresponding engine cylinder goes out completely. Another aspect of the mechanical injection pump is that it may possibly limit the rotational speed of the engine. The '; jiungers are lifted by a cam and are returned by a spring, •*the mechanism being very similar to a normal valve tappet. The profile of the cam is such that a very quick lift is obtained, and it is probable that at high speeds the plunger might not follow the cam accurately. In this respect it may be noted that the diving r.p.m. of the Jumo 21 iD. engine is limited to 2,400, whereas the Merlin is cleared for 3 600. Servicing of injection pumps would require skilled per- sonnel and special equipment, and it is unlikely that it could be done "in the field." A photograph of a dismantled Junkers injection pump is attached and, for comparison, a photograph of the Merlin X carburettor components. In addition, there is a photo- graph of the parts of the Junkers air separator, injection pipes and nozzles. The right-hand side of this photograph shows also the Merlin Amal valve and fuel pipes. Conclusions It is probable that some simplification of controls might '«-ult from the use of fuel injection pumps in place of car- ' -irettors, but this simplification seems out of all propor- '••'•'•n to the considerably increased complication of the com- . i '••••te system •-., No greater fuel economy is shown, and it is not con- wkred that production-engine fuel consumptions could be hvid to any closer limits than carburettor engines work to. 120 BO !40 B.M.E.R-LB./IN? 180 Fig. 4. Comparative fuel consumptions of Rolls-Royce Merlin X and Junkers Jumo 21 iD. in M.S. ratio. Consumptions represent actual automatic fuel settings. 0. r CD 900 800 MAXIMUM POWER FORLEVEL FUpHT | —ROLLS-ROYCE MERUN X ABCD.E.--3000R.RM. " ''" JUNKERS JUMP 211D eoosr Se 700^- FGH.JK.L7-240pRPM+47LB/a"B0OSr- M.N.RQ.RS--2400RRM.+47L8/o'B005r-PC«T CNGINEI I I I I 2000 4000 6000 S000 10000 12000 14000 16000 16000 20000ALTITUDE - FEET Fig. 5. Rolls-Royce Merlin X and Jumo 21 iD. power avail- able in level flight at maximum r.p.m. at altitudes. Merlin X capacity, 27 litres ; Jumo 21 iD. capacity, 35 litres. (All cal- culations are in accordance with A.P. 840—Brook's formulae.) In this respect it is i;oted from a maker's technical cata- logue for the Daimler-Benz 601 Series engines, which also employ direct fuel injection, that production-engine econo- mical consumption limits are quoted with a +5 per cent, tolerance. This is a normal tolerance for carburettor engines. It is probable that good distribution of the air to the indi- vidual cylinders of the engine is of more importance in obtaining low fuel consumptions than any slight improve- ment in fuel distribution which might, or might not, be obtained with the injection pump. Injection pumps have no control over the air distribution, and with fully super- charged carburettor engines it is known that the fuel distri- bution is very good. 87 Octane Fuels Used The claim that rather lower-grade fuels can be used with injection engines is not substantiated by the analysis of samples of German fuel, which indicated that it is some- what better than our 87 octane fuel. Other claims often put forward in support of direct petrol injection are immunity from freezing and cutting-out during aerobatics. With suitable heating there is no fundamental reason why carburettors cannot be made immune from freezing, and. cutting-out troubles during aerobatics can be overcome by careful design of the carburettor. The use of induction pipe flame dampers obviates the- risk of fire which used to be associated with carburettors and not with injection pumps. A careful examination of the merits of both carburettors and injection pumps indicates that nothing would be gained, at least on conventional types of engines, by the use of injection pumps, and the carburettor has the advan- tage of simplicity, relatively low cost and ease of manu- facture and maintenance. [The arguments in favour of the retention of the carburettor, and the comparison with the injection equipment, are very ably put by the author, who is a member of the Rolls-Royce tech- nical staff, and we are very glad to publish the article. At the same time, we would point out that the views expressed are by no weans shared universally. No less an authority than M. Massuger, of the French Hispano-Suiza company, has voted in favour of direct injection.—ED.]
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
