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Aviation History
1948
1948 - 1761.PDF
•f- F LIGHT 487 RUS BOMBARDIER nalysis of the Latest Cirrus Engine : Direct Fuel Injection : Low Specific Consumption : Excellent Power/Weight Ratio IK introducing their new rangeof Cirrus engines, BlackburnAircraft, Ltd., have, very wisely, held to the classic formula of inverted, air-cooled, in-line configuration upon which the Cirrus reputation has been so firmly based. However, a breakaway from former practice is made by the use of direct fuel injection in place of the more conventional carburation system of induction. This development has made possible an increase in power of 25 b.h.p. over the well-known Cirrus Major Series III of almost identical size, but with normal induction. There are three engines in the new range, respectively, the 180 b.h.p., Bombardier 4-cylinder, normally aspirated unit; the 265 b.h.p., Musketeer 6-cylinder, normally aspirated unit; and the 320 b.h.p. Grenadier 6-cylinder supercharged unit. Of these, the Bombardier is the furthest advanced, although work on the Grenadier is also proceeding rapidly; however, the Musketeer is not at present receiving quite the same meed of attention as, so far, the demand for a unit of this power category is less marked than for its smaller and larger brothers. In view of the state, of development, we propose in this article to deal only with the Bombardier. Quite some time ago, the engine division up at Brough began to devote their attention to the subject of direct 'fuel injection, and, having met with some degree of success, were given a development contract by the Ministry of Supply. Rather than produce a single-cylinder test unit, useless for anything other than the establishment of test data, it was decided that a full-scale, 4-cylinder engine should be, produced, for which some useful future might be .foreseen. Official approval of this course was given. Design and Performance Improvements Having a sound design of established reliability already in existence in the Major Series III engine, there seemed little point in going to the extravagance of creating an 'entirely new design. However, the opportunity was taken to effect refinement in details and, at the same time, to change over to English measurement from the erstwhile Cirrus precedent of metric dimensions. Thus it is that although the Bombardier and Major Series III engines are similar in type, size and weight, the new engine is dimen- sioned to the nearest English '' round figure'' equivalent above the earlier engine's metric size. For example, the 120-mm cylinder bore of the Major gives place to a 4.8m (approximately 122 mm) bore in the Bombardier. Strokes remain the same at 5.5m and 140 mm. There are many other points of difference between the two engines, but they are so alike in essentials that the disparity in their power outputs would seem to argue a telling case for the employment of fuel injection. Cer- tainly, the compression ratio has been increased from 6.5 to 7.0:1, but in addition to achieving an increase in take-off power from 155 b.h.p. to 180 b.h.p., a worthwhile reduc- tion has also been made in specific fuel consumptions. The values baldly stated by the performance and consumption curves tell, without embellishment, of an achievement of which those up at Brough may well feel proud. The Bombardier is the first British engine employing direct fuel injection to have passed the Air Registration Board's approval tests. A total of 250 hours' running time under type test conditions was divided as 150 hours for the engine and 100 hours for the airscrew, but, apart from this, something like 750 hours' running has been done on the bench. Flight trials are due to start almost immediately and, for this purpose, the engine will be installed m a Miles Messenger airframe. Elektron magnesium alloy is used extensively in the Bombardier, the crankcase, front cover, wheelcase, valve- carrier plate and valve chest all being sand-cast in this very light material—a major contributory factor toward the high power / weight ratio which characterizes the engine. The crankcase is split horizontally on the crankshaft axis, with waisted studs on each side of the main bearings for retention of the top cover, in addition to through-bolts uniting the meeting flanges. Integral webs carrying the main bearings are pierced with limber holes at the base and with lightening-cum-breathing holes through the main web diaphragms. The main bearings themselves are normal, lead-bronze lined, split steel shells. Although the crankshaft is a conventional enough E.N. 25C/S. 81 forging with hollow journals and crank- pins, it has novel features in its lubrication tracts and thrust bearing. The transference of oil from the inter- mediate main bearings to the crankpins is made via 22 gauge iVin bore copper pipes which, after insertion, are subjected to an internal pressure of 5,000 lb/sq in which causes them to swell and so become locked by expansion in the bores of the main journals and crankpins. This frontal view of the engine complements the side elevation shown on the following page, and well illustrates the compactness of the design.
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