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Aviation History
1933
1933 - 1281.PDF
FLIGHT, DECEMBER 21, 1933 Fig. 18 : A Bristol compression-ignition cylinder compared with a " Jupiter " cylinder. considerably higher temperatures and eliminate some of the sludging problems. When one considers the extent to which the steam- cooled engine constructor is asking the aircraft designer to go in respect of bulk, weight, and expense, on leading edge coolers, it would appear that the air-cooled engine maker has every right to ask for a considerably larger capacity and more efficient low drag oil radiator than is at present standard practice. " Fig. 17 shows an attempt along these lines, which has given excellent results from the oil-cooling standpoint. Better oil straining and centrifuging is a logical request to make for high-speed air-cooled engines employing straight mineral oil. Compression-Ignition Engines When reviewing the problems of fuel consumption of aircraft engines, certain aspects of the compression-ignition engine must inevitably present themselves. The Bristol Company have been working on compression- ignition development for a number of years, on behalf and with the assistance of the British Air Ministry, but it is not possible here to go into details of the work that has been accomplished. It is sufficient to say, however, that some promising results have been obtained in England, particularly in regard to the fuel consumption of aircraft engines operating on this cycle ; 0.360 lb./brake horse power /hr. can be obtained consistently at cruising powers with steady running and clean exhaust, and 0.39 lb./brake horse-power/hr. at normal power, but this is not possible with as high a maximum brake mean effective pressure, or anything like as high a power/weight coefficient as with a petrol engine. Some eight years ago the Bristol Company commenced development on a large four-cycle compression-ignition air- cooled single cylinder of 8-|- litre (503 cu. in.) capacity (see Fig. 18). A number of dif ferent cylinders and pistons were developed, and promising results were ultimately obtained from this unit. Altogether over 2,000 hours' research work were carried out on this size of cylinder, with brake mean effective pressures in the neighbourhood of 90 to 100 and a consumption of less than 0.4 lb./brake horse-power/hr. Some three years ago it was de cided that there was no immediate future for a large compression- ignition engine with cylinders of f ig.'•**.,•» Jffuref-quarter front and three-quarter rear view of the Bristol " Phoenix " compression- ignition engine. this capacity, and, in consequence, experimental work was commenced on a cylinder of the " Pegasus " size, i.e., just over 3 litre (183 cu. in.) capacity, embodying in it the results of previous experience. The Bristol " Phoenix " engine, which has been de veloped by the Bristol Company as a result of this later experimental work, is of the nine-cylinder radial air- cooled type, and of the same swept volume as the " Pega sus." This engine has completed 200 hours of bench testing and flying to date, and many thousands of hours of single-cylinder research work. Fig. 19 shows two views of this engine. The flight reports obtained have been far beyond our expectations, and the bogey of Diesel " thump " has been entirely dismissed, the reports stating that the engine was comparable with a normal petrol engine as regards quality of running. It is suggested that the power/weight ratio that has been obtained on the " Phoenix " engine is probably as good as can be expected for a four-cycle compression- ignition engine. I think it may safely be said that a suc cessful air-cooled four-cycle compression-ignition engine will weigh 30 to 40 per cent, more than its petrol-engine counterpart, but will have a cruising consumption 30 per cent, less than a corresponding petrol engine. Throughout their experience, the Bristol Company have adhered to a maximum pressure of 850 lb. /sq. in., and have employed standard Navy fuel oil of 0.870 specific gravity, and of a specification readily obtainable all over the world. Prior to the flight tests on the Bristol " Phoenix " engine, it was thought that this increased weight would be a serious obstacle to the extensive use of compression- ignition engines, until such time as aircraft were regularly operated on longer non-stop stages than at present, but that such engines would give considerable advantage in weight-carrying capacity on long flights in excess of six to eight hours. The flight tests referred to, however, have shown that the power-maintenance factor at altitude, with the compression-ignition engine, falls off much more slowly than that for the petrol engine, to such a degree, in fact, that it is suggested that the four-cycle compression-ignition engine will show an advantage in all-up weight on a flight of four hours' duration, when the cruising range is con sidered at a height of approximately .10,000 ft. Fig. 20, shown by the courtesy of the Westland Com pany, illustrates this point. One of the greatest obstacles to the use of the com pression-ignition engine has been the inability to obtain a high brake mean effective pressure for take-off without increasing the fuf-1 charge to a point resulting in a foul exhaust. Single-cylinder research work has proved that supercharging will assist this problem and will provide a four-cycle compression-ignition engine such as the "Phoenix " with sufficient power for take-off to make the engine a commercial possibility. It is, of course, obvious that the advantages of the com pression-ignition engine, from the aspect of fuel consump tion, are further reduced by every step-up in octane value of standard fuels, while its claims from the aspect of fire risk are also somewhat nullified by the use of a high flash point hydrogenated fuel which may be used with the direct fuel-injection engine. 1291
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