The adoption of this two-shaft design , and airless injection through several nozzles , and with intensive turbulent scavenging through large inlet and exhaust ports , enabled the fuel consumption to be reduced from 0.385 lb .sx to 0.350 lb .sx per horse-power hour , and were important features in the early Junkers heavy-oil aero-engines .sx The engine , illustrated in Figs .sx 47 and 48 , is of the vertical type , the cylinder block and gear case are made in a single casting , in order to secure strength and accuracy in the assembly of the cylinders and gearing .sx The engine is shown on the test bed in Fig. 49 , which also illustrates the trial of a strut suspension , which has proved of great advantage in aeroplane work .sx After it had been in service , this engine gave an output of 830 H.P. during a half-hour run , the speed being 1,200 r.p.m. , and the mean pressure 8.3 atm .sx ( 122 lb .sx per sq .sx ) .sx The weight of the engine complete was 930 kg .sx , or 1.1 kg .sx per horse-power .sx It was replaced by a six-cylinder design , the dimensions of which were slightly reduced in proportion , the cylinder diameter being 120 m.m. ( 4 11/16 in .sx ) compared with 140 mm .sx ( 5 ) .sx To obtain as high an output per cylinder as possible , attempts were made to maintain the working volume of the cylinder at a maximum in combination with the most effective scavenging .sx Fig. 50 is an indicator diagram of a two-cycle engine , the cycle of operations shown being the result of the simultaneous movement of both pistons , while Fig. 51 shows the hypothetical diagrams for both ends of the cylinder .sx In a high-speed engine the gases should be fully expanded .sx This can be effected on a two-shaft engine by slightly advancing the crank on the exhaust side , as indicated on the left in Fig. 51 .sx This not only enables the exhaust to be opened earlier , but the scavenging ports to be closed correspondingly later , i.e. , after the exhaust ports have closed .sx The result is that the cylinder is filled with fresh air at scavenging pressure .sx Experimentally , the dimensions of the admission and exhaust passages , the exhaust and admission lead , the angle of the scavenging port , and also the scavenging pressure were altered , systematically , and the effect on the combustion , the fuel consumption , and the maximum output , were measured .sx Fig. 52 shows the results obtained by varying the port angle , and gives the effect of turbulent scavenging on the combustion .sx An increased output of 10 percent .sx was obtained by altering the port angle , as shown in Fig. 53 .sx The correct dimensioning of the port-openings , and adjustment of the air flow permitted the scavenging pressure at full load to be reduced to 0.2 atmosphere without affecting the combustion .sx The difficulty in bringing each minute particle of fuel into contact with the air necessary for its combustion lies in the shortness of the time available .sx The operation of fuel injection and atomisation controlled by the pump , has to be effected in about 1/1000 sec .sx , and is repeated 25 times per second in each cylinder .sx The method of effecting this was as follows :sx The Junkers nozzle injects a solid stream of fuel in the form of a fan , with an apex angle of about 120 .sx This fan is as evenly distributed over the flat combustion chamber as it would be in still air .sx Distribution is much more complete , however , if several nozzles round the periphery of the space are used , and is rendered still more efficient by inclining or crossing the planes of the individual fans .sx An important factor in complete combustion is the degree of atomisation .sx If the individual particles are too large , they burn too slowly ; if they are too fine , the fuel spray is deficient in penetrating power , and does not extend sufficiently far into the space .sx With a given arrangement , the degree of atomization is determined by the size of the nozzle opening ; the smaller the nozzle opening the finer the atomization .sx The injection pressure which would give a short injection time , in conjunction with good atomization , was determined .sx The lay-out of the pump nozzles and connections depends on this pressure .sx The smaller the amount of fuel and the shorter the connections , the more satisfactory are the conditions regarding pressures for a short injection time .sx As the fuel in each cylinder was supplied through four nozzles by two pumps placed as close as possible to the point of delivery , small pumps and short connections could be used .sx This reduced the friction loss and avoided late injection through the nozzles .sx In normal operation , two pumps were used for each cylinder ; neither worked at its full capacity on full load .sx The pressures obtained were not high , and are often exceeded in stationary engines when the pipes are long .sx Developments in the construction of the fuel pump have been the principal factor in making the Diesel aero engine possible .sx It had to correctly supply the smallest quantities of fuel against the maximum pressure .sx There must be no leakage of oil outwards , or of air inwards , and the inertia of the moving parts should be a minimum , since the period of useful stroke only lasted 1/1000 sec .sx , the weight and space occupied by the whole pump should be as small as possible , and it should be easily operated and adjusted .sx These conditions have to make the construction of the few parts used as simple as possible , and to produce them with as great a precision as modern workshop methods would allow .sx The only valves are in the leads to the atomisers , and these are of the simplest form .sx Regulation of the fuel supply is effected by turning the plunger in relation to the liner .sx All the pumps are connected by a rod , and can be adjusted by the pilot .sx If one set of pumps is cut out , the engine can still be run satisfactorily .sx The turbo-scavenging pump employed was entirely new , both as regards design and construction .sx It was necessary to obtain scavenging pressure of about 0.2 atmosphere in one stage .sx The peripheral speed of the rotor had to be about 200 m.m. per second , so that considerable centrifugal force was set up .sx Duralumin was chosen for the purpose .sx The wheel was produced from the solid , a very light and rigid design .sx being thereby secured .sx The output and working conditions obtained with this single-stage blower were satisfactory .sx A coupling , which acted as a slipping clutch , was arranged between the crank and blower shafts , to prevent the acceleration , at starting , and during rapid changes in speed , being too high .sx Suitably designed springs were incorporated in this clutch , no dangerous oscillations could take place between the rotating parts of the engine and the rotor within the working speed range .sx This clutch is incorporated in the driving gear on the crankshaft .sx In the general design of the engine , the 'advantages of the .sx single-piece type of cast housing were fully utilised for counteracting the forces set up during running .sx If each cylinder is first considered , the forces generated by the two pistons are transmitted through the main bearing to the vertical continuous cross wall of the housing .sx Strong eye bolts are provided at each end of the engine , to form attachments for the external supports .sx The cross walls , which run from front to back , provide transverse stiffness in the horizontal plane , and also enable the crankpit , cooling water , scavenging and exhaust spaces , to be made as compact as possible .sx Fig. 54 illustrates one of the crankshafts and its bearings , as well as the transmission gearing .sx The crank-shaft bearings are of the roller type , and have worked well in practice .sx The dynamical stresses on the gearing were also the subject of exhaustive investigation .sx It is realised that it is impossible to keep the wide speed range of an aero engine entirely free from critical speeds .sx An important question was whether the two-shaft system was more dangerous , from the point of view of vibration , than the single shaft .sx It appears , however , that while there is a greater possibility that vibration will be generated , as far as can at present be ascertained , this will not occur to any dangerous extent .sx The essential elements in the balancing arrangements adopted are shown in Fig. 55 .sx The simplest type of vibration , in which the masses of both crankshafts oscillate in the contrary sense to the large mass of the propeller , can easily be investigated .sx In this case , either the fundamental with one node , or a harmonic .sx with two nodes , may occur .sx In addition , only one oscillation of the two crankshafts in opposition is possible .sx This is generated by differences in the torques of the upper and lower shafts , which result from the angular displacement necessary to give the lead .sx As regards the values of the vibratory forces , reliance is now often placed on measurements made on the running engine by means of torsion-meters , and a special device for this purpose was designed by Messrs .sx Junkers in 1925 , in connection with the development of an oscillation damper for aero-engines .sx Owing to the somewhat complicated measurements necessary on the two-shaft engine , it was essential to observe the movement of the upper and lower shafts at both ends simultaneously .sx Figs .sx 49-56 show the arrangement of the torsionmeters used for this purpose .sx The design of this instrument enables oscillations with frequencies up to 15,000 per minute to be registered .sx The pistons are connected by rods of the ordinary type to the two shafts , coupled together by gearing .sx The propeller is supported in a combined thrust and roller bearing .sx The cylinders are provided with inlet ports for the scavenging air at the lower end , and with exhaust ports at the upper end .sx The former are arranged tangentially , as shown in Fig. 53 , so that the fresh air passes upwards with a turbulent motion .sx The single-stage scavenging pump is placed at the rear end of the crankcase , and is driven from the lower crankshaft through a slipping clutch .sx Each cylinder is fitted with four fuel nozzles , through which the fuel is injected in a finely atomized condition direct into the hot central core of the combustion air , without coming in contact with the cool walls .sx Owing to the turbulence , the air carries the fuel in suspension , and distributes it evenly over the combustion chamber .sx The nozzles are fixed to the cylinder so that they can easily be dismantled .sx The nozzle passages are expanded into a cone .sx They can easily be cleaned when the needle has been removed .sx If one nozzle should become choked , the engine will continue to run , as there are four-nozzles per cylinder .sx If the injection .sx to one cylinder entirely fails , the engine can still be run , the vibration not being sufficiently great to prevent its operation .sx The fuel pumps , each of which supplies two nozzles , are arranged directly in front of the admission ports on both sides of the cylinder , as shown in Figs .sx 48-56 .sx The pumps are all regulated from a central position by a single lever .sx The water and lubricating-oil pumps are arranged at the rear end of the crank-case , shown in Fig. 48 .sx Forced lubrication is provided for .sx The big-end bearings receive oil through the crankshaft .sx Great care has been taken to ensure adequate lubrication and cooling of the gearing .sx Fig. 57 shows the engine ready to be mounted in the aeroplane .sx Its maximum output on the first test flight was 650 H.P. , with the fuel pumps working on a reduced stroke , so that it is considered possible that this output will be obtained even at altitudes of 9,900 to 11,600 ft .sx without any shortage of scavenging air .sx The crankshaft speed at 700 H.P. was 1,600 r.p.m. , and the speed of the propeller 1,130 r.p.m. The fuel consumption curves show that the best results are obtained in flight with an engine speed of about 1,500 r.p.m. , the consumption at this speed being from 0.363 lb .sx to 0.374 per horse-power-hour .sx The weight of the engine complete is 1,852 lb .sx The M.A.N. High Speed 4-cycle Diesel Engine is shown in sectional view at Fig. 58 .sx