FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1912
1912 - 0572.PDF
(T^GHT AERO CHALLENGER. THE By G. H Weight and Strength—For simplicity the powers developed have been assumed as proportional to the volumes of mixture supplied, i.e., that the thermal efficiencies are the same—reference to tests 9, 10 and n, Table III, will show that they will not be widely different. In the last three items dealing with weights, it has been assumed that the strength and weights of all parts of the motor could be cut down in the proportion of maximum pressures. This would obviously not be practicable. Carburettors, induction pipes, magneto, plugs, valves, cam-shaft and gears, &c, would be a constant quantity in both cases. The small massic power ratio shown by the low compression engine without forced feed is due to the fact that all parts would have to be sufficiently strong to withstand the maximum explosion pressures at lower piston speeds when the ratio of mixture at atmospheric pressure to the volume swept by the piston is nearer unity (see Table V). Propagation of Flame.—The combustion of mixture is not instantaneous at the moment of ignition ; a fraction of time intervenes from the instant of ignition to the instant of attainment of maximum pressure. The time of combustion is controlled by dilution compression and temperature as well as the rate of compression and the mode of firing. The influence of dilution without previous compression can be seen by Table VII which gives experimental results obtained by Mr. Dugald Clerk with mixtures of Glasgow gas exploded at constant volume in a closed vessel. TABLE VII. Mixtures. Gauge Computed Time of p^JT Tempera- Explosion. J££ hire. I volume gas to 13 volumes air ... 0-28 52 1,916 I „ 11 „ ... 018 63 2,309 I ,» 9 ••• 0-13 69 2,523 1 7 » ••• °'°7 89 3,236 I „ 5 ... 0-05 96 3,484 The results taken from Hiscox, of some experiments made in France to determine the influence of the rate of compression on the time of complete combustion are given in Table VIII. TABLE VIII. Mixture. Time of Explosion. I volume coal gas to 9-4 volumes J air ("1093 cubic ft. mixture) I volume coal gas to6"33 volumes J air ("073 cubic ft. mixture)... | Computed Piston Work Speed. from Diagram. •53 I-I8I 70-8 •40 1-64 85-3 •25 301 105-5 •16 4-55 125-8 •15 5-57 127-2 •09 9-51 289-9 •06 14-1 364-4 Combustion and Piston Speed.—The piston speed in feet per minute is obtained by multiplying the number of strokes per minute by the length of stroke in feet. The time of combustion at high piston speed is of great interest. Professor Callendar made some interesting experiments in 1904 with a small motor cycle engine, 60 bore by 70 stroke. A diagram taken at 2,430 revolutions per minute running light with a weak mixture showed that the suction on the induction stroke was between 7 and 8 lbs., and that the piston had travelled through half the compression stroke before the mixture reached atmospheric pressure. Ignition took place at about nine-tenths of the compression stroke, but the combustion was not complete until the piston was about half way down the explosion stroke, although the times occupied in complete combus tion was only about T|0th of a second. The maximum explosion pressure attained (47 lbs.) was very little more than the pressure due to compression. The explosion did not take place with sufficient rapidity to maintain the pressure in the increasing space created by the rapid movement of the piston, with the curious result that actually during the explosion the pressure was diminishing. Expansion.—Expansion of mixture during combustion lengthens the time required. Compression during combustion diminishes the time required. It will be seen that if advantage is to be taken of the diminished time of combustion during compression, ignition has to take place during the compression stroke, and the increasing rise of pressure due to the explosion will exert an effect in opposition to the rotation of the motor. Too much advance of ignition shows itself in loss of power and with a rich mixture in knocking due to high rate of combustion. Ignition Experiments.—In some ignition experiments carried out by Professor Watson some interesting results were obtained. JUNE 22, 1912. ENGINE. Continued from page 548.) A motor was run at 1,000 r.p.m. with a full mixture and the spark advanced as much as advisable, i.e., when no knock could be distinguished—the i.h.p. was 2*36. The extra air valve of the carburettor was afterwards opened as far as possible and the ignition further advanced to allow for the slower burning of the weak mixture—the i.h.p. at 1,000 r.p.m. was 2-76, an increase of nearly 17 per cent, in power. The actual time of combustion was approximately the same in each case ; -r/27r sec. for a piston speed of 660 f.p.m. practically the whole of the combustion of the rich mixture taking place during expansion, and in the weak mixture the greater portion of combustion taking place during compression. The former attained a maximum pressure of about 105 lbs. per sq. in. when the piston had travelled o-2i of its explosion stroke, whilst the latter showed a maximum of about 151 lbs per sq. in. when the piston had travelled o-o6 of its explosion stroke, the power lost in compressing the mixture during part of the time of combustion being more than retrieved by the higher mean pressure during the explosion stroke. Tinie for Combustion.—A little consideration will show that the quicker the combustion of a given mixture the higher the efficiency because if the ignition has to be advanced to ignite during the compression stroke, the combustion exerts a back pressure on the piston and if the ignition is retarded to fire on dead centre, combustion taken place during expansion necessitating an increased time with consequent decrease in the mean pressure available for useful work. Maximum efficiency would be obtained if combustion was instantaneous upon ignition. The time required for complete combustion governs to a large extent the efficiency of the cycle and this is particularly accentuated with increase in revolution speed and consequently piston speed since they are so intimately interconnected. Piston Velocities.—The term piston-speed represents an average of the varying velocities of the piston in performing its strokes. Actually the velocity of the piston at dead centre is zero. From 0° to 90° of the crank pin (neglecting the effect of obliquity of the connecting rod) the velocity increases with diminishing acceleration per second per second until the maximum velocity is reached at 90°, where the acceleration per second per second is zero. From 900 to 1800 the piston velocity decreases with increasing retardation per second per second until at 1800 the piston velocity is again zero. An average piston speed of 1,260 ft. per minute in a motor of 160 mm. stroke running at 1,200 r.p.m. means that the piston is stationary for an instant of time on dead centres, whilst a maximum velocity of 1,978 ft. per minute is attained at mid-stroke. The piston velocity at any instant can be obtained from the formula V = irLn sin a. where V — velocity in feet per minute, L = stroke in feet, n = revolutions per minute, a = angle of crank-pin. Table IX has been prepared to show the effect of varying piston velocity on the volume swept by the piston during equal increments of time, taking ithe total volume as unity and the angular velocity of the crank-pin as being uniform. The fractions of volume swept have been calculated from the formula versm a - versin j8 = volume. Figures for 0° to 900 only are given, from 90° to 1800 the volumes swept would be simply reversed. ( To be concluded.) ® ® ® ® The Berlin-Vienna Flight. To Hirth is the honour of being the only competitor to com plete the journey between Berlin and Vienna within the time limit which expired on June 12th. Bergmann, who is Lieut. Muller in real life, succeeded in flying on from Breslau, but had to land at Troppau, while Krieger, who had arrived at Breslau, deemed it inadvisable to go on in view of the treacherous weather. Lieut. Blaschke, who left Breslau on the nth inst., only succeeded in getting as far as Gaenserndorff, where he made a bad landing and was compelled to give up. An Aerial Mail in Japan. FLYING on his hydro-aeroplane, Mr. W. B. Atwater, on June 1st, carried a bag of mails as well as a letter from the Mayor of Tokio to the Mayor of Yokohama between these two places, a distance of twenty miles. He intends to better this performance by flying between Japan and Korea. 572
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
