The first commercially successful engine operating on the two-stroke cycle was invented by Mr Dugald Clerk, who in 1881 proved the design feasible. As is more or less generally understood, the exhaust gases448 of this engine are discharged from the cylinder5 during the time that the piston is passing the inner dead centre, and the compression, combustion, and expansion of the charge take place in similar manner to that of the four-stroke cycle engine. The exhaust period is usually controlled by the piston overrunning ports in the cylinder at the end of its working stroke, these ports communicating direct with the outer air—the complication of an exhaust valve is thus obviated6; immediately after the escape of the exhaust gases, charging of the cylinder occurs, and the fresh gas may be introduced either through a valve in the cylinder head or through ports situated7 diametrically opposite to the exhaust ports. The continuation of the outward stroke of the piston, after the exhaust ports have been closed, compresses the charge into the combustion chamber8 of the cylinder, and the ignition of the mixture produces a recurrence9 of the working stroke.
Thus, theoretically, is obtained the maximum of energy with the minimum of expenditure10; in practice, however, the scavenging of the power cylinder, a matter of great importance in all internal combustion engines, is often imperfect, owing to the opening of the exhaust ports being of relatively11 short duration; clearing the exhaust gases out of the cylinder is not fully12 accomplished13, and these gases mix with the fresh charge and detract from its efficiency. Similarly, owing to the shorter space of time allowed, the charging of the cylinder with the fresh mixture is not so efficient as in the four-stroke cycle type; the fresh charge is usually compressed slightly in a separate chamber—crank case, independent cylinder, or charging pump, and is delivered to the working cylinder during the beginning of the return449 stroke of the piston, while in engines working on the four-stroke cycle principle a complete stroke is devoted14 to the expulsion of the waste gases of the exhaust, and another full stroke to recharging the cylinder with fresh explosive mixture.
Theoretically the two-stroke and the four-stroke cycle engines possess exactly the same thermal15 efficiency, but actually this is modified by a series of practical conditions which to some extent tend to neutralise the very strong case in favour of the two-stroke cycle engine. The specific capacity of the engine operating on the two-stroke principle is theoretically twice that of one operating on the four-stroke cycle, and consequently, for equal power, the former should require only about half the cylinder volume of the latter; and, owing to the greater superficial area of the smaller cylinder, relatively, the latter should be far more easily cooled than the larger four-stroke cycle cylinder; thus it should be possible to get higher compression pressures, which in turn should result in great economy of working. Also the obtaining of a working impulse in the cylinder for each revolution of the crankshaft should give a great advantage in regularity16 of rotation17—which it undoubtedly18 does—and the elimination19 of the operating gear for the valves, inlet and exhaust, should give greater simplicity20 of design.
In spite of all these theoretical—and some practical—advantages the four-stroke cycle engine was universally adopted for aircraft work; owing to the practical equality of the two principles of operation, so far as thermal efficiency and friction21 losses are concerned, there is no doubt that the simplicity of design (in theory) and high power output to weight ratio (also in theory)450 ought to have given the ‘two-stroke’ a place on the aeroplane. But this engine has to be developed so as to overcome its inherent drawbacks; better scavenging methods have yet to be devised—for this is the principal drawback—before the two-stroke can come to its own as a prime mover for aircraft.
Mr Dugald Clerk’s original two-stroke cycle engine is indicated roughly, as regards principle, by the accompanying diagram, from which it will be seen that the elimination of the ordinary inlet and exhaust valves of the four-stroke type is more than compensated22 by a separate cylinder which, having a piston worked from the connecting-rod of the power cylinder, was used to charging, drawing the mixture from the carburettor past the valve in the top of the charging cylinder, and then forcing it through the connecting pipe into the power cylinder. The inlet valves both on the charging and the power cylinders23 are automatic; when the power piston is near the bottom of its stroke the piston in the charging cylinder is compressing the carburetted air, so that as soon as the pressure within the power cylinder is relieved by the exit of the burnt gases through the exhaust ports the pressure in the charging cylinder causes the valve in the head of the power cylinder to open, and fresh mixture flows into the cylinder, replacing the exhaust gases. After the piston has again covered the exhaust ports the mixture begins to be compressed, thus automatically closing the inlet valve. Ignition occurs near the end of the compression stroke, and the working stroke immediately follows, thus giving an impulse to the crankshaft on every down stroke of the piston. If the scavenging of the cylinder were complete, and the cylinder were to451 receive a full charge of fresh mixture for every stroke, the same mean effective pressure as is obtained with four-stroke cycle engines ought to be realised, and at an equal speed of rotation this engine should give twice the power obtainable from a four-stroke cycle engine of equal dimensions. This result was not achieved, and, with the improvements in construction brought about by experiment up to 1912, the output was found to be only about fifty per cent more than that of a four-stroke cycle engine of the same size, so that, when the charging cylinder is included, this engine has a greater weight per horse-power, while the lowest rate of fuel consumption recorded was 0.68 lb. per horse-power per hour.
Dugald Clerk’s Two-stroke Cycle Engine.
In 1891 Mr Day invented a two-stroke cycle engine which used the crank case as a scavenging chamber, and a very large number of these engines have been built for industrial purposes. The charge of carburetted air is drawn24 through a non-return valve into the crank chamber during the upstroke of the piston, and452 compressed to about 4 lbs. pressure per square inch on the down stroke. When the piston approaches the bottom end of its stroke the upper edge first overruns an exhaust port, and almost immediately after uncovers an inlet port on the opposite side of the cylinder and in communication with the crank chamber; the entering charge, being under pressure, assists in expelling the exhaust gases from the cylinder. On the next upstroke the charge is compressed into the combustion space of the cylinder, a further charge simultaneously25 entering the crank case to be compressed after the ignition for the working stroke. To prevent the incoming charge escaping through the exhaust ports of the cylinder a deflector is formed on the top of the piston, causing the fresh gas to travel in an upward direction, thus avoiding as far as possible escape of the mixture to the atmosphere. From experiments conducted in 1910 by Professor Watson and Mr Fleming it was found that the proportion of fresh gases which escaped unburnt through the exhaust ports diminished with increase of speed; at 600 revolutions per minute about 36 per cent of the fresh charge was lost; at 1,200 revolutions per minute this was reduced to 20 per cent, and at 1,500 revolutions it was still farther reduced to 6 per cent.
So much for the early designs. With regard to engines of this type specially26 constructed for use with aircraft, three designs call for special mention. Messrs A. Gobe and H. Diard, Parisian engineers, produced an eight-cylindered two-stroke cycle engine of rotary27 design, the cylinders being co-axial. Each pair of opposite pistons28 was secured together by a rigid29 connecting rod, connected to a pin on a rotating crankshaft which was mounted eccentrically to the axis30 of rotation453 of the cylinders. The crankshaft carried a pinion31 gearing with an internally toothed wheel on the transmission shaft2 which carried the air-screw. The combustible32 mixture, emanating33 from a common supply pipe, was led through conduits to the front ends of the cylinders, in which the charges were compressed before being transferred to the working spaces through ports in tubular extensions carried by the pistons. These extensions had also exhaust ports, registering with ports in the cylinder which communicated with the outer air, and the extensions slid over depending cylinder heads attached to the crank case by long studs. The pump charge was compressed in one end of each cylinder, and the pump spaces each delivered into their corresponding adjacent combustion spaces. The charges entered the pump spaces during the suction period through passages which communicated with a central stationary34 supply passage at one end of the crank case, communication being cut off when the inlet orifice to the passage passed out of register with the port in the stationary member. The exhaust ports at the outer end of the combustion space opened just before and closed a little later than the air ports, and the incoming charge assisted in expelling the exhaust gases in a manner similar to that of the earlier types of two-stroke cycle engine. The accompanying rough diagram assists in showing the working of this engine.
The Gobe and Diard Co-axial Two-stroke Engine.
Exhibited in the Paris Aero Exhibition of 1912, the Laviator two-stroke cycle engine, six-cylindered, could be operated either as a radial or as a rotary engine, all its pistons acting35 on a single crank. Cylinder dimensions of this engine were 3.94 inches bore by 5.12 inches stroke, and a power output of 50 horse-power454 was obtained when working at a rate of 1,200 revolutions per minute. Used as a radial engine, it developed 65 horse-power at the same rate of revolution, and, as the total weight was about 198 lbs., the weight of about 3 lbs. per horse-power was attained36 in radial use. Stepped pistons were employed, the annular37 space between the smaller or power piston and the walls of the larger cylinder being used as a charging pump for the power cylinder situated 120 degrees in rear of it. The charging cylinders were connected by short pipes455 to ports in the crank case which communicated with the hollow crankshaft through which the fresh gas was supplied, and once in each revolution each port in the case registered with the port in the hollow shaft. The mixture which then entered the charging cylinder was transferred to the corresponding working cylinder when the piston of that cylinder had reached the end of its power stroke, and immediately before this the exhaust ports diametrically opposite the inlet ports were uncovered; scavenging was thus assisted in the usual way. The very desirable feature of being entirely38 valveless was accomplished with this engine, which is also noteworthy for exceedingly compact design.
The Lamplough six-cylinder two-stroke cycle rotary, shown at the Aero Exhibition at Olympia in 1911, had several innovations, including a charging pump of rotary blower type. With the six cylinders, six power impulses at regular intervals39 were given on each rotation; otherwise, the cycle of operations was carried out much as in other two-stroke cycle engines. The pump supplied the mixture under slight pressure to an inlet port in each cylinder, which was opened at the same time as the exhaust port, the period of opening being controlled by the piston. The rotary blower sucked the mixture from the carburettor and delivered it to a passage communicating with the inlet ports in the cylinder walls. A mechanically-operated exhaust valve was placed in the centre of each cylinder head, and towards the end of the working stroke this valve opened, allowing part of the burnt gases to escape to the atmosphere; the remainder was pushed out by the fresh mixture going in through the ports at the bottom end of the cylinder. In practice, one or other of the456 cylinders was always taking fresh mixture while working, therefore the delivery from the pump was continuous and the mixture had not to be stored under pressure.
The piston of this engine was long enough to keep the ports covered when it was at the top of the stroke, and a bottom ring was provided to prevent the mixture from entering the crank case. In addition to preventing leakage40, this ring no doubt prevented an excess of oil working up the piston into the cylinder. As the cylinder fired with every revolution, the valve gear was of the simplest construction, a fixed41 cam lifting each valve as the cylinder came into position. The spring of the exhaust valve was not placed round the stem in the usual way, but at the end of a short lever, away from the heat of the exhaust gases. The cylinders were of cast steel, the crank case of aluminium42, and ball-bearings were fitted to the crankshaft, crank pins, and the rotary blower pump. Ignition was by means of a high-tension magneto of the two-spark pattern, and with a total weight of 300 lbs. the maximum output was 102 brake horse-power, giving a weight of just under 3 lbs. per horse-power.
One of the most successful of the two-stroke cycle engines was that designed by Mr G. F. Mort and constructed by the New Engine Company. With four cylinders of 3.69 inches bore by 4.5 inches stroke, and running at 1,250 revolutions per minute, this engine developed 50 brake horse-power; the total weight of the engine was 155 lbs., thus giving a weight of 3.1 lbs. per horse-power. A scavenging pump of the rotary type was employed, driven by means of gearing from the engine crankshaft, and in order to reduce weight to a minimum the vanes were of aluminium.457 This engine was tried on a biplane, and gave very satisfactory results.
American design yields two apparently43 successful two-stroke cycle aero engines. A rotary called the Fredericson engine was said to give an output of 70 brake horse-power with five cylinders 4·5 inches diameter by 4·75 inches stroke, running at 1,000 revolutions per minute. Another, the Roberts two-stroke cycle engine, yielded 100 brake horse-power from six cylinders of the stepped piston design; two carburettors, each supplying three cylinders, were fitted to this engine. Ignition was by means of the usual high-tension magneto, gear-driven from the crankshaft, and the engine, which was water-cooled, was of compact design.
It may thus be seen that the two-stroke cycle type got as far as actual experiment in air work, and that with considerable success. So far, however, the greater reliability44 of the four-stroke cycle has rendered it practically the only aircraft engine, and the two-stroke has yet some way to travel before it becomes a formidable competitor, in spite of its admitted theoretical and questioned practical advantages.
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1
combustion
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| n.燃烧；氧化；骚动 | |
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shaft
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| n.(工具的)柄，杆状物 | |
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piston
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| n.活塞 | |
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inefficient
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| adj.效率低的，无效的 | |
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cylinder
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| n.圆筒，柱(面)，汽缸 | |
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obviated
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| v.避免，消除（贫困、不方便等）( obviate的过去式和过去分词 ) | |
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situated
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| adj.坐落在...的，处于某种境地的 | |
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chamber
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| n.房间，寝室；会议厅；议院；会所 | |
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recurrence
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| n.复发，反复，重现 | |
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expenditure
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| n.(时间、劳力、金钱等)支出；使用，消耗 | |
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relatively
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| adv.比较...地，相对地 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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accomplished
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| adj.有才艺的；有造诣的；达到了的 | |
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devoted
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| adj.忠诚的，忠实的，热心的，献身于...的 | |
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thermal
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| adj.热的，由热造成的；保暖的 | |
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regularity
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| n.规律性，规则性；匀称，整齐 | |
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rotation
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| n.旋转；循环，轮流 | |
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undoubtedly
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| adv.确实地，无疑地 | |
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elimination
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| n.排除，消除，消灭 | |
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simplicity
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| n.简单，简易；朴素；直率，单纯 | |
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friction
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| n.摩擦，摩擦力 | |
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compensated
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| 补偿，报酬( compensate的过去式和过去分词 )； 给（某人）赔偿（或赔款） | |
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cylinders
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| n.圆筒( cylinder的名词复数 )；圆柱；汽缸；（尤指用作容器的）圆筒状物 | |
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drawn
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| v.拖，拉，拔出；adj.憔悴的，紧张的 | |
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simultaneously
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| adv.同时发生地，同时进行地 | |
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specially
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| adv.特定地；特殊地；明确地 | |
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rotary
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| adj.(运动等)旋转的；轮转的；转动的 | |
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pistons
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| 活塞( piston的名词复数 ) | |
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rigid
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| adj.严格的，死板的；刚硬的，僵硬的 | |
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axis
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| n.轴，轴线，中心线；坐标轴，基准线 | |
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pinion
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| v.束缚；n.小齿轮 | |
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combustible
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| a. 易燃的,可燃的; n. 易燃物,可燃物 | |
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emanating
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| v.从…处传出，传出( emanate的现在分词 )；产生，表现，显示 | |
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stationary
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| adj.固定的，静止不动的 | |
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acting
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| n.演戏，行为，假装；adj.代理的，临时的，演出用的 | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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annular
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| adj.环状的 | |
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entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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intervals
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| n.[军事]间隔( interval的名词复数 )；间隔时间；[数学]区间；（戏剧、电影或音乐会的）幕间休息 | |
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40
leakage
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| n.漏，泄漏；泄漏物；漏出量 | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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aluminium
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| n.铝 (=aluminum) | |
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43
apparently
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| adv.显然地；表面上，似乎 | |
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reliability
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| n.可靠性，确实性 | |
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