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Diesel Engine

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DIESEL ENGINE. In 1892 Dr. Rudolf Diesel (q.v.) patented the type of internal-combustion engine with which his name is now inseparably associated, though it was not until that the first real "diesel" was built. Independent tests of an engine were conducted by Prof. Schroter at Augsburg in 1897, and diesel engines were first publicly exhibited at the Munich exhibi tion of 1898. In a paper read before the Congress at Paris in 1900 Diesel stated that the cycle of operations finally adopted by him after extended experiments was as follows: (I) A suction stroke during which air alone at atmospheric pressure was drawn into the cylinder.

(2) A compression stroke in which this air was next compressed "adiabatically" to a pressure of 500-600 lb. per square inch.

(3) From, and for a short period after, the end of the com pression stroke, regulated admission of the fuel in the form of a fine spray in such manner as to cause combustion to occur at (approximately) constant pressure.

(4) Expansion of the ignited mixture of fuel spray and air during the remainder of the working stroke.

(5) Expulsion of the burnt products during the next stroke.

The cycle thus described was of the ordinary four-stroke type with stroke-sequence of suction, compression, working and ex haust; and the engine contemplated was of the normal single acting type furnishing one working impulse in each two complete revolutions of the crankshaft.

The special feature of the diesel cycle is the regulated intro duction of the fuel spray giving a combustion at approximately constant pressure, so that, in normal conditions of running, the maximum pressure of compression is not exceeded. In other internal-combustion engines the combustion or "explosion" of the working charge at the end of the compression stroke results in an almost instantaneous rise of pressure to a peak value much in excess of that of the compression.

It is stated above that the air is considered to be compressed, and that the burning gases are expanded "adiabatically" ; by this is meant that the air and gases are supposed for the sake of sim plicity to be compressed and expanded respectively, without any loss or gain of heat to the containing cylinder. Actually of course some heat is lost to the cylinder during the compression and ex pansion (working) strokes; nevertheless the rapid squeezing-up of the air during the compression stroke results in increase of its temperature to about I,000° F, which is sufficient to cause spontaneous ignition of the fuel spray during its injection; thus no igniting apparatus is required with Neither the suction and subsequent compression of air alone, nor the absence of igniting apparatus in the diesel engine was novel. Akroyd Stuart (1886-9o) injected the fuel spray into a hot-bulb prolongation at the combustion chamber end of the cylinder, and atmospheric air, during compression, formed with this heated spray the working charge, which ignited spontaneously and explosively, at the end of the compression stroke. The char acteristic feature of the diesel procedure is the regulated admis sion of the fuel spray by which combustion at constant pressure is realized.

Diesel Engine

Definition of a Diesel Engine.

The accepted definition of a diesel engine is as follows :—A diesel engine is a prime mover actuated by the gases resulting from the combustion of a liquid or pulverized fuel injected in a state of fine subdivision into the engine cylinder at or about the end of the compression stroke. The heat generated by the compression of air in the cylinder is the sole means of ignittng the charge. Combustion of the charge proceeds at, or approximately at, constant pressure.

A sectional view of a simple f our-stroke diesel engine appears in fig. ; the inverted vertical frame design shown is almost universally adopted. The piston reciprocates within the cylinder and drives the crankshaft through connecting rod. On the first down stroke air is drawn through the air inlet and valve into the cylinder ; this air is next compressed on the return up stroke of the piston, and the fuel spray is then forcibly injected into this compressed and consequently highly heated mass of air through the fuel valve; spontaneous ignition takes place and the piston is driven down and performs the "working stroke." Near the bottom of this stroke the exhaust valve is opened and the burnt gases are discharged through the exhaust into the atmos phere during the final up stroke of the piston ; this completes the cycle.

The inlet and exhaust valves are usually of the "poppet" or "mushroom" type as shown, and all valves are cam-driven from a "half-speed" shaft in the manner indicated in the illustration substantially as in an ordinary motor-car engine. It will be noted that the cylinder is well water-jacketed, and that the engine generally is of massive design.

In normal working the maximum pressure, as already stated, does not exceed that of compression, i.e., soo-600 lb. per sq.in., but the engine has to be built safely to resist occasional excessive pressures that occur if, e.g., a leaky fuel valve permits admission of oil dufing suction with subsequent vaporization during the compression stroke, and pre-ignition near the end of the com pression; in this way momentary pressures of fully i,000 lb. per sq.in. not uncommonly occur, and have to be provided against. Accordingly, in general, the diesel is a heavy internal-combustion engine in relation to its power development.

Ideal Diesel Indicator Diagram.

An ideal indicator dia gram of a four-stroke diesel engine is given in fig. 2. Horizontal measurements represent volumes, and vertical measurements pressures in lb. per sq.in., each to some convenient scale. Thus at the commencement of the cycle, the combustion chamber, of volume Vo is, in the simplest theory, filled with air at atmosphere pressure po. The piston performs the suction stroke, and the line cm is traced on the diagram ; at the end of the suction stroke the cylinder contains a volume Vi of air at atmospheric pressure. The inlet valve is now closed and the returning piston compresses the entrapped air adiabatically at constantly increasing pressure as indicated by the curve 1 2 ; at the end of this stroke the air, now at high temperature, has a volume V2( = Vo), and pressure P2 lb. per square inch. The regulated charge of fuel is next sprayed into this mass of highly heated and compressed air during the first part of the downward movement of the piston on its working stroke; spontaneous ignition takes place, and continues at con stant pressure to some point, as 3, indicated by the horizontal line 23 ; the fuel is then cut off and the working mixture expands at rapidly falling pressure and temperature, driving the piston downwards, until the point 4, the end of the working stroke, is reached.

The exhaust valve is now suddenly opened and the burnt gases escape into the atmosphere, falling instantly from pressure p4 to Pi( = po). Finally, during the return of the piston from to o, the remaining exhaust gas is expelled. This cycle is then repeated indefinitely.

Simple Theory.

The working fluid is regarded as a perfect gas, conforming strictly to the relation : PV = c.T (I) where T denotes absolute temperature (say, in ° F) and c is a constant, being equal to the difference (Cp— Cy) between the specific heat of the fluid at constant pressure and that at constant volume. The compression and expansion curves are regarded as truly adiabatic, so that the fluid neither gains nor loses heat, qua heat, during these periods. Hence, on these simplifying assumptions, the heat given to the working fluid per cycle is H=Kp(T3—T2) British thermal units. While the heat rejected per cycle (see the indicator diagram, at 4.1) is That is, denoting as usual the ratio of the specific heats, viz.

by 7: K,,' I T4 Thermodynamic efficiency = • • (3) 7 T3—T2 This expression gives the value of the ideal thermodynamic efficiency of the diesel cycle in terms of the absolute temperatures at the four corners of the indicator diagram; these temperatures are not, however, readily ascertainable; but the efficiency can, fortunately, also be expressed in terms of magnitudes immediately measurable from the indicator diagram, as follows: The volume ratio of compression, commonly termed the "com pression ratio," 172 (fig. 2), is usually denoted by r; it will be V4 VI V4 seen that this is the same as —.T., ; thus r= — = — • v 2 V2 V2 V3 Also the ratio of expansion at constant pressure (i.e., of combustion at constant pressure) is commonly denoted by p; V3 thus p= • ,v 2 V3 V3 V2 p Note here also that = — • — = — • Now it may be shown V 4 V2 V4 r that when any perfect gas is compressed or expanded adiabatic ally, the product TV-Y—' remains constant along the curve; hence by aid of Eqs. (I) and (3), and the above results we have from fig. 2 : T2V27-1 = Ti1717-'. Therefore -722, = ) 7-1= r7—i.

v 2 T3 V3 Thus T2 = r7-1T1-,T, = = p. Therefore T3 = P T2; 1 2 V 2 and thus T3= pr7-7i; and accordingty T3- T2= Again: T4V47—l— T3V37—i=p0 1V37-71.

(2) The charge of fuel is then forcibly injected at a regulated rate into this compressed and heated air just as in the four-stroke cycle engine, and spontaneous ignition occurs and continues at approximately constant pressure during the early part of the working down-stroke.

(3) When about 8o% of the down-stroke has been completed, the piston over-runs a ring of ports in the lower part of the and thus the diesel cycle efficiency is expressed in terms of the compression and expansion ratios only. Eq. (6) is the exact ex pression for the efficiency, on the ordinary simple theory, and as Table I. below shows, the efficiency falls as p, i.e., the period of constant-pressure combustion, increases. When the constant pressure combustion period is extremely short, i.e., when p =1, y -1 the expression (6) above reduces to i — . Now it is shown in the article on gas engines (q.v.) that for the type of four-stroke cycle in general use in gas and petrol engines known as the "constant-volume" cycle, the thermodynamic efficiency, with the usual simplifying assumptions, is expressed by the formula 1 — —1 _) , where r is, again, the ratio of adiabatic compression. A committee of the Institution of Civil Engineers in 1905 after careful consideration recommended the adoption of this formula for estimating the ideal maximum of thermodynamic efficiency of the internal-combustion engines in general use at that date, all such engines being assumed as working with a perfect gas for which the value of 7 was taken as 1.4. Thus the i905 Air Standard= i — (1)0.4 (7)r and this has been largely used from that date in connection with all internal-combustion engines, including diesels. The 19o5 Air Standard makes the efficiency increase with the compression ratio, thus: For r = 3. 4. 5. 6. 7. 8. io. 12. 14.

The Air Std=-356 •426 '541 •565 •602 .630 •652 and the diesel engine owes its position as one of the most eco nomical engines in fuel consumption to the high compression ratio (14—I5) employed. This high compression ratio is practicable on account of air alone being compressed; in the case of gas engines, and of many oil engines, much lower compression ratios must be used in order to avoid detonation of the working charge, and in such cases the value of r ranges from about 3 to 7.5 only. The ideal diesel efficiency is, however, somewhat over-estimated if Eq. (7) be used, as is shown in Table I. below, which exhibits the true efficiency values from Eq. (6) for a compression ratio of 14, and with p ranging from its minimum value I to the maximum, in practice, of about 2.

Two-stroke Cycle Diesel Engines.

All the earliest diesel engines were operated on the four-stroke cycle, but two-stroke cycle engines are now (1928) common and are increasing in favour. The sequence of operations is as follows : (I) On the first up-stroke of the piston, air alone is adia batically compressed into the combustion chamber, becoming in consequence highly heated.

cylinder through which the burnt gas discharges itself into the atmosphere. Simultaneously air, supplied by a so-called scavenge pump under a pressure of 1.5 to 3 lb. per sq.in. enters the cylinder through a valve, or ports, and causes the expulsion of any re maining exhaust gas, so that at the end of the working stroke the cylinder is again filled with air alone. This completes the cycle. The piston on the return up-stroke first closes the rings of air inlet and exhaust ports, and then compresses the now entrapped air as in operation (I) above. A diagrammatic illustration of one of Messrs. Sulzer's designs of two-stroke cycle diesel engines appears in fig. 3. The piston works in the cylinder. When near the bottom of its stroke the piston over-runs, and so opens the ring of exhaust ports, which extends half-way round the cylinder; the burnt gas at once escapes into the exhaust outlet; cylinder.

A slightly further descent of the piston then uncovers a ring of ports, through which fresh air at 1.5 to 3 lb. per sq.in. pressure (supplied by the scavenge pump) at once enters the cylinder from the air-pipe, and drives out the remaining ex haust gases through the ports. The piston then commences its up-stroke and after the air-ports are masked, but before the exhaust ports are closed, further air is admitted through the ring of small ports, by which the final "scavenging" of the residual exhaust gases is effected. The ports continue to deliver air into the cylinder until just after the exhaust ports have been closed, so that in this ingenious manner the cylinder is charged with fresh air at slightly above atmospheric pressure.

The supply of air to the ports is regulated by a reciprocating valve operated from the crankshaft. The only valves in the cylinder head in this type of engine are (I) the fuel injection valve and (2) the air-starting valve. The massive strength of the design and the liberal water-jacketing provided should be noticed.

The ideal two-stroke cycle engine would develop twice the power of a four-stroke of equal size ; actually, as only about 8o% of the stroke can be utilized on account of the presence of the scavenging and exhaust ports, and as some power is absorbed in driving the scavenging pump, the two-stroke diesel in practice is usually a little less efficient than the four-stroke. Another difficulty with the two-cycle is that it is practically impossible to get complete scavenging, especially at high speeds, where the scavenging pressures and consequent losses are necessarily much higher. It possesses, however, the important advantages (I) that it is lighter for the same power output ; (2) that it possesses fewer moving parts, and is thus simpler; and (3) that it is very readily reversed. With reference to (3), it is obvious that a two stroke cycle engine will work equally well in whichever direction it may be started ; it is therefore only necessary to move the crankshaft operating the fuel inlet valve through a small angle in order to change the injection-period to that suitable for reverse running.

Fuel : Fuel Injection : Fuel Consumption.

Although many liquid and even some solid (pulverized) fuels, as coal dust, have been tried in diesel engines, practically the only fuels employed are (I) crude and residual petroleum oils and (2) coal-tar oils obtained from the large-scale distillation of coal. Suitable petroleum oils have a specific gravity (water = I) of •85–•95 at 60° F, a flash point of 150-250° F (by close test), and a lower heat value of 18,000-19,000 B.T.U. per pound. Oils having an asphalt base are liable to gum-up the valves, and are thus less favoured.

Coal-tar oils often show a flash point of 400° F or more, a sp.gr. of I •0-1 • I and a (lower) heat value of about 16,000 B.T.U. per lb. These oils require in general a very high tem perature to produce spontaneous ignition and this proved at first an obstacle to their employment in the diesel engine ; the difficulty was completely removed by injecting through the fuel inlet valve into the cylinder a minute quantity of some more readily ignitable oil (as gas oil), immediately before, or simultaneously with, the main charge of tar oil; by aid of this "pilot jet" combustion is initiated, and entirely satisfactory performance obtained.

The injection of the charge of fuel oil into the compressed and heated mass of air in the combustion chamber at and for a short period after the instant when the piston has reached the top of its stroke is effected through a spring-seated needle valve (the fuel inlet valve) located in the cylinder head, in one of two ways: (I) By a blast of air supplied from a special air reservoir at a pressure of up to i,000 lb. per sq.in. Air blast injection has given, in general, a somewhat better distribution of the sprayed and "pulverized" fuel throughout the combustion chamber, with consequent improvement in fuel economy. It is thought that one of the principal contributions of air-blast injection is the turbu lence thus introduced into the combustion space. However, it involves the provision of bulky and costly air-compressing ap paratus which absorbs in its op eration 6-7% of the whole power output of the engine. Accordingly great attention was devoted to the discovery of a means of dis pensing with the air-blast, and recent engines are provided in increasing numbers with : (2) "Airless" or "Solid" Injec tion.—In this method the charge of fuel is forced through a cam-operated, or automatic, spring loaded fuel injection valve into the cylinder by a small quick acting mechanically-operated pump at a pressure of 4,000-7,000 lb. per sq.in. The fuel consumption per indicated horse-power hour is usually a little greater than with air-blast injection; on the other hand the elimination of the air-blast apparatus in creases the mechanical efficiency of the engine, and in conse quence the fuel consumption per brake-horse-power-hour with airless injection compares quite favourably with that when an air blast is used.

A diagrammatic illustration of a fuel injection valve and fuel pump for airless injection appears in fig. 4; the small solid-plung er eccentric-driven fuel pump is shown on the right by which the charge of oil is delivered at high pressure into the space sur rounding the needle valve which is simultaneously raised just off its seat, against the pressure of its spring, by a cam-operated lever; the oil immediately enters the combustion chamber in a finely pulverized condition in the form of a hollow cone, and instant ignition occurs; in true diesel engines the inlet is held open during an appreciable portion of the down stroke, and the continued supply of oil then causes the combustion to continue at approximately constant pressure until cut-off. In the impor tant class of so-called "semi-diesel" engines (see OIL ENGINES), the fuel inlet valve is not cam-operated, but is forced open, against the pressure of its closing spring, by the pump delivery pressure, and the whole charge of oil is suddenly sprayed into a heated chamber, usually a prolongation or extension of the com bustion chamber; in such engines the combustion is practically instantaneous, with resulting rise of pressure considerably above that of compression.

The diesel, of all internal-combustion engines, is the most economical in fuel consumption, and Table II. illustrates in a general way not only the high economy of the type but also the progress that has been made since its introduction; all the figures given relate to single-acting four-stroke cycle inverted-vertical engines at full load, using petroleum oils as fuel: These figures show a steady increase in the brake thermal efficiency; the highest value included, viz., 35.5%, is that of a 6-cyl. 1,75o b.h.p. engine installed in 1927 at Charing Cross for the Charing Cross Electricity Supply Co., Ltd. ; this was, at that date, the largest stationary oil engine built in Britain. An external view of this engine is given in fig. 5. With special types of diesel even higher values are obtained ; for example, tests conducted by the marine oil engine trials committee on a Scott Still diesel-steam engine of 1,25o b.h.p. showed a consumption of only .354 lb. of fuel per b.h.p. hour corresponding to a brake thermal efficiency of 36.9%. According to H. R. Ricardo, "al most, if not quite, the highest thermal efficiency ever yet re corded on a diesel engine, namely 38.8% on the net shaft horse power, was obtained by the Royal Aircraft Establishment on a high-speed diesel engine of 8 in. bore running at I,000 revolu Lions per minute." It is of interest to compare these figures with the best attained with other types of prime mover, and the figures on p. 351 (Table III.) include some given by A. B. Chalkley, Diesel Engines (6th ed., 1927).

General.

A fundamental disadvantage of the four-stroke cycle engine, especially when of the predominating single-acting type (see fig. 1), is the infrequency of the working impulses, and a reasonable degree of uniformity of revolution speed can only be obtained in single-cylindered or few-cylindered engines by the provision of heavy fly-wheels. The drawback is minimized in practice by constructing engines having two, three, four, six, eight, and even more, cylinders arranged in line and acting upon a common crankshaft, but the one impulse per cylinder in every two crankshaft revolutions is still an objection, as such engines necessarily remain very bulky and costly in relation to their power output.

The two-stroke engine at once doubles the impulse frequency, but for long the design of completely satisfactory engines of this type proved difficult, largely from the serious piston- and cylinder cooling troubles encountered ; these difficulties were in due course entirely overcome, and the attention of designers was next con centrated upon the problem of the production of double-acting engines, i.e., engines in which working impulses are caused to occur on each side of the piston ; this problem was obviously of extreme importance in relation to marine applications on account of the great value of space and weight saved on a ship. The double-acting engine, however, raised again in an acute form the problems of piston- and cylinder-cooling, and many experimental engines were built before success was finally attained. By the end of 1927, however, large double-acting diesel engines both of the four-stroke and two-stroke type were running regularly in commercial service ; in all these the pistons are necessarily cooled by the continuous circulation through them of a current of water or oil. The piston rod terminates in an external crosshead—as in the normal type of steam engine—by which the piston is en tirely relieved of the side thrust of the connecting-rod, this thrust being taken by a well-lubricated and water-cooled crosshead guide.

British engineers for long favoured the single-acting type of engine with uncooled pistons ; with these the cylinder diameter is limited to a maximum of about 26 in. and is usually considerably less than this limit; with water- or oil-cooled pistons the diameter is independent of cooling considerations.

Of land installations in 1926 the largest was a 15,00o b.h.p. double-acting two-stroke diesel engine built for the Hamburg electricity works; this engine had nine cylinders, each 33.86 in. bore and 59.05 in. stroke, and ran normally at 94 revs. per minute.

An idea of its size may be formed from the statement that it had an over-all length of 77 ft., width of 14 ft. and height of 331 ft. The fuel consumption (using gas oil of .875 sp.gr.) on trial was only 0.392 lb. per b.h.p. hour at full load. Among large marine installations may be mentioned:— 0 ) The engines of the 23,90o ton, 19-knot, motor ship "Satur nia," launched late in 1927; this vessel is propelled by two 10,000 horse-power eight-cylindered double-acting four-stroke super charged diesel engines of 33 in. bore and 59 in. stroke; and (2) The engines of the 3 2,65o ton, 19-knot, motor ship "Augus tus,'' also built in 1927, and fitted with four double-acting two stroke diesels aggregating 25,000 b.h.p. The report of Lloyd's Register at the end of June 1927 showed that throughout the world at that date there were under construction 268 motor ships aggregating 1,459,595 tons, and 3o steam ships aggregating 1,366,809 tons; and that for the first time the motor tonnage under construction throughout the world exceeded that of steam.

In this necessarily limited article only a general sketch can be presented. The diesel engine forms already a highly specialized branch of engineering calling for great knowledge and skill in designing and extreme accuracy and refinement in construction. An instructive series of articles on earlier developments will be found in the pages of The Engineer frorn April to Oct. 1913 ; development since then has been so phenomenally rapid that reference must mainly be made to the many technical papers dealing with the type: The Engi neer, Engineering, The Motor Ship, The Marine Engineer, Gas and Oil Power, etc. Of special treatises wholly devoted to the subject there are but a few, the principal, in Britain, being that of A. B. Chalkley, Diesel Engines for Land and Marine Work (6th ed., 1927), in which a very clear and complete account is given. (G. A. By.) United States.—American practice in diesels is tending dis tinctly towards supercharging. The practice at this date is super charging which is measurable in ounces of super-atmospheric pressure. This has two very beneficial tendencies: (I) it contrib utes in an engine of a given size a larger combustion space, there fore smaller ratio of chilled walls; (2) more oxygen with its com plement of fuel demand per stroke. The difficulty with super charging is that the exhaust valve or exhaust passages open at somewhat higher temperatures and pressures. It is a logical se quence that, when supercharging rises to really considerable pres sures, the corresponding increased pressures at the point of ex haust opening should be utilized by farther expansion in a cylinder provided for this purpose. One group of engineers in America has been pursuing this type of design for some time with results which indicate ultimate success.

The increased temperatures and pressures which the exhaust valves are compelled to handle in case of supercharging engines en tail difficulties in connection with the heat erosion of the valve seats. A complete remedy for this has been found in com pounding, where proper cushion ing in the low pressure cylinder does away entirely with high velocity in gases passing through the exhaust port, which opens under conditions of equal pressure on both sides, the velocity being reduced to a value due to piston movement, and erosion is suppressed.

Fig. 9 gives a typical card from such an engine, the super charged induction pressure being indicated at Ai, the compression proper at BC, the fuel injection at Ci, the double expansion curve in the high pressure cylinder being indicated at C" and that in the low pressure from E to D, the final exhaust passage opening at point Di near the atmospheric line.

Sleeve Valve.

Ricardo succeeded in adapting the sleeve valve to diesel operation in England in 1926. Since that time consid erable advance has been made in the utilization of this type of valve for high speed engines. The great advantage of its adoption consists in giving sufficient port opening to permit the transfer ence of gases to the low pressure cylinder. Further, the in ternal shape of the combustion chamber permits the supercharging pressures to be utilized by its tangential inlet direction to secure an extremely high state of turbulence, by a simple solid fuel injection nozzle. This achieves two important functions : ) almost instantaneous co-mingling of the fuel and oxygen; (2) it allows the solid injection to reach a very much more increased amount of the oxygen present than ever before. The design lends itself, therefore, to high fuel efficiency and high speed operation.

(E. A. SP.)

fuel, pressure, air, engines, stroke, cylinder and compression