HEAVY OIL ENGINES The success of the Otto and Langen engine, described above, and the final establishment of internal combustion engines on a coln mercial basis by the Otto silent gas engine of 1876, directed the close attention of many engineers to improving further such engines particularly in relation to the fuels employable. Town's gas was an expensive fuel, and available only in large centres of population. The increasingly wide distribution of petroleum oils, particularly the paraffin oils, lamp-oils, or kerosenes, no doubt suggested that if such oils could be rendered available as fuel a much wider area of usefulness would be opened to the internal combustion engine. The earliest of such engines to achieve a considerable measure of success was the Priestman Oil engine introduced in 1885.
Heavy Oils.—This is a general name for inflammable liquid hydrocarbons, particularly petroleums, of flash point exceeding about 7 5 ° F and ranging in specific gravity from o.78 to i •o ; with such liquids a considerable preliminary heating is necessary to obtain an explosive mixture of their vapour with air, and the earliest heavy oil engines were accordingly of the vapourizing class, and used as fuel American and Russian kerosenes, Scotch paraffin and shale oils, and even crude and residual petroleum oils.
The Early Priestman Oil Engine.—This engine was built (1885) in four-stroke cycle single-cylindered types up to II horse power, and gave best results when using as fuel kerosenes of o.8 sp.gr. and flash point about 1 oo° F. The oil fuel was delivered by air pressure through a minute orifice, in the form of a very fine spray or mist, into an exhaust-heated vessel or "vapourizer," where it immediately became gaseous; in this condition, during the suction stroke of the engine, it entered the cylinder together with a correct proportion of air drawn in through an automatic valve ; this mixture was then compressed and fired and the work ing stroke followed as usual. To start the engine the vapourizer received a preliminary heating from a blow-lamp; a small hand pump then furnished air for the initial spraying of the fuel into the vapourizer ; the flywheel next being turned by hand, the engine started running. As soon as all was well warmed up, the blow-lamp was extinguished, the engine exhaust thereafter main taining the vapourizer at its necessary high temperature. A test of a 4.5 horse-power, 8.5 in. bore x 12 in. stroke x 18o rev. per min., Priestman engine in 189o, using as fuel Broxburn lighthouse oil of •81 sp.gr. and flash point 152° F, showed a consumption of 1•243 lb. of oil per B.H.P. hour. In later tests (1892) of an engine of the same size, running at 204 rev. per minute, and using as fuel Royal Daylight oil of • 79 sp.gr. and flash point 77° F, the consumption was reduced to 0•842 lb. per B.H.P. hour. The pressure of compression in these tests was 35 lb./sq.in., and of explosion, 151.4 lb./sq.in. above atmosphere; and the total heat value of the Royal Daylight oil was stated to be 21,490 B.Th.U. per lb.
Gardner Paraffin Engine.—Priestman was quickly followed by other inventors, as Griffin, Samuelson, Thornycroft, etc., who used exhaust-heated vapourizers, and by Gardner, Howard, Smith-Dudbridge, Crossley, etc., who used lamp-heated vapour izers. In the earlier Gardner paraffin engines, for example, the paraffin was supplied, either by gravity or under pressure, to the "mixer" diagrammatically shown in fig. 9. The paraffin, entering through K, passed into the small duct D through a hand regulated needle-valve N. The air entered through the branch A.
The spring-supported disc, V, borne on a spindle terminating upwards in a needle valve P automatically closed the fuel supply when the engine ceased running. The branch C com municated with the lamp-heated vapourizer.
During each suction stroke of the engine, air drawn in through A depressed the disc V, so per mitting delivery of a sprayed charge of fuel through D. The mixture of the air and fuel-spray next passed via C into the vapourizing vessel which was continuously heated by an external Bunsen-flame oil lamp. As fuel it was found practicable to use kerosenes of .825 sp.gr. and up to 200° F flash (by close test). Four-stroke vapourizing engines of this type were built having from 1 to 8 cylinders, and developing from 5 to 200 horse-power.
In the vapourizer, heavy oil fuels were rendered gaseous, mixed with a suitable volume of air, admitted to the cylinder and then exploded exactly as in an ordinary gas engine. Simple though this would be were heavy oils hydrocarbons of one definite chemical composition, it has proved practically troublesome with actual heavy oils, which are in general composed of a mixture of many different liquid hydrocarbons, boiling at temperatures ranging from 200° F to 600° F; thus completely to vapourize such fuel its temperature must be raised at least to 600° F. But at this high temperature the lighter constituents of the fuel are liable to be "cracked," i.e., decomposed into free hydrogen and carbon; the carbon deposits in the vapourizer, in time choking it up. In practice a fuel temperature in the vapourizer of about soo° F was found to be the best to use. This necessary pre heating of the fuel involved three serious disadvantages :—(1) The fresh charge of air and vapour being heated before entering the cylinder, had expanded thus reducing the mass of the charge, and hence the power output of the engine. (2) The fresh charge being heated, only a low compression ratio, usually 3.o to 3.5, could be employed in order to avoid detonation and pre-ignition troubles. (3) The use of a low compression ratio kept the ther modynamic efficiency of such engines low.
Accordingly although the heat value of kerosene usually some what exceeds that of petrol, vapourizing oil engines designed to use either fuel usually give only some 85 % of their maximum power output with paraffin, and show a reduced economy.
In small powers (5 H.P. or so), vapourizing engines are still used to a small extent for domestic lighting and light agricultural work, but are rapidly being replaced (a) by the much more cleanly and convenient small petrol engine, and (b) by more modern and simple heavy oil engines as described later herein.
The action is as follows :— The hot bulb receives a preliminary heating from an external blow—lamp; the engine flywheel being next turned by hand, the piston during the suction stroke draws a charge of air directly into the cylinder through an air inlet valve ; at the same time the charge of oil is delivered by the small f orce-pump into the hot bulb where it vapourizes and the vapour, mixed with residual exhaust gas from the previous cycle thus fills the bulb, the cylinder containing air alone. On the compres sion stroke of the piston the air passes through the narrow passage B into the bulb and then mixes with the oil vapour; the mixture is at first too rich to ignite, but the engine is so adjusted that just as the compression is completed a correct mixture is attained ; the heat of the hot bulb then causes automatic igni tion, and the resulting pressure drives the piston outwards, thus performing the working stroke. The bulb temperature is main tained by heat derived from successive explosions during run ning. After prolonged heavy-load running the bulb tends to be come too hot, which causes pre-ignition of the charge, and also "cracking" of the oil, with resulting deposit of "coke"; on the other hand if the engine be run light for some time the bulb tends to become too cool to ignite the charge. A suitable bulb temperature is maintained by varying the supply of cooling water to the jacketed forward end of the bulb by means of the hand controlled valve C. With reference to the automatic ignition of the charge, Sir D. Clerk observes (The Gas, Petrol and Oil En gine, vol. ii.) : "It is a peculiar fact that oil vapour mixed with air will explode by contact with a metal surface at a comparatively low temperature, and this accounts for the explosion of the compressed mixture in the hot bulb A, which is never really raised to a red heat. It had long been known to engineers conversant with gas engines that in certain conditions of the internal surface a gas engine will run and ignite with great regularity without any special form of ignitor, if only some portion of the interior surface of the cylinder or combustion chamber be so arranged that its temperature is moderately raised. Although that temperature may be too low to ignite the mixture at atmospheric pressure, yet when compression is complete the mixture will often ignite quite regularly." In an old early two-stroke cycle engine (circa 188o) Clark screwed a bolt into the end of the piston, the bolt being long enough to project well into the explosive charge. 'After running the engine for about 15 minutes in the usual way he found that if the flame-igniting device were put of action the engine still continued to run regularly, the mixture being fired by the heated end of the bolt which caused ignition just as the mixture became fully com pressed ; in this case, however, the bolt attained a high red heat. Later, "hot-bolt" ignition was for a time used in some oil engines by, e.g., Clayton and Shuttleworth and Crossley.
Test of a single-cylindered 32. B.H.P. Hornsby-Akroyd engine in 1908 gave results as follows:— Compression pressure, 85 lb./sq.in.
Explosion pressure, 26o lb./sq.in.
Lbs. of fuel used per B.H.P. per hour (full load), 0.613.
The fuel used was "Russolene" oil having a sp. gr. of .82, a flash point of 88° F (close test), and a heat value of 18,450 l.'I'h.U. per lb.; the Brake Thermal Efficiency was therefore •613 X 45p X loo = 25.5%. The engine ran at 23o rev. per minute.
Many other "hot bulb" engines soon appeared, e.g., the Black stone, Crossley, National, Robey, Petter, Ruston, etc. Thus, in the Crossley Lampless oil engine, the oil is vapourized by injec tion into a hot bulb forming a prolongation of the combustion chamber ; ignition is, however, here effected automatically by a hot tube which projects into the bulb and is maintained at the high temperature necessary to cause ignition by heat derived from the successive explosions ; many early hot-bulb engines were similarly fitted. During prolonged running at heavy load a small spray of water could be injected into the exploded mixture through the "water sprayer"; the practice of water injection was at one time somewhat extensively adopted, but it is preferable to avoid it if possible.
Numerous hot-bulb engines have appeared since about 1886 varying only in the details of vapourization of the fuel, their mode of operation being, in general, not essentially different from that of Akroyd Stuarts' engine.
Appearance of the Diesel engine (q.v.) in 1895, with its remark ably low fuel consumption and high efficiency of performance, attracted engineers to the problem of obtaining this low fuel con sumption without being compelled to have recourse to the ex tremely high compression and high-pressure injection-air-blast of the Diesel, which involve a heavy and costly engine, necessitating great refinement in design and manufacturing processes to ensure satisfactory continuous running.
This problem has been very well solved by combining the hot bulb engine with a moderate compression and airless or "solid injection of the fuel at or about the instant of maximum com pression in a large class of engines of both four-stroke and two stroke cycle, now termed semi-Diesels.
The accepted definition of a semi-Diesel engine is as follows:— "A semi-Diesel engine is a prime mover actuated by the gases resulting from the combustion of a hydrocarbon oil. A charge of oil is injected in the form of spray into a combustion space open to the cylinder of the engine at or about the time of maximum com pression in the cylinder. The heat derived from an uncooled portion of the combustion chamber, together with the heat generated by the compression of air to a moderate temperature, ignites the charge. The combustion of the charge takes place at, or approximately at, constant volume." By 1928 the great majority of hot bulb semi-Diesel engines op erated on the two-stroke cycle, with or without crank chamber compression (see above) ; among the few examples of four-stroke cycle hot bulb semi-Diesels the Ruston and Hornsby "Class M" engine may be noted. These, built in single-cylindered horizontal type from 6 to 4o B.H.P., are run on crude, standard Diesel, or refined oils, and show a consumption from .48 to .6 lb. per B.H.P. hour. The hot bulb receives a preliminary heating by lamp, and the oil fuel is injected by a small force-pump as usual. Starting is effected by hand, or compressed air if preferred. Four-stroke hot bulb semi-Diesels were also then built by Fielding and Platt, and by Tangye ; the four-stroke type though more costly to build, is somewhat more economical in fuel consumption than the two stroke, and requires also in general considerably less oil for lubrication.
The great simplicity, low production cost, and considerable fuel economy of this type caused it to grow rapidly in favour with power users, and among prominent builders in 1928 may be mentioned:—Allen, Babcock, Beardmore, Bolinders, Campbell, Gardner, Marshall, Mirrlees, Petter and Robey.
A typical two-port, two-stroke, hot-bulb, semi-Diesel heavy oil engine with crank-chamber compression (see above) is the (Swedish) Bolinders engine as largely used for marine purposes in motor trawlers, etc., which is built in robust four-cylindered designs up to 500 brake horse power. In this engine air enters an enclosed crank-chamber during the up-stroke of the piston through automatic valves, and near the end of the down-stroke, under slight pressure, rushes into, and charges, the cylinder through an inlet port, and assists also in the expulsion of the burnt gases through, an exhaust port on the other side. On the succeeding up-stroke the air is compressed to 125-150 lb./sq.in. into the combustion chamber and hot bulb and at the end of com pression the fuel pump sprays the charge of oil into the bulb, the explosion automatically following. The hot bulb receives a preliminary heating by lamp, and the engines are started by corn pressed air.
To reverse the marine Bolinders engine it is slowed nearly to the stopping point, when a special device sprays a charge of oil into the bulb before the piston has quite completed its up-stroke; a premature explosion follows, and reversal of motion takes place, the engine thereafter running equally well in the new direction.
Vickers-Petter Heavy Oil Engines.—These engines are also solid-injection, two-port, two-stroke cycle semi-Diesels with crank chamber compression, but with the bulb water-jacketed, as shown in fig. 11. By extended tests the makers are enabled to obtain automatic ignition of the charge using only a moderate compres sion pressure, and thus eliminate the hot-bulb. These engines are built of single-cylindered 25 B.H.P. type running at 375 rev. per min. to six-cylindered 600 B.H.P. running at 27o revs. per minute, and use as fuel refined, crude and even low-grade residual petroleum oils; the consumption is as low as 0.43 lb. of fuel per B.H.P. hour. Starting from cold is effected by compressed air in about one minute.
Though there is no "uncooled portion of the combustion cham ber" in this and the following type, they are still usually classed by engineers as "semi-Diesels" as the combustion of the fuel is, in general, of the "constant volume" type.
"Cold Starting" Heavy Oil Engines.—Lastly is to be noticed a large modern class of heavy oil engine mostly of four-stroke type in which the bulb is entirely eliminated. The type depends for ignition solely upon the high temperature attained by the air on compression ; it employs compression pressures higher than those used in the hot bulb class, yet lower than those in the true Diesels; the fuel is sprayed by airless or "solid" injection into the combustion space at, or near, the instant of maximum com pression, the resulting combustion being, in general, partly at constant volume and partly at constant pressure. Such are styled "cold starting" engines and are usually included among "semi Diesels," as in a true Diesel engine the combustion pressure should not rise above the maximum pressure of compression.
This is very well illustrated by fig. 12, which is a reproduction of diagrams taken from a i so B.H.P. Blackstone cold starting heavy oil engine. It will be observed that the maximum com pression pressure was 38o lb./sq.in., while the maximum explosion pressure was 55o lb./sq.in. at full load, and that combustion continued constant at this maximum explosion pressure for a short portion of the working stroke.
Four-stroke cold-starting engines were built in 1928 by, among others, the following prominent British makers :— Blackstone, Campbell, Crossby, National, Premier, Robey, Ruston & Tangye ; these were built in units of from 6 to over 30o B.H.P.
The table below shows roughly the progressive increase in fuel economy in heavy oil engines from the introduction of the early Priestman vapourizing paraffin engine of 1885 to the Scott-Still combined Diesel-steam type of i928: