CHAPTER VIIOIL ENGINES

The Diesel engine marks the greatest progress in the internal combustion field made in the last few years. It marks a distinct advance in both thermal efficiency, and in the character of the fuel that it has made a commercial possibility. By the use of cheap fuel heretofore unavailable for any type of prime mover, such as the asphaltum residual oils, coal tar, etc., it has lowered the cost of power production to a point where it is unapproached by any type of heat engine. Besides its thermal efficiency, the engine is free from the annoyances due to delicacy of the auxiliary appliances such as the carburetor, and ignition system which are indispensable with the ordinary type of gasoline engine.

This engine belongs to that type of engine in which combustion takes place at constant pressure (Brayton Cycle), that is the combustion pressure is maintained at a constant value for a considerable distance on the working stroke of the piston. This method differs from the Otto cycle in which the combustion proceeds at a constant volume, or the type in which combustion is completed before the piston moves forward on the working stroke.

In the Diesel cycle the first stroke of the piston draws pure air into the cylinder; the piston then moves forward on the compression stroke, compressing the air to 500 or 600 pounds per square inch and raising the temperature of the air to about 1,000 degrees C, the exact temperature and pressure depending on the character of the fuel used in the engine. The high pressure is obtained by using a small clearance space in the end of the cylinder. At the end of the compression stroke a spray of oil is injected into the cylinder which is instantly ignited by the high temperature of the compressed air.

The oil continues to burn as long as it is sprayed into the cylinder, this period being from one-quarter to one-third of the working stroke. After the oil is cut off, the hot gas is expandedto the end of the stroke at which point the pressure is very considerably reduced due to the mechanical work performed. It should be noted that the type of engine just described performs the complete cycle in four strokes, the fourth stroke being the scavenging stroke as in the ordinary four stroke cycle engine. While the four stroke cycle type of Diesel engine is by far the most common type, it is also built as a two stroke cycle that is similar to the two stroke cycle gas engine previously described except that pure air is received and compressed in the air compressor in place of the combustible mixture.

Fig. 9. Cross Section of Four Stroke Cycle Diesel Engine. The Center Valve is the Fuel Admission Valve.

It will be noted, that as there is no fuel in the cylinder during the compression stroke that there is no danger from preignition from an over heated charge, nor is there trouble from decomposed fuels due to a gradually increasing temperature so often met with in oil engines that compress the entire mixture. As the clearance space is exceptionally small there is a minimum of residual gas held in the cylinder after the explosion with the result that the fuel is completely consumed, and that a full charge is taken into the cylinder.

The speed and output are regulated by controlling the point in the working stroke at which the oil spray is cut off, and as this has no effect on the maximum pressure developed in the cylinder, as in the case of the ordinary gas engine control, the pressure charge under varying loads is not so severe. Because of the high compression, and the continued combustion, there is a very gradual increase of pressure. Since the amount of pure air admitted to the cylinder is the same at no load as at full load there is always sufficient air for the complete combustion of the fuel, and as there is a constant compressionpressure there is a constant ignition temperature and constant quantity of the working medium. Because of the high compression obtained by the Diesel type, it has an efficiency that is far beyond that of any other form of internal combustion motor.

Fuel Nozzle of the Koerting Diesel Engine Showing Operating Cam and Lever, and Compressed Air Connection.

Since the fuel is introduced gradually into the combustion chamber the combustion pressure rises very slowly so that it is not an explosive engine in any sense of the word, the combustion pressure rising steadily from the compression pressure to the maximum in proportion to the supply of fuel. In the ordinary type of gas engine with a compression pressure of from 60 to 70 pounds per square inch the pressure rises abruptly to about three and one-half times the compression pressure,with a correspondingly rapid drop in the pressure on the expansion stroke. In the Diesel engine the drop of pressure in expansion is much more gradual, the indicator diagram expansion curve being nearly horizontal. The uniform pressures thus obtained result in smooth action and even driving power, obtained with no other type of engine.

Fuel Pump of Koerting Diesel Engine with Operating Cam.

As the fuels used vary from the lightest hydrocarbons to the heaviest crude oils, there are many types of oil injection valves in use, the valves being in general divided into two classes, those in which the oil is vaporized mechanically by the pressure of a force pump, and those in which the fuel is vaporized by the atomizing effect of compressed air. Atomization by compressed air is however, the most common method since less trouble is experienced with the air pumps than with the liquid force pumps. The compressed air is supplied by pumps thatare either operated by the main engine or by an independent compressor engine.

The fuel valve is a plug screwed into the cylinder containing an inwardly opening check valve in the inward end. The hole in the center of the plug receives the oil charge under a few pounds pressure from the tanks, during the compression stroke of the engine, and at the end of the compression stroke, a blast of air at a pressure of about 250 pounds above the compression pressure blows it into the cylinder in the form of a fine spray. Injection valves of the forced feed type consist of a plug with a small passage and a needle valve for regulating the spray. Fuel is pumped into the valve at about 250 pounds above the compression pressure of the engine by a small single acting pump which is built so that the length of the stroke may be adjusted to meet the load. In practice the length of stroke is regulated by the governor, so that the full contents of the pump are delivered at full load, and a reduced amount with a short stroke at small loads. On issuing from the fuel nozzle, the liquid strikes a gauze screen by which it is broken up into very fine spray.

Fluidity is practically the only factor that governs the quality of fuel that may be used with the engine, since exceptionally heavy oils and tars cannot be successfully sprayed. In Fig. 9 is shown a cross-section of a Diesel engine cylinder in which the center valve in the cylinder head is the fuel valve, and the valves to the right and left are the air inlet and exhaust valves respectively. The two latter valves correspond to the inlet and exhaust valves of the Otto cycle engine.

Compressed air is used in starting the engine, which is admitted to the cylinder through an auxiliary valve which is operated by a starting cam on the cam shaft. By this mechanism, high pressure air is furnished to the cylinder during a portion of the working stroke, turning it over on the first few revolutions as a common air engine. As soon as the engine picks up speed, the starting valves are thrown out of operation, and the engine proceeds on its regular working cycle with the oil fuel.

When used for marine purposes in sizes over 100 horse-power, where it is not possible to use reverse gears, the Diesel engine whether of the two stroke cycle or four stroke cycle type must be made reversible. This may be accomplished by either of two methods, first, by changing the angular position of the cams in regard to the piston position, and second by using twosets of cams, one being for right hand rotation and the other for left hand. When a single cam is used, the relation of the cam shaft on which the oil pump cams and oil valve cams are located, is advanced or retarded in respect to the crank shaft by means of sliding the two spiral gears that drive the cam shaft, over one another, in a direction parallel to their axes. The spiral gears are moved back and forth by a hand controlled reverse lever. This type is used principally on the two stroke cycle type of engine as there are not so many factors to contend with as on the four stroke cycle.

With double cams, the system almost invariably used with the four stroke cycle engine, the cams may be mounted either on one shaft, or the ahead cams on one cam shaft and the reverse cams on another. When two shafts are used they are arranged so that either set of cams may be swung under the valve lifters by swinging the shafts in a radial direction by brackets. The single type of cam shaft is usually moved back and forth in a direction parallel to its axis, the ahead cams coming under the valve lifts at one position, and the reverse cams at the other. In the four stroke cycle Diesel it is evident that not only the relations of the oil pump and oil valves must be changed in respect to the piston position but the relations of the air inlet and exhaust valves must be changed as well. This necessitates double cams for the inlet and exhaust valves in order to reverse rotation.

Compressed air for starting and injection is generally supplied by a three stage air compressor or a compressor in which the pressure is built up in three different steps, the second cylinder taking the air from the discharge of the first, and the third cylinder taking the air from the second, and compressing it to about 250 pounds above the compression pressure of the engine. Perfect scavenging is possible with this engine because of the large excess of air supplied during the suction stroke and the period of injection. On the marine type the air pumps and water circulating pumps occupy about the same amount of space as the condenser and circulating pumps of a steam engine having the same outputs. In a recent test made with an Atlas-Diesel engine it was found that 11 per cent of the output was lost in driving the air pumps or more than 50 per cent of the total loss by friction and impact.

Fig. 67. Cross-Section Through the Working Cylinders of the M. S. Monte Penado Two Stroke Cycle Diesel Engine. From theMotor Ship, London.

Unlike the ordinary gasoline engine in which an increase of speed increases the output in an almost direct proportion, the output of the Diesel engine decreases when the speed risesbeyond a certain limit due to imperfect combustion at speeds much over 350 revolutions per minute. Because of this fact it has been practically impossible to apply the type to automobile service which ordinarily requires a speed of from 400 to 800 revolutions per minute under ordinary conditions. In addition to the speed limitations, the Diesel engine weighs approximately 70 pounds per horse-power against an average weight of 17 pounds per horse-power with the ordinary type of gasoline automobile motor. Of course these objections may be overcome in time, as the engine is only in its infancy, and the two stroke cycle Diesel has not yet been fully developed, but at the present time it does not seem probable that this engine will ever be an active competitor of the gasoline automobile motor, at least from the standpoint of flexibility.

As the Diesel engine depends entirely upon compression for its operation, it is necessary that all of the parts such as the pistons, valves, etc., shall be perfectly fitted and air tight under extremely high pressures. The careful workmanship required for such fitting and the adjustments make the Diesel much more expensive to build than the ordinary type of gas engine, and for this reason the first cost and overhead charges cut into the fuel item to a considerable extent. A description of the Diesel engines will be found in the chapter devoted to oil engines.

As a practical example of a Diesel engine, which was described in Chapter III, we will give a brief description of the two 850 horse-power Diesel engines installed in the cargo vessel “M. S. Monte Penedo,” which were built by Sulzer Brothers of Winterthur, Switzerland. We are indebted to theMotor Ship, London, for the details.

The engines are of the two stroke cycle, single acting type, with four working cylinders, a double acting scavenging pump cylinder, and a three stage ignition compressor cylinder. The bore of the working cylinders is 18.8 inches, and the stroke 27 inches. While the crank case is of the enclosed type, there are two sets of covers which can be easily removed for inspection while the engine is running, for as the scavenging pump performs the work of the crank case of the ordinary two stroke cycle engine there is no need of a tight case to retain the compression.

Fig. 68. Cross-Section Through the Air Cylinders of the Two Stroke Diesel Motors on the M. S. Monte Penado.

The scavenging pump is mounted on one end of the engineand is driven from the crank-shaft, the cross-head of the pump forming one piece with the piston of the low pressure cylinder of the injection air cylinder. All of the compressor stages are water cooled and fitted with automatic valves. The double acting scavenging pump has a piston valve driven by a link motion for reversing it when the engine is reversed. The air enters the pump through the top valve chamber from a pipe leading into the engine room. The air discharges a pressure of about 3 pounds per square inch in a header that passes in front of all four working cylinders. By means of a valve the air entering the low pressure stage of the compressor can be taken either from the atmosphere or from the discharge of the scavenging pump; taking the air from the latter allows of a greater weight of air taken by the compressor and consequently a higher compression for use in emergencies.

As in the ordinary type of two stroke cycle engine, two independent sets of exhaust ports are used, one set being for the scavenging air and the other for the exhaust gases, both sets being at the end of the stroke as usual. The air inlet ports are divided into two groups, however, one group being controlled by the piston of the working cylinder, and the other group by an independent piston valve driven from the cam-shaft. Both sets of ports connect with the main scavenging air header. By means of the valve controlled ports it is possible to admit scavenging air even after the other ports are closed by the piston, which greatly increases the scavenging effect. With the air at 3 pounds pressure the air from the valve controlled ports throw the scavenging air to the top of the cylinder even after the exhaust ports are closed. This valve is provided with a reverse mechanism. A single cam is used for operating the fuel inlet valve and the air starting valve, and the reversal of the engine is obtained by turning the cam shaft through a small angle relative to the crank-shaft, which of course also reverses the lead of the fuel valve. Starting is accomplished by compressed air, with the air valve lever on the cam, and the fuel valve lever off. After turning through a few revolutions, the air valve levers are raised, and the fuel levers dropped back on the cams which results in the engine taking up its regular cycle.

By moving the tappet rod of the fuel valve out of or into a vertical position, the time of the fuel valve opening is regulated and the amount of air is controlled. This movement is normally performed by a compressed air motor, but in an emergency hand wheels may be used.

One of these serves to rotate the camshaft through the required angle in order to set the cams in the positions for astern or ahead running and also reverses the link motion of the scavenging pump valve by the rotation of shaft, as mentioned above. The other auxiliary motor operates the fuel and starting air valves by moving the small spindle longitudinally to bring the tappet lever of the air valve about the required cam for ahead or reverse and also lifts this or the fuel valve tappet rod off its cam, according as it is desired to run on fuel or air.

The spindle on which the valve levers are pivoted is in two parts, divided at the center. This is to allow two of the cylinders to run on air whilst the other two are running on fuel, and, as can be seen from the dial where the pointer indicates the position, in starting up, whether astern or ahead, first two cylinders are put on air, then four on air, next two on air and two on fuel, and finally all four on fuel. This allows very rapid attainment of full speed.

The amount of fuel entering each cylinder can be regulated separately by small hand wheels.

Below the fuel pumps are arranged three auxiliary pumps, two of these being oil pumps for the oil circulation, whilst the other is of the piston cooling water. On the left of the engine and driven in a similar manner from the cross-head by links are three other pumps, one for the circulating water and the other for the general water supply of the ship.

Lubrication for the cylinders is furnished by 8 small pumps, just above the water pumps, two oil pumps being provided for each cylinder. As the supply pipe is divided into two parts, the oil reaches the cylinder at four points in its circumference. Four oil pumps are provided for the air compressor.

Four steel columns are provided for the support of each cylinder in addition to the cast iron frame of the base, and by this means the explosion stresses are transmitted directly to the bed plate. The cast iron columns provide guide surfaces for the cross-head shoes. The guides are all water cooled.

The Maschinenfabrik Augsburg-Nürnberg, A. G., a German firm have built some remarkably large Diesel engines both of the vertical and horizontal types. The peculiar merits of the horizontal type of Diesel engine of which the M.A.N. company are pioneers are still open to discussion at present, but there is no doubt but what this type will be the ultimate form ofvery large engines when certain alterations are made in the design.

Fig. 69. Horizontal M. A. N. Diesel Engine at the Halle Municipal Plant.

Fig. 70. High Speed Mirlees-Diesel Engine.

In Fig. 69 is shown a 2,000 brake-horse-power horizontal M.A.N. Diesel engine of the four stroke cycle type which is installed at the Halle Municipal Electricity Works, Halle, Germany. It is of the double acting type with twin-tandem cylinders giving four working impulses per revolution. This engine was installed in addition to the six producer gas engines already in place to take the peak load of the station at different times during the day, the gas engines meeting the normal steady demand.

This firm has built many thousands of the vertical type of Diesel engine of all sizes, and has recently installed 13 engines of 4,500 brake horse-power for operating the Kreff tramways. The company is now building cylinders giving outputs of from 1,200 to 1,500 brake horse-power per cylinder, giving outputs of from 5,000 to 6,000 horse-power in tandem twin type engines. As will be seen from the cut, the horizontal Diesel engine is remarkably free from complicated valve gear.

The Mirlees-Diesel engine is built by the English firm, Mirlees, Bickerton and Day both for stationary and marine service. A generating plant consisting of two, 200 horse-power Mirlees engines direct connected to Siemens generators has been installed in the municipal plant at Dundalk as shown by Fig. 71. On test these units consumed 0.647 pounds of oil per horse-power at full load and 0.704 pounds per horse-power at half load with a regulation of 3.24 per cent from full load to no load. All of the engines built by this firm are of the four stroke cycle type.

Fig. 71. Mirlees-Diesels at Dundalk.

The Willans-Diesel engines built by the Willans and Robinson Company of Rugby, England, are in sizes up to 400 brake horsepower,and run at speeds up to 250 revolutions per minute. They are all of the four stroke cycle type and are applied principally to the driving of electric generators. The cut shows one of the four, 280 horse-power units supplied to the Alranza Company and the Rosario Nitrate Works in South America.

Fig. 72. Willans Vertical Diesel Engine.

Unlike the Mirlees engine, the Willans has an individual frame for each cylinder as in steam engine practice. Like the steam engine frame, the bottom is left open for the inspection of the connecting rod ends and the main bearings which is a most desirable feature. The air compressor and pumps are arranged in a most compact form at the left end of the crank-shaft from which the pipes may be seen issuing to the four cylinders. The valves and over head gear are of the conventional type, which, with the exception of a few minor details are the same as those on the recently developed Sulzer-Diesel. The individual grouping of the cylinder units has many desirable features and should, we believe, be more extensively copied.

An English gas-electric station was completed at Egham, England, that is a good example of the changes that have been made recently in the electricity supply abroad, with Diesel power.

The generating plant comprises two 94 K. W. Diesel engines built by Mirrlees, Bickerton and Day, direct connected to single phase alternators generating at 2,000 volts. The exciters are direct connected to the main shaft, and the plant is capable of generating an overload of 10 per cent for two hours. Space has been left for the installation of two more units of a larger size.

The following fuel consumption was guaranteed for a load of unity power factor, and the official tests show slightly better figures than the guarantee.

Cross-Section Through Egham, England Municipal Plant.

Particular attention has been given to the water supply for the jackets of the engines; the circulation being by two electrically driven, direct connected centrifugal pumps, one of which is a spare. A Little Company’s cooler has been installed, which consists of a horizontal cylindrical chamber, the lower part of which contains water. In the tank are arranged a number of concentric metal cylinders spaced about ¼-inch apart, and in several sections, that are carried on a slowly revolving shaft, driven from the fan shaft. The cylinders are all of the same length, and are open at both ends.

The lower half of the cylinders dips into the water in the casing, and as they revolve, a thin film of water on each side of the plate is carried into the upper portion of the casing where it meets a blast of cold air from the fan. The fan is driven from the circulating pumps, and passes the air throughthe chamber in a direction opposite to that of the water, baffles being placed so that correct circulation is maintained.

The small loss is made up by connecting the ball cock in the tanks with another tank charged from the works well by means of a self-starting rotary pump, electrically driven. Very little power is required for the pumps and cooler. Fuel oil is stored in a tank outside the building, the oil being supplied to the tanks from an oil wagon by means of a small hand pump.

Oil is taken from the tanks and forced into the engine room by a rotary pump, from which it enters two graduated tanks located in the roof of the station. The graduations on the tanks allow the consumption of oil to be carefully recorded by alternately filling and emptying the two auxiliary fuel tanks.

The entire building is electrically heated, and the kitchen of the flat above the station is equipped with an electric cooking-stove for the use of one of the engineers who make it his residence.

P. A. Holliday, in theEngineer, derives a new formula for computing the horse-power of the four stroke cycle, single-acting engine. For each horse-power developed by these engines about 21,000 cubic inches of displacement is necessary, per minute.

For high speed, low ratio (R), four stroke cycle engines, approximately 22,000 cubic inches displacement per minute is required.

In both formulae, the air compressor for fuel injection is included.

(32) Semi-Diesel Type Engine.

In the “Semi-Diesel” Type Engine the oil is injected into the cylinder at the point of greatest compression in the same manner as in the Diesel engine, and like the Diesel it compresses only pure air. In regard to the compression pressure, however, it stands midway between the pressure of the Diesel engine and that of the ordinary “aspirating” type oil engine, as the compression averages about 150 pounds per square inch. While this is a much higher pressure than that carried by the ordinary kerosene engine which compresses a mixture of kerosene vapor and air, it is not sufficiently high to ignite the oil spray by the increase in temperature due to the compression, but ignites the charge by means of a red hot bulb or plate placed in the combustion chamber.

This type of engine is built both in the two stroke and four stroke cycle types, the events occurring in the same order as in the two stroke and four stroke Diesel types, that is, pure air is drawn into the cylinder on the suction stroke (four stroke cycle) or is forced in at the beginning of the compression stroke (two stroke cycle), and is compressed in the combustion chamber. At the end of the compression stroke, the fuel is injected against the red hot bulb or plate by which the charge is ignited. Expansion follows on the working stroke after the fuel is cut off, and release occurs at the end of the stroke.

Fuel oil is supplied to the spray nozzles by a governor controlled pump having a variable stroke or by compressed air as in the Diesel engine, making the supply of fire proportional to the load. A separate pump is generally supplied for each cylinder, which is capable of developing a pressure of about 400 pounds per square inch. Several of the Semi-Diesel type engines have water sprayed into the cylinder for the purpose of cooling the cylinder and piston, and as an aid in the combustion. This water spray increases the output of a given size cylinder by the amount of the steam formed by the heat of the cylinder and piston walls, and by the increased rate of combustion. The amount of water supplied to the cylinder is equal, approximately to the amount of fuel oil. The water connection is made in the air intake pipe so that the water spray and the intake air are drawn into the cylinder at the same time.

There is very little difference in the efficiency of the Diesel and Semi-Diesel in favor of the true Diesel type for both have accomplished records of a brake horse-power hour on .45 pound of crude oil in units of the same capacity. Neglectingthe question of efficiency the Semi-Diesel has many advantages which are due principally to the differences in compression pressures. Valve and piston perfection in regard to leakage is not as essential with the semi-type as with the Diesel, as the former is not dependent on compression for its ignition. This means that the Semi-Diesel has a lower first cost and a lower maintenance expense. Its low compression pressure makes starting possible without the use of compressed air with engines of a considerable horse-power. As the explosion pressure is much lower than with the Diesel type there is less strain on the working parts and lubrication is much more easily performed.

Compared with the ordinary type of kerosene engine the Semi-Diesel is much more positive in its action as the oil is sure to ignite when sprayed on the hot surface of the bulb or plate when under the comparatively high compression. In the engine where the air is mixed with the vaporized fuel before it is drawn into the cylinder, it is difficult to obtain perfect combustion because of the uncertain mixtures obtained on varying loads by the throttling method of governing. At light loads the only difficulty encountered with the Semi-Diesel type is that of keeping the igniting surface hot enough to fire all of the charges.

In the majority of cases the two stroke cycle type of Semi-Diesel engines compress the scavenging air in the crank chamber in the same way that a two stroke cycle gasoline motor performs the initial compression, although there are several makes that compress the air in an enlarged portion of the cylinder bore by what is known as a “trunk” piston. This initial compression determines the speed of the engine, the pressure limiting the time in which the air traverses the cylinder bore and sweeps out the burnt gases of the previous explosion.

Two types of four stroke cycle oil engines are built by the De La Vergne Machine Company, which differ principally in the method and period of injecting the fuel into the cylinder. While both types compress only pure air in the working cylinder, the oil is injected in a heated vaporizer during the suction stroke in the smaller engine (type HA), and is injected directly into the combustion chamber of the larger engine (type FH) at the point of greatest compression. This fuel timing classifiesthe type FH as a semi-Diesel, while type HA comes under the head of that class of engines known as aspirators.

76-a. Elevation of De La Vergne Oil Engine, Semi-Diesel Type. Class F H.

Semi-Diesel (Type FH)

During the suction stroke, air is drawn into the cylinder through the inlet valve located on the top of the cylinder head, and on the return, or compression stroke, the air is compressed to about 300 pounds per square inch in the combustion chamber. The compression heats the air to a high temperature which is still further increased by contact with the hot walls of a cast iron vaporizer D, shown by Fig. 76-b. At the completion of the compression, the fuel is injected in a highly atomized state by compressed air through the spray nozzle F, the spray being thrown into the vaporizer.

76-b. Cross-Section of Type F H, De La Vergne Oil Engine.

The vapor formed by the contact of the spray with the wallsof the vaporizer mixes with the compressed air in the combustion chamber and is ignited at the instant of fuel admission by the combined temperatures of the vaporizer and compression pressure.

As the fuel is not injected until the proper instant for ignition, it is possible to obtain a relatively high compression without danger of the charge preigniting. The oil is supplied to the nozzle by a fuel pump under pressure. The atomizing air takes the oil at pump pressure and performs the actual injection. Details of the spray valve are shown by Fig. 76, in which the oil and air are entered at a pressure of about 600 pounds per square inch.

Fig. 76. De La Vergne Spray Nozzle.

The air and oil enter the nozzle at opposite sides of the cylinder B which fits snugly into the valve body A. As the air and oil proceed side by side along the outside of B, they are forced to pass through a series of chambers connected by a system of fine diagonal channels on the surface of B which results in a very fine subdivision and intimate mixture. The charge is admitted to the cylinder by a sort of needle valve about one-half inch in diameter which is provided with a spring that holds it closed on its seat as shown by C, in Fig. 76. The needle is so constructed that it may be readily removed at any time for inspection. The spray valve is located on the right hand side of the valve chamber directly oppositethe vaporizer and is operated by an independent cam on the camshaft.

Fig. 76-c. De La Vergne Governor and Fuel Pump.

The vaporizer consists of an iron thimble having ribs on the inside to increase the radiating surface. In starting, the vaporizer is heated for a few moments until it reaches the temperature necessary for vaporizing the fuel, but after the engine is running, the blast lamp is removed and the temperature is maintained by the heat generated by the combustionof the successive charges. Since the fuel is ignited at the instant that it makes contact with the vaporizer, it is possible to accurately adjust the point of ignition by adjusting the position of the fuel cam on the camshaft.

Air for spraying the fuel is supplied by a two stage air compressor that is driven from the crankshaft by an eccentric. The air compressed by the first stage is stored in tanks at about 150 pounds pressure for starting the engine. The second stage compresses the air to about 600 pounds pressure, but is correspondingly small in volumetric capacity since it handles only enough air to spray the oil which amounts to about 2 per cent of the cylinder volume. A governor controlled butterfly valve in the air intake pipe regulates the amount of air taken in on the second stage to suit the varying charges of oil injected at each load.

In starting by compressed air, a quick opening lever operated valve on the cylinder head is used to admit air from the tanks to turn the engine over until the first explosion takes place. If the vaporizer is sufficiently heated by the torch, the explosion occurs during the first revolution of the crank shaft. At a point about 85 per cent of the expansion stroke, the exhaust valve is opened, and the products of combustion are expelled into the atmosphere. When starting, the compression may be relieved by shifting the starting lever from the exhaust cam to the auxiliary starting cam provided for that purpose.

Speed regulation is affected by a Hartung governor, driven from the camshaft, which actuates the oil supply pump through levers by shifting the point of contact between the pump levers and its actuating cam. This lengthens or shortens the stroke of the pump in accordance with the requirements of the load. The type FH engines are built in both single and twin cylinders ranging from 90 to 180 horse-power in the single cylinder type to 360 horse-power in the twin.

Since the fuel injection of the smaller engine type HA differs from that just described, it will be described separately in the following section.

In the small four stroke cycle De La Vergne Oil Engine, the fuel is injected into a heated vaporizer during the suction stroke in such a way that the vapor and intake air do not form a mixture in the cylinder proper. On the return stroke ofthe piston, the compression of the pure air takes place which forces the air into the vaporizer and into intimate contact with the oil vapor. This forms an explosive mixture which ignites and forces the piston outwardly on the working stroke. The release and scavenging are performed in a similar manner to that of a four stroke cycle gas engine. Both the inlet and exhaust valves are of the mechanically operated poppet type, and as both the inlet and exhaust gases pass through the same passage, the entering air is heated to a comparatively high temperature.

The injection pump receives the fuel from a constant level stand pipe or tank, located near the engine and injects the fuel into the vaporizer through a spray nozzle. The vaporizer is a bulb shaped vessel that is connected with the cylinder through a short post and really forms a part of the combustion chamber. Since no water jacket surrounds the vaporizer, it remains at a high temperature and vaporizes the oil at the instant of its injection. Because of the residual gases remaining in the chamber, ignition does not occur until air is forced through the passage by the compression. The air inlet valve and the fuel injection valve are opened at the same instant by a cam lever that also operates the pump.

On the compression stroke, the air which is at a pressure of approximately 75 pounds per square inch enters the vaporizer, and ignition occurs, partly because of the increased heat due to the compression and partly because of the supply of additional oxygen. Internal ribs provided in the vaporizer greatly increase the heat radiating surface and add to the thoroughness with which the atomized oil is vaporized. Since no mixture of air and fuel takes place in the cylinder proper, sudden changes in the load do not affect the ignition of the charge as the heated surfaces are surrounded with comparatively rich gas under all conditions.

Before the engine is started, the vaporizing chamber is heated to a dull red heat by means of a blast torch in order to vaporize the oil for the first stroke. As soon as the engine is running, the lamp is cut out and the temperature is maintained by the heat of the successive explosions. The combustion attained by this method is very complete even with the heaviest fuels, and whatever carbon deposit is formed occurs in the vaporizer from which it is easily removed. The contracted opening of the vaporizer passage effectually prevents the solid matter from working in the bore or valves.

A Porter-type fly ball governor maintains a constant speed at varying loads by regulating the quantity of fuel supply to the vaporizer, the air intake remaining constant. A by-pass valve, controlled by the governor divides the oil supplied by the pump, into two branches, one of which leads to the spray nozzle and the other to the supply tank. In the case where all of the oil is not supplied to the vaporizer because of a light load, the by-pass valve will return the surplus to the tank, thus maintaining a constant pressure at the spray nozzle.

When operating under ordinary loads, the governor opens only the small inside valve which regulates the amount of oil injected into the vaporizer. But should the engine speed up, due to a sudden change in the load, the governor will not only open the small valve but also the large concentric valve, in which case all of the oil will return to the tank. The makers guarantee the following speed variation limits under the different loads.

As the semi-Diesel type engine will operate successfully on the lowest grades of crude oils, with an efficiency that compares favorably with the true Diesel type, the operating expenses are very much lower than with the gas or gasoline engine. With the same fuels, the semi-Diesel will show greater net saving than the Diesel with a low load factor, as the fuel saving is not eaten up by the high first cost, and overhead charges of the true Diesel. Western crude oils with a specific gravity of .960 (16° Beaumé) are being used daily with this type of engine while nearly every builder of the semi-Diesel type will guarantee results with oils up to 18° Beaumé (.948 Specific Gravity). Fuel of this grade will cost anywhere from 1½ cents to 3½ cents per gallon in tank car lots, depending on the distance of the engine from the wells or refinery.

With fuel oil weighing 7½ pounds per gallon, an engine consuming .65 pounds per horse-power hour (a usual guarantee) at full load, the cost of a horse-power hour delivered at the shaft will be .26 cent with fuel at 3 cents per gallon. This the lowest fuel expense of any prime mover even with steam or gas units of great power. In a twenty-four hour test of aDe La Vergne oil engine running on 19° Beaumé oil, the consumption was considerably below the figure assumed above, being .508 pounds per horse-power hour. Even the engine was exceeded in a test made on a 175 horse-power engine by Dr. Waldo, which gave a consumption of .347 pounds of oil per horse-power hour with oil of .86 Specific Gravity.

The following is a tabulation of reports received by the De La Vergne Machine Company from the Snead Iron Works, giving the cost of power at their plant under actual working conditions extending over a period of twenty-four months. The plant consisted of a 17 × 27½ inch De La Vergne semi-Diesel type engine of 180 horse-power rated capacity, the load factor being 54.2 per cent. The total power produced during the record was 552,217 horse-power hours, with a working period of 588 days. Fuel = 28.8° Beaumé = 7.35 pounds per gallon.

Fuel oil used = .761 pounds per K. W. hour = .508 pounds per horse-power hour. Computing from the load factor of 54.2 per cent, the cost of power produced under the above conditions would be $9.30 per horse-power year, or $13.98 per kilowatt year. This result is obtained by assuming that the horse-power hours would be increased from 552,217 to 1,077,354, or in proportion to the actual load factor, the period, of course being the same in both cases.

A type of semi-Diesel type oil engine has been recently developed by the Elyria Gas Power Co., Elyria, O., that presents many features of interest. It operates on the two stroke cycle principle, and with the exception of the spray nozzle has no valves in the working cylinder. The principle of the semi-Diesel type cycle as distinguished from the true Diesel engine, was described in Chapter III, as havingthe following characteristics. (1) Fuel injection. (2) Medium compression pressure. (3) Hot plate ignition. (4) An efficiency approximating that of the true Diesel type.

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