Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(31) Diesel Oil Engine.

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 expanded to 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 compression pressure 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 that are 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 two sets 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 the Motor 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 rises beyond 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.