CHAPTER IVINDICATOR DIAGRAMS

A brief description of the indicator as a means of recording the pressures in the cylinder of a simple heat engine in relation to the piston position was given in paragraph (6), Chapter I, and as this instrument is so peculiarly adapted to locating the events taking place in the cylinder we will devote some space on its application to the practical gas engine cycles described in the preceding chapter. Since each event in the cycle is accompanied by a corresponding increase or reduction in pressure, the beginning or end of an event will be indicated on the diagram by a change in the vertical height of the curve above the atmospheric line, at some particular piston position. The piston position will be in the same relation to the total stroke as the pencil position will be to the horizontal length of the card.

If the event, for example, as indicated by a drop in pressure, be at the center of the card, it will show that the drop in pressure took place when the piston was in the center of the cylinder or at mid-stroke. Should the pressure change at a point one-quarter of the card length from the starting point of the pencil, it shows that the event took place in the cylinder when the piston had accomplished the first quarter of its stroke, and so on. It should be noted that horizontal distances on the indicator card denote piston positions, and the vertical distances, pressures.

As explained in a former paragraph the length of the vertical lines represents certain definite pressures, each inch of length representing so many pounds as per square inch, the exact amount per inch depending on the indicator spring strength or adjustment. To make this point clear, all of the indicator diagrams shown in this chapter will be provided with a scale of pressures at the left of the diagram by which the pressure at any point may be accurately measured off for practice. It should be noted that points on the curves which are above the atmospheric linerepresent positive pressures above the atmosphere, and that the points lying below the atmospheric line represent partial vacuums which may be expressed as being so many pounds per square inch below the atmosphere. The vacuum pressures indicate the extent of the “suction” created by the piston when drawing in a charge of air and gas.

Straight vertical lines show that the increase of pressure along that line has been practically instantaneous in regard to the piston velocity, for if the pressure increased at a slow rate this line would be inclined toward the direction in which the piston was moving, as the piston would have moved a considerable distance horizontally while the pencil was moving vertically. This inclination of the vertical line gives an idea of the rate at which the pressure increases in relation to the piston speed, the greater the inclination, the slower is the rate of pressure increase. Straight horizontal lines that lie parallel to the atmospheric line denote a constant pressure or vacuum.

The rate at which horizontal lines descend or incline to the atmospheric line represents the rate at which the pressure increases or decreases, in respect to the piston position (not piston velocity). A steep curve represents a rapid expansion or compression from one piston position to the next. A waving or rippling line indicates vibration due to valve chattering or explosion vibrations. A straight inclined line shows that the pressure is decreasing or increasing in direct proportion to the piston position.

By referring to paragraph 25, Chapter III, it will be seen that the five events of suction, compression, ignition, expansion and exhaust are accomplished in four strokes, in the following order:

These events with the pressures incident to each drawn to some relative scale are shown graphically in Fig. 10 by four lines representing the four strokes of the piston. In order to show the relation between the diagram and the piston, a sketch of the cylinder with a stroke equal to the length of the diagram is shown directly beneath the curve. The vertical line IJis the scale of pressures (somewhat exaggerated in order that the small vacuum and scavenging pressures shall be clearly shown). The line marked “atmosphere” represents atmospheric pressure and it is from this line that all measurements of pressure are taken.

Figs. 10–11–12. Showing Respectively a Typical Four Stroke Diagram, Retarded Combustion and Retarded Spark.

Consider the piston starting on the suction stroke, the piston moving from the position L to K, or from left to right. The movement creates a partial vacuum in the combustion chamber N which is shown on the diagram as the distance OA, equal to 2 pounds below atmosphere according to the pressure scale. The suction line remains at this distance below the atmospheric line until within a short distance of the end of the stroke when it rises to meet the atmospheric line at B when the piston reaches the end of the stroke at K. This rise at the end of the stroke is due to the fact that the piston moves more slowly when approaching the end of the stroke while the velocity of the incoming gases remains nearly constant so that the piston exerts no pull nor suction. On the diagram the entire suction stroke is represented by AB.

The piston now returns on the compression stroke from K to J compressing the mixture in the combustion chamber N. On the diagram this stroke is shown beginning at B, with the pressure slowly rising until the pressure is a maximum at the point C at the end of the stroke. During the compression, the pressure has risen from that of the atmosphere at B to 125 pounds pressure at C as shown by the scale. At a point slightly before C is reached, ignition occurs, and the pressure rapidly rises from C to D, due to the expansion of the heated gas. In this case the combustion is practically instantaneous as shown by the straight, vertical combustion line CD.

At D the piston starts on the working stroke from left to right increasing the volume of the gas and at the same time diminishing the pressure because of the expansion until the maximum pressure of 400 pounds per square inch at D is reduced to 30 pounds per square inch at E, the line DE being called the expansion line. During this time the heated gas has been performing work on the piston. At E the exhaust valve opens and the pressure drops from E to T, a point still about 10 pounds above atmospheric pressure. Theoretically the pressure should drop instantly from E to atmosphere, or from 30 pounds per square inch to zero, but practically this is impossible because of the back pressure due the slow escape of the exhaust gases through the comparatively small valve openings and exhaust pipes.Since considerable pressure is exerted by the piston on the return stroke in forcing the gases out of the exhaust valve, the exhaust line TO on the diagram is nearly 10 pounds above the atmospheric pressure from T to O. At a point near O, the piston slows up on nearing the end of the stroke so the gases have more time to escape through the valves, and the pressure drops to the atmosphere, readyfor the succeeding suction stroke.

It should be noted that the points A, B, E, and F represent periods of valve action. At A the inlet valve opens; at B the inlet closes; at E the exhaust opens; at F the exhaust closes, and at A the inlet again opens at the beginning of the suction stroke AB. That this is true is apparent from the fact the inlet must open at the beginning of the suction stroke, and both valves must be closed from the point B to the point E in order to prevent the escape of the compressed charge and expanded gases from the cylinder. At the end of the working stroke the exhaust valve must liberate the gases and remain open to the end of the scavenging stroke to eliminate the residual gas while the closed inlet valve prevents the burnt gases from being forced through the inlet pipe and carburetor.

As shown on the diagram, the exhaust valve closes at the same time that the inlet opens, as F, and O both occur on the same vertical line DL. This is true theoretically, but owing to the different conditions met in practice, the actual setting of the valves may vary slightly from that shown on the diagram. Some makers of high speed engines open the inlet slightly before the exhaust closes as it is claimed that the inertia of the exhaust gas passing through the exhaust pipe creates a slight vacuum that is an aid in filling the cylinder with a fresh charge. It should be borne in mind that this condition only exists when the piston has come to rest and exerts no pressure on the exhaust gas. The vacuum is due to the velocity inertia of the gas after it has been reduced to atmospheric pressure. Other makers close the exhaust valve a very little before the inlet opens, but no matter what the setting, the difference in the time of opening and closing is very small, and the results obtained probably differ by an almost negligible amount.

During the suction and scavenging strokes, the fly wheel of the engine is expending energy on the gas since it is moving a considerable volume at a fairly high pressure. In the case of the scavenging stroke, the piston is working against 10 pounds back pressure, which on a 10 inch piston would amountto a force of 785 pounds. With the 2 pound vacuum the drag on the piston would amount to 157 pounds, no small item when the velocity of the piston is considered. Of course the pressure of 10 pounds per square inch is rather high, but it is often attained with long and dirty exhaust pipes. It is items of this nature that cut into the efficiency of the engine, and increase the fuel bills, and it is only by the indicator that we can determine the extent of such “leaks” and remedy them.

Since the area of the indicator card represents the power of the engine, it is evident that we lose the power represented by the area included in the rectangle FEBO on the scavenging stroke plus the area BOA on the suction stroke. The area included in BCO represents the work taken from the engine in compressing the charge, but this is returned to us during the next stroke plus the benefits gained by compressing the mixture. The arrows show the direction in which the piston is moving during that event.

An actual engine does not follow the form of the diagram shown by Fig. 10 exactly because of certain conditions met with in practice such as imperfect mixtures, faulty valve and ignition timing, small valve areas or leakage. The combustion in the real engine is neither instantaneous nor complete but it approximates the “IDEAL” cycle just described more or less closely with a high compression and a fairly well proportioned mixture.

For the best results the gas must be completely ignited at the point of maximum compression, and the pressure must be established on the dead center, so that the indicator card will show a straight and vertical combustion line. As all gases require a certain length of time in which to burn, the ignition should haveLEAD, that is, should be started before the end of the stroke so that combustion will be complete at dead center. The amount of ignition lead required depends on the fuel and the compression. In Fig. 10 the point of ignition (I) is shown as occurring before the end of the compression at (C), which insures a straight combustion line CD.

With a lean or slow burning gas, that is, a gas slower than used on the diagram, combustion would not be complete at the end of the stroke if the same point of ignition were used. This effect is shown by Fig. (11), in which the full line diagram BCDE represents the ideal diagram (Y), and BCFG represents the slow burning mixture with the same point of ignition (X).The compression curves of both diagrams are coincident as far as C, the ideal diagram shooting straight up at this point and the weak mixture diagram staying at the same level. When under the influence of the mixture (X) the piston starts from left to right and reaches the point F before the slow burning gas reaches its maximum pressure. During this part of the stroke there has been very little pressure on the piston and it will be noticed that the maximum pressure is far below that of the ideal diagram. This low maximum is due principally to the reduced compression under which the gas has been burning, from C to F.

Figs. 13–14. The First Diagram (13) Shows a Two Port Two Stroke Diagram, the Second Shows a Typical Diesel Card.

As the gas has but a small part of the stroke left in which to expand, the pressure at the point of release is much higher than the release pressure of the ideal diagram, which means that a considerable amount of heat and pressure have been wasted through the exhaust pipe. Besides the heat loss, thehigh temperature of the escaping gas has a bad effect on the exhaust valve and passage. The great volume of gas passing through the exhaust valve also increases the back pressure on the scavenging stroke.

Delayed or retarded ignition will cause a low combustion pressure and slow combustion with any type of fuel or compression pressure as will be seen from Fig. 12. In this case the compression pressures of the ideal diagram Y and the diagram X showing the retarded spark are of course the same, the compression line extending from B to C in the direction of the arrows. At C the ignition occurs for curve Y, and the pressure immediately rises to D. In the case of curve X, ignition does not occur until the point I is reached, the compression falling on the line CI with the forward movement of the piston as far as the point I. At this point the compression pressure is very low which results in the slow combustion indicated by the slant of the combustion line IF. The point of maximum pressure F is much below D of the ideal curve, and as there is no opportunity for complete expansion during the rest of the stroke, the release pressure is high causing a great heat loss. If running on aLATEorRETARDEDspark is continued for any length of time the excessive heat that passes out of the exhaust will destroy the valves.

It is apparent that for the best results, the spark should occur slightly before ignition in order to gain the effects of the compression, and a high working pressure on the piston. It is also evident that the point of ignition should be varied for different mixtures that have different rates of burning. With engines that govern their speeds by throttling or by changing the quality of the mixture it is necessary for the best results, to vary the point of ignition with each quality of fuel that is admitted by the governor. The retard and advance of the ignition is very necessary on an automobile engine because of the throttling control and constant variation of the load and speed. All automobilists know of the heating troubles caused by running on a retarded spark.

In the two stroke cycle diagram, the lines showing the suction and scavenging strokes are missing if the indicator is applied only to the working cylinder.

Starting at the beginning of the working stroke as at A in Fig. 13, the gas expands during the working stroke until thepiston uncovers the exhaust port at B where the pressure drops to C. A slight travel uncovers the inlet port with the pressure still above atmosphere due to the pressure in the crank case filling the cylinder. The crank case pressure continues from C to D or to the end of the stroke, the pressure dropping slightly at the latter point.

The compression stroke now takes place with the piston moving from right to left, the compression pressure reaching a maximum at F. Ignition occurs slightly before the point of greatest compression, at I, and the expanded gas increases in pressure until the point A is reached. From this point the same cycle of events is repeated. Because of the dilution of the charge by the burnt gases of the preceding combustion, the mixture burns slowly as will be seen from the inclined combustion line FA. Due to this delayed combustion, the piston travels the distance S on the working stroke before the pressure reaches a maximum. This diagram is typical of the small marine type of two stroke cycle engine which has no further scavenging than that performed by the rush of the entering mixture. The diagram of the pressures and vacuums in the crank case are similar to those of suction and compression in the four stroke cycle type.

A diagram of the Diesel engine is different in many particulars from that of an ordinary gas engine, as will be seen from the diagram in Fig. 14. The pressures rise in an even, gradual line from the end of the compression curve, and instead of having a sharp peak at the end of the combustion, as in a gas engine, the top of the curve is broad and greatly resembles the indicator diagram of a steam engine. The compression curve constitutes a greater proportion of the pressure line than that of a steam engine, the rise of pressure due to the ignition being very slight in comparison to the height of the compression curve. There is no explosion in the usual sense of the word, only a slight increase in pressure as distinguished from the rapid combustion in the gas engine.

Starting at the beginning of the compression stroke at H, the pressure of the pure air charge increases to about 500 pounds to the square inch at I, the point at which the fuel is injected. From I to C is the increase of pressure due to the combustion. The pressure stays at a constant height from C to D as the fuel supply is continued between these points, and is cut off whenthe piston reaches the position D. It will be seen that the admission of the fuel through the distance A covers a considerable proportion of the working stroke, and that the points of fuel injection and ignition are coincident.

From the point of fuel cut-off at D expansion begins and is continued in the usual manner to F, the point of release.

When the load is increased, the period of oil injection is also increased, the other events remaining constant. Should the light load require an oil injection period as shown by A, the greater load would require injection for the period B. In the latter case, the expansion line would be EG, which would produce a diagram having a greater area than the line DF, and there would be a great increase in the release pressure GH as well.

It will be seen from the diagram that the quantity of air taken into the cylinder and the compression pressure remain constant with any load, and that for this reason it is possible to have a constant point of ignition, or rather point of fuel injection. As there is no mixture compressed, there are no difficulties encountered at light loads due to attenuated mixtures. An excess of air over that required to burn the fuel is also present at every load within the range of the engine. For the sake of simplicity, the suction and scavenging lines on the Diesel engine have been omitted, but they are the same in all respects as the corresponding lines shown in the diagram, Fig. 14.

In the attempt to gain mechanical simplicity, small weight, and diminutive size of the steam turbine, many able experimenters have endeavored to obtain an internal combustion motor in which the energy of the expanding gas is converted into mechanical power by its reaction on a bladed wheel, but so far the problem is far from being solved. In 1906 two experimental turbines were built by René Armengand and M. Lemale, of the constant pressure type, one of which developed 30 Brake horse-power and the other 300 horse-power.

A 25 horse-power De Laval steam turbine was altered by Armengand says Dugald Clerk so that it operated with compressed air instead of steam. The compressed air was passed into a combustion chamber together with measured quantities of gasoline vapor, and the mixture was ignited by an incandescent platinum wire as it entered the chamber, thus maintaininga constant pressure with continuous combustion. Around the carborundum lined combustion chamber was imbedded a coil in which steam was generated by the heat of the burning gas, the steam being used to reduce the temperature of the gas from 1800°C to about 400° as it issued from the orifice and came into contact with the running wheel. The working medium was therefore composed of two elements, the products of combustion and the steam at the comparatively low temperature of 400°C.

The constant pressure maintained in the combustion chamber was about 10 atmospheres, and the hot gases were allowed to expand through a conical Lava jet in which the expansion produced a high velocity, and reduced the temperature of the fluid. At this reduced temperature and high velocity the gases impinged upon the Laval wheel, and rotated the wheel in the same way as steam would have done. The experiments showed that under these conditions the total power obtained from the turbine separate from the compressor was double that necessary to drive the compressor.

In the large 300 H. P. turbine the first part of the combustion chamber was lined with carborundum, backed by sand, but the second part was surrounded by a coil through which water was circulated. The water kept the temperature of the combustion chamber within safe limits, and after absorbing heat, it passed also around the jet nozzle, and was discharged into the passage leading to the jet, and there converted into steam by the hot gases. A mixture of products of combustion and steam thus impinged upon the turbine wheel. The expanding jet was arranged to convert the whole of the energy into motion before the fluid struck the wheel; the temperature was thus reduced to a minimum before the gases touched the blades. Notwithstanding this, the wheel itself had passages through which cooling water flowed, and each blade was supplied with a hollow into which water found its way. In the large turbine the compressor was mounted on the turbine spindle; it was of the Rateau type, and consisted of an inverted turbine of four stages, which delivered the compressed air finally to the combustion chamber at a pressure of 112 lb. per sq. in. absolute. The efficiency of this turbine compressor was found to be about 65 per cent. The total efficiency of the combined turbine and compressor was low, as the fuel consumption amounted to nearly 3.9 lb. of gasoline per B. H. P. hour. An ordinary gasoline engine with a moderate compression canreadily give its power at the rate of 0.5 lb. of gasoline per B. H. P. hour. The combined turbine and compressor was stated to have run at 4,000 R. P. M. and to have developed 300 H. P. over and above the negative work absorbed by the compressor.

A gas turbine in which there was no compression was built in the following year by M. Karovodine which gave 1.6 horsepower at a speed of about 10,000 revolutions per minute.

It contained four explosion chambers having four jets actuating a single turbine wheel, which wheel was of the Laval type, about 6 inches diameter, having a speed of 10,000 R. P. M. The explosion chambers were vertical, and had a water jacket surrounding the lower end. The upper portion contained the igniting plug on one side, and the discharge pipe connecting with the expanding jet on the other. In the lower water-jacketed part there was provided a circular cover, held in place by a screwed cap. This circular plate was perforated with many holes, and it carried a light steel plate valve of the flap or hinging type, which pulled down by a spring contained within the admission passage. This spring could be adjusted, and the lift of the valve was regulated by means of a set screw passing diagonally through the water jacket. Air was admitted at one side by a pipe leading into the valve inlet chamber and a corresponding passage or pipe admitted gasoline and air or gas to mix with the air before reaching the thin plate valve. Adjusting contrivances were supplied in both air and fuel ducts. To start the apparatus, an air blast was forced through the valve, carrying with it sufficient gasoline vapor to make the mixture explosive. The electrical igniter was started, and the spark kept passing continuously. Whenever the inflammable mixture reached the upper part of the combustion chamber ignition took place, and the pressure rose in the ordinary way, due to gaseous explosion. The gases were then discharged through the pipe and nozzle on the Laval wheel. The cooling of the flame after explosion and the momentum of the moving gas column reduced the pressure within the explosion chamber to about 2 lb. per sq. in. below atmosphere. Air and gasoline vapor then flowed in to fill up the chamber, and as soon as the mixture reached the igniter, explosion again occurred. In this way a series of explosions was automatically obtained, and a series of gaseous discharges was made upon the turbine wheel. Diagrams taken from the explosion chamber showed a fall in pressure during suction of 2 lb. per sq. in.; ignition occurredwhile the pressure was low, and the pressure rapidly rose to about 1 1–3 atmospheres absolute. The pressure propelling the gas column and jet was thus only 5 lb. per sq. in. above atmosphere. The pressure rapidly fell, and the whole process was repeated again. According to the diagrams taken, a complete oscillation required about 0.026 second, so that about 40 explosions per second were obtained.

Fig. 15. Cross-Section of the Combustion Chamber of the Holzwarth Gas Turbine. From the Scientific American.

The most promising type of turbine that has been built to date is that designed by Hans Holzwarth, an engineer of some prominence in the steam turbine field. A 1000 horse-power machine has been built at this writing and as experimental machines go has made most remarkable performance.

The turbine in general arrangement outwardly resembles the Curtis steam turbine, in that the turbine wheel rotates in a horizontal plane, the spindle or shaft is vertical and a dynamo is mounted on this spindle above the turbine. In the Holzwarth turbine ten combustion chambers are provided, each of a pear or bag shape. They are arranged in a circle around the wheel,and are cast so as to form the base of the machine. The wheel is of the Curtis type, with two rows of moving and one row of stationary blades.

In this turbine the energy of the fuel is liberated intermittently by successive explosions, instead of by continuous combustion, and in much the same way that the explosions occur in a reciprocating engine. Tests made on the new machine have shown that it is in no way inferior in efficiency to the ordinary type of motor, and that at full load, the weight per horse-power is only about one-quarter of that of the reciprocating engine. The weight factor, as is well known, is of the utmost importance in marine service and should prove of value to the marine engineer, if this alone were its only characteristic.

Any of the ordinary power gases may be used with success, as well as vaporized liquid fuels, and the lower grade oils such as crude and kerosene have given much better results in the turbine, than in reciprocating engines, even at this early stage of its development. As the heat losses are much smaller than met with in ordinary practice, the temperature is higher, which, of course, greatly facilitates the vaporization of the lower grade liquids.

Mr. Holzwarth does not give the dimensions of his turbine wheel, but from the drawings and some of the velocities given by him it appears to be about 1 m. in external diameter. The lower part of each combustion chamber carries gas and air inlet valves, and the upper part carries a nozzle arranged to cause the gases to impinge upon the first row of moving blades. This nozzle is connected to and disconnected from the combustion chamber by means of an ingeniously operated valve. The explosion chambers are charged with a mixture of gas and air, which appears to attain a pressure of about two atmospheres within the chamber before explosion. The air and gas are supplied under sufficient pressure from turbine compressors, actuated by steam raised from the waste heat of the explosion and the gases of combustion, so that whatever work is done in compression is obtained by this regenerative action, and does not put any negative work upon the turbine itself. The combustion chambers are fired in series, by means of high-tension jump spark ignition.

Referring to the cut, the explosion chamber A is filled intermittently with the explosive mixture at a low pressure (about 8 to 12 pounds per square inch). When ignition has occurred, the pressure of explosion opens the nozzle valve F, allowingthe compressed gases to flow through the nozzle G to the bladed turbine H, on which the energy is to be expended. The expansion of the heated gases in the nozzle reduces the pressure to that of the exhaust, with the resulting increase in the velocity of the gas. By means of fresh air, the nozzle valve F is kept open throughout the expansion and scavenging periods.

After the expansion has been completed, the air that is forced through the valve D, at a low pressure, thoroughly scavenges or removes the residual burned gases left in the combustion chamber and nozzle, forcing it into the exhaust. When the scavenging has been completed, the nozzle valve and the air valve D are closed. The combustion chamber A is now filled with pure cold air, which not only enables a fresh charge of gas to be introduced into the chamber but which also aids in keeping the chamber cool.

Pure fuel gas, or atomized oil, is now injected through the fuel valve E, forming an explosive mixture ready for the ensuing cycle of events. A number of these chambers are arranged around the turbine wheel in order to have a uniform application of power, by having the several chambers working intermittently. This is in effect, the same proposition as increasing the number of cylinders on a reciprocating engine.

Back to Index Next