Gas-Engines and Producer-Gas Plants Part 18

CHAPTER XV

THE SELECTION OF AN ENGINE

The conditions which must be fulfilled both by engines and gas-producers in order that they may industrially operate with regularity and economy have been dwelt upon at some length. Unfortunately it often happens that engines are not installed as they should be, with the result that they run badly and that the reputation of gas-engines suffers unjustly. The use of suction gas-producers in particular caused considerable trouble at first owing to inexperience, so that even now many hesitate to adopt them despite their great economical advantages. The reason a.s.signed for this hesitation is the supposed danger attending their operation.

The factory proprietor who intends to install a gas-engine in his plant is not usually able to appreciate the intrinsic value of one engine when compared with another, or to determine whether the plans for an installation conform with the best practice. The innumerable types of engines offered to him by manufacturers and their agents, each of whom claims to have a better engine than his rivals, plunges the purchaser into hesitation and doubt. Not knowing which engine to select, he usually buys the cheapest. Very often he learns, as time goes by, that his installation is far from being perfect. Finally he begins to believe that he ought to consult an expert. The author's personal experience has convinced him that eight times out of ten the factory owner who has picked out an engine for himself has not obtained an installation which meets the requirements which the manufacturers of gas-engines should fulfil. Many of these requirements could be complied with were it not for the fact that the manufacturer has dropped certain details which appeared superfluous, but which were in reality very important in obtaining perfect operation. The author therefore suggests that the services of a competent expert be retained by those who intend to install a gas-engine in their plants.

=The Duty of a Consulting Engineer.=--An expert fills the same office as an architect, and impartially selects the engine best suited to his client's peculiar needs. His examination of the engines offered to him will proceed somewhat according to the following programme:

1. He will first study the installation from the mechanical point of view, and also the local conditions under which that installation is to operate, in order that he may not order an engine too large or too small, or a type incompatible with the foundations at his disposal, or unable to fulfil all the requirements of his client.

2. He will examine the precautions which have been taken to avoid or reduce to a minimum certain inconveniences which attend the operation of explosion-engines.

3. He will draw up specifications, with the terms of which gas-engine makers must comply, so that he can compare on the basis of these specifications the merits of the engines submitted to him.

4. He will prepare an estimate of cost and also a contract which is not couched in terms altogether in the gas-engine maker's favor, and which gives the purchaser important warranties.

5. He will supervise the technical installation of the engine or plant.

6. He will make tests after the engine is installed and see to it that the maker has fulfilled his warranties.

=Specifications.=--Since engines and gas-producers are constructed for commercial ends, it naturally follows that their manufacturers seek to make the utmost possible profit in selling their installations. Prices charged will necessarily vary with the quality of material employed, the care taken in constructing the engine and generator, the number of apparatus of the same type which are manufactured, the arrangement of the parts and that of the installations. Since there is considerable rivalry among gas-engine builders, selling prices are often cut down so far that little or no profit is left. It is very difficult--indeed impossible--to convince a purchaser that it is to his interest to pay a fair price in order to obtain a good installation, especially when other manufacturers are offering the same installation at a less price with the same warranties. As a result of this state of affairs, engine builders, in order that they may not lose an order, are willing, to reduce their prices, hoping to make up in the quality of the workmanship and the material what they would otherwise lose. Often they will deliver an engine too small in size but operating at a higher speed than that ordered; or they will select an old type, or carry out certain details with no great care.

This, to be sure, is not always the case; for there are a few builders of engines who place their reputation above everything else and who would rather lose an order than execute it badly. Others, unfortunately, prefer to have the order at all costs.

By retaining a consulting engineer, all these difficulties are overcome.

In the first place, the engineer draws up a scale of prices and specifications which must be complied with in their entirety as well as in all details. Rival engine builders are thus compelled to make their estimates according to the same standard, so that one engine can readily be compared with another with the utmost fairness. In these specifications, penalties will be provided for by the engineer which will be levied if the warranties of the maker are not fulfilled.

Otherwise the warranties are worth nothing.

The first consequence of engaging a consulting engineer is to render the matter of cost a secondary one. A factory owner who employs a consulting engineer and pays him for his services, is impelled chiefly by the desire to obtain a good installation which will perform what he expects of it. For that reason necessary sacrifices will be made to comply with the client's wishes.

If the purchaser considers the question of cost most important to him, he need not engage an expert to supervise the installation of his engines. He has simply to pick out the cheapest engine. Unfortunately, however, the money which he will save by such a procedure will be more than compensated for by the trouble which he will later experience when his motor stops or when it breaks down, because it has been cheaply built in the first place.

The advice of a consulting engineer is therefore of importance to the purchaser, because an engine will be installed which will in every way meet his requirements. The gas-engine builder will also prefer to deal with an engineer, because the engineer can appreciate at their true worth good material and good workmanship and place a fair valuation upon them. The specifications of a gas-engine and gas-producer expert are accepted by most engine builders, because an expert will not introduce conditions which cannot be fulfilled. Some manufacturers refuse to consider the conditions imposed by specifications seriously, or else they fix different prices and make tenders on the basis of these with or without specifications. In either case the purchaser may be sure that he is not receiving what he has a right to exact.

=Testing the Plant.=--When the engine has been selected the consulting engineer supervises its installation, and, after this is completed, carries out tests in order to determine whether or not the guaranteed power and consumption are attained. The methods employed in testing a gas-engine are both complex and delicate. The quality of the gas, the proportions of the elements forming the mixture, the time and the method of ignition, the temperature of the cylinder-walls, the temperature and the pressure of the gas drawn into the cylinder, all these are factors which have a decided bearing upon the results of a test. If these factors be not carefully considered the conclusions to be drawn from the test may be absolutely wrong.

Indicators of any type should not be indiscriminately employed; only those specially designed for gas-engine purposes should be used.

Indicator cards are in themselves inadequate, and should be supplemented by the records of explosion-recorders.

The calorific value of the gas should be measured either by the Witz apparatus or by means of any other calorimeter.

In interpreting the diagrams and records some difficulty will be encountered. Sometimes it happens that a particular form of curve is attributed to a cause entirely different from the real one. It happens not infrequently that engineers, whose experience is confined to engines of one make and who have not had the opportunity to make sufficient comparisons, draw such erroneous conclusions from cards.

To recapitulate what has already been said, the testing of gas-engines requires considerable experience and cannot be lightly undertaken.

Special instruments of precision are necessary. The author has very often been called upon to contradict the results obtained by experts whose tests have consisted simply in ascertaining the engine power either by means of a p.r.o.ny brake, or by means of a brake-strap on the fly-wheel. The brake gives but crude results at best; it is a means of control, and not an instrument of scientific investigation.

Something more than the mere power produced by an engine should be ascertained. The tests made should throw some light upon the reasons why that power cannot be exceeded, and show that the necessary changes can be made to cause the engine to operate more economically and to yield energy of an amount which its owner has a right to expect. The indicator and the recorder are testing instruments which clearly indicate discrepancies in operation and the means by which they may be corrected.

The tests made should determine whether the power developed is not obtained largely by means of controlling devices which cause premature wearing away of the engine parts.

It is not the intention of the author to describe indicators of the well-known Watt type. It is simply his purpose to call attention to the explosion-recorder which he has devised to supplement the data obtained by means of the indicator.

[Ill.u.s.tration: FIG. 145.--Mathot explosion-recorder.]

=Explosion-Recorder for Industrial Engines.=--The explosion-recorder ill.u.s.trated in Fig. 145 can be adapted to any ordinary indicator. It is composed of a supporting bracket _B_ upon which a drum _T_ is mounted.

This drum is rotated by a clock-train, the speed of which is controlled by means of a special compensating governor. The entire system is pivotally mounted upon the supporting screw _O_, so that the drum _T_, about which a band of paper is wound, may be swung against a stylus _C_, which records upon the paper the number and power of the explosions. These explosions are measured according to scale by a spring connected with an indicator. The records obtained disclose for any given cycle the amount of compression as well as the force of the explosion, and render it possible to study the phenomena of expansion, exhaust, and suction. They are, however, inadequate in showing exactly how an engine runs in general. Indeed, in most gas-engines, as well as oil and volatile hydrocarbon engines, each explosion differs from that which follows in character and in power; and it is absolutely essential to provide some means of avoiding these variations. The explosion-recorder gives a graphic record from which the number of explosions can be read, and also the initial pressure of each explosion, the number of corresponding revolutions, the order in which the explosions succeed one another, and consequently the regularity of certain phenomena caused by secondary influences, such as the section of the distributing members, the sensitiveness of the governor, and the like.

The explosion-records can be taken simultaneously with ordinary diagrams. In order to attain this end, the recorder is allowed to swing around the pivot _O_, so that the drum carrying the paper band is brought into engagement, or swung out of engagement with the stylus, as it is influenced by each explosion, thereby leaving its record on the paper. The ordinary diagram may be traced on the drum of the indicator, as it continues to operate in its usual way. Thus the explosion-recorder renders it possible to control the operation of engines, to obtain some idea of the cause of defects and to attribute them to the proper force. Improvements can then be made which will ensure a greater efficiency. A number of records herewith reproduced ill.u.s.trate the defects in the controlling apparatus and in the construction of certain engines, and also the result of improvements which have been made on the basis of the records obtained. The smaller lines indicate the compression, which is usually constant in engines in which the "hit-and-miss" system of governing is employed, while the larger lines indicate the explosions. These records are only part of the complete data normally drawn on the paper in the period of 120 seconds corresponding with an entire revolution of the recorder-drum.

[Ill.u.s.tration: FIG. 146.--Record with automatic starter.]

[Ill.u.s.tration: FIG. 147.--Gas-engine running at one-half load.]

[Ill.u.s.tration: FIG. 148.--Record made after correcting faults.]

[Ill.u.s.tration: FIG. 149.--Record made after correcting faults.]

The first record was taken while starting up an engine provided with an automatic starting device and supplied with explosive mixture without previous compression (Fig. 146). The gradual lessening of the distances of the ordinates or lines representing the explosions shows that the speed of the motor was slowly increasing, and also indicates the time which elapsed before the engine was running smoothly. The records that follow (Figs. 147, 148 and 149) show the results which can be obtained with the recorder by correcting the errors due to faults in installing the engine and its accessories. The fifth record is particularly interesting because it shows the influence of the ignition-tube on the power of the deflagration of the explosive mixture (Fig. 150). This record was obtained with an engine provided with two contiguous tubes.

The communication of each of these tubes with the explosion-chamber could be cut off at will at any moment. The last record (Fig. 151) was obtained at a time when the effective load of the engine was changed at two different intervals. This record shows how regularly the engine was running and how constant were the initial pressures. These pressures, however, which is the case in most engines, manifestly diminish when the explosions succeed one another without idle strokes of the piston. This shows, also, the influence of "scavenging" the products of combustion and the effect it has on the efficiency of explosion-engines.

=a.n.a.lysis of the Gases.=--It has already been stated that one of the tests which should be made consists in measuring the calorific value of the gas. Just what the calorific value of the gas may be it is necessary to know in order to obtain some idea of the thermal efficiency of the installation. If a suction gas-producer be employed (an apparatus in which the nature of the gas generated changes at each instant), calorimetrical a.n.a.lyses are indispensable in appreciating the conditions under which a generator operates.

These a.n.a.lyses are made by means of calorimeters which give the calorific value either at a constant pressure or at a constant volume.

Constant-volume instruments give a somewhat weaker record than constant-pressure instruments; but according to Professor Aime Witz, the inventor of an excellent calorimeter, the constant-volume type is almost indispensable in gaging the efficiency of explosion-engines.

[Ill.u.s.tration: FIG. 150.]

[Ill.u.s.tration: FIG. 150_b_.]

[Ill.u.s.tration: FIG. 151.--Record made when effective load was changed at two different intervals.]

[Ill.u.s.tration: FIG. 152.--The Witz calorimeter.]

=The Witz Calorimeter.=--The accompanying diagram (Fig. 152) ill.u.s.trates Professor Witz's instrument. Its elements are a steel cylinder having an interior diameter of 2.36 inches, about a thickness of 0.078 inch and a height of about 3.54 inches, so that its capacity is about 15.1 cubic inches, and two covers screwed on the cylinder to seal it hermetically, oiled paper being used as a washer. The upper cover carries a spark-exciter; the lower cover is provided with a valve which discharges into a cylindrical member 1.06 inches in diameter. This second cover is downwardly inclined at its circ.u.mference toward the center to insure complete drainage of the mercury used for charging the calorimeter. All surfaces are nickel plated. The proportions of nickel and of steel are fixed by the manufacturer so as to render it possible to calculate the displacement of the apparatus in water. The calorimeter having been completely filled with mercury is inverted in this liquid in the manner of a test tube. The explosive mixture is then introduced, being fed from a bell in which it has previously been prepared. A rubber tube connects the bell with the instrument. The gas is forced from the bell to the calorimeter by the pressure in the bell. The conical form of the bottom causes the calorimeter to be emptied rapidly and to be refilled completely with explosive gas at a pressure slightly above that of the atmosphere. Equilibrium is re-established by manipulating the valve, during a very short interval, so as to permit the excess gas to escape.

During this operation the calorimeter must be maintained in the vertical position shown in the diagram. The atmospheric pressure is read off to one-tenth of a millimeter (0.003936 inches) on a barometer. The temperature of the gas may be taken to be that of the mercury-vessel.

The explosive mixture is prepared in the water reservoir, the gla.s.s bulb shown in the accompanying ill.u.s.tration being employed. This bulb is closed at its upper end by means of a c.o.c.k and is tapered at its lower end. The gas or air enters at the top by means of a rubber tube and gradually displaces the water through the lower end. The bulbs have a volume varying from 200 to 500 cubic centimeters (12 to 30 cubic inches), and the error resulting from each filling of a bulb is certainly less than 15 cubic millimeters (0.0009 cubic inches). The contents are emptied into a bell by lowering the bulb into the water and opening the c.o.c.k. If seven bulbfuls of air be mixed with one bulbful of gas, an explosive mixture of 1 to 7 is produced, this being the proportion commonly employed for street-gas. For producer-gases the preferred proportion is 1 to 1, oxygen being often added to the air in order to insure complete combustion.

The calorimeter, after having been filled, is placed in a vessel containing a liter (1.7598 pints) of water so that it is completely immersed. A spark is then allowed to pa.s.s. The explosion is not accompanied by any noise; the temperature rises a fixed number of degrees, so that the quant.i.ty of heat liberated can easily be computed.

Each division of the thermometer is equal to 0.01502 C. The scale reading is minute, each interval being divided by ten, so that readings to the 1,500th part of a degree can be taken.

It should be observed that the mixture generated in the reservoir is saturated with water vapor at the temperature of the reservoir.

Consequently, the vapor generated by the explosion must condense in the calorimeter if the final temperature of the calorimeter is the same as that of the water reservoir. If, on the other hand, the temperature be slightly different, a correction must be made; but the error is negligible for differences in temperature of from 2 to 3 degrees C. (3.6 to 5.4 degrees F.). This, however, is never likely to occur if the operation is conducted under favorable conditions.

This apparatus is exceedingly simple and practical. It does not require the manipulation of a pump. The pressure of the mixture is read off on the barometer; the calorimeter is entirely immersed in the water of the outer vessel, so that all corrections of doubtful accuracy are obviated.