Gas-Engines and Producer-Gas Plants Part 8

[Ill.u.s.tration: FIG. 74.]

[Ill.u.s.tration: FIG. 75.--Ignition-tubes provided with needle valves to facilitate starting.]

The mere enumeration of defects caused by leakage is sufficient to indicate the remedy to be adopted. It may be well to recall in this connection the important part played by the ignition-valve. If it be leaky, or if its free operation be impeded, starting will always be difficult.

=Electric Ignition by Battery or Magneto.=--If the electric ignition apparatus, whatever may be the method by which the spark is produced, be imperfect in operation, the first step to be taken is to ascertain whether the spark is produced at the proper time, in other words, slightly after the dead center in the particular position given to the admission device at starting. If a coil and a battery be employed, it is advisable to remove the plug and to place it with its armature upon a well-polished metal surface to produce an electrical contact, preventing, however, the contact of the binding post with this metallic surface. The same method of inspection is adopted with the make-and-break apparatus of an electric magneto. In both cases it should be ascertained whether or not there is any short-circuiting. The contacts should be cleaned with a little benzine if they are covered with oil or caked grease.

If no spark is produced at the plug or at the make-and-break device it may be inferred that the wires are broken or that the generating apparatus is out of order. A careful examination will indicate what measures are to be taken to cure the defects.

=Premature Ignition.=--It has several times been stated that the moment of ignition of the gaseous mixture has a p.r.o.nounced influence on the operation of gas-engines and upon their economy.

Premature ignition takes place when there is a violent shock at the moment when the piston leaps from the rear dead center to the end of the compression stroke. The violent effects produced are all the more harmful because they tend to overheat the interior of the engine and thereby to increase in intensity.

Premature ignition may be due to several causes. If a valveless hot tube be employed it may happen that the incandescent zone is too near the base. If the tube be provided with a valve, it very frequently happens that the valve leaks or that it opens too soon. In the case of electric ignition, the circuit may be completed before the proper time, because of faulty regulation. The suggestions made in the preceding chapters indicate the method of remedying these defects.

Faulty ignition may have its origin not only in the method of ignition employed, but also in excessive heating of the internal parts of the engine, caused by continual overloading or by inadequate circulation of water.

Pa.s.sing to those cases of premature ignition of a special nature which are not due to any functional defect in the engine, but which are purely accidental in origin, such as the uncleanliness of the parts within the cylinder or the presence of some projecting part which becomes heated to incandescence during compression, it should first be stated that these ignitions, usually termed spontaneous, often occur well in advance of the end of the compression stroke. They are characterized by a more marked shock than that caused by ordinary premature ignition and usually result in bringing the engine to a complete stop in a very short time. These spontaneous explosions counteract to such an extent the impulse of the compression period, during which the piston is moving back, that they have a tendency to reverse the direction in which the engine is running. In such cases a careful inspection and a scrupulous cleaning of the cylinder and of the piston should be undertaken.

The bottom of the piston is particularly likely to retain grease which has become caked, and which is likely to become heated to incandescence and spontaneously to ignite the explosive mixture.

=Untimely Detonations.=--The sound produced by the explosions of a normally operating engine can hardly be heard in the engine-room.

Untimely detonations are produced either at the exhaust, or in the suction apparatus, near the engine itself. These detonations are noisier than they are dangerous; still, they afford evidence of some fault in the operation which should be remedied.

Detonations produced at the exhaust are caused by the burning of a charge of the explosive mixture in the exhaust-pipe, which charge, for some reason, has not been ignited in the cylinder, and has been driven into the exhaust-pipe, where it catches fire on coming into contact with the incandescent gases discharged from the cylinder after the following explosion.

Detonations produced in the suction apparatus of the engine, which apparatus is either arranged in the base itself or in a separate chest, are often noisier than the foregoing. They are caused by the accidental backward flowing of the explosive mixture, and by its ignition outside of the cylinder. The accident may be traced to three causes:

1. The suction-valve of the mixture may not be tight and may leak during the period of compression, allowing a certain quant.i.ty of the mixture to pa.s.s into the suction-chest or into the frame. When the explosion takes place in the cylinder that part of the mixture which has pa.s.sed back is ignited, as we have just seen, thereby producing a very loud deflagration. The obvious remedy consists in making the suction-valve tight by carefully grinding it.

2. It may happen that at the end of the exhaust stroke incandescent particles may remain in the cylinder, which particles may consist of caked oil or may be retained by poorly cooled projections. The result is that the mixture is prematurely ignited during the suction period.

3. The engine is so regulated, particularly in the case of English-built engines, as to effect what is technically called "scavenging" the products of combustion. In order to obtain this result, the mixture-valve is opened before the end of the exhaust stroke of the piston and the closing of the exhaust-valve. Owing to the inertia and the speed acquired by the products of combustion shot into the exhaust-pipe after explosion, a lowering of the pressure is produced in the cylinder toward the end of the stroke, causing the entrance of air by the open admission-valve and consequently effecting the scavenging of the burnt gases, part of which would otherwise remain in the cylinder.

It is evident that if a charge of the mixture has not been normally exploded, either because its const.i.tuents have not been mingled in the proper proportion, or because the ignition apparatus has missed fire, this charge at the moment of exhausting will pa.s.s out of the cylinder without any acquired speed, and will flow back in part at the end of the exhaust stroke past the prematurely opened admission-valve, thereby lodging in the air suction apparatus. Despite the suction which takes place immediately following the re-entrance of the gas into the cylinder, a certain quant.i.ty of the mixture is still confined in the suction-pipe and its branches, where it will catch fire at the end of the exhaust stroke after the opening of the mixture-valve.

In order to avoid these detonations it is necessary simply to see to it that the mixture is regularly ignited. This is accomplished by mixing the gas and air in proper proportions or by correcting the ignition time.

=r.e.t.a.r.ded Explosions.=--r.e.t.a.r.ded explosions considerably reduce the power which an engine should normally yield, and sensibly increase the consumption. They are due to three chief causes: (1), faulty ignition; (2), the poor quality of the mixture; (3), compression losses. The existence of the defect cannot be ascertained with any certainty without the use of an indicator or of some registering device which gives graphic records. Nevertheless, it is possible in some degree to detect r.e.t.a.r.ded explosions, simply by observing whether there is a diminution in the power or an excessive consumption, despite the perfect operation and good condition of all the engine parts.

In order to remedy the defect it should be ascertained if the compression is good, if the supply of gas is normal, and if the conditions under which the mixture of air and gas is produced have not been changed. Lastly, the ignition apparatus is gradually adjusted to accelerate its operation until a point is reached when, after explosion, shocks are produced which indicate an excessive advance. The ignition apparatus is then adjusted to a point slightly ahead of the corresponding position. Recalling the descriptions already given of the various systems of ignition, the manner of regulating the moment of ignition in each case may be summarized as follows:

1. For the valveless incandescent tube, provided with a burner the position of which can be varied, ignition can be accelerated by bringing the burner nearer to the base. r.e.t.a.r.dation is effected by moving the burner away from the base.

2. In the case of the incandescent tube of the fixed burner type, the moment of ignition will depend upon the length of the tube. The r.e.t.a.r.dation will be greater as the tube is shorter, and _vice versa_.

3. If the tube be provided with an ignition-valve, the time of ignition having been regulated by the maker, regulation need not be undertaken except if the valve-stem be worn or the controlling-cam be distorted.

If these defects should be noted, the imperfect parts should be repaired or renewed.

4. In electric igniters the controlling apparatus is generally provided with a regulating device which may be manipulated during the operation of the motor. If the manual adjustment of the regulating apparatus be unproductive of satisfactory results, it is advisable to ascertain whether the spark is being produced normally. Before the engine has come to a stop, one of the valve-casings is raised, and through the opening thus produced it is easily seen whether the spark is of sufficient strength, the engine in the meanwhile being turned by hand. Care should always be taken to purge the cylinder of the gas that it may contain, in order to prevent dangerous explosions. If the spark should prove to be too feeble, or if there be no spark at all, despite the fact that every part of the mechanism is properly adjusted, it may be inferred that the fault lies with the current and is caused by

1. Imperfect contact with the binding-posts, with the conducting wire, or with the contact-breaking members;

2. A short circuit in one of the dismembered pieces;

3. The presence of a layer of oil or of caked grease forming an insulator, injurious to induction, between the armature and the magnets;

4. A deposit of oil or moisture on the contact-breaking parts;

5. The exhaustion of the magnets, which, however, occurs only after several years of use, except when the magneto has been subjected for a long time to a high temperature.

The mere discovery of any of these defects sufficiently indicates the means to be adopted in remedying them.

=Lost Motion in Moving Parts.=--Lost motion of the moving parts is due to structural errors. Its cause is to be found in the insufficient size of the frictional bearing surfaces, and improper proportioning of shafts, pins, and the like. The result is a premature wear which cannot be remedied. Imperfect adjustment, lack of care, and bad lubrication, may also hasten the wear of certain parts. This wear is manifested in shocks, occurring during the operation of the engine,--shocks which are particularly noticeable at the moment of explosion.

Besides the inconveniences mentioned, wearing of the gears and of the moving parts leads to derangement of the power-transmitting members.

So far as the admission and exhaust valves are concerned, the wearing of the cams, rollers, and lever-pivots is evidenced by a r.e.t.a.r.dation in the opening of these valves and an acceleration in their closing.

The ignition, whatever may be the system employed, is affected by lost motion and is r.e.t.a.r.ded. The engine appreciably loses in power, and its consumption becomes excessive.

=Overheated Bearings.=--Apart from the imperfect adjustment of a member, it may happen that the bus.h.i.+ngs of the main bearings of the ends of the connecting-rod, and of the piston-pivot, may become heated because of excessive play, or of too much tightening, or of a lack of oil, or of the employment of oil of bad quality. The overheating may lead to the binding of frictional surfaces and even to the fusion of bus.h.i.+ngs if they be lined with anti-friction metal. In order to avoid the overheating of parts, it is advisable, while the engine is running, to touch them from time to time with the back of the hand. As soon as the slightest overheating is felt, the temperature may be lowered often by liberal oiling. If this be inadequate and if for special reasons it is impossible to stop the engine, the overheated part may be cooled by spraying it with soapy water.

If the overheating has not been detected or reduced in time, a characteristic odor of burnt oil will be perceived, accompanied by smoke. The part overheated will then have attained a temperature so high that it cannot be touched with the hand. Should this occur, it is inadvisable to employ oil, because it would immediately burn up and would only aggravate the conditions. Cotton waste should be carefully applied to the overheated member, and gradual spraying with soapy water begun.

In special cases where the lubricating openings or channels are not likely to be obstructed, a little flowers of sulphur may be added to the oil, if this be very fluid. Castor oil may also be successfully employed.

If the binding of the rubbing surfaces should prevent the reduction of the overheated member's temperature, the engine must necessarily be stopped, and the parts affected detached. All causes of binding are removed by means of a steel sc.r.a.per. The surfaces of the bus.h.i.+ngs and of the shaft which they receive are smoothed with a soft file and then polished with fine emery paper. Before the parts are replaced, the precaution of ascertaining whether they touch at all points should be taken. Careful inspection and copious lubrication should, of course, be undertaken when the engine is again started.

=Overheating of the Cylinder.=--The overheating of the cylinder may be due to a complete lack of water in the jacket or to an accidental diminution in the quant.i.ty of water supplied. If this discovery is made too late, and if the cylinder has reached a very high temperature, the circulation of the water should not be suddenly re-established, because of the liability of breaking the casting. It is best to stop the engine and to restore the parts to their normal condition.

It is well to recall at this point that if the calcareous incrustation of the water-jacket or the branch pipes should hinder the free circulation of water, cleaning is, of course, necessary. The jacket may be washed several times with a twenty per cent. solution of hydrochloric acid. After this treatment the jacket should, of course, be rinsed with fresh water before the piping of the water-circulating apparatus is again connected.

=Overheating of the Piston.=--If the overheating of the piston is not due to faulty adjustment, it may be caused by lack of oil or to the employment of a lubricant not suitable for the purpose. In a previous chapter the importance of using a special oil for cylinder lubrication has been insisted upon. The overheating of the piston can also result from that of the piston-pin. Should this be the case it is advisable to stop the engine, to ascertain the condition and the degree of lubrication of this member and its bearing. Overheating of the piston is manifested by an increase of the temperature of the cylinder at the forward end. If this overheating be not checked, binding of the piston in the cylinder is likely to result.

=Smoke Arising from the Cylinder.=--This is generally a sign either of overheating, which causes the oil to evaporate, or of an abnormal pa.s.sage of gas, caused by the explosion. Abnormal pa.s.sage of gas may result from wear or from distortion of the cylinder, or from wear or breakage of the piston-rings. The result is always the overheating of the cylinder and a reduction in compression and power.

If the engine is well kept and shows no sign of wear, leakage may be caused simply by the fouling of the piston-rings, which then adhere in their grooves and have but insufficient play. This defect is obviated by cleaning the rings in the manner explained in Chapter VII.

Lubrication is faulty when the quant.i.ty of lubricant supplied is either insufficient or too abundant, or when the oils employed are of bad quality. It has already been shown that insufficient lubrication and the utilization of bad oils leads to the overheating of the moving parts.

Insufficient lubrication may be caused by imperfect operation of the lubricators, or, particularly during cold weather, by too great a viscosity or congelation of the oil. If a lubricator be imperfect in its operation, the condition of its regulating mechanism should be ascertained, if it has any, and an examination made to discover any obstruction in the oil-ducts. Such obstructions are very likely to occur in new devices which have been packed in cotton waste or excelsior, with the result that the particles of the packing material often find their way into openings.

An oil may be bad in quality because of its very nature, or because of the presence of foreign bodies. In either case an oil of better quality should be subst.i.tuted.

The freezing of oil by intense cold may be r.e.t.a.r.ded by the addition of ordinary petroleum to the amount of 10 to 20 per cent.

An excess of oil in the bearings results simply in an unnecessary waste of lubricant, and the splas.h.i.+ng of oil on the engine and about the room.

If too much oil be used in the cylinder, grave consequences may be the result; for a certain quant.i.ty of the oil is likely to acc.u.mulate within the cylinder, where it burns and forms a caky ma.s.s that may be heated to incandescence and prematurely ignite the explosive mixture. Especially in producer-gas engines is an excess of cylinder-lubricant likely to cause such accidents. Indeed, the temperature of explosion not being as high as in street-gas engines, the excess oil cannot be so readily removed with certainty by evaporation or combustion. On the other hand, the compression of the mixture being generally higher, premature ignition is very likely to occur.