PERTURBATIONS IN THE OPERATION OF ENGINES AND THEIR REMEDY
In this chapter will be discussed certain perturbations which affect the
operations of gas-engines to a more marked degree than lack of care in
their construction. In previous chapters defects in operation due to
various causes have been dwelt upon, such as objectionable methods in
the construction of an engine, ill-advised combination of parts, defects
of installation, and the like; and an attempt has been made to determine
in each case the conditions which must be fulfilled by the engine in
order to secure efficiency and economy at a normal load.
=Difficulties in Starting.=--The preliminary precautions to be taken in
starting an engine having been indicated, it is to be assumed that the
advice given has been followed. Nevertheless various causes may prevent
the starting of the engine.
=Faulty Compression.=--Defective compression, as a general rule,
prevents the ignition of the explosive mixture. Whether or not the
compression be imperfect can be ascertained by moving the piston back to
the period corresponding with compression, in other words, that position
in which all valves are closed. If no resistance be encountered, it is
evident that the air or the gaseous mixture is escaping from the
cylinder by way of the admission-valve, the exhaust-valve, or the
piston. The valves, ordinarily seated by springs, may remain open
because their stems have become bound, or because some obstruction has
dropped in between the disk and the seat. In a worn-out or badly kept
engine the valves are likely to leak. If that be the case grinding is
the only remedy. If a valve be clogged, which becomes sufficiently
evident by manipulating the controlling levers, it is necessary simply
to clean the stem and its guides in order to remove the caked oil which
accumulates in time. If the engine be new, the binding of the
valve-stems is often caused by insufficient play between the stems and
their guides. Should this prove to be the case, the defect is remedied
by rubbing the frictional surface of the stem with fine emery paper and
by lubricating it with cylinder-oil. The exhaust-valve, however, should
be lubricated only with petroleum.
It is not unlikely that the exhaust-valve may leak for two other
reasons. In the first place, the tension of the spring which serves to
return the valve may have lessened and may be insufficient to prevent
the valve from being unseated during suction. Again, the screw or roller
serving as a contact between the lever and the valve-stem, may not have
sufficient play, so that the lengthening of the stem on account of its
expansion may prevent the valve from falling back on its seat. The
first-mentioned defect is remedied by renewing the spring, or by the
provision of an additional spring or of a counterweight in order to
prevent the stoppage of the motor. The second defect can be remedied by
regulating the contact.
Leakage past the piston may be caused by the breaking of one or more
rings, by wear or binding of the rings, or by wear or binding of the
cylinder. The whistling caused by the air or the mixture as it passes
back proves the existence of this fault.
=Presence of Water in the Cylinder.=--It may sometimes happen that water
may find its way into the cylinder with the gas by reason of the bad
arrangement of the piping. It may also happen that water may enter the
cylinder through the water-jacket joint. Again, the presence of water in
the cylinder may be due to condensation of the steam formed by the
chemical union of the hydrogen of the gas and the oxygen of the air,
which condensation is caused by the cool walls of the cylinder. The
water may sometimes accumulate in the exhaust pipe and box, when they
have been improperly drained, and may thus return to the cylinder.
Whatever may be its cause, however, the presence of water in the cylinder
impedes the starting of the engine, because the gases resulting from the
explosion are almost spontaneously chilled, thereby diminishing the
working pressure.
If electric ignition be employed, drops of water may be deposited
between the contacts, thereby causing short circuits which prevent the
passing of the spark.
If there be no drain-cock on the cylinder, the difficulty of starting
the engine can be overcome only by ceaseless attempts to set it in
motion. The leaky condition of a joint as well as the presence of a
particle of gravel in the cylinder-casting, through which the water can
pass from the jacket, is attested by the bubbling up of gas in the
water-tank at the opening of the supply tube. These bubbles are caused
by the passage of the gas through the jacket after the explosion. If
such bubbles be detected, the cylinder should be renewed or the defect
remedied. In order to obviate any danger, the stop-cocks of the
water-jacket, which have already been described in a previous chapter,
should be closed while the engine is idle.
=Imperfect Ignition.=--The difficulties encountered in starting an
engine, and caused by imperfect ignition, vary in their nature with the
character of the ignition system employed, whether that system, for
example, be of the electric, or of the incandescent or hot tube type.
Frequently it happens that in starting an engine a hot tube may break.
If the tube be of porcelain the accident may usually be traced to
improper fitting or to the presence of water in the cylinder. If the
tube be of metal, its breaking is caused usually by a weakening of the
metal through long use--an accident that occurs more often in starting
the engine than in normal operation, because the explosions at starting
are more violent, owing to the tendency of the supply-pipes to admit an
excess of gas at the beginning.
A misfire arising from a faulty tube in starting may be caused by an
obstruction or by leaks at the joints or in the body of the tube
itself, thereby allowing a certain quantity of the mixture to escape
before ignition. This defect in the tube is usually disclosed by a
characteristic whistling sound.
A tube may leak either at the bottom or at the top. In the first case,
starting is very difficult, because the part of the mixture compressed
toward the tube will escape through the opening before it reaches the
incandescent zone. In the second case, ignition may be simply retarded
to so marked an extent that a sufficient motive effect cannot be
produced. An example of this retardation, artificially produced to
facilitate the starting and to obviate premature explosions, is found in
a system of ignition-tubes provided with a small cock or variable valve
(Figs. 74 and 75).
[Illustration: FIG. 74.]
[Illustration: 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 pronounced 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.
Passing 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 quantity of the mixture to
pass into the suction-chest or into the frame. When the explosion takes
place in the cylinder that part of the mixture which has passed 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 constituents have not been mingled in the
proper proportion, or because the ignition apparatus has missed fire,
this charge at the moment of exhausting will pass 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 quantity 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.
=Retarded Explosions.=--Retarded 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
retarded 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. Retardation 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
retardation 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 retardation 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 retarded. 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 bushings 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 bushings 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 scraper. The surfaces of the bushings 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 quantity 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
passage of gas, caused by the explosion. Abnormal passage 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 quantity 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 substituted.
The freezing of oil by intense cold may be retarded 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 splashing 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 quantity of the oil is likely to accumulate within
the cylinder, where it burns and forms a caky mass 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.
=Back Pressure to the Exhaust.=--How the pipes and chests for the
exhaust should be arranged in order not to exert a harmful influence on
the motor has already been explained. Even if the directions given have
been followed, however, the exhaust may not operate properly from
accidental causes. Among these causes may be mentioned obstructions in
the form of foreign bodies, such as particles of rust, which drop from
the interior of the pipes after the engine has been running for some
time and which, accumulating at any place in the pipe, are likely to
clog the passage. Furthermore, the products of combustion may contain
atomized cylinder oil which finds its way into the exhaust-pipe. This
oil condenses on the walls of the elbows and bends of the pipe in a
deposit which, as it carbonizes, is converted into a hard cake and which
reduces the cross-section of the passage, thereby constituting a true
obstacle to the free exhaust of the gases.
These various defects are manifested in a loss in engine power as well
as in an abnormal elevation of the temperature of the parts surrounding
the exhaust opening.
=Sudden Stops.=--Sudden stops are occasioned by faulty operation of the
engine, and by imperfect fuel supply. Among the first class the chief
causes to be mentioned are the following:
1. Overheating, which has already been discussed and which may block a
moving part.
2. Defective ignition.
3. Binding of the admission-valve or of the exhaust-valve, preventing
respectively suction or compression.
4. The breaking or derangement of a member of the distributing
mechanism.
5. A weakening of the exhaust-valve spring, so that the valve is opened
by the suction of fresh quantities of mixture.
These faults are due to carelessness and improper inspection of the
engine.
So far as the fuel supply of the engine is concerned, the causes of
stoppage will vary if street-gas or producer-gas be employed. In the
former case the difficulty may be occasioned by the improper operation
of the meter, by the formation of a water-pocket in the piping, by the
binding of an anti-pulsator valve, by the derangement of a
pressure-regulator, or by a sudden change in the gas pressure when no
pressure-regulator is employed. If producer-gas be used, stoppages may
be occasioned by a sudden change in the quality, quantity, or
temperature of the gas. These defects will be examined in detail in the
chapter on Gas-Producers.