PRODUCER-GAS ENGINES
Thus far only street-gas or illuminating-gas engines have been
discussed. If the engine employed be small--10 to 15 horse-power, for
instance--street-gas is a fuel, the richness, purity and facility of
employment of which offsets its comparatively high cost. But the
constantly increasing necessity of generating power cheaply has led to
the employment of special gases which are easily and cheaply generated.
Such are the following:
Blast-furnace gases,
Coke-oven gases,
Fuel-gas proper,
Mond gas,
Mixed gas,
Water-gas,
Wood-gas.
The practical advantages resulting from the utilization of these gases
in generating power were hardly known until within the last few years.
The many uses to which these gases have been applied in Europe since
1900 have definitely proved the industrial value of producer-gas engines
in general.
The steps which have led to this gradually increasing use of
producer-gas have been learnedly discussed and commented upon in the
instructive works and publications of Aime Witz, Professor in the
Faculty of Sciences of Lille, in those of Dugald Clerk, of London, F.
Grover, of Leeds, and Otto Gueldner, of Munich, and in those of the
American authors, Goldingham, Hiscox, Hutton, Parsell and Weed, etc. The
new tendencies in the construction of large engines may be regarded as
an interesting verification of the forecasts of these men--forecasts
which coincide with the opinion long held by the author. Aime Witz has
always been an advocate of high pressures and of increased piston speed.
English builders who made experiments in this direction conceded the
beneficial results obtained; but while they increased the original
pressure of 28 to 43 pounds per square inch employed five or six years
ago to the pressure of 85 to 100 pounds per square inch nowadays
advocated, the Germans, for the most part, have adopted, at least in
producer-gas engines, pressures of 114 to 170 pounds per square inch and
more.
=High Compression.=--In actual practice, the problem of high pressures
is apparently very difficult of solution, and many of the best firms
still seem to cling to old ideas. The reason for their course is,
perhaps, to be found in the fact that certain experiments which they
made in raising the pressures resulted in discouraging accidents. The
explosion-chambers became overheated; valves were distorted; and
premature ignition occurred. Because the principle underlying high
pressures was improperly applied, the results obtained were poor.
High pressures cannot be used with impunity in cylinders not especially
designed for their employment, and this is the case with most engines of
the older type, among which may be included most engines of English,
French, and particularly of American construction. In American engines
notably, the explosion-chamber, the cylinder and its jacket, are
generally cast in one piece, so that it is very difficult to allow for
the free expansion of certain members with the high and unequal
temperatures to which they are subjected (Fig. 22).
Some builders have attempted to use high pressures without concerning
themselves in the least with a modification of the explosive mixture.
The result has been that, owing to the richness of the mixture, the
explosive pressure was increased to a point far beyond that for which
the parts were designed. Sudden starts and stops in operation,
overheating of the parts, and even breaking of crank-shafts, were the
results. The engines had gained somewhat in power, but no progress had
been made in economy of consumption, although this was the very purpose
of increasing the compression.
High pressures render it possible to employ poor mixtures and still
insure ignition. A quality of street-gas, for example, which yields one
horse-power per hour with 17.5 cubic feet and a mixture of 1 part gas
and 8 of air compressed to 78 pounds per square inch, will give the same
power as 14 cubic feet of the same gas mixed with 12 parts of air and
compressed to 171 pounds per square inch.
"Scavenging" of the cylinder, a practice which engineers of modern
ideas seem to consider of much importance, is better effected with high
pressures, for the simple reason that the explosion-chamber, at the end
of the return stroke, contains considerably less burnt gases when its
volume is smaller in proportion to that of the cylinder.
In impoverishing the mixture to meet the needs of high pressures, the
explosive power is not increased and in practice hardly exceeds 365 to
427 pounds per square inch. With the higher pressures thus obtained
there is consequently no reason for subjecting the moving parts to
greater forces.
[Illustration: FIG. 76.--Method of cooling the cylinder-head.]
=Cooling.=--The increase in temperature of the cylinder-head and of the
valves, due wholly to high compression, is perfectly counteracted by an
arrangement which most designers seem to prefer, and which, as shown in
the accompanying diagram (Fig. 76), consists in placing the mixture and
exhaust-valves in a passage forming a kind of antechamber completely
surrounded by water. The immediate vicinity of this water assures the
perfect and equal cooling of the valve-seats. This arrangement, while it
renders it possible to reduce the size of the explosion-chamber to a
minimum, has the additional mechanical advantage of enabling the builder
to bore the seats and valve-guides with the same tool, since they are
all mounted on the same line. From the standpoint of efficiency, the
design has the advantage of permitting the introduction of the explosive
mixture without overheating it as it passes through the admission-valve,
which obtains all the benefit of the cooling of the cylinder-head,
literally surrounded as it is by water.
In large engines the cooling effect is even heightened by separately
supplying the jackets of the cylinder-head and of the cylinder. In
engines of less power the top of the cylinder-head jacket is placed in
communication with that of the cylinder, so that the coldest water
enters at the base of the head and, after having there been heated,
passes around the cylinder in order finally to emerge at the top toward
the center. The water having been thus methodically circulated, the
useful effect and regularity of the cooling process is increased.
Notwithstanding the care which is devoted to water circulation, it is
advisable to run the producer-gas engine "colder" than the older
street-gas types, in which the more economic speed is that at which the
water emerges from the jacket at about a temperature of 104 degrees F.
It would seem advisable to meet the requirements of piston lubrication
by reducing to a minimum the quantity of heat withdrawn by the
circulating water. Indeed, the personal experiments of the author bear
out this principle.
For street-gas engines, however, the cylinders should be worked at the
highest possible temperature consistent with the requirements of
lubrication. It should not be forgotten that, in large engines fed with
producer-gas, economy of consumption is a secondary consideration,
because of the low quantity of fuel required. The cost, moreover, may
well be sacrificed to that steadiness of operation which is of such
great importance in large engines furnishing the power of factories; for
in such engines sudden stops seriously affect the work to be performed.
For this reason engine builders have been led to the construction of
motors provided with very effective cooling apparatus. Since the
circulation of the water around the explosion-chamber and the cylinder
is not sufficient to counteract the rise of temperature, it has become
the practice to cool separately each part likely to be subjected to
heat. The seats of the exhaust-valves, the valves themselves, the
piston, and sometimes the piston-rod, have been provided with
water-jackets.
=Premature Ignition.=--Returning to the causes of the discouragements
encountered by some designers who endeavored to use high pressures, it
has already been mentioned that premature ignition of the explosive
mixture in cylinders not suited for high pressures is one reason for the
bad results obtained. An explanation of these results is to be found in
the high theoretical temperature corresponding with great pressures and
in the quantity of heat which must be absorbed by the walls of the
explosion-chamber. These two circumstances are in themselves sufficient
to produce spontaneous ignition of excessively rich mixtures, compressed
in an overheated chamber unprovided with a sufficient circulation of
water. A third cause of premature ignition may also be found in the old
system of ignition which, in most English engines, consists of a
metallic or porcelain tube, the interior of which communicates with the
explosion-chamber, an exterior flame being employed to heat the tube to
incandescence. In tubes of this type which are not provided with a
special ignition-valve, the time of ignition is dependent only on the
moment when the explosive mixture, driven into the tube, comes into
contact, at the end of the compression stroke, with the incandescent
zone, thereby causing the ignition. This very empirical method leads
either to an acceleration or retardation of the ignition, depending upon
the temperature of the tube, the position of the red-hot zone, its
dimensions, and the temperature of the mixture, which is determined by
the load of the engine. Although this system, the only merit of which is
its simplicity, may meet the requirements of small engines, there is not
the slightest doubt that it is quite inapplicable to those of more than
20 to 25 horse-power, for in such engines greater certainty in operation
is demanded. Even if only the more improved of the two types of hot-tube
ignition be considered, with or without valves, it must still be held
that they are inapplicable to high compression engines. The
ignition-valve is the part which suffers most from the high temperature
to which it is subjected. Its immediate proximity to the incandescent
tube, and its contact with the burning gas when it flares up, render it
almost impossible to employ any cooling arrangement. Although with the
exercise of great care it may work satisfactorily in engines of normal
pressure, it is evident that it cannot meet the requirements of high
pressure engines, because the temperature of the compressed mixture is
such that the charge is certain to catch fire by mere contact with the
overheated valve. In industrial engines of small size, premature
ignition has little, if any, effect except upon silent operation and
economic consumption. This does not hold true, however, of large
engines. Besides the inconveniences mentioned, there is also the danger
of breaking the cranks or other moving parts. The inertia of these
members is a matter of some concern, because of their weight and of the
linear speed which they attain in large engines. Some idea of this may
be obtained when it is considered that in a producer-gas or
blast-furnace-gas engine having a piston diameter of 24 inches and an
explosive pressure of 299 pounds per square inch, the force exerted at
the moment of explosion is about 132,000 pounds. Naturally, engine
builders have adopted the most certain means of avoiding premature
ignition and its grave consequences.
The method of ignition which at present seems to be preferred to any
other for producer-gas is that employing a break-spark obtained with
the magneto apparatus previously described. Some builders of large
engines, particularly desirous of assuring steadiness of running, have
provided the explosion-chamber with two independent igniters. It may be
that they have adopted this arrangement largely for the purpose of
avoiding the inconveniences resulting from a failure of one of the
igniters, rather than for the purpose of igniting the mixture in several
places so as to obtain a more uniform ignition and one better suited for
the propagation of the flame.
=The Governing of Engines.=--Various methods have been adopted for the
purpose of varying the motive power of an engine between no load and
full load, still preserving, however, a constant speed of rotation.
These methods consist in changing either the quantity or the quality of
the mixture admitted into the cylinder. Thus it may happen that an
engine may be supplied:
1. With a mixture constant in quality and in quantity;
2. With a mixture variable in quality and constant in quantity;
3. With a mixture constant in quality and variable in quantity.
1. _Mixture Constant in Quality and Quantity._--This method implies the
use of the hit-and-miss system of admission, in which the number of
admissions and explosions varies, while the value or the composition of
each admitted charge remains as constant as the compression itself (Fig.
34). This system has already been referred to and its simplicity fully
set forth. By its use a comparatively low consumption is obtained, even
when the engine is not running at full load. On the other hand, it has
the disadvantage of necessitating the employment of heavy fly-wheel to
preserve cyclic regularity.
2. _Mixture Variable in Quality and Constant in Quantity._--The
governing system most commonly employed to obtain a mixture variable in
quality and constant quantity is based upon the control of the
gas-admission valve by means of a cam having a conical longitudinal
section, as shown in Fig. 35. This cam, commonly called a "conical cam,"
is connected with a lever actuated from the governor. As the lever
swings under the action of the governor, the cam is shifted along the
half-speed shaft of the engine. The result is that the gas-admission
valve is opened for a longer or shorter period.
In another system a cylindrical valve is mounted between the chamber in
which the mixture is formed and the gas-supply pipe, the valve being
carried on the same stem as the mixture-valve itself. The cylindrical
valve is displaced by the governor so as to vary the quantity of gas
drawn in with relation to the quantity of air.
When the engines are fed with producer-gas the parts which have just
been described should be frequently inspected and cleaned; for they are
only too easily fouled.
Engines thus governed should be run at high pressure so as to insure
the ignition of the producer-gas mixtures formed when the position of
the cam corresponds with the minimum opening of the gas-valve. Powerful
governors should be employed, capable of overcoming the resistance
offered by the cylindrical valve or the cam.
It may often happen that variations in the load of the engine render it
necessary to actuate the air valve, so as to obtain a mixture which will
be ignited and exploded under the best possible conditions.
3. _Mixture Constant in Quality and Variable in Quantity._--In supplying
an engine with a mixture constant in quality and variable in quantity,
the compression does not remain constant. The quantity of mixture drawn
in by the cylinder may even be so far reduced that the pressure drops
below the point at which ignition takes place. For that reason engines
of this type should be run at high pressures.
The variation of the quantity of mixture may be effected in various
ways. The simplest arrangement consists in mounting a butterfly-valve in
the mixture pipe, which valve is controlled by the governor and
throttles the passage to a greater or lesser degree. A very striking
solution of the problem consists in varying the opening of the
mixture-valve itself. To attain this end the valve is moved by levers.
The point of application of one of these levers is displaced under the
action of the governor so as to vary the travel of the valve within
predetermined limits. Under these conditions a mixture of constant
homogeneity is introduced into the cylinder, so proportioned as to
insure ignition even at low pressures.
[Illustration: FIG. 76_a_.--Governing system for producer-gas engines.]
In recent experiments conducted by the author it was proved that with
this governing system ignition still takes place even though the
pressure has dropped to 43 pounds per square inch. This system has the
merit of rendering it possible to employ ordinary governors of moderate
size, since the resistance to be overcome at the point of application of
the lever is comparatively small. In the accompanying illustration the
Otto Deutz system is illustrated.