OIL ENGINES
The small oil engine is practically the same as the gas engine, with the
addition of a vaporiser for converting the oil into gas, or vapour, to
be exploded in the cylinder; consequently the one may be converted into
the other in many cases without much trouble. The difficulty of
producing an efficient oil engine lies principally in devising a
satisfactory and reliable vapouriser--one which will work equally well
under all loads. The heat supplied to the chamber must be sufficient to
vaporise the oil, but not great enough to decompose it. There are
various methods of vaporising the oil, and many types of vaporisers are
employed to attain the same end. There are some in which a charge of oil
is drawn by suction into a hot chamber in which it is converted into
vapour and at the same time mixed with a small quantity of hot air; this
rich mixture is then passed into the combustion chamber of the engine,
in the same manner as coal-gas would be, where it is further diluted
with more air drawn in through the air valve. Other arrangements cause
a jet of oil to be injected into a chamber containing hot air, in the
form of spray, which immediately converts the oil into vapour, and is
then passed into the cylinder, compressed, and fired. Then, again, we
can pump oil through a spraying nipple into the vapouriser (which is
kept at a suitable temperature) whilst the cylinder is being filled with
air on the suction stroke. On the following compression stroke the air
is driven into the vapouriser, which communicates with the cylinder
through a narrow neck, and mixes intimately with the oil vapour.
Gradually, as the pressure rises, due to compression, the charge becomes
more and more explosive, until at the completion of this stroke it has
attained the proper proportions of air and oil vapour, and is fired by
the temperature of the vapouriser and that caused by a high compression;
that is, the charge is fired automatically; and once the engine is
running, no heating lamp is required to keep the vapouriser at the
correct temperature. It is necessary, however, to raise it to the
workable temperature at starting. This is known as the Hornsby-Akroyd
method.
Capel's arrangement is also simple and efficient, and has the additional
advantage of being capable of being fitted to their existing gas
engines, the conversion being made in a very short time. This vapouriser
consists primarily of a tubular casting A, on the outside of which are
formed a series of vertical ribs, shown in plan, fig. 46, running to
within a short distance of the flange at one end, as shown in the
section, fig. 45, thus providing an annular space C between the upper
ends of the ribs and the flange. This casting is enclosed by an outer
casing B, which fits well over the inner tube. It has also a number of
small holes drilled near the lower end communicating with the channels
between the ribs. Thus it will be seen that when the gas valve is opened
and suction takes place, air is drawn in through these holes, passes up
into the annular space C below the top flange, from there travels to the
opposite side of vapouriser, and mixes with the oil which is also being
drawn in through a small nipper at N, fig. 45. Both then pass between a
series of pegs, where they become thoroughly mixed, and finally pass on
to the inlet valve V, fig. 47, and so into the cylinder, where the
complete charge is mixed up and compressed and fired in the usual
manner. Iron ignition tubes may be used, and one heating lamp serves a
double purpose in keeping the tube and vapouriser hot at the same time.
This lamp is fed by means of a pump actuated from the side shaft. The
plunger of the pump is loaded with a spiral spring, which may be
adjusted to give any desired pressure, and is kept constant and steady
by means of an air vessel. This pump is shown in fig. 48. It is actuated
by means of a rod and lever from the side shaft of engine. The plunger P
works in a barrel B, which is carried by a small reservoir R, the latter
being in communication with the main oil tank by means of the pipe H.
[Illustration: FIG. 46.]
[Illustration: FIG. 45.]
[Illustration: FIG. 47.]
[Illustration: FIG. 48.]
[Illustration: FIG. 49.]
[Illustration: FIG. 50.]
[Illustration: FIG. 51.]
The plunger is loaded with a spiral spring, and has a ball valve, as
shown. Intermediate between this small reservoir and the main oil tank
is another set of valves, shown in fig. 49. It will be seen that the
suction of the pump will draw the oil up, the small and lower ball
valve, of course, allowing it to pass freely. On the down stroke the
lower valve will be automatically closed, and the oil will be put under
pressure, this being determined by the load on the plunger valve, which
is adjustable by means of the screw S, fig. 48. When the required
pressure in the pipe P, figs. 45 and 49, has been attained, the plunger
valve lifts on each stroke and the surplus oil flows through the plunger
into the small reservoir R. The latter is at about the same level as
another still smaller reservoir M (shown in figs. 47 and 50), a flow of
oil being established between the two by means of a pipe Q (see figs. 48
and 50). In the reservoir R is fitted an overflow pipe, so that the oil
cannot rise beyond a certain level; hence the head of oil in the smaller
one M is always constant. On the suction stroke a partial vacuum is
formed in the engine cylinder, consequently the pressure in the
vapouriser drops somewhat below that of the atmosphere, and this small
difference in pressure is enough to cause the oil to rise in the small
passage X, fig. 45, beyond its normal level, and overflow into the
vapourising chamber, as previously described. The valve or nipper N is
shown open in the diagram, fig. 45, and all that is required to stop the
engine when running is to drop the small handle L, fig. 45, when the
valve will close, due to the spring S. The air vessel shown in fig. 49
is in communication with the pipe leading to the blow lamp. A pressure
gauge can also be fitted, although it is not in any way a necessity.
The ratchet wheel and pawl shown in fig. 48 are part of the lubricator.
The wheel drives a brass or gun-metal plug, producing an intermittent
rotary motion. The plug has a small hole in its periphery, which becomes
filled with oil when it is at the upper part of its travel, and empties
the oil out into a discharge pipe T, when it is inverted, and is then
led away and applied to the piston at the required spot. Fig. 51 shows
this arrangement in section.