LUBRICATION
=Properties of Oils.=—Owing to the very high speed at which the modern
petrol engine runs great attention must be paid to lubricating the
moving parts, otherwise undue wear or even _seizure_ will result. We
must be very careful to choose a suitable oil, one which is chemically
pure and retains its lubricating properties at high temperatures. A
considerable amount of oil finds its way into the cylinder, where it
comes into direct contact with the hot gases. If an oil is heated
a temperature will sooner or later be attained, when the oil will
give off an _inflammable_ vapour, i.e., one which will burn. This
temperature is called the _flash point_ of the oil. If the oil is
likely to get into the cylinder of a petrol engine it should have a
very high flash point; in fact, most of these oils do not flash until
well over 400° Fahrenheit. Also when the oil is burnt it must not leave
any appreciable residue. Some oils are very defective in this respect,
and leave large quantities of _carbon deposit_ on the metal walls of
the cylinder and the valves; others again are gummy or too _viscous_
even at high temperatures. Such oils must be avoided equally with those
which lose their _viscosity_ too much under heat.
=Splash System of Lubrication=.—One method of lubricating the working
parts is known as the splash system. In this system oil is poured
into the crankchamber and the moving parts dip into it, splashing it
all over the interior of the crankcase and the lower portions of the
cylinder walls. Oil holes are drilled in such positions that as the
oil drops down again after being splashed upwards some of it will
fall into these holes and lubricate the bearings. This is a very
cheap method of lubrication in _first cost_, but very wasteful and
unsatisfactory in regular use, hence it has practically died out. As
the oil is used up a fresh supply must be admitted by some form of
continuous drip-feed arrangement, the oil being forced over very often
from a small tank on the footboard by means of air pressure or the
pressure of the exhaust gases from the engine. It is very difficult
under these circumstances to estimate how much oil is present in
the crankchamber at any given instant, so that there was usually
alternately too much or too little. Too little oil meant undue wear
on bearings (perhaps seizure), and too much oil meant a smoky exhaust
which became very obnoxious when the engine was suddenly accelerated.
[Illustration: FIG. 55.—IMPROVED SYSTEM OF SPLASH LUBRICATION.]
=Improved System of Splash Lubrication.=—This is a combination of the
splash system and the _forced_ system, and is shown in Figs. 55 and
56. In these figures A_{2} and A_{3} represent two of the main engine
bearings which support the crankshaft; C_{1}, C_{2}, C_{3} are three
of the crankpins; F_{1}, F_{2}, F_{3} are oil troughs placed under
the crankpins; D_{2}, D_{3} are oil feed pipes to the main bearings.
Generally speaking, the oil is drawn from the bottom of the crankcase
by means of a pump, and this pump delivers the oil to some form of
indicator mounted on the dashboard of the car. After passing through
the indicator the oil flows by two main pipes, one of which feeds the
main bearings by means of branches D_{2}, D_{3}, etc., and the other
feeds oil troughs by means of branches such as G_{2}. When the troughs
are _full_ the oil overflows into the bottom of the crankchamber, and
so there is always a constant depth of oil for the scoops attached to
the connecting rod ends to dip into, and one great drawback to the
splash system is overcome; also the main bearings are always sure of
being amply supplied. The oil pump may be an ordinary plunger type pump
or a rotary pump.
[Illustration: FIG. 56.—SECTIONAL VIEW OF END OF CONNECTING ROD,
SHOWING ARRANGEMENT OF SCOOP AND OIL TROUGH.]
=Forced Lubrication.=—One system of forced lubrication is shown in
Fig. 57. The general arrangement of the system is very similar to the
preceding one, except that there are no troughs in the crankchamber and
_all the bearings_ receive an ample supply of oil under _pressure_ so
that the journals are supported in their bearings on a _film of oil_
and the metals never come in direct contact with each other. After
entering the main bearings the oil passes through holes drilled in
the crankshaft and thus positively lubricates the crankpin bearing,
passing up the connecting rod either internally as shown or by an
external pipe it lubricates the gudgeon pin and then falls down
into the crankchamber. On its way down it gets splashed about and
thus lubricates the cylinder walls and piston; sometimes these are
positively lubricated by leading the oil through the centre of the
gudgeon pin direct to the surface of the cylinder walls—but this often
gives an excess of oil and causes a smoky exhaust. In Figs. 58 and
59 we show two views of a very popular form of oil pump for forced
lubrication systems. It consists of two gear wheels, one of which is
driven by a spindle from the engine crankshaft, and it drives the
second wheel by means of the projecting teeth. The oil is picked up by
the teeth and passed round from the suction to the delivery side of
the pump on the _outer_ edge of the wheels; no liquid can pass direct
across between the teeth which are in mesh, and hence the direction of
rotation is as shown by the arrows.
[Illustration: FIG. 57.—FORCED LUBRICATION SYSTEM.]
[Illustration: FIG. 58.]
[Illustration: FIG. 59. TWO VIEWS OF A ROTARY OIL PUMP FOR FORCED
LUBRICATION.]
The difficulty of securing a really good lubricant for petrol engines
must be apparent from a study of the prices of the various oils. It
will be observed that they are all considerably more expensive than
petrol, and therefore we must economize in their use. The old splash
system was very wasteful and consumed oil at the rate of one gallon
every hundred miles at least, but a modern system of forced lubrication
will not require more than one gallon of oil every thousand miles.
Perhaps an average everyday figure for ordinary motor-car engines would
be one gallon every 250 miles. The pressure of the oil in a forced feed
system varies in different makes of engines from 5 up to 40 pounds per
square inch—a very common figure, however, is 10 pounds per square
inch. The speed of the oil pump also varies considerably, and ranges
from 500 up to 2,000 revolutions per minute at normal engine speed.
Generally a small relief valve is fitted in the pump casing, which
returns oil to the crankchamber if the pressure tends to rise above the
desired limit due to the engine speed increasing. We have mentioned
already that the flash point is generally over 400° Fahrenheit when the
oil is new, but after it has been in the crankcase some time and got
used over and over again it is found that the petrol vapour leaking
past the piston rings of the engine condenses when the engine cools
down after a run and drops into the oil in the sump, thus lowering its
viscosity and its flash point. According to Mr. Morcom it may come
down as low as 200° Fahrenheit (about), but if the oil is heated and
the petrol driven off the flash point goes up again. Therefore it is
a good plan with forced lubrication systems to empty the old oil out
periodically and fill up entirely with fresh oil.