DESCRIPTION OF A TYPICAL PETROL ENGINE
For the purpose of explaining the cycle of operations we have
considered only a diagrammatic sketch of an imaginary motor-car
engine, but in Fig. 7 we illustrate an up-to-date motor-car engine.
In the first place we note the position and arrangement of the four
water-cooled cylinders, A_{1}, A_{2}, A_{3}, A_{4}, containing their
pistons and mushroom type valves. These are conveniently placed in a
vertical position and mounted on top of the crankchamber C, to the
bottom of which is attached the oil-base B. At the front of the engine
are shown the timing wheels in their casing E, and at the rear end the
flywheel F. The starting-handle connexion is at S, the fan pulley being
shown at M. The high tension magneto which supplies the current to the
sparking plugs is shown at H, and I is the induction pipe connected to
the carburettor K. The water circulating pump is on the off side of the
engine and does not appear in the illustration, but L_{1} is the inlet
water pipe leading from the radiator (not shown) to the water pump, and
L_{2} is the delivery pipe from the pump to the respective cylinder
jackets, L_{3} being the outlet water pipe. The exhaust pipe is shown
at D, and the oil pump at P. The valve springs, valve tappets and
guides can also be clearly seen. In examining the several parts of the
engine in detail we must not lose sight of their respective positions
in the general arrangement view of Fig. 7.
[Illustration: FIG. 7.—GENERAL ARRANGEMENT OF A MODERN PETROL ENGINE.]
=The Cylinder.=—Probably one of the most important parts of an engine
is the cylinder. As we have already seen, it is inside the cylinder
that the charge of petrol vapour and air is exploded and completely
burnt. The heat energy of the petrol mixture which is liberated by
the explosion is immediately transformed into mechanical work and
propels the piston forward like a projectile from a gun. But we must
also notice that our present-day arrangements (clever as they are) are
by no means perfect, and we cannot, even under the most favourable
circumstances, convert more than about _one-third_ of the heat energy
of the petrol mixture into the mechanical energy of the moving piston.
Of the remaining two-thirds of the heat, part is used up in heating the
cylinder walls, the piston and the valves, and the remainder goes out
with the exhaust gases to the silencer, finally escaping to the outside
air. Thus two important facts are brought to our notice:—
(1) The reason why we use petrol to drive our motor-cars is because
petrol (and certain other liquid fuels such as benzol, etc.) contains
within itself a store of energy which can be liberated as heat when the
fuel is burnt or exploded in the presence of air in the engine cylinder.
(2) At the present day, even with our most up-to-date contrivances,
we cannot make use of two-thirds of the available heat in our petrol.
Instead of being able to turn this heat into useful mechanical work,
we are compelled to throw it away—to waste it. Further than that, we
have to make special provision to ensure that it shall be wasted as
quickly as possible and as easily as possible. We take out the greatest
amount that we can possibly turn into work and then hasten to dissipate
the remaining two-thirds. We cast hollow chambers on the outside of
our cylinders through which we circulate cold water to keep down the
heat in the cylinder walls; if our cylinder walls and piston get too
hot our engine may seize up, therefore we must cool them to ensure
satisfactory running. Again we make large exhaust valves and provide a
free escape through the silencer for the exhaust gases, so that when we
have snatched our useful one-third of the heat supply we may throw the
remainder away into the atmosphere as rapidly as possible.—this part is
of no use to us, we cannot turn it into work, then why let it stay here
and heat our cylinder walls and piston still further?
It is a good plan to extend this hollow chamber, containing the water
in circulation, at least round the whole of the combustion chamber
and all round the inlet and exhaust valve passages and down the
barrel of the cylinder as far as the walls are likely to come into
contact with the hot gases from the explosions. We refer to this
hollow chamber, with its circulating water, as the _water-jacket_ of
the cylinder. It is not absolutely _essential_ to have our cylinder
water-jacketed, especially with small engines for motor-cycles and
engines for aeroplanes which have revolving cylinders, but it is
practically essential in nearly all other cases. Even in the special
cases mentioned it is found necessary to form special heat radiating
fins on the outside of the heated walls to assist in dissipating or
getting rid of the surplus heat and preventing seizure of the piston
within the cylinder. These fins are clearly seen on the cylinder of the
motor-cycle engine shown in Fig. 13.
Thus we may say that motor-car engine cylinders are bound to be
water-jacketed, i.e., to have a hollow space round them containing
water in circulation. The cylinders themselves are nearly always made
in the form of iron castings and the jacket spaces form part of the
_cylinder casting_ as a general rule, but occasionally the water-jacket
space is formed by attaching plates or tubes to the cylinder casting by
means of bolts or screws—not an easy thing to arrange successfully, as
it requires water-tight joints.
The procedure for manufacturing a motor-car cylinder is first of all
to design and calculate the proportions of the various parts and get
out a set of working drawings. From these drawings we get _patterns_
and _core-boxes_ made in wood. The patterns are the exact shape of the
finished cylinder on the _outside_, and the core-boxes are the exact
shape of the _inside_ of the finished cylinder (except in so far as
allowance has to be made for parts which must afterwards be machined).
The patterns are pushed down into the moulding sand in the foundry,
and when withdrawn leave their impression, thus forming _moulds_. The
core-boxes are filled with sand, which when withdrawn furnishes us with
masses of sand that are the counterpart of the interior of the cylinder
in shape. These _cores_ are supported centrally in the mould (which is
usually in halves, or _more_ than two parts), while the molten iron
is poured into the intervening space to form the iron _casting_. When
the casting has cooled down the sand can be cleaned off quite easily.
One set of patterns and core-boxes will thus produce quite a number of
cylinder castings, each being similar in every respect to the other,
the process being a quick and fairly cheap method of reproduction.
Later on the cylinder barrel has to be machined and bored out true to
very fine limits by the use of boring tools and some kind of boring
machine or lathe. The flanges or flat faces have to be planed true in
a planing machine and the valve stem guides and valve seatings must be
carefully and truly machined to correct size and shape.
[Illustration: FIG. 8.—SECTIONAL DRAWING OF A T-HEADED WATER-JACKETED
CYLINDER. VALVES ON OPPOSITE SIDES OF CYLINDER.]
[Illustration: FIG. 9.—OUTSIDE VIEW OF A WATER-JACKETED CYLINDER.]
Figs. 8 and 9 show two views of a single motor-car engine cylinder,
the water-jacket forming part of the cylinder casting. In the figures
C is the cylinder barrel or bore; J the water-jacket; I the inlet for
the jacket water; O the outlet for the jacket water; D is for the
compression tap; S for the sparking plug; V_{1}, V_{2} are the valve
seats; G_{1}, G_{2} are the valve stem guides; H_{1}, H_{2} are caps
which may be removed when the valves are being put in or taken out;
f_{1}, f_{2}, f_{3}, f_{4}, f_{5} are called flanges. The flange f_{1}
is used for attaching the cylinder to the crankchamber; while it is
quite true that the force of the explosion within the cylinder drives
the piston downwards, it is equally true that it also tends to force
the cylinder head off or to blow the cylinder casting upwards off the
crankchamber, unless it is securely fastened to it by means of screws
or bolts passing through the flange f_{1}. The flanges, f_{2}, f_{3}
are for the inlet and outlet water pipe attachments, and f_{4}, f_{5}
are for the induction pipe and exhaust pipe connexions. Generally the
pipes will have flanges and be held tight against the flanges on the
cylinder casting by means of screws or studs. Figs. 10, 11, and 12 show
how two metal flanges are held in contact by means of screws or studs
or bolts, and they also show the _packing materials_ between the metal
surfaces which keep the joint _tight_ and prevent water or gas leaking
across the flanges and escaping to the outside air, or air leaking in
if the internal pressure is below that of the atmosphere.
[Illustration: STUD FIG. 10.]
[Illustration: BOLT FIG. 11.]
[Illustration: SETSCREW FIG. 12.]
[Illustration: FIG. 13.—MOTOR-CYCLE ENGINE WITH AN L-HEADED AIR-COOLED
CYLINDER. VALVES BOTH ON SAME SIDE OF CYLINDER.]
[Illustration: FIG. 14.—SECTIONAL DRAWING OF A BUILT-UP CYLINDER
SUITABLE FOR AN AEROPLANE ENGINE.]
In Figs. 8 and 9 the valves are placed one on each side of the
cylinder, this form of cylinder being known as a =T=-headed cylinder,
but it is rather more usual here in England to place both valves on
the same side of the cylinder and next to each other as indicated in
Fig. 13, this form of cylinder being known as an =L=-headed cylinder.
The chief object is of course to avoid the use of two valve shafts and
also to produce a neater looking engine, but the _T_-headed design is
better cleaned or scavenged by the passage of the inlet and exhaust
gases. When a motor-car engine has two cylinders we frequently find
them both in a single casting, having a common water-jacket, and then
we say they are _cast in pairs_. A four-cylinder engine may thus have:
(1) Cylinders _cast separately_; (2) Cylinders _cast in pairs_; (3)
Cylinders _cast en bloc_; or all four in a single large casting. The
third method is cheapest in first cost, but in the event of breakage
will become the most expensive. The second method is a sound compromise.
An example of a _built-up_ cylinder and water-jacket is shown in Fig.
14, the cylinder barrel being of steel tube with steel flanges, and
the water-jacket being formed by copper tube slipped over the outside
of the steel cylinder. Its great advantage lies in the reduction of
weight, and it is thus largely used for aeroplane work. The valves
would then be fitted in the top cover of the cylinder and driven by
overhead gearing.