THE VALVES
=Poppet Valves.=—Valves are provided for the purpose of controlling
the admission of the mixture to the cylinder and also for controlling
the exhaust or ejection of the burnt gases at the end of the firing
stroke. The most common form of valve is the mushroom or _poppet_ type
of valve shown in Fig. 29, in which A is the valve head, B is the
valve stem, C is the valve seating, and D is the cotter hole for the
cotter E. It will be seen that the general appearance of the valve is a
disc of steel with a fine stem to it similar to a mushroom in general
outline—hence its name. The valve has a coned face which is kept
pressed down on a coned seating by means of the pressure of a powerful
spring F acting on the washer G, which bears against the cotter E and
thus presses down the valve stem. To ensure that the valve shall always
come down correctly on its seating and make a gas-tight joint, the
valve stem guide M is provided.
[Illustration: FIG. 29.—GENERAL ARRANGEMENT OF A POPPET VALVE (A) WITH
TAPPET (K) AND CAM (H).]
The cam H raises the valve off its seat at the required instant when
the motion of the camshaft brings the cam under the roller R. The cam
lifts the roller vertically and with it the tappet or push rod K, which
slides vertically upwards in the guide P and lifts the valve. The
tappet is provided with an adjustable head S kept in position by the
locknut T. To adjust the _clearance_ between the head of the tappet and
the underside of the valve stem the locknut T must first be slackened
back and then the head S can be screwed up or down as desired, the best
clearance being about 1/64 of an inch; the locknut is then tightened
down again. During this operation the valve must be down on its seat.
Sometimes to reduce the noise arising from the tappet striking the
valve stem, the head of the tappet is padded with some material such as
hard vulcanite fibre, but this wears down more quickly than steel and
requires frequent adjustment. The latest device for reducing the noise
arising from the valve mechanism consists in totally enclosing the
valve gear and springs either by metal plates bolted to the cylinder
casting or by extending the crankchamber to cover it all in, and then
it is certain to be well lubricated. The exhaust valve is always liable
to give trouble either from leakage or seizure or other causes due to
the great heat of the exhaust gases, so that the valves are often made
now of tungsten steel alloy which is not much affected by heat. If a
mushroom type valve leaks it can be _ground in_ and made a tight fit
on its seating, provision usually being made for this in the form of
a slot cut in the valve head, as shown in Fig. 32, for the insertion
of a screwdriver or special tool. To grind in a valve, remove the cap
Q by unscrewing it, raise the spring F by pushing up the washer G and
then withdraw the cotter E. Lift out the valve and smear the coned face
with fine emery powder and oil (or water). Put the valve back and turn
it to and fro on its seating by means of the screwdriver, keeping a
firm pressure down on it; continue the operation until by examining the
valve you ascertain that it touches on the seating all the way round,
then couple up the spring again, after carefully removing all traces of
the emery powder.
[Illustration: FIG. 30.—SECTIONAL DRAWING OF THE CYLINDER OF A SLEEVE
VALVE ENGINE, WITH INLET FORTS UNCOVERED.]
[Illustration: FIG. 31.—SECTIONAL DRAWING OF THE CYLINDER OF A SLEEVE
VALVE ENGINE, WITH _EXHAUST_ PORTS UNCOVERED.]
=Sleeve Valves.=—Another form of valve which has come very much into
favour is the sleeve valve, two views of which are shown in Figs. 30
and 31. In this case the gases enter the cylinder through ports or
slots P cut in the cylindrical cast iron sleeves S_{1}, S_{2}, which
are placed between the piston K and the walls of the water-jacketed
cylinder C. These sleeves are moved up and down inside the cylinder,
while the piston travels up and down inside the inner sleeve S_{2} just
as though it constituted the cylinder C. Some engines have two sleeves,
as shown in the figure, but others have only one sleeve, and there is
very little to choose between the two types on the score of efficiency.
The great claim made for the sleeve valve is that it is almost
noiseless in action and gives very much fuller openings for inlet and
outlet of the gases. The piston has the usual number of packing rings
to keep it gas-tight, and there is also a deep packing ring provided
in the head of the cylinder H to keep the sleeve S_{2} gas-tight and
prevent loss of compression pressure. The head of the cylinder is
usually detachable, and has often separate water connexions in the form
of pipes leading from the cylinder jackets. The sleeves receive their
reciprocating motion from eccentrics and rods attached to pins shown
at the bottom right-hand corner of each sleeve. It might be expected
that the sleeves would get very hot or very dry and seize up, or the
piston might seize, but in actual practice this has not occurred to any
great extent, and on the whole they have been very successful. It is,
however, necessary to keep the engine well lubricated, especially when
the sleeves get worn, as the oil prevents loss of gas by leakage past
the sleeves and piston. In Fig. 31 the two sleeves have come together
in such a position that the ports coincide with the exhaust ports cut
in the cylinder walls and therefore the exhaust is _full open_, and
as the sleeves travel at times in opposite directions quick opening
and closing of the ports is secured. The cylinder head is held down to
the cylinder casting by screws or bolts and can be readily detached
for cleaning or inspecting the interior of the cylinder. The great
objection raised against the sleeve valves is their excessive weight
and the unmechanical manner in which they are operated.
[Illustration: FIG. 32.—SKETCH SHOWING SLOT IN A POPPET VALVE HEAD FOR
GRINDING-IN PURPOSES.]
[Illustration: FIG. 33.—INLET (A) AND EXHAUST (B) VALVE CAMS FOR A
SLOW-RUNNING PETROL ENGINE.]
=The Camshafts and Eccentric Shafts.=—These are usually made from the
same material as the crankshaft and machined from the solid bar, the
projecting cams or eccentrics being afterwards cut to the correct
shape. In the case of a camshaft it is very important that the shape
of the cams should be such that they lift the valves quickly off their
seats to the full extent of their opening (or _lift_), keep them open
for as long a period as desirable, and then allow them to close quickly
but without shock. Cams which have straight sides are more in favour
than those with curved sides, but if the action of the cams is to be
theoretically correct the side of the cam should be curved in such a
manner that the valve is lifted at first with a uniformly increasing
speed and afterwards with a uniformly decreasing speed, so that it will
be at rest in its top position. If this is not done the valve tappet
may jump a little above the cam each time the valve is lifted. In
Fig. 33 the cam A is intended for the _inlet_ valve and the cam B for
the _exhaust_ valve, the essential difference being that the exhaust
valve must be kept open longer than the inlet valve, and therefore
the exhaust valve cam is the wider of the two. The _timing_ of the
inlet and exhaust valves of an up-to-date engine may be explained by
considering the crankpin circle as divided into 360 parts or _degrees_.
If there were no _lag_ or _lead_ in the opening of the valves, then
they would open when the crank was on its dead-centre and close when
the crank was on its dead-centre. The inlet valve would open when
the crank was on its top dead-centre and close when it had reached
its bottom dead-centre, this representing the _suction_ stroke of
the engine. Then would follow compression and explosion, giving two
strokes or one revolution before the exhaust valve commenced to open.
The exhaust valve would then open when the crank was on its bottom
dead-centre and close when the crank reached its top dead-centre
corresponding to the completion of the exhaust stroke. It is very
important that the pressure of the gases above the piston when it
commences to move upwards on the exhaust stroke should be as low as
possible, and this can only be secured by opening the exhaust valve
towards the end of the _explosion_ or power stroke, thus allowing
the bulk of the gases to escape and leaving the piston with little
resistance to encounter on its upward exhaust stroke. Therefore we give
the exhaust valve a _lead_ of about 30 _degrees_, which means that it
begins to open when the engine crank is 30 degrees from the bottom
dead-centre on the downward explosion stroke, and we give it a _lag_
of about 5 degrees in closing. This means that we keep the exhaust
valve open until the crank has moved 5 degrees over the top centre, so
that we may fully utilize the _momentum_ of the gases to clear out the
cylinder or _scavenge_ it. As the piston moves rapidly up the cylinder
on the exhaust stroke it pushes the gases in front of it out through
the exhaust opening, but when it gets to the top of its stroke the
piston stops and then comes down again for the suction stroke, whereas
the gases will tend to keep on moving if they are not unduly restricted
in their passage through the exhaust system, so that we can generally
obtain some slight advantage by giving the exhaust valve a small amount
of _lag_ in closing.
The pressure of the gases in the cylinder after the exhaust valve
closes will nearly always be a little above atmospheric pressure, and
therefore nothing is gained by opening the inlet valve immediately the
exhaust closes—we generally allow an interval of 5 degrees, which means
that the total _lag of the inlet_ valve is 10 degrees in opening, or
the inlet valve does not begin to open until the crank has moved 10
degrees off its top dead-centre on the downward suction stroke. At the
end of the suction stroke the piston will again come to rest before
moving up on the compression stroke, but the gases will continue to
rush into the cylinder from the carburettor owing to their _momentum_
if we leave the inlet valve open a little longer, hence we generally
give it a _lag_ of 20 degrees in closing, which means that the inlet
valve does not close until the crank has moved 20 degrees up from the
bottom dead-centre on the compression stroke.
[Illustration: FIG. 34.—ECCENTRIC SHEAVE (A) AND ECCENTRIC ROD (B) FOR
OPERATING A SLEEVE VALVE.]
The camshaft requires to be well supported in bearings to prevent
it from sagging or bending under its load. If the shaft and the
cams are not made from nickel steel or high-grade steel alloy, they
require to be case-hardened (hardened on the surface) to prevent wear
on the surfaces due to the pressure of the valve springs, which is
considerable and may reach 100 lb. per valve easily; the same applies
to the rollers of the tappets. When sleeve valves are fitted to the
engine, _eccentric sheaves_ must be used instead of cams, as no springs
are employed. An eccentric sheave with its _strap_ and rod are shown
in Fig. 34. The valve shaft or lay shaft is shown at C, and the sheave
with the hole bored eccentrically is shown at A, and B is the combined
eccentric strap and rod. The pin D operates the sleeve valve, giving it
a reciprocating motion in a vertical direction, the angular movement
being taken up by the oscillation of the rod about the pin D, which
would be fixed into the sleeve. Sometimes a groove is formed round
the periphery of the eccentric disc or sheave to keep the strap in
position and prevent end movement. As the weight of the sleeves is very
considerable, the pin D and the eccentric rod must be well proportioned
to prevent breakage or undue wear.
[Illustration: FIG. 35.—A PAIR OF TIMING WHEELS.]
=The Timing Wheels=.—As there is only one suction stroke and one
exhaust stroke in every two revolutions of the engine crankshaft, it
will be clear that the camshaft or eccentric shaft must be driven at
_half the speed_ of the engine crankshaft. This may be done by the use
of two _gear wheels_ or wheels having teeth cut on their periphery,
such wheels when used for this purpose being called _timing wheels_,
because the positions of the cams on the camshaft (or the eccentrics on
the eccentric shaft) _relative_ to the engine crankshaft when the teeth
of the timing wheels are put into mesh determines the _timing_ of the
inlet and exhaust valves, i.e., the instant at which they will open or
close. A pair of timing wheels is shown in Fig. 35. The pinion A has
twelve teeth and is keyed to the engine crankshaft, but the wheel B,
which is keyed to the valve shaft, has twenty-four teeth, and hence
the valve shaft runs at half the speed of the crankshaft. The wheels
shown are _spur_ gears, and the teeth run straight across the rim of
the wheel; it is, however, quite common to find wheels with curved or
_helical_ teeth, as these run quieter. Sometimes when spur gearing
is used, one of the wheels is made of fibre and the other of steel,
but when helical gears are used the wheels are generally made from
nickel steel of high tensile strength. The finer the _pitch_ of the
teeth (i.e. the distance between the centres of consecutive teeth) the
quieter the gears will run, but the question of strength and the cost
of production must also be considered. The latest practice is to use
a silent _chain_ drive; this originated with the introduction of the
sleeve valve and eccentric shaft. When chains are used for the timing
wheels provision must be made for taking up slack in the chain owing
to stretching of the links, and as this cannot be done in the usual
manner (by sliding the _sprocket wheels_ further apart) owing to the
centres of the crankshaft and the valve shaft being rigidly fixed by
the bearings, a small jockey pulley (with teeth on it similar to those
on the chain sprocket wheels) is provided attached to a short shaft
or spindle, which can be raised or lowered at will, and thus keep the
correct tension on the chain. The chain drive must be more expensive
and require more attention; moreover, it cannot be so very much quieter
in action than good well-cut helical gearing.
[Illustration: FIG. 36.]
[Illustration: FIG. 37. TWO VIEWS OF A CRANKCHAMBER.]
=The Crankchamber.=—The crankchamber, as its name implies, is the
receptacle which contains and supports the crankshaft and also the
camshaft. It is generally an aluminium casting, but frequently for
commercial vehicle engines the top portion is made of cast iron and
the bottom portion of sheet steel. In either case brass or gunmetal
bearings, often lined with white metal, are fitted for the shafts
to revolve in, and the engine cylinders are mounted on the top of
the chamber. Provision should be made on the sides and ends of the
crankchamber for fitting the _magneto_ and _oil pump_ and also the
_water pump_, if required. There must also be some form of housing or
extension of the chamber to enclose the timing wheels, and sometimes
the whole of the valve gear is contained within the crankchamber to
ensure proper lubrication for it and stop any noise from it reaching
the outside world. It is also important that there should be large
inspection openings fitted with proper oil-tight covers and some
provision for easily pouring large quantities of oil down into the
lower portion of the chamber. The design of a crankchamber necessitates
careful forethought to ensure ample provision for all the necessary
attachments and fittings and to secure the maximum accessibility of all
parts. One or two _vent_ pipes, consisting of upwardly projecting pipes
having their outer end covered with wire gauze and screened from dust
should be provided to allow hot air and gas to escape from the chamber.
Two views of a crankchamber of modern design are shown in Figs. 36
and 37. In these figures A is the top half of the crankchamber which
rests upon the _chassis_ or framework of the car, being bolted to an
_underframe_ at B and C. The cylinders are attached to the chamber at
the flange H by means of studs and nuts. This portion, the top half of
the crankchamber, requires to be very strong and stiff, because the
upward pressure of the explosions acts on the crown of the cylinder and
tends to tear the cylinder off the flange H, while at the same time it
exerts a great force on the piston, pushing it downwards and tending to
force the crankshaft down out of its bearings. In the best practice the
whole weight of the crankshaft is supported from the top half of the
crankchamber and is carried on the bearing bolts as shown at S, so that
they also receive the downward thrust of the piston and in their turn
transmit it to the main casting.
The bottom half of the crankchamber then becomes merely an oil
container, or reservoir, and dust cover; it should be so arranged and
situated that it may be readily removed for inspection of shaft and
bearings from underneath. Sometimes the crankchamber has long arms,
which can be attached directly to the side members of the chassis, or
it may be supported in the chassis by a tubular cross member.
In Fig. 37 the camshaft is shown at T; the magneto would be carried on
the bracket E and driven by gearing from the crankshaft. The facing at
G is for the water pump, which, in this case, is intended to be mounted
on an extension of the camshaft T. The oil pump would be fixed at F,
preferably towards the rear of the engine, so as to secure an adequate
supply of oil for the pump when the car is climbing a steep hill.
The oil could be drawn off and the reservoir emptied by unscrewing
the large plug shown in the centre of D in Fig. 37. The timing wheel
housing or casing is shown at Q; the oil ducts and connexions for
supplying the main bearings with oil are not shown in these drawings,
nor are the inspection openings and covers. The upper half of the
crankchamber frequently becomes very hot, due to conduction of heat
from the metal of the cylinders, and for this reason it has from time
to time been proposed to draw the air supply of the carburettor through
the crankchamber to serve the dual purpose of cooling the bearings and
heating the air supply to the carburettor; but the idea has not found
favour, as there is considerable risk of dust and grit finding its way
into the bearings and causing trouble due to abrasion.