THE POINTS OF A GOOD ENGINE
=Choosing the Number of Cylinders.=—It is a very difficult problem to
select the _best_ engine for a particular purpose, as there are so
many factors which influence one’s choice. A single cylinder engine
would only be used for a motor-cycle or a small car of low power; the
vibration and noise resulting from the use of a single cylinder petrol
engine of even six horse-power are most objectionable, and difficulties
of starting and risk of engine unexpectedly pulling up and stopping
are greatly enhanced. The two-cylinder engine offers better prospects,
and was for some time considered quite good enough for most purposes,
but owing to its _comparatively_ bad balance and its low torque it
has fallen into disfavour. We have seen how the rotating parts of the
engine can be balanced, but we have not considered the reciprocating
parts. To understand this question of balancing we must talk about
“_inertia forces_.” All bodies possess inertia, that is, they resent
changes in their state of rest or motion. If a body is moving uniformly
it tends to keep on doing so, whereas if it is at rest it tends to
remain so. To start the body off from rest, or to stop the body and
bring it to rest, requires a _force_ to be exerted, and this force
may be called the _inertia force_. When a petrol engine is running at
high speed the piston has to be started and stopped at the top and
bottom of its stroke every time the crankshaft revolves once, and to
do this very large forces are needed, because it has to be done so
quickly. These inertia forces take the form of pushes or pulls on the
shaft and framework of the engine, and thus cause _vibrations_ to be
set up. If the periodicity or _frequency_ of these forced vibrations
happens to coincide with the natural period of vibration of the shaft
material the shaft will commence to _whip_, and may possibly break
under the excessive strain. In a two-cylinder engine with cranks 180
degrees apart (or half a revolution) one piston is moving upwards and
the other piston is moving downwards, both at very high speed; and both
have to be brought to rest when the cranks come on their respective
dead-centres. The piston which is moving up tends to lift the shaft up
with it, and the one which is moving down tends to pull the shaft down
with it, because the connecting rods check the progress of the pistons
and bring them to rest at the top and bottom of their strokes. If these
two pulls acted in line with each other they would _balance_, but the
cylinders are usually mounted side by side, and then the two pulls
virtually act at the ends of a bar whose length is the longitudinal
distance between the vertical centre lines of the two cylinders.
Thus these two inertia forces tend to rotate the whole engine first
in a clockwise direction and then in a counter-clockwise direction,
according to which piston is moving up or down. The only way to balance
these forces under these conditions is to extend the crankshaft
longitudinally and place another pair of cylinders and cranks in line
with the first, but so arranged that the inertia forces tend to turn
the engine in the opposite direction to the first pair. This gives
us the well known four-cylinder arrangement so much in evidence at
the present time, the arrangement of cranks being shown in Fig. 21. A
six-cylinder engine gives _perfect balance_ if all the parts are of
equal weight, and the cranks at 120 degrees to each other in opposed
pairs.
Again, a single-cylinder engine gives one power stroke in every two
revolutions of the shaft, a two-cylinder gives a power stroke in
every revolution, a four-cylinder gives two power strokes, and a
six-cylinder gives three power strokes in every revolution of the
shaft. Hence a six-cylinder engine is very _flexible_ (i.e., can
accommodate itself easily to varying loads), is perfectly balanced,
and can be made both powerful and economical. One objection to the use
of engines with multiple cylinders (exceeding, say, _four_ in number)
is that the crankshaft is more liable to vibrate and cause very harsh
running at high speeds on account of the fact that the _periodicity
of the power impulses_ imparted to the shaft approaches the natural
period of vibration of the shaft. This effect arises from _torsional_
oscillations and is distinct from the _periodicity due to inertia
forces_ which acts in the vertical plane. A four-cylinder engine is
nearly as good as a six-cylinder of equal power, and is of course much
cheaper in first cost, takes up less room, and weighs less. A good
four-cylinder engine will often prove more economical in running costs
than a six-cylinder, as it will probably be running a greater length of
time at or near its full output, and the work done on the idle strokes
of the cycle will be less owing to the smaller number of cylinders.
Another feature to consider is the _arrangement_ of the cylinder
castings. A monobloc casting (cylinders all in one casting) gives a
very short engine and reduces the length of the crankshaft, but in the
event of one cylinder bore being damaged the advantage lies with the
separate cylinder construction.
_The Question of the Valves._—The question as to which is the better
engine, the sleeve valve or the poppet valve, cannot be said to have
been definitely decided yet. The great feature of the poppet valve used
to be its very quick opening and closing, but nowadays engines turn
over so fast that very strong springs are needed to close the valves
in a reasonable time. One complete revolution of the engine means that
the crank has turned through 360 _degrees_, and the inlet valve is open
while the crank turns through 190 degrees (on the average), but during
part of this time it is _being_ lifted or _opened_, and during an
equal period it is _being closed_. The question then is, “How long does
it remain _fully open_?” The answer is—not more than ten degrees at
the most! To keep the inlet valve open longer than this would require
excessively stiff springs and throw a great strain on the valve gear.
Now this is where the sleeve valve managed to get a look in—as one
might say. With _two_ sleeves moving in opposite directions, or one
sleeve receiving a special form of motion, we can open and close the
ports and keep them fully open for just as long period or even longer
than the poppet valve. If it were not for the fact that sleeve valves
are heavy and not so easy to keep gas-tight as poppet valves, it is
perfectly obvious that the poppet valve would have disappeared or taken
second place long before this.
Another great advantage of the sleeve valve is that by making large
ports we can easily secure larger valve openings than are possible,
for practical reasons, with a poppet valve. It is now claimed also
that the interior of the cylinders keeps free from carbon deposit much
longer with sleeve valves than with poppet valves, this carbon deposit
being due chiefly to the use of too rich a mixture which causes the
combustion to be imperfect and results in the deposit of solid carbon
on the walls and sides of the combustion chamber. =Carbon deposit= is
also caused by using unsuitable lubricating oil, but it principally
arises from the use of too rich a mixture for the purposes of securing
quick acceleration. _Perfect_ combustion is only secured by the use
of a relatively _weak_ mixture, which would prevent the maximum power
being developed and give rather a feeble acceleration. Modern engines
have to be very carefully designed to reduce this nuisance of the
carbon deposit to a minimum, and also with a view to its speedy and
efficient removal when it does take place. Detachable cylinder heads
have been introduced principally to allow of rapid removal of carbon
deposit from pistons and valves and the combustion chamber. If the
carbon deposit is allowed to accumulate, _pinking_ or sharp knocking
commences, due to pre-ignition of the charge by red-hot particles
of carbon. This results in loss of power, and is first noticed by
inability to climb steep hills that were formerly negotiated with
ease. Mention must also be made of the great claim for _silence_ of
running of sleeve valve engines, and this is thoroughly justified with
_high-class engines_ of the sleeve valve type, provided they are in
the hands of skilled drivers. In unskilled hands one finds that the
poppet valve is safer, and will stand more knocking about without much
increase in noise resulting. Rotary valves have fallen into disuse on
account of the difficulty of keeping them gas-tight. There is nothing
to choose between poppet and sleeve valves on the score of economy in
running.
=Economy and Durability.=—A good modern petrol engine of reasonable
size—say over 3 in. bore—will give one brake horse-power for an hour
from the consumption of two-thirds of a pint of petrol. This means
that an engine giving 12½ horse-power on the brake would use a gallon
of petrol every hour. But economy in petrol consumption is not the
only desirable feature of a petrol engine. There must be economy in
lubricating oil and in cost of replacements or repairs. Nowadays the
tendency with high grade steel alloys and other modern metals of high
strength and durability is to cut everything down to its minimum size
with a view to reducing _the cost of production_. This often leads to
many serious troubles in running on the road. In choosing an engine
one should carefully examine such points as provision for wear and
adjustment, strength and rigidity, and whether the engine impresses one
with a sense of its _durability_ and also its general _accessibility_.