TWO-STROKE ENGINES
In the two-stroke type of petrol engine the cycle of operations is
completed in two working strokes of the piston instead of the four
required by the “Otto” cycle; there is thus one explosion or power
stroke in every revolution of the crankshaft. Theoretically this
represents a great advance over the “Otto” cycle, but difficulties
and complications arise in the practical carrying out of the cycle.
The cycle on which it is desired to operate the engine is: _1st
stroke_—Compression; _2nd stroke_—Explosion. The charge would be
introduced on the compression stroke and exhausted towards the end of
the explosion stroke.
Now the charge of gas required by the engine consists of a mixture of
petrol vapour and air, and it must either be sucked in or pushed in
under _pressure_. In the “Otto” cycle the charge is _sucked_ in, and in
the two-stroke cycle it is delivered to the cylinder under _pressure_;
hence in the two-stroke cycle some form of pump is required which will
suck in the charge of air and gas, compress it a small amount, and
deliver it to the working cylinder at a pressure of 5 or 6 lb. per
square inch above atmospheric pressure. This is where the complications
commence; if we fit a separate pump for each cylinder, which is
what would generally be done, or if we made one pump serve for two
cylinders, we have to provide pump cylinders, pistons, rods and valves,
and therefore there is practically no gain over the four-stroke engine.
Hence it is that inventors all try to avoid the use of a separate
charging pump and turn their attention to the production of an engine
in which one or more of the existing portions is made to serve as
a pump for charging the working cylinder or cylinders with gas. A
favourite and fairly successful device is to make the crankchamber
gas-tight and use it as the cylinder of the pump, the underside of the
engine piston then forming the pump piston which draws the charge from
the carburettor into the crankchamber on its upstroke and compresses
it on its downstroke, delivering it to the working cylinder through
the inlet port as soon as the piston has uncovered it by its downward
movement.
_FIG. 63.—TWO PORT TYPE OF TWO-STROKE ENGINE WITH CRANKCASE
COMPRESSION._
There is _no exhaust valve_, as the piston uncovers the exhaust ports
a little before the inlet ports are opened. To prevent the new charge
escaping directly across the top of the piston from the inlet ports to
the exhaust ports, a deflector is fitted on the top of the piston equal
in height to the height of the exhaust opening and situated immediately
in front of and facing the inlet ports.
A two-stroke engine of the type referred to is shown diagrammatically
in Fig. 63. E is the gas-tight crankchamber, upon which the
water-cooled cylinder A is mounted in the usual manner and fixed by
studs or bolts. The piston P carries the deflector H, which is equal
in height to the height of the exhaust opening G. The piston rings
are prevented from turning by pins so arranged that the joint of the
rings does not pass across the ports. The connecting rod D is of
usual form, and also the crankshaft C. The carburettor, or induction
pipe leading from the carburettor, would be attached to the flange L,
and the automatic valve F controls the admission of gaseous mixture
from the carburettor to the crankchamber. The inlet ports N are often
only half the height of the exhaust ports. On the upstroke of the
piston a partial vacuum will be formed in the air-tight crankchamber,
which will allow the atmospheric pressure to force open the valve F
against the pressure of the spring and enable the air to flow into the
crankchamber through the carburettor and induction pipe, carrying the
charge of petrol vapour with it. We must note, however, that no vacuum
can be formed until the port N has been covered up by the piston, so
that a portion of the stroke is _idle_. On the downward stroke of the
piston the charge in the crankchamber is compressed, and as soon as
the piston uncovers the ports N the charge from the crankchamber flows
up into the working cylinder, displacing the burnt gases as it comes
into the cylinder. Exactly what happens next it is difficult to say;
the probability is that this new charge rises in the cylinder a short
distance (but not a sufficient amount to displace all the dead gases
from the top end of the cylinder) and that some of it gets squeezed
out of the exhaust opening as the piston rises and before it has had
time to cover the exhaust ports. Thus, owing to the idle portion of the
stroke during admission to the crankchamber and to the low compression
pressure adopted in the crankchamber, the pumping portion of the
engine has what is termed a very low _volumetric efficiency_.
It can be proved that this type of engine which endeavours to draw
sufficient gas to fill its working cylinder into the crankchamber by
means of a piston having only the _same_ diameter as the diameter of
the working cylinder itself, and which cannot avoid some idle movement
during the operation together with further loss from the exhaust
opening, is incapable of retaining more than a little over one-half a
cylinder full of fresh combustible gas at the instant when compression
commences; the remainder of the contents must be dead exhaust gas.
Thus, even allowing for the double number of power impulses resulting
from the use of the two-stroke cycle, it is difficult to see how this
form of engine could ever give more than about one and a quarter times
the power of a four-stroke engine having the same bore and stroke
even when the many difficulties experienced in the practical working
of two-stroke engines have been overcome. To use a high compression
pressure in the crankchamber would increase the volumetric efficiency,
but would result in lost work during the pumping process, besides
being undesirable at the delivery stage of the process; it is much
better for the transfer of the gases to take place as gently as
possible. If too high a delivery pressure is used the fresh gas will
enter in a sharp gust and get badly contaminated by mixture with the
foul exhaust products instead of gently displacing them in bulk. The
use of an automatic valve is very desirable for the gas inlet to the
crankchamber, but unfortunately it limits the speed of the engine
and also its _flexibility_ or ability to pull well at all speeds. An
engine with an automatic valve runs best at that speed for which the
tension of the spring is most suitable. If the spring is weak the
speed will be low. Tightening the tension on the spring will allow the
engine to speed up, but will prevent it running well at low speeds.
At high speeds and with correspondingly high tension the valve does
not give enough opening, and therefore limits the power of the engine.
It will, therefore, readily be seen that when a two-stroke engine
with automatic inlet valves is pitted against a four-stroke engine
with mechanically-operated inlet valves, the comparison is unfair
to the two-stroke cycle. With the position and arrangement of ports
shown in the drawings, one must have a deflector on the piston head
to prevent excessive loss of fresh gas through the exhaust opening.
After the engine has been running for some time at a high speed this
deflector becomes very hot, and as a general rule the cooling effect
of the incoming gases is not sufficient to prevent it attaining a _red
heat_ on the compression stroke, thus igniting the charge before the
piston reaches the top of the stroke. This defect, which is called
_pre-ignition_, causes the engine to _knock_, and results in a loss
of power; it may be partly overcome by admitting lubricating oil with
the charge, the oil then serving to cool the deflector as the charge
enters the cylinder. At high engine speeds there is great risk of
the hot exhaust gases in the working cylinder setting fire to the
incoming charge in the inlet ports, thus causing _backfiring_ into the
crankchamber. To avoid all possibility of backfire, some form of _air
scavenging_ must be adopted, but the general arrangement of this form
of two-stroke engine does not lend itself to such an addition—it would
merely reduce still further the quantity of gas reaching the cylinder.
A difficulty that is peculiar to multi-cylinder engines of the
two-stroke type arises from the use of open exhaust ports. The several
cylinders generally discharge their exhaust gases into a common
exhaust pipe or box, so that if one cylinder happens to be _missing_
fire the exhaust from another cylinder may set fire to the wasted
charge—this is usually referred to as _flashing-back_ from the exhaust
and results in irregular and spasmodic knocking. It will be clear from
the foregoing that this cycle of operations, which is so attractive
from the theoretical point of view, is not by any means so encouraging
from the practical standpoint, as many inventors have discovered.
The difficulties and failures of the early inventors which were so
discouraging for them have only encouraged their successors and spurred
them on to further efforts. After a time the attempt to produce a
_simple_ two-stroke engine was abandoned generally, and inventors
turned their attention to improved forms of two-stroke engines, some of
which were very costly and complicated, and none of which have survived
for motor-car purposes.
The writer of this volume became interested in the problem of the
two-stroke in connexion with one of these inventions for an improved
engine, and at a later stage patented and designed an improved engine
of the two-stroke _air scavenging_ variety, which by that time had
become a recognized type of two-stroke engine. This engine was
constructed and exhibited at one of the motor shows held in London
some years ago. A vast amount of experimental and research work was
carried out on it by the writer, but the work had to be abandoned when
incomplete owing to the Syndicate which financed the venture having
exhausted its resources. The promoters of the Syndicate were anxious to
produce an engine that would give _double_ the power of a four-stroke
engine, but their early attempts were not at all successful. One
of their four-cylinder engines, which would have been rated at 35
horse-power on the four-stroke cycle, only gave 12 brake horse-power
when tested by the writer. The engine designed by the writer, which
we may call the Kean two-stroke engine, would have been rated at 25
horse-power on the four-stroke cycle, and gave approximately 35 brake
horse-power. Although this result was excellent, so much advance had
been made in the four-stroke engine that it did not quite come up to
the best results obtained on that system, and hence we were unable
to show any marked advantage to be gained from its adoption. My
experiments clearly pointed out the road to further success, but owing
to the partial failure of my attempt to beat the four-stroke engine we
could not influence sufficient capital to reorganize and reconstruct
the Syndicate. My engine had not been designed to secure a high speed
of rotation but rather for strength and durability, but it exceeded
my expectations by turning up to 1,500 revolutions per minute. The
four-stroke had, however, got well ahead of me by that time, and 2,000
was becoming common for it, hence I was heavily handicapped in the race
for horse-power.
[Illustration: FIG. 64.—DIAGRAMMATIC SKETCH SHOWING HOW THE DUPLEX TYPE
OF TWO-STROKE ENGINE OPERATES WITH AIR SCAVENGING.]
A description of my engine will probably prove of interest. To
understand the principle of the engine we must turn to the diagrammatic
sectional view of Fig. 64. Instead of using the crankchamber of the
engine as a gas pump, this type of engine has a _duplex_ piston, and
the pump chamber is formed by an annular extension of the main engine
cylinder. At first sight one would say this resulted in a very high
engine, but as a matter of fact the increase in height is not more than
about 25 per cent. in the cylinders, and there is no difference in the
crankchamber height to that of a four-stroke engine. The outstanding
feature of the invention is the provision of a pump piston of larger
effective diameter than the main piston and the arrangement of transfer
pipes by which one pump feeds its neighbour’s power cylinder, and
_vice versa_. These are the basis of the invention, and were being
used a long time before the writer had even heard of this type of
engine, but it was left for him to seize upon their capabilities and
correctly proportion the area of the annulus with respect to the main
engine piston. A careful study of the two-stroke problem revealed the
inherent defect of the _low volumetric efficiency_ and the tremendous
possibilities of having an unlimited volume for the pump chamber
by simply increasing the area of the lower or annular piston. Then
followed the writer’s attempt to tackle the outstanding practical
difficulties enumerated above. The engines already employed _air
scavenging_, but could not really use it effectively until proper
proportions had been fixed upon for the respective pipes, valves, and
ports. The cycle of operations is as follows:—On the downstroke of No.
1 piston the annular portion draws a charge of gas from the carburettor
into the annular chamber D_{1} (Fig. 64) through the inlet valve B_{1}
and at the same time pure air is drawn into the transfer pipe by the
valve A_{2}. On the upstroke the charges of air and gas are compressed
into the transfer pipe, and as soon as the piston P_{2} uncovers the
inlet ports the air and gas enter the working cylinder. In my engine
I used a relatively high compression pressure for the transfer of the
charge and curved the inlet ports up towards the head of the cylinder
as shown. The head of cylinder I made curved, and the exhaust ports
were carefully rounded and curved also. The deflector on the head of
the piston I inclined, to curl the gases back against the wall of the
cylinder, and I reduced the height of the deflector to that of the
_inlet_ port (instead of the exhaust port). My ultimate aim was to
abolish the deflector entirely by suitably shaping the inlet ports, and
I estimated that the path of the gases would be in the direction of the
arrows. The object of raising the compression pressure in the lower
cylinder was twofold. First of all I aimed at an increase of volumetric
efficiency there, and secondly I hoped to propel the scavenging air and
the new charge right up to the head of the cylinder and so clear out
all the dead gases. Then by suitably curving the head of the cylinder
I expected to compel the scavenging air to keep going ahead of the
gaseous mixture and curl round and down, then following the exhaust
gases out of the exhaust port.
My efforts in this direction were very unfortunately frustrated to a
large extent by the fact that the cylinders of my engine had already
been cast before I fully realized the _tremendous_ importance of
_curving the cylinder head_ and giving a very _steep inclination_ to
the inlet ports. We did our best to rectify matters in the machining
and finishing stages, but any engineer will understand the limitations
now imposed upon us. It was impossible to get new cylinders cast
owing to lack of time and funds, as we were intending to exhibit the
completed engine. Thus I cannot say that my ideas were ever given a
really satisfactory test; the inlet ports _were_ curved and inclined
and the cylinder head _was_ rounded off, but not to such an extent
that I can feel certain no further improvement could ever be made
in those directions. Other improvements which I introduced were an
improved automatic inlet valve for the gases, which was fitted inside
the induction pipe and whose spring tension could be adjusted while
the engine was running without letting any air leak into the induction
pipe; also an improved air scavenging valve, which could be set to
give the full amount of air to the engine and yet be controlled from
the dashboard of the car to give any desired quantity of scavenging
air from _no air_ up to _full air_. _Very_ large inlet valves were
fitted, but when indicator diagrams were eventually obtained from the
engine they showed that they were not nearly large enough and that the
carburettor opening was too restricted, thus cutting down the power
(and very likely the speed) of the engine by probably over 25 per cent.
High tension magneto ignition was fitted and thermo-syphon cooling.
Arrangements were made to carry 80 lb. of water in the system, so that
the engine never showed any tendency to boil even when the car had
been running for long periods on the low gear. A pump was afterwards
fitted, but it did not effect the cooling of the water any better than
the natural circulation, which was quite satisfactory. The range of
speed was from 150 revolutions per minute up to 1,500 revolutions per
minute; the lower figure is very good indeed, and can be attributed to
the large number of impulses obtained due to the two-stroke cycle. At
the highest speed the crankshaft received 6,000 impulses per minute,
or equivalent to a four-stroke engine running at 3,000 revolutions per
minute. The effective pressure in the cylinder was, however, only just
over 40 pounds per square inch, due to the throttling at inlet already
explained. In a four-stroke engine we would expect just double that
figure. The extraordinary thing about this was that, under heavy load,
when the speed was brought down to about 300 revolutions per minute,
the effective pressure had risen to nearly 200 lb. per square inch, but
this appears to be due to imperfect scavenging (or cleansing) of the
cylinder under these conditions.
The question of silencing the exhaust from the engine had caused me
some difficulty in the earlier experiments, so that I now tackled this
problem and designed a special form of silencer in which the gases
were first expanded to remove their pressure and then afterwards their
velocity was taken up without shock. This answered so well that a
_cut-out_ made no difference whatever, and on taking diagrams with the
optical indicator I discovered that the exhausting process was divided
into equal periods of slight pressure and slight vacuum with an average
of zero pressure (just atmospheric). We have seen in the earlier part
of this chapter how the fitting of automatic inlet valves is liable to
hamper the engine and reduce its flexibility, and this impressed me
very much with the earlier engines so that at one time I adopted dual
springs for the inlet valves. These springs were mounted one above the
other, the lower one being much stiffer than the upper one. The idea of
the invention was that the weak springs would serve for slow running
and all loads up to _say_ half the lift of the valve, and then the
stiffer springs would secure correct action at high speeds. Further
than this, I had them all coupled on a bar which was controlled from
the driver’s seat, and by means of which I could cut out the weaker
springs or reduce their effect at will. It certainly answered well
in the older engines, but my new engine, shown in Fig. 65, was so
satisfactory that I abandoned the idea. About five different systems of
lubrication were experimented with and many lubricating oils. Finally,
forced lubrication was employed for all the bearings and a drip
sight-feed for the pistons.
[Illustration: FIG. 65.—GENERAL ARRANGEMENT OF THE “KEAN” FOUR-CYLINDER
HIGH SPEED HIGH COMPRESSION DUPLEX TWO-STROKE ENGINE EMPLOYING AIR
SCAVENGING. IN THIS ENGINE THERE IS _NO_ CRANKCHAMBER COMPRESSION.]
Much trouble was caused at one time in the new engine by _knocking_ of
various kinds, and many hours were spent in locating these troubles
and curing them. The first kind of knocking was most violent and
almost made one hold one’s breath in anticipation of seeing parts of
the engine go skywards. This turned out to be partial seizure of a
piston owing to a hard spot in the cylinder. After curing this, general
knocking from all cylinders began, and was found to result from worn
gudgeon pins. These had been mild steel and case-hardened; they were
discarded for _Ubas_ steel of slightly larger diameter, and this
trouble disappeared. Then _pre-ignition_ was discovered. When the
magneto was switched off the engine slowed down and nearly stopped,
then began to run on again, knocking and hammering in a most diabolical
manner. All cylinders were taken off again, all parts ground up, and
corners well rounded off, but still it continued. At first it seemed
to be due to the deflectors, but on several very careful examinations
(which of course meant dismantling the whole engine every time and
removing the cylinders) no trace of overheating or burning could be
found on these or anywhere else in the interior of the cylinder.
Then the trouble was traced to the electrodes of the sparking plugs.
This was followed by two or three weeks’ continuous experiments on
fitting different types of plugs, and the same type of plug was tried
in four different positions inside the cylinder. Then the device of
fitting the plugs to an adapter and so keeping them at the top of a
small hole instead of projecting into the cylinders was tried. They
still showed signs of overheating, and strange to say no loss of power
or flexibility was noticeable. Finally, I fitted a water tank on the
dashboard and allowed the engine to suck water into the induction pipe
while it drew its mixture from the carburettor in the usual manner. I
had previously fitted separate drip-feed of water to the air scavenging
valves with a view to effecting cooling of the engine, but abandoned it
owing to lack of results. Very soon I discovered that for every gallon
of petrol the engine consumed I could let it take nearly half a gallon
of water into the induction pipe. The engine ran much quieter and very
smoothly, and for a time I thought I had succeeded, although the water
gave me trouble in restarting if I happened to stop the engine while
it was in use. It meant that the water had to be shut off some minutes
before the engine was going to be stopped. The day after I thought
I had effected a cure for the pre-ignition _intermittent knocking_
began, and there was also general knocking _always_ for a second or two
when _accelerating quickly under load_. After much loss of time and
the expenditure of a large sum of money on experiments, I persuaded
the Syndicate to let me take some diagrams from the engine with an
optical indicator, and eventually after nine months they consented,
but they would not agree to my taking the engine out of the chassis
and putting it on the bench for a proper _power test_. Therefore my
diagrams were taken while the engine was in the garage in its chassis,
and the load was applied by the propellor shaft brake, the shaft itself
being withdrawn. Anyone who has attempted even in a well-equipped
laboratory and with the aid of a proper brake to take diagrams from
a petrol engine when the indicator is driven by a _flexible shaft_,
will understand and appreciate my work in securing thirty photographic
records under as many conditions of load and speed. After carefully
analyzing my diagrams, I came to the conclusion that the intermittent
knocking was undoubtedly _flashing back_ from the exhaust, and
the acceleration knocking was due to a _cushion of hot gas_ which
accumulated in the head end of the cylinder at times when the engine
speed was low and the load on the engine was heavy.
Having explained these things to the Syndicate and pointed out the
need for still larger valves, they set about attempting to raise fresh
capital for the final attempt at success. They were not successful, and
up to the present nothing more has been done. The Syndicate was wound
up, the members drifted apart, and the patents were allowed to lapse.
The engine and chassis were eventually sold, and are still doing good
service somewhere in the North of England. Meantime the writer has
not rested, but has steadily formulated his ideas for the improvement
of the engine, which have resulted in the securing of a fresh patent
early this year. In the new engine the charge enters at the head end
of the cylinder, there is a special transverse combustion chamber, and
many improvements are introduced in the scavenging and flow of gases;
also there is no deflector at all on the piston head. Funds have not
yet been secured to enable an experimental engine to be constructed,
but it is to be hoped they will be forthcoming, for the benefit of the
motor-car industry generally, as the future undoubtedly lies with the
two-stroke.
During the whole of this time the writer was engaged as Chief Assistant
in the Engineering Department of Leeds University, being in charge of
the experimental work of the students in the laboratories there. Many
of the drawings were made by students in their vacation, and the writer
is greatly indebted to his friend, Professor John Goodman, for so
kindly allowing him the necessary freedom during vacation times when
there is often much miscellaneous work that requires attention.
Before closing this chapter one may add a few words on carburation and
ignition for two-stroke engines. A four-cylinder two-stroke engine
should have cranks at right angles to secure the maximum torque on the
shaft. Looked at in end view the cranks form the four arms of a cross
and thus four impulses are given every revolution, but as the ordinary
magneto only gives two sparks in every revolution it must be driven at
_twice_ the crankshaft speed. This puts a great strain on the machine
at top speed, and also on the insulation of the windings and the plugs,
so that the plugs require constant attention. Magneto troubles were
found to be eliminated by the use of the special _racing_ pattern
magneto supplied by some manufacturers and the choice of high grade
sparking plugs.
Carburation troubles were not so easily dealt with. A multi-cylinder
two-stroke engine should undoubtedly have a multiple jet carburettor
and some form of hand-controlled _extra-air_ inlet valve on the
induction pipe; also the mixing chamber of the carburettor should be
water-jacketed by _hot_ water. It was also found necessary to fit a hot
water-jacket round a portion of the induction pipe, as the demand for
petrol vapour was so great and the rate of evaporation so high that
frost readily formed on the induction pipe unless the weather was very
warm. The two-stroke engine requires its petrol much faster than the
four-stroke, so that the float of the carburettor should be delicately
balanced and the height of the petrol in the jet should be quite level
with the top of the orifice, although this often leads to flooding.
Reviewing the description of what we have designated the Kean
two-stroke engine, we may sum up the results of these experiments by
saying that the engine _could_ have developed considerably more power
than it did had diagrams been taken from it in the first instance and
the severe throttling in the carburettor and automatic inlet valves
been discovered; moreover, the flashing back from the exhaust would
have been located much sooner and probably cured by a re-arrangement
of the exhaust manifold. If the exhaust manifold had been arranged so
that there was a separate branch for at least each pair of cylinders,
it would very likely have been stopped, or at any rate greatly reduced.
But what could not have been altered was the _acceleration knocking_.
It must not be imagined because I have been very frank in the criticism
of my own work that the engine was a failure; it was a great _success_,
but not sufficiently successful to represent an improvement on the best
four-stroke practice. The car ran well, was very reliable and efficient
in petrol consumption; the engine was quiet and extremely flexible; but
it had one very objectionable feature in that every time you pressed
the accelerator pedal down sharply, either to put on a spurt for the
purpose of passing slower traffic or in _rushing_ a short gradient, a
peculiar knocking or hammering arose from the engine cylinders—this
is what I describe as acceleration knocking and must not be confused
with the knocking or hammering of a four-stroke engine when labouring
on a gradient. My engine would be _full of life_ all the time it was
knocking like this, and gradually as the speed increased the noise
would ease-off, even though no change of gear had been made.
The diagrams proved to me that this knocking was due to _pre-ignition_
caused by a cushion of hot gases remaining in the top of the working
cylinder, and in my opinion no alteration of the ports or cylinder head
would have influenced this defect to any marked extent; therefore I
should never attempt again to feed the new charge in at the _bottom_
end of the cylinder of a two-stroke engine if I wished to obtain the
maximum amount of power from it. It seems to me that other people
must also have been impressed with similar misgivings, for in one or
two types of engine using crankchamber compression we see a special
attempt made to overcome it, although the method adopted leads to a
rather undesirable arrangement of the engine mechanism. In the type of
engine I refer to the charge may be drawn into the crankchamber in the
usual manner, if desired, but the working cylinder is a casting with
two bores having two separate pistons and a common combustion chamber.
The charge enters above one piston while the crank is on its bottom
dead-centre and is exhausted from the space above the other piston
simultaneously, and the path of the gases is from the inlet port up
to the top of No. (1) bore, then down to No. (2) bore, and out of the
exhaust. This ensures that there shall be no cushion of hot exhaust
gases left in the combustion chamber (or top end of the cylinder).
These engines have given quite good results, and would be much more
extensively used but for the fact that there is double compression
to overcome in starting, and their running torque, due to the number
of impulses given to the crankshaft, is no better than a four-stroke
engine. Fig. 66 shows the arrangement of the cylinders and the path
of the gases. A_{1} and A_{2} are the twin pistons working in the
water-jacketed cylinder casting B, and having the common combustion
chamber C. The connecting rods may drive separate cranks in opposite
directions or both be coupled together and work a single crank. It will
be seen that in this type of engine the piston does not require any
deflector.
The simple two-stroke engine described at the beginning of this chapter
is often constructed in such a form that no automatic inlet valve is
required on the crankchamber.
In this case the induction pipe is connected to a third set of ports
just below and a little to one side of the inlet ports to the working
cylinder, and these are uncovered by the piston towards the completion
of its upstroke, thus allowing the carburetted air to enter the
crankchamber. Such an arrangement constitutes a _three-port_ two-stroke
engine, which is of course _less efficient_ than a two-port engine
with automatic valve, but has the great merit that it is entirely
_valveless_, and therefore extremely simple and cheap to manufacture.
It is much used for motor boat work, both in this country and in
America, on account of its relatively low speed of rotation.
[Illustration: FIG. 66.—TWIN-CYLINDER TWO-STROKE ENGINE WITH
CRANKCHAMBER COMPRESSION.]