The way an Engine
Explanation of a Piston-Type Engine
Our piston type engine transforms the kinetic energy
of exploded fuel into mechanical energy by confining the
expanding gases in a cylinder so that they will thrust a
piston (fitted into one end of this cylinder) outward.
Each explosion of fuel results in a straight-line outward
movement of the piston which we call a power stroke.
This power structure is a single, forceful thrust that
is limited in duration and also limited to the distance
that it is practical to allow the piston to travel in it
straight-line movement. In order to develop mechanical
power (the capacity for continuous work) it is necessary
for our engine to accomplish two things:
1.It must transform the straight-line piston movement
into a movement which can be harnessed to various types
2. It must provide a series of power strokes that will
continue as long as desired.
The basic design of a piston-type
We harness the straight-line piston movement by
transforming it into a rotary movement (of a shaft) that
becomes continuous in one direction of rotation. The
piston is connected to one end of a simple lever that has
a fixed fulcrum at the other end. During its power stroke
the piston revolves this lever through a half circle (180
degrees), at which point the lever stops the piston
movement. With the power cloak thus ended, the rotary
movement of the lever carries it on round the remaining
180 degrees of a full circle-and the leader returns to
the level position which also returns the piston to its
In a piston-type engine, the cylinder is a cylindrical
bore inside a cylinder block. The piston is a metal
cylinder fitted into this cavity and encircled by piston
rings which seal it tightly (so that the expanding gases
cannot escape around it) while also serving as bearings
for the piston to slide on when it moves in the cylinder.
One end of the cylinder is sealed closed by a cylinder
head which contains a mated cavity called the combustion
chamber (in which fuel is exploded). This cylinder head
may be a separate parts bolted to the cylinder block with
a cylinder-head gasket in between (to seal it tightly);
or it may be an integral part of the cylinder block. The
opposite end of the cylinder opens into an area enclosed
by a plan, called the crank-case, that is fastened to
this end of the cylinder block.
Instead of a simple lever with a fixed fulcrum point we
use a crankshaft held in a stationary support by one or
more main bearings which allow it to revolve. The
"lever" is an offset (throw) of the crank shaft
(which "juts out" at a 90 degree angle to the
shaft center line). Our piston is connected to the shaft
(crankpin) of this offset by a connecting rod that is
held to the crankpin by a connecting rod bearing. A wrist
pin with bearings serves to attach the piston to this
For each single power stroke to occur there must, of
course, be a refresh supply of fuel (a charge) in the
combustion chamber when. This charge must be compressed,
to obtain its maximum power. When it is ready, combustion
must take place-followed by the power stroke. Finally,
the used charge must be expelled to make room for the
next fresh charge.
There are five events that must occur in any piston type
internal-combustion engine; they constitute what we call
a cycle. They are:
filling of the cylinder to capacity with a fresh charge
2. Compression-the compression of this
charge into as small a space as practicable.
3. Ignition - the firing of the
4. Power stroke-the resulting outward
thrust of the distance.
5. Exhaust-the discharge of the burnt
gases from the cylinder.
The vacuum principle of intake.
If a piston were designed to completely fill its cylinder
when in it, and it was pulled partially out of the
cylinder without any air meeting passed it - the space
that it had occupied would be completely empty ... what
we call a 100 percent sign vacuum. Actually, the piston
of an engine does not fill its cylinder (It never goes
into the compression chamber); and there is always a
little leakage around it, even when the piston rings are
in good condition. Therefore, when a piston is moved
outward, only a partial vacuum is created.
This partial vacuum, however, contains many fewer
molecules of gas, ... than would normally occupy the
space. In short, there is room for many more molecules.
If an opening is provided to connect this relatively
empty space with the upside atmosphere (which, we must
remember, is under 15 lbs. per square inch pressure at
sea level), as much air as the 15 pounds . per square
inch pressure can force in will flow into this space.
This is what we call the suction of a vacuum. The amounts
of air (number of molecules) that will be
"sucked" in per second depends upon the outside
pressure (which, as already noted, will be less at higher
altitude) and upon the density of the outside air (which
is also less at higher altitude).
In an engine, intake is effected in this manner. The
piston is called outward by revolution of the contrast to
create a vacuum instead of pure air, however, a mixture
of air and petrol (under atmospheric pressure) is forced
in through an opening into the "emptied"
cylinder. At sea level more mixture will be forced in -
in a given time then at higher attitudes; less then at
explanation of compression
To repeat what was previously said, in a slightly
different manner, compression increases the force of
It increases it, first, because compressing the charge
very quickly super-heats it throughout so that every
molecule of the charge is approaching its flash point at
the instant combustion starts. Hence, when combustion
does occur it is practically instantaneous and compete
for all of the charge.
It increases it, second, because the tightly packed,
highly activated molecules (all "striving" to
move apart with considerable pressure, even before
combustion occurs) served as millions of tiny
"spring - boards" to thrust the piston back
An engine will run on on compressing charges. The
earliest engines were so operated. However, the power
output was exceedingly low and wasteful of fuel. we have
found that even though some energy is used to
"build-up" compression, the power advantage
gained far exceeds the power spent. Modern engines are
being designed for higher and higher compression ratios.
Explanation of Ignition.
As already mentioned, our piston-type engines to not use
compression as the means of firing the charge. They do
not, chiefly, because the very high degree of compression
required to create combustion would make it necessary to
build over bulky and heavy engine parts to withstand the
strain of the high pressure involved. There are other
reasons, also, concerned with the types of fuel, its
handling, performance-factors, etc.
Apparent in gasoline engines we use an electrical spark,
created across the electrodes of a spark plug, to ignite
the charge made ready by compression. Electrical energy
(a charge) is made to jump the gap between the two
electrodes. In jumping the gap it lessens the molecules
of the charge that are in its path with sufficient speed
to burn them. The "fire" then spreads
throughout the charge.
Actually, then, combustion starts at a single point. This
point (the location of the spark plug) is carefully
plotted for maximum efficiency-that is:
1) so that the combustion will spread accordingly to a
plan that will ensure continued, even pressure all over
the piston top throughout the power stroke; and
2) so that the fuel charge will be indicted and expanded
during the incredibly short interval of time allowed.
The principles involved in exhaust
The "law of inertia".
We have found that any body that is at rest tends to
remain at rest, while any body that is in motion tends to
continue moving along the same straight line ... and it
requires an equal and opposing force to change either
condition. This is called the law of inertia. It means
simply that if you start an object moving (assuming there
is no friction or gravity to interfere), it will take the
same force to stop it that was used to start it.
This rule applies to molecules as well as to whole
objects. Consequently, once a gas (for instance) is
started moving in a direction, each and every molecule of
the gas will tend to continue moving in this
direction-until stopped by other forces (such as gravity
or collisions with other molecules).
Inertia hopes produce a
The inertia principle is used to discharge burnt gases
from an engine cylinder. As we have said, an engine
piston does not fill its cylinder, even when all the way
in. If it did fill the cylinder, it would be easy to
squeeze out all the burnt gases. However, it never enters
the combustion chamber; yet it is desirable to completely
empty the cylinder and combustion chamber of burnt gases,
to make room for a new charge.
When the crankshaft returns a piston to its starting
point, the fast moving piston starts the burnt gases in
the cylinder moving at high velocity. Each molecule of
the burnt gases is given a very forceful "push"
straight towards the combustion chamber. We provide an
opening in the cylinder head-and the "resting"
molecules flows out of this opening. If the opening is
properly positioned, nearly all of the molecules in both
the cylinder and the combustion chamber will flow
out-even though the piston does not actually
"squeeze" them all out.
The "valves" of a
A 2-stroke-cycle engine requires two valves per
cylinder-the same as a 4-stroke cycle engine - plus one
additional valve to admit fresh charges into the crank
case. Hence, there are three valves for a one cylinder
engine, and 5 for two cylinders.
The first two are always opening through the cylinder
which are covered and uncovered by the piston. These are
called the transfer and exhaust ports. For the third
valve, a third port, a reed-valve, a rotary valve, or a
poppet valve may be used. Hence, we generally speak of a
2-stroke-cycle engine as being a 3-port, read-valve,
rotary valve, or poppet valve type. (Note - twin
cylinders are an exceptions to this rule).
The transfer and exhaust port
These ports on are generally
located at opposite sides of the cylinder - and the
exhaust port opens to the "outside", while the
transfer port opens into a passage way to the crank case
enclosure. Piston length is such that the piston will
still fully cover both parts when at T D C - but will
fully uncover both port when at the BDC. The exhaust port
is usually just a little higher in the cylinder then the
transfer port, so that on a down stroke-exhaust begins a
fraction ahead of cylinder entered. This also results -
on an up stroke - in closing the exhaust port a fraction
later than closing of the transfer port . . . which
permits the fresh charge in the cylinder to "push
out" practically all the remaining exhaust gases.
Either one or more openings may be provided to serve the
purpose of each port.
A reed valve is simply a flap,
fastened along one edge, that will flop open under
pressure to expose an opening. The flap is called a reed.
It is mounted on a reed plate provided with a suitable
opening. This plate is fastened to the crank case (at any
convenient position) with the reed inside - so that when
pressure inside is highest the reed is pressed against
the plate to close the opening; but when pressure inside
becomes less than atmospheric pressure the valves open.
One, two or more reeds of an "springy" material
may be used; and the holes in the reed plate are smoothly
surfaced so that the reed (s) will close them tightly. On
high-speed engines - likely to develop constable suction
inside and therefore considerable outside pressure to
open the reeds - each read is provided with a stiff metal
stop that limits its opening (to prevent damage to the
reed and malfunction). Stops are usually omitted from
A rotary valve is generally of the integral Type -
meaning that it is attached directly to one end of the
crankshaft - though some are run on a separate shaft
geared to the crankshaft. The valve proper is called the
rotary valve - and it rotate against a wear plate which,
in turn, is secured stationary to the crankcase side.
When the valve and plate holes are aligned, the valve is
"open"; and when they are not, the valve makes
a tight seal against the wear plate.
The valve may be secured to the crankshaft in any manner.
One manner is to employ a key way so that the valve will
turn with the shaft, yet be free to move slightly forward
or back on the shaft. A spring washer on the shaft
between the valve and a shoulder of the shaft serves to
continually thrust the valve firmly against the wear
There are no timing problems with ports or a the reed
valve; and the only timing required for a rotary valve is
to position it correctly on the shaft.
A reed valve does have replaceable parts - and these are
such that it is better to replace damaged ones then to
attempt repairs. The openings through the plate and reed
must be kept clean, and the contact surface of the plate
must be smooth to effect a tight seal .A reed must have
the flexibility intended, or it will not function if bend
or damaged. If reed stops are used they must not be bent
from their original shape.
All parts of a rotary valve are also replaceable - and
are generally best replaced rather then repaired. The
openings must be kept clean, smooth and a tight seal is
preferable. If there is a spring device to hold the
contact parts they must be readily changed or checked.
If we were to compress the air-fuel mixture to
its flash point, it is not true that the electrical
ignition could be termed an "explosion". Though
ignited internal combustion may appear to occur in one
single flash, the burning actually starts at the
electrical spark - and "spreads out" through
the balance of the mixture.
For this reason, there is a slight lag between the
instant in which the spark occurs and the instant when an
explosion reaches it peak force (the peak of combustion).
Combustion lag and Engine speed
This combustion lag affects an engine in several
ways which must be compensated for - and the compensation
is called advancing the spark (that is, dining the spark
to occur ahead of TDC, while the piston is still
completing it's compression stroke). When the spark is
"moved back" to occur at all behind TDC, we
therefore call all this a retarded spark.
A retarded spark is desirable when starting an engine or
when it is idling. During starting, if the peak of
combustion (or even the initial force of combustion) were
to occur before TDC, the comparatively small force
applied to start the engine would be overcome - and the
piston would be driven back down before it could reach
TDC . . . in what we call a kickback.
At idling speed, there is insufficient momentum to carry
a piston through to TDC against the full peak of
combustion - and an "idling" piston is still
moving slowly enough for the combustion lag to be
negligible in comparison.
As an engine is revved (speeded), however, the speed with
which a piston travels soon reaches a point at which the
piston will move a noticeable distance during the
split-second interval of combustion lead lag. At 5000
revs per minute, a piston can travel to 10% of its stroke
during the interval of combustion lag. then, too, at such
a speed the pistons momentum is quite enough to overcome
the relatively slight force exerted by the gases burnt
prior to the peak of combustion. therefore, it is
desirable to advance the spark to that point at which the
peak of combustion will occur when it will do the most
good - that is, just after TDC.