In modern ages many of us ride motorcycle. If we know the
basic function of engine then it will be helpful for all of us.
Engine Unit Basics
If one studies an exploded of a modern multi-cylinder engine
with its gearbox built on as one unit, one’s initial reaction is that it is a
highly complicated device. However, by considering each part in turn and
appreciating the basic method of operation, the principles and practice will
become clear.
The Basic Engine
Cycle
All types of internal combustion engine, regardless of the
mechanical method of converting the fuel energy into power, operate on the same
basic cycle. This cycle comprises five elements as follows:
Induction: Air
or an air-fuel mixture is taken into the working chamber.
Compression: The
charge is compressed.
Burning: The
charge is ignited and burns, thus raising the chamber pressure and temperature.
Work: The
pressure is changed into useful work.
Exhaust: The
spent burnt gas is removed from the chamber.
The mechanical methods used to convert the pressure into
work are the reciprocating piston engine as used in the great majority of
machines and a very few rotary engines. The main essentials of the
reciprocating engine are a piston which slides up and down inside a round
cylinder closed above the piston. Below the cylinder is mounted a crankshaft
carrying a connecting rod to join the crankshaft to the piston. This is the
basic arrangement of an engine, and to this has to be added some means of
allowing the combust able mixture into the space above the piston to complete
the elements of a single cylinder engine. Two basic methods of allowing the
gases in and out are used, the first being poppet valves in the closed end of
the cylinder and the second by holes or ports in the cylinder wall which are
covered or uncovered by the movement of the piston. There are refinements to
these methods, details of which are given later.
It is apparent from the above that the burning of mixture
will generate a force pushing down on the piston which will be transmitted
through the connecting rod to the crank, thus turning it. It is also apparent
that for every turn of the crank, the piston will make two strokes, one up and
one down. It is these strokes which are referred to when reciprocating engines
are classified as two-stroke or four-stroke engine ,there is a power stroke in
every two piston strokes and thus for each single turn of the crank.
The piston has two extreme positions, one when it is at the
top of the cylinder, known as top dead centre (TDC) ;the other when it is at
the bottom of the cylinder, known as bottom dead centre(BDC). The space left
above the piston at TDC. forms the combustion chamber. The piston is attached
to the connecting rod with a gudgeon pin and that end of the rod is called the
small or little end, which pivots on the crank, is called the big end.
The Four-stroke Cycle
Figure 2.2 shows the stages of the cycle. In (a) the piston
is descending and the inlet valve open. The movement of the piston creates a
partial vacuum in the cylinder, so that the charge is sucked in. When the
piston reaches its lowest position, the valve is shut, so that the cylinder is
sealed.
In (b) the piston is rising and compressing the charge. As
the piston reaches its highest position, an electric current is fed to a
sparking plug and a spark jumps across the plug points, igniting the charge.
In (c) the piston is descending on the power stroke as the
burnt gases expand and at the end of this stroke the exhaust valve opens.
In (d) the piston is rising, the exhaust valve is open, and
the burnt gases are pushed out of the cylinder. At the end of the stroke, the
exhaust valve shuts, the inlet valve opens and the cycle starts again.
As only one working stroke occurs during two revolutions of
the crankshaft, a flywheel is attached to the crankshaft to keep the engine turning
during the three idle strokes. It strokes up energy during the working stroke
and its momentum is given up to turn the engine round to the next one.
Fig 2.2
The valves shown in figure 2.2 are mounted above the piston
so that the engine is known as an overhead valve type (o.h.v) the are opened by
a system of cams, pushrods and rockers and since each valve only open during
each cycle, the camshaft has to be
driven at half the speed of the crankshaft . this is done with a system of
chains, Belts gearwheels or shafts . in addition to the basic o.h.v engine, it is now common practice to
mount the camshaft itself above the valves and fot it to open the valves by
rocker arms as shone in figure 2.3. the arrangement is referred to as overhead
camshaft (o.h.c) and may be further
refined by the use of a separate camshaft for each valve, this arrangement
being know as twin or double overhead camshaft (d.o.h.c)
4 stroke engine valve mechanism fig 2.3
Many other variants have been used , amongst them the high
camshaft type which is an o.h.v engine with the camshaft mounted higher than is
usual which is an o.h.v engine with the camshaft mounted higher than is usual
to gibe very short pushrods. Another type used on much older engines is the
side valve type where the valves are mounted in a chamber alongside the cylinder
and are pushed up by direct action cams into a combustion space carried over
them from the region above the cylinder .
In addition to the mechanism
to open and shut the valves, a means of ignition the charge is also
required and the device for this is usually driven a half crankshaft speed.
However , as well be soon , this is not always so as many four-stroke engines
are arranged that this ignition occurs once for every crankshaft revolutions
this assists manufacture and has no detrimental effect on the running of the
engine.
The two stroke cycle
Figure 2.4 the two stroke engine cyle showing( a) exhaust
and transfer, (b) induction and secondary compression , c(0 powre expansion and
primary compression
Figure 2.4 shows the various stages of the cycles and the
disposition of the three ports in a simple engines. In (a)the piston is near
b.d.c so that both exhaust and transfer ports are open and the inlet is closed.
The brunt gas is leaving the cylinder while while a fresh charge is being
transferred from the crankcase to the cylinder, driving the burnt gases before
it.
In (b), the piston has traveled up the cylinder, closing the
exhaust and transfer ports and compressing the fresh charge in the cylinder.
The inlet port has just opened so that the partial vacuum formed in the
crankcase by the rising piston sucks a fresh charge into the crankcase. When
the piston reaches top dead center, the spring plug ignite the charge in the
cylinder and the piston descends on the
power stroke, shutting the inlet port and compressing the charge in the
crankcase .
In (c) the piston is
one third of the way down the cylinder, so that the exhaust port will shortly
open allowing the burnt gases to escape
to atmosphere, and while the fresh
charge in the crankshaft is at its greatest pressure, the transfer port will then open to allow
this charge to enter the cylinder.
It is apparent that there is two comprehensive stages in the
two stroke engine: primary in the crankcase and secondary in the cylinder.
Because the two-stroke has to complete six operation in one revolution of the
crankshaft no one operation has a complete stroke and various phases overlap to
some extent. These operation are : induction inti crankcase, primary
compression , transfer , secondary compression , power stroke and exhaust .
The two strock engine, in its most use form, dose not have
any valves, as the entry and exit the charge
to and form the cylinder is controlled by the piston covering ports in
the cylinder wall during its stroke. The requirement of the basic internal
combustion engine cycle are met by using the volume of the crankcase as a
primary stage where the charge can be slightly compressed and then passed to
the working chamber. As this primary chamber is used during the cycle, it has
to be sealed relative to the atmosphere.
Most tow stroke engines are the three port type , these
ports being the inlet connection the crankcase to the atmosphere , the transfer
connecting the crankcase to the combustion chamber, and the exhaust from the
combustion chamber. In practice the basic two stroke three port engine has two
transfer ports, one either side of the exhaust port.
From this simple layout have developed tow other intake
systems and the use of additional transfer ports.
The intake systems are the rotary valve and the reed valve.
The first is shown in figer 2.5 and a crankshaft mounted disc, partly cut away,
that open and close a hole in the side of the crank chamber to allow the
mixture to enter. The reed valve shown in figer 2.6 is a thin diaphragm that
opens under crankcase depressions to allow the mixture into the engine and
then springs shut to prevent it
escaping. Reed valves may be fitted to an inlet pipe with piston controlled
opening or direct to the crankcase as shown in figer 207
Figure 2.5 rotary valve
Fig 2.6
Figure: 2.7
