26-09-2016, 10:44 AM
1456157625-icEngines.pdf (Size: 531.44 KB / Downloads: 16)
Introduction
Internal combustion engines are seen every day in automobiles, trucks, and
buses. The name internal combustion refers also to gas turbines except that
the name is usually applied to reciprocating internal combustion (I.C.) engines
like the ones found in everyday automobiles. There are basically two
types of I.C. ignition engines, those which need a spark plug, and those that
rely on compression of a
uid. Spark ignition engines take a mixture of fuel
and air, compress it, and ignite it using a spark plug. Figure 1.1 shows a
piston and some of its basic components. The name `reciprocating' is given
because of the motion that the crank mechanism goes through. The pistoncylinder
engine is basically a crank-slider mechanism, where the slider is the
piston in this case. The piston is moved up and down by the rotary motion
of the two arms or links. The crankshaft rotates which makes the two links
rotate. The piston is encapsulated within a combustion chamber. The bore
is the diameter of the chamber. The valves on top represent induction and
exhaust valves necessary for the intake of an air-fuel mixture and exhaust
of chamber residuals. In a spark ignition engine a spark plug is required to
transfer an electrical discharge to ignite the mixture. In compression ignition
engines the mixture ignites at high temperatures and pressures. The lowest
point where the piston reaches is called bottom dead center. The highest
point where the piston reaches is called top dead center. The ratio of bottom
dead center to top dead center is called the compression ratio. The compression
ratio is very important in many aspects of both compression and spark
ignition engines, by dening the eciency of engines.
Compression ignition engines take atmospheric air, compress it to high
pressure and temperature, at which time combustion occurs. These engines are high in power and fuel economy. Engines are also divided into four stroke
and two stroke engines. In four stroke engines the piston accomplishes four
distinct strokes for every two revolutions of the crankshaft. In a two stroke
engine there are two distinct strokes in one revolution. Figure 1.2 shows a
p-v diagram for the actual process of a four stroke internal conbustion (IC)
engine. When the piston starts at bottom dead center (BDC) the intake valve
opens. A mixture of fuel and water then is compressed to top dead center
(TDC), where the spark plug is used to ignite the mixture. This is known
as the compression stroke. After hitting TDC the air and fuel mixture have
ignited and combustion occurs. The expansion stroke, or the power stroke,
supplies the force necessary to drive the crankshaft. After the power stroke the piston then moves to BDC where the exhaust valve opens. The exhaust
stroke is where the exhaust residuals leave the combustion chamber. In order
for the exhaust residuals to leave the combustion chamber the pressure needs
to be greater than atmospheric. Then the piston preceeds to TDC where the
exhaust valve closes. The next stroke is the intake stroke. During the intake
stroke the intake valve opens which permits the air and fuel mixture to enter
the combustion chamber and repeat the same process.
Otto Cycle
The Otto cycle is a model of the real cycle that assumes heat addition at top
dead center. The Otto cycle consists of four internally reversible cycles, that
describe the process of an engine. Figure 2.1, shows the p-v and T -s diagram
for the Otto cycle.
Process 1-2 is an isentropic compression of air and fuel, which occcurs
when the piston moves from bottom dead center (BDC) to TDC. In this
process air and fuel are compressed and ready for the second process. Process
2-3 is a constant volume heat addition process where the air to fuel mixture
11
is ignited. Process 3-4 is an isentropic expansion, where work is done on the
piston, but no heat is added. This process is referred to as the power stroke.
The nal process, 4-1, is a constant volume heat removal that ends at BDC.
Work and heat are important aspects of engines, that can be represented
by Figure 2.1. On the T -s diagram the area 1-4-a-b-1 corresponds to the
heat rejected per unit of mass. Area 2-3-a-b-2 corresponds to the heat added
per unit of mass. The enclosed area shown represents the net heat added
during the process. The area 1-2-a-b-1 in the p-v diagram corresponds to
the work input per unit mass and area 3-4-b-a-3 corresponds to work output
per unit mass. The net work done is interpreted by the enclosed region in
Figure 2.1, in the T -s diagram. In the Otto cycle there are therefore two
processes that involve work but no heat transfer and two dierent processes
that involve heat transfer but no work.
Dual Cycle
The dual cycle is a better description of the actual pressure variation in
the engine. There are several dierences though with the Otto and Diesel
cycle. In the dual cycle there are ve processes. Process 1-2 is an isentropic
compression where there is no heat transfer but there is work done. Process
2-3 is a constant volume heat addition process where there is no work done.
Process 3-4 is another heat addition process but with constant pressure.
This process is also know as the power stroke. Process 4-5 is an isentropic
expansion that nishes with the remainder of the power stroke. Finally, process 5-1, is a constant volume heat rejection process. Figure 2.3 shows a
p-v and T -s diagram of the dual cycle
Spark Ignition Engines
Internal combusiton engines are divided into spark ignition engines and compression
ignition engines. Almost all automobiles today use spark ignition
engines while trailers and some big trucks use compression ignition engines.
The main dierence between the two is the way in which the air to fuel mixture
is ignited, and the design of the chamber which leads to certain power
and eciency characteristics.
Spark ignition engines use an air to fuel mixture that is compressed at high
pressures. At this high pressure the mixture has to be near stoichiometric
to be chemically inert and able to ignite. Stoichiometric means that there
is a one to one ratio between the air and fuel mixture. So the mixture in
order to ignite needs not to be either with too much fuel or too much air
but rather have an overall even amount. There are several components to
the spark ignition engine. Chamber design, mixture and the injection system
are some of the most important aspects of the spark ignition engine. The
importance of the chamber design will be discussed. The four basic designs
for combustion chambers are as follow:
the distance travelled by the
ame front should be minimised
the exhaust valve and spark plug should be close together
there should be sucient turbulence
the end gas should be in a cool part of the combustion chamber.
The rst design requires that the distance between the end gas and the spark
plug be close in order for combustion to progress rapidly. If combustion is
sped up then, (i) the engine speed is increased and therefore power output
is higher, and (ii) the chain reactions that lead to knock are reduced. From
the second design criteria the exhaust valve, since it is very hot, should
be as far from the end gas in order to prevent knock or pre-ignition. The
third design criteria suggest that there should be enough turbulance in order
to \promote rapid combustion", through mixing. (Stone, p.126) Too much
turbulance, however, will lead to excessive heat transfer from the chamber
and too rapid combustion which causes noise. Turbulance in combustion
chambers is generated by squish areas or shrouded inlet valves. The fourth
design requires that the end gas be in a cool part of the combustion chamber.
The cool part of the combustion chamber forms between the cylinder head
and piston. There are many types of designs for combustion chambers. Four
common combustion chambers are
wedge chamber
hemispherical head
bowl in piston chamber
bath-tub head
The wedge design is simple giving good results. In the wedge design the
\valve drive train is easy to install, but the inlet and exhaust manifold have
to be on the same side of the cylinder head." (Stone, p.127) The second
type of combustion chamber is the hemispherical head. The advantage of
a hemispherical chamber is its angled valves which are used in high performance
engines. This design is expensive with twin overhead camshafts. The
design allows for cross
ow from inlet to exhaust, with cross
ow occuring at
the end of the exhaust stroke and at the beginning of the induction stroke
while both valves are open. The third combustion chamber is a cheaper design
that has good performance. The last combustion chamber design has
a \compact combustion chamber that might be expected to give economical
performance." (Stone, p.128)
The process by which the air to fuel mixture is prepared and put in the
combustion chamber is through carburetors and fuel injectors. Spark plugs are part of all spark ignition engines. In order to start one of these engines
a spark has to ignite a mixture into a
ame. The way in which this spark
is rst initiated is through the car battery and a circuit directly leading to
the spark plug. The battery supplies the electic current to initiate a spark
in the spark plug. The Spark then ignites the air and fuel mixture. The
type of fuel injectors used divide into multi-point and single-point injection.
Carburettors divide into xed and variable jet carburettors. The air and fuel
mixture is analysed as either a lean or rich mixture depending on the content
of fuel. A stoichiometric mixture is one in which there is a perfect ratio of air
and fual molecules. A lean mixture would be decient in fuel where a rich
one would be saturated with fuel. To achieve economic status and yet receive
the maximum power the engine would have to use a lean mixture and a rich
one at full throttle. When the throttle is fully opened and a lean mixture
is used the power output is economical because of the weak fuel. When the
throttle is opened the combustion chamber needs the air to fuel mixture.
Since a stream of air is generated extra fuel is needed to compensate for the
insucient
ow of fuel. In order to obtain maximum power a rich mixture is
needed. For good fuel economy all the fuel should be burnt and the \quench
area where the
ame is extinguished should be minimised." (Stone, p.126)
3.2 Compression Ignition Engines
Compression ignition engines dier from spark ignition engines in a variety
of ways but the most obvious one being the way in which the air and fuel
mixture is ignited. As stated above a spark plug is used to create a spark in
the combustion chamber which ignites the mixture. In a compression ignition
engine there is no spark to create the
ame but rather high temperatures and
pressures in the combustion chamber cause a
ame to initiate at dierent sites
of the combustion chamber. Combustion increases with increasing pressure
and temperature. Compression ignition engines are divided into direct and
indirect ignition engines. Diesel engines require fuel injection systems to
inject fuel into the combustion chamber. Fuel injection systems are either
linear or rotary. Rotary fuel injectors are used in indirect ignition engines
because of low pressures.
Direct injection engines use pressures of up to 1000 bars to inject fuel into
the combustion chamber. High pressure is needed because the heat addition
process takes place at a compressed state, so in order for the fuel to inject well the pressure has to be greater than the one that has been accumulated
through compression. There are several engineered direct injection combustion
chambers. This goes to show that the actual design of compression
ignition engines is not as critical as the design considered for spark ignition
engines. Swirl is the most important air motion in the Diesel engine. The
importance of swirl is that it mixes the air and fuel so that combustion can
increase. The direction of swirl is at a downward angle so that proper mixing
can take place. The compression ratio for direct ignition engines is usually
between 12 : 1 and 16 : 1.
Indirect ignition engines have a pre-combustion chamber where the air
to fuel mixture is rst stored. The purpose of the separate chamber is to
speed up the combustion process in order to increase the engine output by
increasing the engine speed. The two basic combustion systems are the swirl
and pre-combustion chambers. Pre-combustion chambers depend on turbulance
to increase the combustion speed and swirl chambers depend on the
uid motion to raise combustion speed. In divided chambers the pressure
required is not as high as the pressure required for direct ignition engines.
The pressure required for both type of divided chambers is only about 300
bars.
With all Diesel engines there is some type of aid to help combustion.
Electrical components aid in the initiation of the combustion process by using
an electrical source, such as a car battery, to heat themselves and transfer the
energy to the mixture for combustion. Cold starting a Diesel engine is very
dicult without the use of these tabs that conduct an electric current. When
electrical elements heat up and the air to fuel mixture comes in close contact
with the tab then a combustion occurs. The Diesel engine has high thermal
eciencies, and therefore low fuel consumption. The disadvantage of Diesel
engines is their low power output, relative to their weight, as compared with
spark ignition engines.
Two Stroke Engine
The fundamental dierence between the four stroke engine and the two stroke
engine is the way in which the induction and exhaust process takes place.
In the four stroke engine there are separate strokes for the induction and
exhaust processes. In the two stroke engine however, both the induction and
exhaust processes take place with the same stroke. The process that involves
both induction and exhaust is called scavenging, or simply a gas exchange
process.
The two stroke engine can be either made into a spark ignition or compression
ignition engine. The smallest engines used in two stroke engines are
compression ignition engines. The engines are usually used in models and
their power output does not exceed 100 W. The other type of two stroke
engine with power output of up to 100 kW is spark ignition engine. Some
of these engines output high power relative to their weight and bulk. Some
applications of these engines are in motorcycles, chain saws and small generators.
A two stroke engine is seen in Figure 4.1. Some of the important parts of
this engine are the exhaust, inlet, and crankcase port, and spark plug. The
de
ector is also an important design of the engine. The inlet port is where
the charge is drawn from. The charge is a mixture of mainly air and fuel but
may contain some exhaust. The exhaust port is where the exhaust leaves
the piston, and the crankcase port provides the mixture. The combustion
process for the two stroke engine goes through various processes. Following
are the steps for combustion:
1) At 60 before hitting BDC the piston uncovers the exhaust port (EO),
and the exhaust leaves the cylinder chamber while attaining atmospheric pressure.