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INTERNAL COMBUSTION ENGINES
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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.) en-
gines 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 piston-
cylinder 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.
Ideal Engine Cycles
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.
Diesel Cycle
The diesel cycle is similar to the Otto cycle, except that heat addition and
rejection occur at dierent conditions. The diesel cycle is also an ideal cycle
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meaning that it does not give an exact representation of the actual process.
The diesel cycle consists of four internally reversible processes. Process 1-2 is
an isentropic compression. Process 2-3 is a constant pressure heat addition.
This process makes the rst part of the power stroke. Process 3-4 is an
isentropic expansion, which makes up the rest of the power stroke. Process
4-1 nishes the cycle with a constant volume heat rejection with the piston
at BDC. Figure 2.2 shows the p-v and T-s diagram for the diesel cycle.
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 and Compression Ignition Engines
Spark Ignition Engines
Internal combusiton engines are divided into spark ignition engines and com-
pression 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 mix-
ture 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.
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.
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 com-
pression 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 gener-ators.
Induction and Exhaust
In internal combustion engines the induction and exhaust processes give im-
portance to the performance and eciency of the engine. In the two stroke
engine the
ow is regulated by the piston covering and uncovering ports, but
in the four stroke engine the induction and exhaust processes are controlled
through valves. The four types of valves used are poppet, sleeve, rotary, and
disc valves.
Valves
The most commonly used valve is the poppet valve. The poppet valve has a
straight cylinder rod and its end has the shape of a mushroom. The advan-
tages of the poppet valve are that it is cheap, it has good
ow properties,
good seating, it is easy to lubricate, and it has good heat transfer to the
cylinder head. Rotary and disc valves are sometimes used, but contrary to
poppet valves, they have heat transfer, lubrication, and clearance problems.
The other type of valve is the sleeve valve. The sleeve valve has some ad-
vantages over the poppet valve, but its disadvantages discontinued the use
of it. The use of sleeve valves was best suited for aerospace engines before
the introduction of the gas turbine engine. The advantages of sleeve valves
were that they eliminated the \hot spot associated with the poppet valve."
(Stone, p.232) Other advantages were that it produced higher outputs and
higher eciencies due to a higher compression ratio, which was due to the
use of low octane fuel. The disadvantages of the sleeve valve were the cost
and diculty to manufacture, the lubrication and friction between the cylin
Thermochemistry and Fuels
Combustion Reactions
Internal combustion engines obtain their energy from the combustion of hy-
drocarbon fuel with air. The chemical energy stored in the fuel is converted
to energy that the engine can use in the through hot gases within the cham-
ber. The combustion process involves the chemical reaction of hydrocarbon
fuel with oxygen to produce water vapor and CO2. The maximum amount
of chemical energy from the hydrocarbon fuel is when it reacts with stoi-
chiometric oxygen. The meaning of stoichiometric oxygen is dened as the
amount of oxygen that is needed to convert all of the carbon in the fuel to
CO2 and all of the hydrogen to H2O. The simplest chemical reaction using
Heat Transfer In IC Engines
Heat transfer in IC engines is a very serious problem since you need high
temperatures to combust the fuel but you also need to keep the tempera-
ture at a controllable level in order to operate the engine safely. Once the
temparature in the engine has reached intolerable values the engine block
and components may suer damage. Therefore it is essential to have a heat
removal process which will maintain the engine at a safe operating condition.
A water jacket or air through nns are two ways that reduce the temperature
in the engine.
Turbocharging
Introduction
In order to increase the power of an engine there has to be an increase in
pressure, and hence force exerted on the piston, during the power stroke.
The amount of work that the power stroke delivers is basically determined
by the air-fuel mixture in the combustion chamber. The combustion that
occurs during the end of the comprssion stroke and throughout the power
stroke is determined by how much air is mixed with the fuel. When air is
compressed its density increases but volume decreases. Hence compressing
air at the beginning of an engine cycle increases the power output by in-
creasing the amount of air that is mixed with fuel. Since the total volume of
occupied space within the cylinder is decreased when compressing air, more
air can be used to combust with the fuel.
Superchargers
The most common supercharger is a mechanical supercharger. Figure 9.1
shows this supercharger and its arrangement with the engine. In this cong-
uration the compressor receives atmospheric air. This air is then compressed
so that the density increases. After the air has been compressed the en-
gine takes this compressed air and mixes it with the fuel in the combustion
chamber. The imiting factor in the maximum power an engine can deliver is
limited by \the amount of fuel that can be burned eciently inside the en-
gine cylinder." (Heywood, p.248) This in turn is limited by the \the amount
of air that is introduced into each cylinder each cycle." (Heywood, p.248) So
Turbochargers
In a turbocharger, shown in Figure 9.3, a combination of a compressor and
turbine are used. Although this requires the use of another shaft, the en-
gine power is not used to provide the work needed to run the compressor.
After the exhaust gases leave the exhaust manifold through the process of
supercharging as described above the exhaust gases go into a turbine. The
turbine uses the energy content in the hot gases to run the shaft that runs
the compressor. The turbine basically expands the gas mixture, which is at
high temperature, and transfers some of the energy into useful work
Friction and Lubrication
Friction
Friction in IC engines is a major problem because it deteriorates the cylinder
and other components in the engine. Eective lubrication is needed in order
to maintain the engine at safe operating conditions. Oil is a good lubricant
since it provides necessary friction lubrication.
Friction presents problems to the engine by reducing the power output
and hence its eciency. Friction is classied as a loss in form of power in
equation 10.1,
Lubrication
The lubrication process for reducing the friction in an engine is essential for
making the engine run well. There are three oil distribution systems. The
rst involves a splash system where oil is splashed into other componensts of
the engine by the rotating crankshaft. The second system involves using an
oil pump to distribute the oil into engine components. The third lubrication
process is a combination of the two above.
In the splash system, the crankcase is used as the oil sump (reservoir). As
the crankshaft rotates oil is splashed to the oter parts of the engine. Many
small four-stroke cycle engines use (lawn mowers, golf carts, etc.) this type
of lubrication process.
Lubrication
Introduction
When there is contact between two surfaces friction develops because of the
relative movement between the two surfaces. Friction between metal sur-
faces causes wear. To decrease friction, the two mating surfaces have to be
separated by a lubricant. A lubricant is a thin
uid lm that separates to
surfaces so as so reduce the friction between them. By reducing the friction
between surfaces wear is reduced, hence increasing the life of the machine.
If a proper lubricant is used, wear of parts will be minimized. This chapter
talks about thin
uid lm lubrication. The two types of
uid lm lubrication
are hydrodynamic and hydrostatic lubrication.
Hydrodynamic Lubrication
In hydrodynamic lubrication pressure is self induced by the relative motion of
the walls. This type of lubrication is the most useful in terms of its applica-
tions. There are two design variables that are considered in the analysis. The
pressure that is self induced by the relative motion of the walls is one design
that is strived for. By having an understanding of the pressures generated
the maximum load applied on the
uid lm can then be determined. The
second design to strive for is the lm thickness. If you have a load applied,
then you know the force applied on one of the walls. If you know the force
that is applied on an area, then the pressure needed to keep the surfaces
from contact is known. The
uid lm can then be calculated from knowing
Hydrostatic Lubrication
The second type of
uid lm lubrication is hydrostatic lubrication. In this
type of lubrication there is very little relative motion between the surfaces. In
this case it may be desirable to introduce a thin
uid lm lubricant from an
external high pressure source through a cut or groove. Since the lubricant is
being pressurized into a clearance the pressure is not self induced but rather
externally induced. This is known as externally pressurized lubrication. In
this type of lubrication the bearing surfaces are kept separated at all times
even if the surfaces are stationary. Relative sliding motion does not occur
Adiabatic Engine
Introduction
An adiabatic process is one in which there is no heat added or removed from
an isolated system. Heat is not transferred into or out of the system. The
amount of work done by the process is therefore equal to the total change in
energy. In an internal combustion engine the engine is the system. There is
work done on the system and by the system. There is also heat transfer from
the engine to the environment, through the coolant system. A system where
the adiabatic process is employed to a certain extent is the adiabatic engine.
In theory the adiabatic engine has no heat loss. The change in energy for
the system, which is the diesel engine, is due to work done by the engine
and work done on the engine. Some advantages of the adiabatic engine are
described below.
Adiabatic Diesel Engine
In practice it is impossible to have a 100% adiabatic engine. At best the
engine can reach 50-60% of adiabatic with advanced ceramics. In many
cases the adiabatic engine is called the low heat rejection engine (LHRE),
which more accurately describes the technology available today. As described
earlier in an adiabatic engine there is no heat added or rejected. Theoretically
one would like to make use of the exhaust that is released by the engine. The
use of a turbocharger idealizes the no heat rejected concept by taking the
high temperature exhaust and transferring work to the engine.
The adiabatic diesel engine with waste heat utilization is a very rewarding
concept since there is energy being extracte from the hot exhaust gases. The
brake fuel consumption is reduced because of the following:
Chemical and Phase Equilibrium.
Introduction
This chapter deals with chemical and phase equilibrium of pure substances
and mixtures. The chemical equilibrium of a reaction in a single phase is
considered. The discussion on hand deals with ideal gas mixtures. Phase
equilibrium is also considered. Gibbs function and its uses is also discussed.
The use of Gibbs function and chemical potential to solve for equilibrium
constants in one and two phase equilibrium reactions is discussed.
Equilibrium Criteria
A system is in thermodynamic equilibrium when it is isolated from its sor-
roundings and there are no \observable macroscopically observable changes."
(Moran, p.684) In order to have equilibrium the temperature needs to be con-
stant throughout the system. If the system is not at a constant temperature
then there will be a variance in temperature. When there is a temperature
variance there is heat transfer within the system. So even if the system is
isolated there can be heat transfer which will make the system not be in
equilibrium. Another way for the system not to be in equilibrium is if it
has unbalanced forces. So the system can be in thermal and mechanical
equilibrium but there still might be the possibility that it is not in complete
equilibrium. The process of a chemical reaction, a transfer of mass, or both
Chemical Potential
The chemical potential is an expression formulated from Gibbs function. Any
extensive property of a single phase, single component system is a function
of two independent intensive properties and the size of the system. The
two indepemdent intensive properties are pressure and temperature. The
size of the system is dened by the number of moles. For a single phase
multi-component system the Gibbs function is expressed through equation