Comparison of Otto & Diesel cycles efficiency on the basis of maximum temperature and maximum pressure:
Both Otto cycle and Diesel cycle are idealised thermodynamic cycles which can convert heat energy into useful work done and hence, be the basis of spark ignition and compression ignition internal combustion engines.
Although, both the cycles produce useful work, they have different efficiencies and they can be compared under different constraints and parameters. The most important of them is the comparison on the basis of same maximum cycle temperature and pressure, which is obviously a design constraint. In both the cycles, maximum temperature occurs at the end of compression ie., the state after compression will be same for both the cycles.
In the figure processes 1-2'-3-4-1 is the p-v diagram for Otto cycle, where as processes 1-2-3-4-1 represents diesel cycle and in both the cycles maximum temperature and pressure occurs at state 3, hence both have same maximum temperature and pressure. From the diagrams it has been seen that heat rejection is same for both the cycles and equal to Q4-1.
Where as heat input in diesel cycle is Q2-3 and in Otto cycle it is Q2'-3.
EfficiencyOtto = 1 - (Q4-1 /Q2'-3)
Efficiencydiesel = 1 - (Q4-1 /Q2-3)
From the T-s diagram, we can say area A-2-3-B represents heat transfer during heating in diesel cycle or Q2-3 and area A-2'-3-B represents heat transfer during heating in Otto cycle or Q2'-3. As the area A-2-3-B is larger than area A-2'-3-B, we can conclude,
Shree Ganpati Institute of Technology; Ghaziabad
From 23rd September, 2013 to 26th September first minor test has been organised. This semester, I am teaching IC Engines and Compressors (EME-505) and Thermodynamics (ME-301).
Here is the Question paper of EME-505
snapshot of the question paper
ME-301; Thermodynamics
3rd Semester; Mechanical Engg
Vapour lock is a problem that mostly affects " Gasoline-fuelled internal combustion engine. " It occurs when liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburettor or fuel injection system resulting in transient loss of power or even complete stalling.
REASONS OF VAPOUR LOCK
The fuel can vapourise due to being heated by the hot engine or by the local hot climate or due to a low boiling point at high altitude.
In regions where higher volatility fuels are used during winter to improve the cold starting, the use of winter fuels during summer can cause vapour lock more easily.
Vapour lock occurs in older type gasoline fuel systems where a low pressure mechanical fuel pump driven by the engine is located in the engine compartment and feeding a carburettor. These pumps are typically located higher than the fuel tank, are directly heated by the engine, and feed fuel. directly to the float bowl or float chamber of carburettor. As in these pumps fuel is drawn from the feedline and fed into the fuel pump under negative pressure, it lowers the boiling temperature of the liquid fuel. As a result fuel gets evaporated fast and totally invades the fuel pump system and carburettor. As the carburettor becomes devoids of liquid fuel, the mixture it prepares will have less amount of fuel as the volume of vapour of fuel is larger than the equal amount of liquid fuel.
The automotive fuel pump is designed to handle a mixture of liquid and vapour phases of fuel, hence it should handle both the phases of fuel. But, if the amount of fuel evaporated in the fuel system is critically high, the fuel pump stops functioning as per the design and started to pump more vapours than liquid fuel and hence, less amount of liquid fuel will go to the engine. The vapours of fuel will invade the fuel pump delivery system which stops the flow of liquid fuel into the engine.
Most carburettors are designed to run at a fixed level of fuel in the flat bowl of carburettor and reducing the level will reduce the fuel to air mixture and hence, will deliver a lean mixture to the combustion chamber which translates into uneven running of the engine or even stalling while idling or sometimes momentary stalling when running.
VAPOUR LOCK AND (V/L) RATIO
The vapour liquid ratio or (V/L) ratio of a gasoline, defined as the amount of vapour released from a gasoline to the amount of liquid remaining at a given temperature directly correlates with the degree of vapour lock likely to be experienced with this gasoline in the fuel system of a car. At V/L ratio = 24, vapour lock may start and at V/L ratio = 36, vapour lock may be severe. Therefore, the volatility of the gasoline should be maintained as low as practical to prevent vapour lock.
6 Critical Characteristics of SI Engine Fuel (Gasoline) | Edunes
THE FUEL CHARACTERISTICS OF INTERNAL COMBUSTION ENGINE:
Edunes Online Education
6 Critical Characteristics of SI Engine Fuel (Gasoline) | Edunes:
The fuel characteristics that are important for the performances of
Internal combustion engines are
• Volatility of the Fuel
• Detonation Characteristics
• Power and Efficiency of Engines
• Good thermal properties like heat of combustion and heat of
evaporation
• Gum Content
• Sulphur Content
• Aromatic Content
• Cleanliness
IMPORTANT CHARACTERISTICS OF SI ENGINE FUELS
SI (spark-ignition) engines, also known as gasoline engines, use a
fuel-air mixture that is ignited by a spark from a spark plug to produce
power. Some of the important properties of SI engine fuel include:
Octane rating: The octane rating of a fuel measures its resistance to knocking,
which is an uncontrolled explosion in the engine cylinder that can
damage the engine. The higher the octane rating, the more resistant the
fuel is to knocking.
Energy content: The energy content of the fuel determines how much power can be
produced from a given amount of fuel. Gasoline has a higher energy
content per unit of volume than ethanol, for example.
Volatility: Volatility refers to the ease with which a fuel evaporates.
High-volatility fuels can vaporize quickly, which is important for good
cold-start performance. However, if a fuel is too volatile, it can also
cause vapor lock in hot weather, which can disrupt fuel delivery to the
engine.
Stability: Fuel stability refers to the ability of a fuel to resist
oxidation and degradation over time. Stable fuels are less likely to
form deposits or gum up fuel injectors, which can negatively impact
engine performance and fuel efficiency.
Chemical composition: The chemical composition of the fuel can affect its combustion
characteristics, including its flame speed and emissions. Gasoline
typically contains hydrocarbons, oxygenates (such as ethanol), and
various additives to improve performance and reduce emissions.
Cost: The cost of fuel is an important consideration for consumers and
businesses alike. Gasoline is typically less expensive than alternative
fuels like diesel or natural gas, but its price can fluctuate depending
on supply and demand, as well as other market factors.
Every SI engines are designed for a particular fuel having some desired
qualities. For a good performance of a SI engine the fuel used must have
the proper characteristics.
The followings are requirements of a good SI engine fuels or
Gasolines.
It should readily mix with air to make a uniform mixture at inlet, ie.
it must be volatile
It must be knock resistant
It should not pre-ignite easily
It should not tend to decrease the volumetric efficiency of the
engine.
It should not form gum and varnish
Its Sulphur content should be low as it is corrosive
It must have a high calorific value
VOLATILITY OF THE FUEL
It is the most important characteristics of a SI engine fuel. Volatility
is a physical concept that loosely defined as the tendency to evaporate at
a temperature lower than their boiling temperature. It is the most
dominant factor that controls the air-fuel ratio inside the combustion
chamber.
One of the most important requirements for proper and smooth combustion is
the availability of a highly combustible air-fuel mixture at the moment of
the start of the ignition inside the combustion chamber.
A highly volatile (of low molecular weight) fuel generates a rich fuel air
ratio at low starting temperature, to satisfy the criteria at the starting
of the ignition. But, it will create another problem during running
operation; it creates vapour bubble which choked the fuel pump delivery
system. This phenomenon is known as vapour lock.
A vapour lock thus created restricts the fuel supply due to excessive
rapid formation of vapour in the fuel supply system of the carburetor.
High volatility of fuel can also result in excessive evaporation during
storage in a tank which will also pose a fire hazards.
Low volatile fuel like kerosene and distillates can be used for SI engines
for tractors.
VOLATILITY AND ITS EFFECT ON ENGINE PERFORMANCES
Volatility greatly affects the engine performances and fuel economy
characteristics. The most important of them are
·Cold and Hot starting
·Vapour Lock in fuel delivery system
·Short and Long trip economy
·Acceleration and Power
·Warm Up
·Hot Stalling
·Carburetor Icing
·Crankcase Dilution Deposit formation and Spark Plug Fouling
When the percentage evaporation of the fuel is 0% ~ 20%, it is called
front end of volatility curves, and there are 3 major problems that we
encounter in this region of volatility curves which is also known
as Distillation curves. They are
• Cold Starting
• Hot Starting
• Vapour Lock
If front end volatility is very low of a SI engine fuel the engine
may show the symptoms of "Cold Starting."
THE CONCEPT OF COLD STARTING
In order to start an engine a highly combustible mixture rich in fuel is
needed at starting temperature near the spark plug.
As the ambient temperature is low during starting condition, hence the
fuel-air mixture must be rich to ensure the start of combustion as
sparking of spark plug is not able to start a chemical reaction of
combustion near the spark plug.
The limit of air-fuel mixture at the start is
• for rich mixture it is 8:1
• for lean mixture it is 20:1
MECHANISMS OF COLD START:
At low ambient temperature, only a small fraction of total fuel fed to
the combustion chamber is able to be effectively evaporated and it creates
a insufficiently lean fuel-air mixture that is unable to combust and
sustain the combustion process. As a result, the combustion never be able
to provide a steady rate of heat supply and engine never starts in this
condition.
This phenomenon is known as cold starting of an IC engine.
To get rid of this problem, we generally apply Choking Process at the
start of an engine at ambient temperature. When an Engine becomes hot
enough to engineered a sufficiently rich fuel air mixture, the combustion
becomes steady and it is known as Warming Up of an IC engine.
Choking is a process generally used to control or regulate air flow into
the carburetor where fuel gets mixed with air homogeneously and been fed
into combustion chamber. By decreasing air-flow rate into the carburetor,
a rich mixture of fuel and air is prepared and fed into the cylinder or
combustion chamber, one can increase the vapour content of fuel in the
mixture as the reduced air makes the mixture fuel rich and the mixture
becomes a combustible inside the combustion chamber.
DETONATION CHARACTERISTICS OF A SI ENGINE FUEL:
The detonation characteristics of a fuel refer to its tendency to
detonate or explode prematurely in the engine cylinder, leading to engine
knock or detonation. This is undesirable as it can cause damage to the
engine and reduce its performance and efficiency.
In spark-ignition (SI) engines, the detonation characteristics of the
fuel are influenced by several factors, including:
Octane rating: The octane rating of a fuel is a measure of its ability
to resist knocking or detonation. Fuels with higher octane ratings are
less prone to detonation and are therefore more suitable for use in
high-performance engines.
Chemical characteristics: Fuels with higher percentages of aromatic
hydrocarbons or olefins tend to have lower resistance to detonation.
Air-fuel ratio: The air-fuel ratio (AFR) is the ratio of air to fuel in
the combustion mixture. AFRs that are too lean (i.e., too much air
relative to fuel) can increase the risk of detonation.
Compression ratio: The compression ratio is the ratio of the volume in
the engine cylinder when the piston is at the bottom of its stroke to
the volume when it is at the top of its stroke. Higher compression
ratios can increase the risk of detonation.
Engine operating conditions: The operating conditions of the engine,
such as load, speed, and temperature, can affect the detonation
characteristics of the fuel.
In general, fuels with higher octane ratings and lower percentages of
aromatic hydrocarbons and olefins are more resistant to detonation and are
therefore preferred for use in SI engines. Additionally, controlling the
air-fuel ratio, compression ratio, and engine operating conditions can
help to reduce the risk of detonation.
FACTORS OF DETONATION CHARACTERISTICS:
THE OCTANE RATING:
The octane rating is a measure of a fuel's ability to resist knocking or
detonation in internal combustion engines. Knocking or detonation occurs
when the air-fuel mixture in the engine's cylinder ignites prematurely or
unevenly, leading to a rapid and uncontrolled burning of the remaining
fuel. This can cause engine damage and reduce overall performance.
Fuels with higher octane ratings have better anti-knock properties and
can withstand higher compression ratios and temperatures before
auto-ignition occurs. High-performance engines, such as those found in
sports cars or high-powered motorcycles, often operate at higher
compression ratios and temperatures, which can lead to a greater tendency
for knocking. Using a fuel with a higher octane rating helps prevent
knocking and maintains engine performance.
On the other hand, some vehicles, especially those with lower compression
ratios or engines designed for regular-grade fuel, do not require
high-octane gasoline. In such cases, using fuel with a higher octane
rating than what the engine needs might not provide any significant
benefits and could be a waste of money.
It's essential to use the fuel recommended by the manufacturer for your
specific vehicle, as using the wrong octane rating can lead to inefficient
combustion and potentially harm the engine. Many modern vehicles have
knock sensors and engine management systems that can adjust the engine's
performance based on the octane level of the fuel being used, but it's
still best to follow the manufacturer's guidelines.
THE CHEMICAL COMPOSITION OF A FUEL:
The chemical composition of a fuel can significantly influence its
resistance to detonation or knocking. Fuels with higher percentages of
aromatic hydrocarbons or olefins tend to have lower resistance to
detonation compared to fuels with higher percentages of paraffins
(saturated hydrocarbons). Let's explore this further:
Aromatic hydrocarbons: Aromatic hydrocarbons, such as benzene, toluene,
and xylene, have a cyclic structure and are known for their high octane
number, which indicates good resistance to knocking. However, when
present in high concentrations in a fuel, they can contribute to
pre-ignition issues and reduce the fuel's overall anti-knock properties.
This is why modern gasoline formulations aim to limit the concentration
of aromatic hydrocarbons to maintain optimal octane ratings.
Olefins: Olefins, also known as alkenes, are unsaturated hydrocarbons
that contain at least one carbon-carbon double bond. Fuels with a higher
content of olefins generally have lower octane ratings and are more
prone to detonation. This is because the presence of double bonds in the
molecular structure makes them more reactive, leading to premature
ignition and knocking in high-compression engines.
Paraffins: Paraffins, also known as alkanes, are saturated hydrocarbons
with single bonds between carbon atoms. Fuels with higher percentages of
paraffins tend to have better anti-knock properties and higher octane
ratings. They are less reactive compared to olefins, which makes them
more resistant to detonation.
To improve the overall quality and anti-knock properties of gasoline,
refiners often use various blending components and additives to achieve
the desired octane rating while keeping the concentration of aromatic
hydrocarbons and olefins within acceptable limits.
It's essential for fuel manufacturers to strike a balance in the chemical
composition of gasoline to ensure optimal engine performance, fuel
efficiency, and emissions control, while also meeting regulatory
requirements and environmental standards.
THE AIR-FUEL RATIO:
The air-fuel ratio (AFR) refers to the ratio of the mass or volume of air
to the mass or volume of fuel in the combustion mixture used by an
internal combustion engine. It is a crucial parameter that significantly
affects engine performance, fuel efficiency, and emissions.
In the context of detonation or knocking, an AFR that is too lean
(meaning there is too much air relative to the amount of fuel) can indeed
increase the risk of detonation. When the mixture is lean, there is an
excess of oxygen compared to the available fuel molecules. This can lead
to higher combustion temperatures and pressures, which can cause the
air-fuel mixture to ignite prematurely or unevenly, resulting in
knocking.
Detonation occurs because the rapid and uncontrolled burning of the lean
mixture generates pressure waves that collide and produce a knocking
sound. This can put excessive stress on the engine components and lead to
engine damage over time.
On the other hand, an AFR that is too rich (meaning there is too much
fuel relative to the amount of air) can also lead to knocking. A rich
mixture tends to burn more slowly, and the unburned fuel can create hot
spots in the combustion chamber, increasing the likelihood of pre-ignition
and knocking.
To minimize the risk of knocking and achieve optimal engine performance,
modern engines are equipped with sophisticated engine management systems
and knock sensors that can adjust the air-fuel ratio in real-time based on
various factors, such as engine load, speed, and temperature. These
systems help maintain the AFR within the appropriate range to ensure
efficient combustion and reduce the risk of detonation.
For high-performance engines or engines modified for increased power
output, tuning the air-fuel ratio carefully is crucial to avoid knocking
and maximize performance. It's important to follow the manufacturer's
recommendations or consult with experienced tuners to ensure that the
engine operates within safe and optimal parameters.
THE COMPRESSION RATIO:
The compression ratio is a crucial parameter in internal combustion
engines, and it represents the ratio of the cylinder volume when the
piston is at its bottom dead center (BDC) to the cylinder volume when the
piston is at its top dead center (TDC). It is typically expressed as a
numerical value, such as 10:1 or 12:1, representing the ratio of the
larger volume (at BDC) to the smaller volume (at TDC).
Higher compression ratios indeed increase the risk of detonation,
especially if the fuel used has a low octane rating or if other factors
that promote knocking are present. Here's why:
Increased Temperature and Pressure: Higher compression ratios compress
the air-fuel mixture more, resulting in increased temperature and
pressure in the combustion chamber. This elevated pressure and
temperature can cause the air-fuel mixture to autoignite prematurely,
leading to knocking or detonation.
Reduced Time for Combustion: With higher compression ratios, the time
available for the air-fuel mixture to burn completely is reduced. This
can lead to incomplete combustion, which leaves unburned fuel and hot
spots in the combustion chamber, increasing the likelihood of
knocking.
Increased Sensitivity to Fuel Properties: Fuels with lower octane
ratings are more likely to experience detonation under higher
compression ratios. The lower the octane rating, the more susceptible
the fuel is to pre-ignition, and the greater the risk of knocking in
high-compression engines.
To mitigate the risk of detonation in high-compression engines, it is
crucial to use fuels with higher octane ratings that can withstand the
elevated pressures and temperatures without prematurely igniting.
Additionally, modern engine management systems with knock sensors can
detect knocking and adjust the engine's timing and air-fuel ratio to
reduce the likelihood of detonation.
Engine designers and tuners carefully consider the compression ratio when
developing or modifying engines to ensure optimal performance while
avoiding harmful knocking or detonation. Following the manufacturer's
recommendations regarding fuel type and engine specifications is essential
to maintain the engine's longevity and performance.
THE ENGINE OPERATING CONDITION:
The operating conditions of an engine, including factors such as load,
speed, and temperature, have a significant impact on the detonation
characteristics of the fuel being used. Let's explore how these factors
can influence the likelihood of detonation:
Engine Load: The engine load refers to the amount of power the engine
is producing to meet the demands of driving or operating the vehicle.
Higher engine loads, such as during acceleration or towing heavy loads,
result in increased pressure and temperature in the combustion chamber.
This elevated pressure and temperature can make the air-fuel mixture
more prone to detonation, especially if the fuel used has a lower octane
rating. As a result, engines under high load conditions are more
susceptible to knocking.
Engine Speed: Engine speed, commonly measured in revolutions per minute
(RPM), determines how frequently the combustion process occurs in the
cylinders. Higher engine speeds mean that the air-fuel mixture is being
compressed and ignited more frequently. If the engine is operating at
high RPM, there is less time for the air-fuel mixture to burn
completely, increasing the chances of knocking.
Engine Temperature: The temperature of the engine components,
particularly the combustion chamber, plays a crucial role in the risk of
detonation. Higher engine temperatures can cause hot spots in the
combustion chamber, which can lead to premature ignition of the air-fuel
mixture. This is especially true when the engine is running under heavy
load or high RPM conditions.
Intake Air Temperature: The temperature of the intake air entering the
engine also affects the likelihood of knocking. Cooler air is denser and
can reduce the chances of knocking, as it allows for a higher
air-to-fuel ratio without increasing the risk of detonation. Engines
equipped with intercoolers or air intake temperature control systems can
optimize the intake air temperature for improved performance and reduced
knocking.
Ignition Timing: The ignition timing refers to the precise moment when
the spark plug ignites the air-fuel mixture in the cylinder. Advanced
ignition timing (igniting the mixture earlier) can increase the risk of
knocking, especially under high load and high temperature conditions.
Retarding the ignition timing (igniting the mixture later) can help
reduce knocking in some cases.
To optimize engine performance and reduce the risk of detonation, modern
engines use sophisticated engine management systems that continuously
monitor various parameters and adjust ignition timing, air-fuel ratio, and
other factors to maintain safe and efficient operation. Additionally,
using high-quality fuels with appropriate octane ratings can also play a
vital role in preventing knocking under varying operating
conditions.
Google Patent wrote on the page as Abstract ideas about stratified charge engine. Google Patent
An internal combustion engine is disclosed having a
cylinder and a reciprocating piston which, together with
the cylinder head, define the boundaries of a combustion
chamber.
A movable septum is supported by the cylinder
head adjacent the combustion chamber for selectively
dividing the combustion chamber into a first and second
combustion chamber. The movable septum may be
formed by a cylindrical plate or by a flat or curved plate
which makes nominal contact with the walls of the
cylinder and the face of the reciprocating piston to
divide the combustion chamber.
The fuel supply system
provides a rich-fuel mixture through a rich mixture
intake valve to the first combustion chamber, and a
lean-fuel mixture through a lean mixture intake valve to
the second combustion chamber.
The movable septum
tracks the movement of the reciprocating piston during
the compression stroke to maintain the division between
the first and second combustion chambers. However,
just prior to the power stroke, the actuator retracts the
movable septum from the combustion chamber, and the
spark plug ignites the rich air-fuel mixture which then
ignites the lean air-fuel mixture to complete the power
stroke. During the exhaust stroke of the reciprocating
piston, the movable septum is maintained in a retracted
position. As a result, effective stratification is achieved
between the first and second combustion chambers.
Lecture Note:
STRATIFIED CHARGE ENGINE
Internal combustion engines or popularly known as IC
Engines are life line of human society which mostly
served as a mobile, portable energy generator and
extensively used in the transportation around the
world.
There are many types of IC Engines, but among them
two types known as petrol or SI engines and diesel or CI
engines are well established. Most of the automotive
vehicles run on either of the engines. Despite their wide
popularity and extensive uses, they are not fault free.
Both SI Engines and CI Engines have their own demerits
and limitations.
Limitations of SI Engines (Petrol Engines)
Although petrol engines have very good full load power
characteristics, but they show very poor performances
when run on part load.
Petrol engines have high degree of air utilisation and
high speed and flexibility but they can not be used for
high compression ratio due to knocking and detonation.
Limitations of CI or Diesel Engines:
On the other hand, Diesel engines show very good part
load characteristics but very poor air utilisation, and
produces unburnt particulate matters in their exhaust.
They also show low smoke limited power and higher
weight to power ratio.
The use of very high compression ratio for better
starting and good combustion a wide range of engine
operation is one of the most important compulsion in
diesel engines. High compression ratio creates
additional problems of high maintenance cost and high
losses in diesel engine operation.
For an automotive engine both part load efficiency and
power at full load are very important issues as 90% of
their operating cycle, the engines work under part load
conditions and maximum power output at full load
controls the speed, acceleration and other vital
characteristics of the vehicle performance.
Both the Petrol and Diesel engines fail to meet the both
of the requirements as petrol engines show good
efficiency at full load but very poor at part load
conditions, where as diesel engines show remarkable
performance at part load but fail to achieve good
efficiency at full load conditions.
Therefore, there is a need to develop an engine which
can combines the advantages of both petrol and diesel
engines and at the same time avoids their disadvantages
as far as possible.
Working Procedures:
Stratified charged engine is an attempt in this direction.
It is an engine which is at mid way between the
homogeneous charge SI engines and heterogeneous
charge CI engines.
Charge Stratification means providing different fuel-air
mixture strengths at various places inside the
combustion chamber.
It provides a relatively rich mixture at and in the vicinity
of spark plug, where as a leaner mixture in the rest of
the combustion chamber.
Hence, we can say that fuel-air mixture in a stratified
charge engine is distributed in layers or stratas of
different mixture strengths across the combustion
chamber and burns overall a leaner fuel-air mixture
although it provides a rich fuel-air mixture at and
around spark plug.
Thermodynamics can be understood with the help of the curves, where each curve represents a specific process. In general curves are plotted in a coordinate system where X axis and Y axis represent thermodynamic variables, often two conjugate variables. The state of a thermodynamic system can be fully specified by the values of any two conjugate thermodynamic properties. Therefore, in a coordinate plane where X and Y axes are replaced by any two conjugate thermodymanic properties, each point will represent an unique thermodynamic equilibrium states. Hence, curves joining any arbitrary two points on this plane will represent a thermodynamic processes.
The curves those are used most:
In thermodynamics, p-v diagrams, T-s diagrams, h-s diagrams are the important diagrams. h-s diagrams of water is also known as Mollier Chart. Curves play a crucial role in studying Thermodynamics.
In thernodynamics all the possible types of processes which are reversible can be represented by a mathemetical relation hence, can be plotted in different thermodynamic planes. It can be represented by a relation pvāæ = constant and called polytropic process.
In the second law analysis, it is useful to plot the process on diagrams for which has one coordinate is entropy. The two diagrams commonly used in second law analysis are temperature-entropy (T-s) and enthalpy-entropy (h-s) diagrams. For some pure substance, like water, the entropy is tabulated with other properties.
The T-s Diagrams and its importance
On a P-v diagram, the area under the process curve is equal, in magnitude, to the work done during a quasi-equilibrium expansion or compression process of a closed system. On a T-s diagram, the area under an internally reversible process curve is equal, in magnitude, to the heat transferred between the system and its surroundings. That is,
The T-s diagram of a Carnot cycle is shown on the above figure. The area under process curve 1-2 (area 1-2-B-A-1) equals the heat input from a source (QH). The area under process curve 3-4 (area 4-3-B-A-4) equals the heat rejected to a sink (QL). The area enclosed by the 4 processes (area 1-2-3-4-1) equals the net heat gained during the cycle, which is also the net work output.
The idea of engines come from heat engines. Expanding steam was the working substance of the primitive kind of Steam Engines. But, locomotion was tough using steam engines as it needed continuous supply of water and coal as fuel. People started to think about a compact engines, light and portable and combustion will be the basis of heat generation. If heat generation could be taken place inside the cylinder, then it will be easier to design a compact engine which could be used to run a locomotive vehicle.
This semester, I am teaching IC Engines and Compressors. The text book is selected as IC Engines by Sharma and Mathur published by Dhanpat Rai Publications. The course is designed by MTU (Mahamaya Technical University, Noida and Gautam Budh Technical University) and it is taught in 5th semester. Although it is a 50 marks paper, still it is a subject which every Mechanical Engineering students must know. It is completely based on the principles of thermodynamics.
The course starts with defining IC Engines, introducing the components used in IC engines, different terms and processes related with IC engines, general working procedures of an IC engine and at last describing the classification of IC engines. Then the thermodynamic analysis of the engine operations along with Air-standard thermodynamic cycles are studied. If any one wants to know the subject deeply, then he should know very basic concepts of thermodynamics.
PRE-REQUISITE KNOWLEDGE:
As air-standard cycles are one of the basic models based on which engines are practically run and is a highly simplified or even oversimplified version of the original engine operation and due to this, the experimental values of the engine efficiencies are much below the value predicted by the air standard cycles. The large amount of deviations of actual cycles from the theoretical air standard cycles are due to assumptions taken during air standard cycle analysis.
DESCRIPTION OF THE IC ENGINE:
While describing IC engines, one should start with the engine cylinder which acts as the combustionchamber which has a variable volume due to a piston which can slide inside the cylinder.
One end of the cylinder is sealed off by cylinder head which provides the space for clearance volume and it also housed the inlet and exhaust valves.
The other end of the cylinder is covered by the piston which can slide along the principal axis of the cylinder.
Inside the cylinder air-fuel mixture is sucked into and then compressed it in case of SI engines, where as in CI engine only air is sucked into the cylinder.
The piston is connected to a link known as Connecting rod by a pin named Gudgeon or Piston or Wrist pin.
This connecting rod has unequal ends. The smaller end is connected to piston by gudgeon pin and the bigger end is connected to the eccentric on the Crank.
It is joined to the eccentric by a pin named Crank pin. Piston, Connecting Rod and Crank constitute a "Slider-Crank Mechanism" which translates a linear "to and fro motion" of the piston into "rotational motion" of the crank.
Here, connecting rod is the element that bears the whole load, hence it fails quite frequently.
Crank is mounted on a crank shaft and crank shaft operates two valve mechanism through poppet valve, rocket arm and cams.
These valve mechanisms are responsible for the opening and closing of inlet as well as exhaust valves.
This valves are regulated by cams. Cams are mounted on a cam shaft which is geared with crankshaft by a step down gear mechanism so that for every two revolutions of crankshaft rotation the camshaft makes one rotation. So, the complete thermodynamic cycle of two crankshaft rotation crankshaft makes only one cycle. The idea behind this step down mechanism, is valves are needed to open and close once in a complete thermodynamic cycle and a cam profile can be designed easily.
A flywheel is mounted on the crankshaft, so that it can absorb and store energy during power stroke or expansion stroke and releases energy to power suction, compression and exhaust stroke.
In SI engine, after the end of compression stroke, the pressure and temperature of the air-fuel mixture becomes sufficiently high to sustain the ignition process after ignition takes place. After the compression pressure becomes 10 to 12 bar and temperature becomes 300o C to 500o C. It is still below the temperature at which spontaneous auto-ignition generally starts. If the temperature after compression is above the temperature at which auto ignition starts, then auto ignition will start during the last phases of compression stroke and it will create an explosion known as knocking and detonation.
Then theoretical basis of an IC engines are discussed. While analyzing any phenomena, the best way is to make an idealized modelling of the phenomena by considering certain assumptions which would reduce
the complexity of the phenomena and make a oversimplified model and then add the complexity one by one.
Similarly, here we oversimplified the model of IC engine operation by considering the working substance an ideal gas like air and study some reversible thermodynamic cycles those resemble with the processes those occurs inside an IC engine.
As those cycles are considered having air as working substance and hence, they are called Air-Standard cycles. But, as Air-Standard Cycle are the idealized version of the real life working principle of an IC engines, its analysis can not be used to gauge the performances of the engine with closest accuracy.
Thermodynamic Air-standard cycles like Otto, Diesel, Dual, Stirling and Ericsson cycles are discussed.
Derivation of total work done, Efficiency, Mean Effective Pressure and graphs in p-v and T-s diagrams are studied.
In the air standard cycles, working substance is assumed to be perfect gas like pure air, but in actual cycles the working substance is different and it is the mixture of air and fuels. In air standard cycle it is assumed that specific heats are constant where as in reality, specific heats are functions of temperature and it increases with the increase of temperature.
Moreover, in air standard cycle, it is assumed that working substance is chemically non-reactive and there is no chemical changes inside the engine cylinder, but in reality, inside the cylinder combustion process takes place and the chemical composition of the working substance rapidly changes during the combustion process which alters the composition as well as number of moles of the working substances also got changed.
The combined effect of both the phenomena is to reduce the temperature and pressure after the end of compression stroke as well as it reduces the maximum cycle temperature and pressure after the end of combustion.
While expanding adiabatically during the power stroke, the temperature and pressure after expansion is higher than the predicted value according to air standard cycle and as a result it increases the value of rejected heat into the thermal sink.
Therefore, the actual cycle efficiency is much lower than the air standard cycle efficiency. Moreover, there are several other losses during the actual cycle due to various other design limitations. The major losses are
(i) burning time losses,
(ii) losses due to incomplete combustion,
(iii) Direct heat losses due to colder cylinder and heat carried away by coolants,
(iv) pumping losses,
(v) friction losses due to rubbing of parts,
(vi) blow down losses during exhaust.
So, we have first idealized the engine operations and oversimplified it to have an idealized version, but its prediction will not be accurate, but we shall get an upper limit of the efficiencies of IC engines. Now, to get more accurate analysis, we shall modified the simplistic assumptions we have considered during the air standard cycles analysis.
The most important assumption of the air standard cycle is the choosing pure air as our working substance, which is in reality a mixture of air with fuel, which has been mixed homogeneously in the carburettor and then supplied into the engine cylinder which acts as combustion chamber. Therefore, we first substitute air with the air fuel mixture in the air standard cycles and it is hence called "Fuel Air Cycles".
Due to the replacement of working substance by air fuel mixture in stead of pure air, our two key assumptions have been changed too. First of all, fuel-air mixture doesn't show a constant specific heats in stead specific heats are functions of temperature, linearly at low temperatures, non linearly at high temperatures.
Internal combustion engines or popularly known as IC Engines are life line of human society which mostly served as a mobile, portable energy generator and extensively used in the transportation around the world.
There are many types of IC Engines, but among them two types known as petrol or SI engines and diesel or CI engines are well established. Most of the automotive vehicles run on either of the engines. Despite their wide popularity and extensive uses, they are not fault free.
Both SI Engines and CI Engines have their own demerits and limitations.
Limitations of SI Engines (Petrol Engines)
Although petrol engines have very good full load power characteristics, but they show very poor performances when run on part load.
Petrol engines have high degree of air utilisation and high speed and flexibility but they can not be used for high compression ratio due to knocking and detonation.
Limitations of CI or Diesel Engines:
On the other hand, Diesel engines show very good part load characteristics but very poor air utilisation, and produces unburnt particulate matters in their exhaust. They also show low smoke limited power and higher weight to power ratio.
The use of very high compression ratio for better starting and good combustion a wide range of engine operation is one of the most important compulsion in diesel engines. High compression ratio creates additional problems of high maintenance cost and high losses in diesel engine operation.
For an automotive engine both part load efficiency and power at full load are very important issues as 90% of their operating cycle, the engines work under part load conditions and maximum power output at full load controls the speed, acceleration and other vital characteristics of the vehicle performance.
Both the Petrol and Diesel engines fail to meet the both of the requirements as petrol engines show good efficiency at full load but very poor at part load conditions, where as diesel engines show remarkable performance at part load but fail to achieve good efficiency at full load conditions.
Therefore, there is a need to develop an engine which can combines the advantages of both petrol and diesel engines and at the same time avoids their disadvantages as far as possible.
Working Procedures:
Stratified charged engine is an attempt in this direction. It is an engine which is at mid way between the homogeneous charge SI engines and heterogeneous charge CI engines.
Charge Stratification means providing different fuel-air mixture strengths at various places inside the combustion chamber.
It provides a relatively rich mixture at and in the vicinity of spark plug, where as a leaner mixture in the rest of the combustion chamber.
Hence, we can say that fuel-air mixture in a stratified charge engine is distributed in layers or stratas of different mixture strengths across the combustion chamber and burns overall a leaner fuel-air mixture although it provides a rich fuel-air mixture at and around spark plug.
If we carefully think about human civilization, one shall notice an wonderful fact about human beings. The thing that made us different from other hominids is the skill to manufacture tools. We just triumphed due to our ability to make primitive tools out of stone and metals during the dawn of the civilizations. Since then much time has passed and we have entered into a Machine Era and man has been still continuously engaged in converting the natural resources into useful products by adding value to them through machining and other engineering activities applying on the raw materials. Manufacturing is the sub branch of Engineering which involves the conversion of raw materials into finished products.
The conversion of natural resources into raw materials is normally taken care of by two sub branches of engineering viz. Mining and Metallurgy Engineering. The value addition to the raw materials by shaping and transforming it to final products generally involves several distinct processes like castings, forming, forging, machining, joining, assembling and finishing to obtain a completely finished product.
Understanding Manufacturing Engineering largely based upon three engineering activities and they are Designing, Production and Development of new more efficient techniques.
At the Designstage, engineering design mainly concentrates on the optimization of engineering activities to achieve most economical way to manufacture a goods from raw materials. It also chooses the raw materials and impart the requisite engineering properties of materials like hardness, strength, elasticity, toughness by applying various heat treatment to them.
During the production stages, the selection of the important process parameters to minimize the idle time and cost, and maximizing the production and its quality is very important.
The New Technologies must be implemented to adapt to the changing scenarios of the markets and demands to make the sales competitive and sustainable.
Q) Write a C programme to check a odd or even number
A) c program to check odd or even:
We will determine whether a number is odd or even by using different methods all are provided with a code in c language. As you have study in mathematics that in decimal number system even numbers are divisible by 2 while odd are not so we may use modulus operator(%) which returns remainder, For example 4%3 gives 1 ( remainder when four is divided by three). Even numbers are of the form 2*p and odd are of the form (2*p+1) where p is is an integer. C program to check odd or even using modulus operator
#include<stdio.h>
main()
{
int n;
printf("Enter an integer\n");
scanf("%d",&n);
if ( n%2 == 0 )
printf("Even\n");
else
printf("Odd\n");
return 0;
}
We can use bitwise AND (&) operator to check
odd or even, as an example consider binary of 7
(0111) when we perform 7 & 1 the result will be
one and you may observe that the least
significant bit of every odd number is 1, so
( odd_number & 1 ) will be one always and also
( even_number & 1 ) is zero.
C program to check odd or even using bitwise
operator
#include<stdio.h>
main()
{
int n;
printf("Enter an integer\n");
scanf("%d",&n);
if ( n & 1 == 1 )
printf("Odd\n");
else
printf("Even\n");
return 0;
}
Find odd or even using conditional operator
#include<stdio.h>
main()
{
int n;
printf("Input an integer\n");
scanf("%d",&n);
n%2 == 0 ? printf("Even\n") : printf("Odd\n");
return 0;
}
C program to check odd or even without using
bitwise or modulus operator
#include<stdio.h>
main()
{
int n;
printf("Enter an integer\n");
scanf("%d",&n);
if ( (n/2)*2 == n )
printf("Even\n");
else
printf("Odd\n");
return 0;
}
In c programming language when we divide two
integers we get an integer result, For example
the result of 7/3 will be 2.So we can take
advantage of this and may use it to find whether
the number is odd or even. Consider an integer
n we can first divide by 2 and then multiply it by
2 if the result is the original number then the
number is even otherwise the number is odd.
For example 11/2 = 5, 5*2 = 10 ( which is not
equal to eleven), now consider 12/2 = 6 and 6 *2
= 12 ( same as original number). These are some
logic which may help you in finding if a number
is odd or not.
Q) Write a C program to check whether input
alphabet is a vowel or not.
A) This code checks whether an input alphabet
is a vowel or not. Both lower-case and upper-
case are checked.
#include <stdio.h>
int main()
{
char ch;
printf("Enter a character\n");
scanf("%c", &ch);
if (ch == 'a' || ch == 'A' || ch == 'e' || ch == 'E' ||
ch == 'i' || ch == 'I' || ch =='o' || ch=='O' || ch ==
'u' || ch == 'U')
printf("%c is a vowel.\n", ch);
else
printf("%c is not a vowel.\n", ch);
return 0;
}
Check vowel using switch statement
#include <stdio.h>
int main()
{
char ch;
printf("Input a character\n");
scanf("%c", &ch);
switch(ch)
{
case 'a':
case 'A':
case 'e':
case 'E':
case 'i':
case 'I':
case 'o':
case 'O':
case 'u':
case 'U':
printf("%c is a vowel.\n", ch);
break;
default:
printf("%c is not a vowel.\n", ch);
}
return 0;
}
Function to check vowel
int check_vowel(char a)
{
if (a >= 'A' && a <= 'Z')
a = a + 'a' - 'A'; /* Converting to lower case or
use a = a + 32 */
if (a == 'a' || a == 'e' || a == 'i' || a == 'o' || a == 'u')
return 1;
return 0;
}
This function can also be used to check if a
character is a consonant or not, if it's not a
vowel then it will be a consonant, but make sure
that the character is an alphabet not a special
character.
Q) Write C program to perform addition,
subtraction, multiplication and division.
A) C program to perform basic arithmetic
operations which are addition, subtraction,
multiplication and division of two numbers.
Numbers are assumed to be integers and will be
entered by the user.
#include <stdio.h>
int main()
{
int first, second, add, subtract, multiply;
float divide;
printf("Enter two integers\n");
scanf("%d%d", &first, &second);
add = first + second;
subtract = first - second;
multiply = first * second;
divide = first / (float)second; //typecasting
printf("Sum = %d\n",add);
printf("Difference = %d\n",subtract);
printf("Multiplication = %d\n",multiply);
printf("Division = %.2f\n",divide);
return 0;
}
In c language when we divide two integers we
get integer result for example 5/2 evaluates to 2.
As a general rule integer/integer = integer and
float/integer = float or integer/float = float. So
we convert denominator to float in our program,
you may also write float in numerator. This
explicit conversion is known as typecasting.
Q) Write a C programme to check a Leap year.
A) C program to check leap year: c code to
check leap year, year will be entered by the
user.
#include <stdio.h>
int main()
{
int year;
printf("Enter a year to check if it is a leap year
\n");
scanf("%d", &year);
if ( year%400 == 0)
printf("%d is a leap year.\n", year);
else if ( year%100 == 0)
printf("%d is not a leap year.\n", year);
else if ( year%4 == 0 )
printf("%d is a leap year.\n", year);
else
printf("%d is not a leap year.\n", year);
return 0;
}
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