COMPARISON OF OTTO AND DIESEL CYCLE EFFICIENCY ON THE BASIS OF SAME MAXIMUM TEMPERATURE AND PRESSURE

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 Q_4-1. Where as heat input in diesel cycle is Q_2-3 and in Otto cycle it is Q_2'-3.

Efficiency_Otto = 1 - (Q_4-1 /Q_2'-3)

Efficiency_diesel = 1 - (Q_4-1 /Q_2-3)

From the T-s diagram, we can say area A-2-3-B represents heat transfer during heating in diesel cycle or Q_2-3 and area A-2'-3-B represents heat transfer during heating in Otto cycle or Q_2'-3. As the area A-2-3-B is larger than area A-2'-3-B, we can conclude,

Q_2-3 > Q_2'-3

Therefore,
(Q_4-1 /Q_2'-3) > (Q_4-1 /Q_2-3)
or, 1 - (Q_4-1 /Q_2'-3) < 1 - (Q_4-1 /Q_2-3)
or Efficiency_otto < Efficiency_diesel

FIRST MINOR TEST: IC ENGINES IN SGIT

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

THE CONCEPT OF VAPOUR LOCK IN IC ENGINES

VAPOUR LOCK

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.

IMPORTANT PROPERTIES OF SI ENGINE FUEL

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THE FUEL CHARACTERISTICS OF INTERNAL COMBUSTION ENGINE:

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

Book For a Trial Class:

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:

 

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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

1. ·         Cold and Hot starting
2. ·         Vapour Lock in fuel delivery system
3. ·         Short and Long trip economy
4. ·         Acceleration and Power
5. ·         Warm Up
6. ·         Hot Stalling
7. ·         Carburetor Icing
8. ·         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:

 

1. 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.
2. Chemical characteristics: Fuels with higher percentages of aromatic hydrocarbons or olefins tend to have lower resistance to detonation.
3. 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.
4. 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.
5. 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:

1. 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.
2. 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.
3. 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:

1. 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.
2. 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.
3. 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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

STRATIFIED CHARGE INTERNAL COMBUSTION ENGINE

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 - THEORY

Curves in Thermodynamics:

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 pvⁿ = 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 (Q_H). The area under process curve 3-4 (area 4-3-B-A-4) equals the heat rejected to a sink (Q_L). 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.

IC ENGINES: A CONCEPTUAL ANALYSIS

• INTRODUCTION: 

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 combustion chamber 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 300^o C to 500^o 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.

C_p = aT² + bT + k

C_v = cT² + dT + k'