AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO

AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO

CHEMICAL COMBUSTION OF FUEL

Subhankar Karmakar
Assistant Professor; SGIT
Jindal Nagar; Ghaziabad

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Chemical Combustion is basically a rapid oxidation process of hydro-carbon fuel inside thekjm combustion chamber in the presence of air. The oxidation of fuel is basically a Exothermic or heat liberating chemical process.

In SI engines generally we use volatile hydrocarbon as fuel. The intermixing of fuel with air takes place outside the engine and the device that prepares air-fuel mixture of required mixture strength is called CARBURETION and the device is known as CARBURETTOR.

Estimation of air quantity needed for complete combustion of a given fuel

We know that any chemical reaction can be represented by the corresponding chemical equation like
CH₄ + 2O₂ = CO₂ + 2H₂O
here molecular weight of CH₄
μ_CH4 = 12 + 4x1 = 16
μ_2O2 = 2x16x2 = 64
μ_CO2 = 12 + 2x16 = 44
μ_2H2O = 2x(2+16) = 36

For complete combustion of CH₄
16 kg CH₄ needs 64 kg O₂ therefore,
1 kg of CH₄ needs (64/16) = 4 kg of O₂
For 23 kg of O₂ air needed is 100 kg
hence, for 4 kg of O₂ air needed is (100/23)x4 = 17.39 kg of air.
Air-fuel ratio will be 17.39 : 1

The ratio of air fuel mixture, needed for the complete combustion of the fuel or the chemically correct ratio of air fuel mixture required for complete combustion of the fuel is called " Stoichiometric Air fuel mixture. "

If the amount of air in the air-fuel mixture is less than the chemically correct amount of air, then the mixture is called rich mixture, where as if the quantity of air is more than the chemically corrected amount of air it is called lean mixture.

The strength of air-fuel mixture has a profound influences on the process of combustion. The required mixture strength for different operation conditions are different.

Different Operating Conditions	Required air-fuel Mixture Strength
For Max. Efficiency	17 : 1, 16.4% weak
For Max. Power	12 : 1 17.8% rich
For Starting, Idling, & Low load running	11 : 1 ~ 16 : 1 very rich mixture
For accelerated motion	13 : 1 rich mixture
For Part Load running Cruising Range	17 : 1 Lean Mixture Strength

RECIPROCATING COMPRESSORS

Q.2) Classify the compressors
(i) On the basis of operations employed
(ii) On the basis of pressure achieved
(iii) On the basis of pressure ratio
(iv) On the basis of capacity of compressors.

A.2) Depending upon different parameters, compressors can be classified on the basis of operations employed, the delivery pressure achieved, pressure ratio and capacity of compressors as follows.

On the basis of operations employed, compressors are classified into two groups:
i) :Reciprocating compressors : It uses piston cylinder arrangement and due to positive displacement of air in the cylinder, the air is compressed and delivered to a vessel called Receiver. These are capable to produce high delivery pressure with low volume flow rate.
ii) Rotary Compressors : These compressors operate at high speeds, therefore, can handle large volume flow rates compared to reciprocating compressors.

In rotary compressors, the dynamic head is imparted to the gas with the help of very high speed impeller rotating at a confined space so that the air is compressed due to centrifugal action.

On the basis of delivery pressure, compressors are classified into three categories
i) Low Pressure Compressors : Delivery pressure upto 1.1 bar
ii) Medium Pressure Compressors : Delivery pressure upto 7 bar
iii) High Pressure Compressors : Delivery pressure between 7 to 10 bar.

On the basis of pressure ratio, we can classify the devices as follows,
Fans : Pressure ratio upto 1.1
Blower : Pressure ratio upto 1.1 to 4.0
Compressors : Pressure ratio above 4.0

On the basis of capacity, compressors can be classified as follows,
Low capacity compressors : Volume flow rate upto 10 m³/min, or less
Medium capacity compressors : Volume flow rate 10 m³/min to 300 m³/min
High capacity compressors : Volume flow rate above 300 m³/min

Q.3) Find an expression for required work done to drive a compressor, when compression is,
adiabatic in nature
isothermal compression
polytropic compression

A.3) During the analysis of the operations of a reciprocating air compressor, we consider some assumptions to simplify the analysis. They are as follows
i) There is no Clearence Volume
ii) Working substance air is an ideal gas
iii) There is no frictional loss.
iv) There is no wire drawing in the valve or pipe lines.

WORK REQUIRED TO DRIVE A COMPRESSOR

Suppose, we are running a single stage air compressor, which draws air at pressure P₁ and temperature T₁ during the suction or induction process. The air thus drawn inside the cylinder then compressed to achieve a delivery pressure, P₂ by adiabatic process. In the adjacent figure, process a - b is the suction process. Process b - c is the adiabatic compression of the air from pressure P₁ to pressure P₂. Process c - d is the delivery stroke, delivering the compressed air at a pressure P₂.

W_ad = P₂V₂ + {( P₂V₂ - P₁V₁)/(γ - 1)} - P₁V₁

=> (P₂V₂ - P₁V₁){1 + 1/(γ - 1)}

=> {γ /(γ - 1)}P₁V₁ {(P₂V₂)/(P₁V₁) - 1}

=> {γ /(γ - 1)}mRT₁{(P₂/P₁)^{{(γ - 1)/γ}} - 1}

POLYTROPIC WORKDONE IN RECIPROCATING COMPRESSORS

W_poly = P₂V₂ + {( P₂V₂ - P₁V₁)/(n - 1)} - P₁V₁

=> (P₂V₂ - P₁V₁){1 + 1/(n - 1)}

=> {n/(n - 1)}P₁V₁ {(P₂V₂)/(P₁V₁) - 1}

=> {n/(n - 1)}mRT₁{(P₂/P₁)^{{(n - 1)/n}} - 1}

ISOTHERMAL WORKDONE REQUIRED TO DRIVE RECIPROCATING COMPRESSOR

W_iso = P₂V₂ + {P₁V₁ln(V₁/V₂)} - P₁V₁

but as the process is isothermal, P₁V₁ = P₂V₂

W_iso = P₁V₁ln(V₁/V₂)

W_iso = mRT₁ln r_p

V₁/V₂ = P₂/P₁ = r_p (pressure ratio)

COMPRESSORS AND COMPRESSED AIR

COMPRESSORS:

Q.1) What is a Compressor? What is the difference between a Compressor and a Pump? What are the practical uses of Compressed Air?

A.1) A compressor is a device which is extensively used to raise the pressure of a compressible fluid like pure air. In a compressor, the pressure is increased at the expense of work done on the fluid, which is generally provided by an electric motor, IC engines or Gas Turbines. In a compressor, fluids are compressed by reducing the specific volumes of the working fluids. Due to compression, the temperature of the fluid is also increased.

If air is used as the working fluid in a compressor and air is compressed into a high pressure by the application of work on the fluid, then it is known as Air Compressor.

PRACTICAL USES OF COMPRESSED AIR

In industry, compressed air is so widely used that it is often regarded as the fourth utility, after electricity, natural gas and water. Compressed air, commonly called Industry's Fourth Utility, is air that is condensed and contained at a pressure that is greater than the atmosphere. The process takes a given mass of air, which occupies a given volume of space, and reduces it into a smaller space. In that space, greater air mass produces greater pressure. The pressure comes from this air trying to return to its original volume. It is used in many different manufacturing operations.

Compressed air is extensively used in industrial applications like pneumatic machines, as well as in the refrigeration and air-conditioning systems or supercharging CI engines to boost the output of the engine.
01) Compressed air is extensively used to operate pneumatic tools like drills, hammers, rivetting machines etc.
02) It is used to drive Compressed Air Engine.
03) Compressed air is used to spray painting.
04) Compressor is a vital component of Air-conditioning and Refrigeration industry.
05) Very often, compressed air can be used as a means of energy storage.
06) It is used in Gas Turbine power plants.
07) It is used to Super-charging an IC engines.
08) It is used to convey or pump to flow the materials like sand or concrete slurries along a pipe line.
09) It can be used as a means to pump water through the pipe lines.
10) It is used to drive minning machineries in a fire risky zone.
11) It is also used in blast furnaces.

Q.2) Classify the compressors
(i) On the basis of operations employed
(ii) On the basis of pressure achieved
(iii) On the basis of pressure ratio
(iv) On the basis of capacity of compressors.

A.2) Depending upon different parameters, compressors can be classified on the basis of operations employed, the delivery pressure achieved, pressure ratio and capacity of compressors as follows.

On the basis of operations employed, compressors are classified into two groups
i) Reciprocating compressors : It uses piston cylinder arrangement and due to positive displacement of air in the cylinder, the air is compressed and delivered to a vessel called Receiver. These are capable to produce high delivery pressure with low volume flow rate.
ii) Rotary Compressors : These compressors operate at high speeds, therefore, can handle large volume flow rates compared to reciprocating compressors.

In rotary compressors, the dynamic head is imparted to the gas with the help of very high speed impeller rotating at a confined space so that the air is compressed due to centrifugal action.

On the basis of delivery pressure, compressors are classified into three categories
i) Low Pressure Compressors : Delivery pressure upto 1.1 bar
ii) Medium Pressure Compressors : Delivery pressure upto 7 bar
iii) High Pressure Compressors : Delivery pressure between 7 to 10 bar.

On the basis of pressure ratio, we can classify the devices as follows,
Fans : Pressure ratio upto 1.1
Blower : Pressure ratio upto 1.1 to 4.0
Compressors : Pressure ratio above 4.0

On the basis of capacity, compressors can be classified as follows,
Low capacity compressors : Volume flow rate upto 10 m³/min, or less
Medium capacity compressors : Volume flow rate 10 m³/min to 300 m³/min
High capacity compressors : Volume flow rate above 300 m³/min

QUESTION PAPER: EME-505, IC ENGINES & COMPRESSORS

INTERNAL COMBUSTION ENGINE COOLING

COOLING:

In general, one can define cooling as the reduction of temperature of an object or system due to heat extraction from it.

NECESSITY OF COOLING IN IC ENGINES:

Where as the energy generated during combustion of fuel inside the cylinder is the source of heat input Q_f. Out of this energy, approximately one third part is converted into useful work, one third has been carried away by the hot exhaust gas. A part of the remaining one third fraction of the energy has been accounted for various losses including frictional power, where as the remaining portion of heat energy flows into cylinder, thus making it hotter. The cylinder itself radiates heat to air, but rate of heat leakage is very small compared to the heat energy the cylinder is receiving from hot gases. Therefore, the temperature of the cylinder will start to increase until it becomes so hot that

average temperature of the cylinder wall equals to the temperature of hot gases inside. As a result, the cylinder metal properties will suffer and it will soon have a mechanical failure.

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

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.

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'

STRATIFIED CHARGE INTERNAL COMBUSTION 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. 

ASSUMPTIONS CONSIDERED IN ANALYZING AIR STANDARD CYCLE:

AIR STANDARD CYCLE:

• In true sense, internal combustion engines in which combustion of fuels occurs inside the engine cylinder can not be defined as cyclic heat engines. The temperature generated during combustion is very high so that engines must be water cooled to prevent the damage of the engine due to thermal shock. The working fluid here is a mixture of air and fuel that undergoes permanent chemical changes due to combustion and the products of combustions must be exhausted and driven out of the cylinder so that fresh charges can be admitted. Therefore, it does not complete a full thermodynamic cycle.
• The engine cycle analysis is an important tool in the design and study of
• Internal Combustion Engines. 
•  A thermodynamic cycle is defined as a series of processes through which the working fluid progresses and ultimately return to the original state. 
•  Although the thermodynamic cycles are closed cycles and actual engine 
• A real thermodynamic analysis of such an engine quite complex. Hence, we simplified the operation of an I.C. Engine by introducing somewhat idealized version of a real thermodynamic processes occur inside an IC Engine, and this idealized thermodynamic cycles are called "Air standard cycle." In an air standard cycle, a certain mass of a perfect gas like air operates in a complete thermodynamic cycle, where heat is added and rejected reversibly with external heat reservoirs, and all the processes in the cycle are reversible. Air is assumed to behave like a perfect gas, and like a perfect gas, its specific heats are assumed to be constant (although they are certain functions of temperature). These air standard cycles are conceived in such a manner that they may correspond to the operations of internal combustion engines.
•  Although, there are numerous such air standard cycles, the important of them are

a) Otto Cycle (used for petrol engine)

b) Diesel Cycle (used for diesel engine)

c) Mixed, limited pressure or Dual Cycle (used for hot spot engine)

d) Stirling Cycle

e) Ericsson Cycle

To make the analysis simpler, certain assumptions are made during the analysis of air standard cycle. They are as following,

• i) The working substance is a perfect gas obeying the gas equation pV = mRT.
• ii) The working fluid is a fixed mass of air either contained in a closed system or flowing at a constant rate round a closed cycle.
• iii) The physical constants of the working fluid will be those of air.
• iv) The working medium has constant specific heats.
• v) The working media doesn't undergo any chemical change throughout the cycle.

OTTO CYCLE:

The Otto cycle is a thermodynamic cycle used in gasoline (petrol) engines to convert the chemical energy stored in fuel into useful work. It is a four-stroke cycle, consisting of four processes: intake, compression, combustion, and exhaust.

During the intake stroke, the fuel-air mixture is drawn into the engine cylinder as the piston moves downward. During the compression stroke, the mixture is compressed by the upward motion of the piston, which raises the temperature and pressure of the mixture. Near the end of the compression stroke, the spark plug ignites the mixture, causing a rapid combustion that generates a high-pressure wave that drives the piston downward, producing power. This is the power stroke. Finally, during the exhaust stroke, the spent gases are expelled from the cylinder as the piston moves upward.

The Otto cycle is an idealized model of the engine, assuming that the combustion occurs instantaneously and that there are no losses due to friction, heat transfer, or other factors. In practice, real engines operate less efficiently than the idealized model, due to these losses.

The Otto cycle is named after its inventor, Nikolaus Otto, a German engineer who patented the four-stroke engine in 1876. The cycle is widely used in modern gasoline engines, which have been refined and optimized over more than a century of development to achieve high levels of performance, efficiency, and reliability.

DIESEL CYCLE:

The Diesel cycle is a thermodynamic cycle used in diesel engines to convert the chemical energy stored in fuel into useful work. It is a four-stroke cycle, consisting of four processes: intake, compression, combustion, and exhaust.

During the intake stroke, air is drawn into the engine cylinder as the piston moves downward. During the compression stroke, the air is compressed by the upward motion of the piston, which raises the temperature and pressure of the air. Near the end of the compression stroke, fuel is injected into the cylinder, which ignites due to the high temperature and pressure of the air. The fuel-air mixture combusts, generating a high-pressure wave that drives the piston downward, producing power. This is the power stroke. Finally, during the exhaust stroke, the spent gases are expelled from the cylinder as the piston moves upward.

The Diesel cycle is similar to the Otto cycle but differs in that it does not rely on a spark plug to ignite the fuel. Instead, the fuel is injected directly into the cylinder and ignites due to the heat of the compressed air. This allows diesel engines to operate at a higher compression ratio than gasoline engines, which leads to higher efficiency and better fuel economy.

The Diesel cycle is named after Rudolf Diesel, a German inventor who patented the diesel engine in 1892. Diesel engines are widely used in a variety of applications, including cars, trucks, buses, ships, and generators. They are known for their efficiency, durability, and reliability.

Mixed, limited pressure or Dual Cycle (used for hot spot engine):

The Mixed or Dual Cycle is a thermodynamic cycle used in hot-spot engines, which are a type of internal combustion engine that combines elements of diesel and gasoline engines. The cycle is also sometimes referred to as the Limited Pressure cycle.

The Dual Cycle is a combination of the Otto and Diesel cycles. It uses the diesel combustion process, where fuel is injected directly into the cylinder and ignited by the heat of compressed air, but also includes a spark plug like in the Otto cycle. During the intake stroke, air is drawn into the cylinder, and during the compression stroke, the air is compressed to a higher pressure and temperature than in the Otto cycle. Fuel is injected into the cylinder, and the spark plug ignites the fuel-air mixture, creating a flame that spreads through the cylinder. The combustion of the fuel-air mixture produces high pressure and temperature, which drives the piston downward, producing power. Finally, during the exhaust stroke, the spent gases are expelled from the cylinder as the piston moves upward.

The Dual Cycle is designed to provide the advantages of both the diesel and gasoline engines, namely high efficiency and low emissions. It allows for a higher compression ratio than the Otto cycle, which leads to better fuel economy, while also reducing the emission of pollutants like nitrogen oxides (NOx) and particulate matter. The Dual Cycle is used in some specialized applications, such as large marine engines and certain military vehicles. However, it is not as widely used as the Otto and Diesel cycles in most everyday applications.

STERLING CYCLE:

The Stirling cycle is a thermodynamic cycle used in Stirling engines, which are a type of heat engine that converts heat energy into mechanical work. Unlike traditional internal combustion engines, Stirling engines operate on an external heat source, which can be supplied by any fuel source that can produce heat, such as wood, coal, or natural gas.

The Stirling cycle consists of four processes: heating, expansion, cooling, and compression. During the heating process, the working fluid (typically a gas such as helium or hydrogen) is heated by an external heat source, causing it to expand and drive a piston outward. During the expansion process, the expanding gas continues to drive the piston outward, producing mechanical work. During the cooling process, the working fluid is cooled by a heat sink (usually air or water), causing it to contract and pull the piston inward. Finally, during the compression process, the compressed gas is pushed back to the starting point, ready to begin the cycle again.

The Stirling cycle is designed to maximize efficiency by minimizing the losses associated with traditional internal combustion engines, such as friction and heat transfer. However, Stirling engines have a relatively low power-to-weight ratio and are less suitable for high-speed applications. They are typically used in specialized applications, such as in submarines, where quiet operation and long running times are important.

The Stirling engine was invented in the early 19th century by Robert Stirling, a Scottish clergyman, and engineer. Despite its potential benefits, the Stirling engine has not been widely adopted in mainstream applications due to its complexity and high cost compared to other types of engines. However, research and development continue to explore ways to improve the efficiency and practicality of Stirling engines.