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

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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.