Fuels and Combustion MCQ Quiz - Objective Question with Answer for Fuels and Combustion - Download Free PDF

Explanation:

Crankcase in a Two-Stroke Petrol Engine:

• In a two-stroke petrol engine, the crankcase serves a critical function in the engine's operation. Unlike in four-stroke engines, where the crankcase primarily stores lubricating oil, in two-stroke engines, it plays a more active role in the engine's air-fuel mixture management.
• In a two-stroke engine, the engine completes a power cycle in two strokes of the piston (one revolution of the crankshaft). The crankcase in such an engine is used as a primary pumping chamber for the air-fuel mixture. This process is facilitated through the movement of the piston, which helps to draw the mixture into the crankcase and then transfer it to the combustion chamber.

In a two-stroke engine, the cycle is divided into two strokes: the compression stroke and the power stroke. During these strokes, the crankcase is used to manage the air-fuel mixture, which is crucial for the engine's operation. The following steps outline the process:

• As the piston moves upwards during the compression stroke, it creates a vacuum in the crankcase. This vacuum helps to draw the air-fuel mixture from the carburetor into the crankcase through the intake port.
• When the piston reaches the top of the compression stroke, the air-fuel mixture in the combustion chamber is compressed and ignited by the spark plug, causing a rapid expansion of gases and forcing the piston downwards during the power stroke.
• As the piston moves downwards, it compresses the air-fuel mixture in the crankcase, which is now ready to be transferred to the combustion chamber. This mixture is pushed through the transfer port into the combustion chamber as the piston uncovers the port towards the end of the power stroke.

Explanation:

Exhaust Stroke in SI Engines:

• In most spark ignition (SI) engines, the intake valve opens a few degrees before the top dead center (TDC) on the exhaust stroke to allow for better scavenging of exhaust gases. Scavenging refers to the process of clearing out exhaust gases from the combustion chamber to make room for the fresh air-fuel mixture. This overlap between the opening of the intake valve and the closing of the exhaust valve is known as "valve overlap." The main purpose of valve overlap is to enhance engine efficiency by ensuring that the combustion chamber is free of residual exhaust gases, which could otherwise dilute the fresh air-fuel mixture and adversely affect combustion.
• During the exhaust stroke, the piston moves upwards, pushing the burnt gases out of the combustion chamber through the exhaust valve. Towards the end of the exhaust stroke, the intake valve begins to open slightly before the piston reaches the TDC. This early opening of the intake valve creates a smoother transition between the exhaust and intake strokes, enabling the incoming air-fuel mixture to help push out the remaining exhaust gases. This process is crucial for maintaining the engine's efficiency and performance.

Advantages of Scavenging:

• Improved Combustion Efficiency: Removing exhaust gases ensures that the fresh air-fuel mixture is undiluted, leading to better combustion and higher power output.
• Reduced Emissions: Effective scavenging helps in reducing the amount of unburnt fuel and harmful emissions, making the engine more environmentally friendly.
• Smoother Operation: The overlap between the intake and exhaust valves minimizes abrupt changes in pressure within the combustion chamber, resulting in smoother engine operation.

Technical Insights:

• The degree of valve overlap is carefully designed based on the engine's operating conditions and intended performance characteristics. High-performance engines may have larger valve overlaps to maximize scavenging and airflow at high speeds, while engines designed for fuel efficiency may have smaller overlaps to minimize the risk of backflow (when exhaust gases re-enter the combustion chamber).

Additional Considerations:

• While valve overlap is beneficial for scavenging, excessive overlap can lead to issues such as backflow or loss of compression, particularly at low engine speeds. Therefore, engine designers must strike a balance between optimizing scavenging and minimizing potential drawbacks

Explanation:

Intake Valve Closing in Low-Speed and High-Speed Four-Stroke Petrol Engines

Definition: In a four-stroke petrol engine, the intake valve controls the entry of the air-fuel mixture into the cylinder during the intake stroke. The timing of the intake valve closing is critical for optimal engine performance, as it impacts the volumetric efficiency, power output, and fuel economy. The closing timing varies depending on whether the engine is operating at low or high speeds.

Correct Option Analysis:

The correct option is:

Option 4: The intake valve closes at 10° after Bottom Dead Center (BDC) for a low-speed engine and at 60° after Bottom Dead Center (BDC) for high-speed four-stroke petrol engines.

Explanation:

The intake valve closing timing is designed to maximize the efficiency of the air-fuel mixture entering the cylinder. Here's why the timings differ for low-speed and high-speed engines:

• Low-Speed Engine: In low-speed engines, the intake valve closes shortly after the piston reaches BDC during the intake stroke. This timing (10° after BDC) ensures that the cylinder is filled adequately with the air-fuel mixture before the piston starts its compression stroke. Since the piston moves relatively slowly, closing the intake valve too late might lead to backflow of the mixture into the intake manifold.
• High-Speed Engine: High-speed engines require more time for the air-fuel mixture to enter the cylinder due to the rapid movement of the piston. To compensate for the inertia of the incoming mixture, the intake valve remains open longer (up to 60° after BDC) even after the piston has started its compression stroke. This delayed closing allows more air-fuel mixture to enter the cylinder, enhancing volumetric efficiency and engine power output at high speeds.

Additional Information:

Importance of Valve Timing:

• Proper valve timing ensures efficient engine operation by optimizing the intake and exhaust processes.
• Incorrect valve timing can lead to reduced engine performance, lower fuel economy, and increased emissions.

Factors Affecting Valve Timing:

• Engine Speed: Higher speeds require adjustments in valve timing to account for the inertia of the air-fuel mixture and the rapid movement of engine components.
• Engine Design: The geometry and design of the intake and exhaust systems influence the ideal valve timing for an engine.
• Desired Performance Characteristics: Valve timing is tuned to achieve specific performance goals, such as maximizing power, improving fuel efficiency, or reducing emissions.

Explanation:

Battery Ignition Systems:

• Battery ignition systems rely on an external battery to provide the electrical energy required to create a spark at the spark plug. This system is commonly used in modern vehicles and offers consistent performance regardless of engine speed. The main components of a battery ignition system include a battery, ignition coil, distributor, and spark plugs.

Working Principle: The battery supplies electrical energy to the ignition coil, which steps up the voltage to a level sufficient to create a spark. The distributor then directs this high-voltage current to the appropriate spark plug at the correct time, igniting the air-fuel mixture in the combustion chamber.

Advantages:

• Consistent spark generation independent of engine speed.
• Reliable performance, especially at low engine speeds.
• Easy to maintain and diagnose issues due to the availability of electrical components.

Disadvantages:

• Reliance on the battery means the system can fail if the battery is discharged or faulty.
• Additional components such as the battery and charging system add weight and complexity.

Magneto Ignition Systems:

• Magneto ignition systems generate electrical energy on-demand using electromagnetic induction. This system is commonly used in small engines, such as those found in motorcycles, lawn mowers, and aircraft, due to its simplicity and reliability. The main components of a magneto ignition system include a magneto, ignition coil, and spark plugs.

Working Principle: The magneto consists of a rotating magnet that induces a high-voltage current in the ignition coil. This current is then directed to the spark plug, creating a spark to ignite the air-fuel mixture. The system generates power on-demand, eliminating the need for an external battery.

Advantages:

• Independence from an external battery, making it more reliable in remote or off-grid applications.
• Simpler and lighter design due to fewer components.
• Consistent performance at high engine speeds.

Disadvantages:

• Performance can be inconsistent at low engine speeds.
• Requires precise alignment and timing of components for optimal performance.

Explanation:

Combustion of Methane

Definition: The combustion of methane (CH₄) is a chemical reaction in which methane reacts with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O). This reaction is exothermic, releasing energy in the form of heat and light. The balanced chemical equation for the complete combustion of methane is as follows:

CH₄ + 2O₂ → CO₂ + 2H₂O

Calculation: To determine the minimum number of kilograms of oxygen needed for the complete combustion of 2 kilograms of methane, let us follow a step-by-step approach:

Thus, the molecular weight of methane is 16 g/mol and that of oxygen is 32 g/mol.

From the balanced chemical equation:

1 mole of CH₄ reacts with 2 moles of O₂.

This means that 16 g of CH₄ requires 2 × 32 = 64 g of O₂ for complete combustion.

The mass ratio of CH₄ to O₂ is:

16 g CH₄ : 64 g O₂ = 1 : 4

For 2 kg (2000 g) of CH₄, the amount of O₂ required is:

2000 g CH₄ × (4 g O₂ / 1 g CH₄) = 8000 g O₂

Converting grams to kilograms:

8000 g O₂ = 8 kg O₂

The minimum number of kilograms of oxygen needed for the complete combustion of 2 kilograms of methane is 8 kg.​

The correct answer is Ghee.

• Ghee has the highest per gram calorific value.
• Milk and egg mainly contain protein, rice is rich in carbohydrates while ghee is rich in fat.
• Calorific value is defined as the amount of energy generated by the combustion of a specific amount of food or fuel.
• Calorific value unit is joule per kilogram or calorie per kilogram.

Key Points

• Calorific values are the amount of heat that gets released after the complete combustion of a specific amount of a substance.
• Defined in terms of Joules/Kg, the higher the calorific value of a food component consumed by humans, the more energy gained by them.
• ‘Fat’ has a calorific value as high as 37 kJ/g.
• The following table gives the various substances and their calorific values ​

• Food component	Calorific Value
  Fat	37 kJ/g
  Carbohydrates	17 kJ/g
  Protein	17 kJ/g

The correct answer is Rapid combustion.

• Rapid combustion is a form of combustion in which large amounts of heat and light energy are released.
• This is used in a form of machinery, such as internal combustion engines, and thermobaric weapons.

Important Points

• Combustion or burning is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.
• Spontaneous burning is when a substance catches fire as soon as its temperature reaches the ignition temperature.
  • White Phosphorus catches fire at room temperature of 35°C.
• Explosion combustion reaction that takes place suddenly with the evolution of heat light and sound.
  • In this case, a large amount of gas is evolved which gets liberated.
  • When a cracker is ignited it suddenly reacts and bursts.
• Incomplete burning or combustion occurs when a combustion reaction occurs without a sufficient supply of oxygen.
  • It releases less energy than complete combustion and produces carbon monoxide (a poisonous gas).

Key Points

• The burning of LPG is an example of rapid combustion.
• Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available.

Concept:

The function of the fuel system in an IC engine is to transfer the fuel from the storage tank to the engine cylinder in proper quantity to meet the load and speed requirement of the engine.

Since the SI engine and CI engine have different types of fuel requirements at the time of injection in the cylinder, there is a considerable difference in the fuel supply of these engines.

Generally, 9 to 10 m³ of air is required for consuming 1 liter of fuel by a four-stroke engine.

Air - Fuel ratio for petrol engine:

• Starting: 4 to 6
• Idling: 11 to 12
• Normal working: 14 to 18
• Acceleration: 9 to 11

Concept:

Combustion: It is defined as the rapid chemical reaction of hydrogen and carbon in fuel with oxygen in the air. When the fuel and oxygen reacts, it releases energy in the form of heat.

Following conditions are necessary for combustion to take place:

• A combustible mixture
• Some means to initiates combustion
• Stabilization and propagation of flame in the combustion chamber

In SI engines, a carburettor generally supplies a combustible mixture and the electric spark from a spark plug initiates the combustion.

• The homogeneous mixture of vaporized fuel and air is compressed and ignited by the single intense, high temperature spark which initiates the flame kernel.
• Flame kernel generated by spark grows, and a turbulent flame propagates throughout the mixture until it reaches the combustion chamber walls.
• The flame speed is very low in non-turbulent (laminar) mixture. A turbulent motion of the mixture intensifies the processes of heat transfer and mixing of the burned and unburned portions in the flame front.

Concept:

Combustion:

• A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
• The substance that undergoes combustion is said to be combustible.
• It is also called fuel.
• The fuel may be solid, liquid or gas. 
• Combustion reactions are exothermic since heat is generated during the process.

There are 3 types of combustion- rapid, explosion, and spontaneous.

Rapid combustion:

• The type of combustion when gas burns rapidly and produces heat and light is known as rapid combustion.

Explosive combustion:

• Explosion combustion is that in which a huge amount of gases are evolved with the production of a large amount of light and sound.

Spontaneous combustion:

• Spontaneous combustion is that in which no external heat is provided.
• When heat released by a substance is higher than the heat required for ignition of the substance is known as Spontaneous combustion.

Explanation:

Cetane number
Cetane number (cetane rating) is an indicator of the combustion speed of diesel fuel and compression needed for ignition. It is an inverse of a similar octane rating for gasoline. The CN is an important factor in determining the quality of diesel fuel, but not the only one; other measurements of diesel’s quality include energy content, density,
lubricity, cold-flow properties and sulphur content.

Octane number
It is a measure to determine the burning quality of the gasoline. It has the tendency to resist knocking in an engine. The higher the octane number the lesser the tendency to knock.

Concept:

By Principle of conservation of mass we can have

m_a + m_f = m_e

where, m_a → mass flow rate of air into the cylinder, m_f → mass flow rate of fuel injected into the cylinder and m_e → mass flow rate of exhaust gas.

Calculation:

Given:

m_f = 1 g/s, air-fuel ratio → 20, therefore ma = 20 g/s

now, applying the conservation of mass we have

m_e = 20 + 1 = 21 g/s

Concept:

A catalytic converter is located within the exhaust system and converts harmful emissions like HC, CO, NOx produced by an IC engine to less harmful elements like H₂O (water), CO₂ (carbon dioxide), and N₂ (Nitrogen).

The term ‘three way’ is in relation to the regulated emissions the converter is designed to reduce:

• Unburnt hydrocarbons are oxidized into water/steam
• Carbon monoxide is oxidized into carbon Dioxide
• Oxides are converted into Nitrogen and Oxygen

Explanation: 

Pour point

The pour point in that temperatures just about which the oil sample will not flow under certain prescribed conditions. In other words, pour point of a liquid is the temperature below which the liquid loses its flow characteristics.

Important Points

• Flash Point: The flash point of a volatile material is the lowest temperature at which vapors of the material will ignite when given an ignition source.
• Fire Point: The fire point of a fuel is the lowest temperature at which the vapor of the fuel will continue to burn for at least 5 seconds after ignition by an open flame. The main difference in fire and flashpoint is that at the flashpoint a substance will ignite briefly, but vapor might not be produced at a rate to sustain the fire.
• Flashpoint and fire points are related to high-temperature characteristics of the fuel and tell the behavior of fuel at high temperatures.
• Cloud Point: Cloud point is the temperature at which oil becomes cloudy or hazy when oil is cooled at a specified rate.
• Pour Point: It is the temperature at which oil just ceases to flow. The pour point of the liquid is the lowest temperature at which it becomes semi-solid and loses its flow characteristics.
• Cloud point and pour point are related to low-temperature characteristics of the fuel and tells the behavior of fuel at low temperatures.

Concept:

In SI engine charge consists of air and fuel, and ignition takes place with the help of spark. When the mixture is lean the fuel will not burn by spark.

However, in case of CI engine the fuel is supplied in fine particles in the end of compression stroke, compressed air having high pressure and temperature and on coming in contact with this air it burns. Hence there is no problem of combustion of fuel even with the lean mixtures.

A mixture that contains just enough air for complete combustion of all the fuel in the mixture is called a chemically correct or stoichiometric fuel – air ratio.

A mixture having more fuel than that in a chemically correct mixture is termed as rich mixture and a mixture that contains less fuel (or excess air) is called a lean mixture.

For most of the hydrocarbon fuels, the stoichiometric air – fuel ratio is 15 : 1.