MDE 8 End of Unit Quiz

The correct answer is:

B) To provide a visual indication of leakage between the shell and tube sides

Explanation:

In a tube-type heat exchanger, a tell-tale ring (or tell-tale hole) is used to provide an early warning of leaks between the shell side and the tube side of the heat exchanger. If there is a leak in one of the tubes or at the tube-to-tube sheet joint, the tell-tale ring allows any fluid that escapes to be visibly detected outside the heat exchanger, preventing cross-contamination between the fluids in the shell and tube sides. This early detection helps in maintaining the integrity of the heat exchanger and preventing more serious failures.

The other options are incorrect because:

• A) To indicate the position of the baffle plates within the heat exchanger: The tell-tale ring does not relate to the position of baffle plates; its function is to detect leaks.
• C) To ensure proper alignment of the tube bundle during assembly: The tell-tale ring is not used for alignment purposes; it is a leakage detection feature.
• D) To improve the flow of fluid through the heat exchanger and enhance heat transfer: The tell-tale ring does not affect fluid flow or heat transfer; its role is to provide a visual indication of leaks.

The correct answer is:

C) The measure of the fuel's ignition delay time and its ease of ignition

Explanation:

In diesel engine terms, the cetane number is a measure of a diesel fuel’s ignition quality. It indicates how quickly and easily the fuel ignites when injected into the combustion chamber. A higher cetane number means the fuel ignites more readily and has a shorter ignition delay, which contributes to smoother engine operation and better performance. This is crucial for efficient combustion in diesel engines, where quick ignition of the fuel is necessary for optimal engine function.

The other options are incorrect because:

• A) The measure of the fuel's ability to resist oxidation over time: This option describes fuel stability or oxidative stability, not the cetane number.
• B) The rating of the fuel's combustion efficiency under high pressure: While related to performance, cetane number specifically measures ignition quality rather than combustion efficiency under pressure.
• D) The standard for the fuel's lubricating properties and its effect on engine wear: This option refers to the lubricating properties of the fuel, which are not related to cetane number.

The correct answer is:

B) The process of removing exhaust gases from the cylinder and replacing them with fresh air

Explanation:

In diesel engines, "scavenging" refers to the process of clearing out the exhaust gases from the cylinder after the power stroke and replacing them with a fresh charge of air. This is crucial for maintaining efficient combustion in the subsequent cycle. Proper scavenging ensures that the engine can operate effectively, with minimal residual exhaust gases remaining in the cylinder, which can otherwise dilute the incoming charge and reduce engine efficiency.

The other options are incorrect because:

• A) The process of cooling the engine cylinders with water or air: This option is incorrect because it describes the cooling process, not scavenging.
• C) The method of injecting fuel into the combustion chamber at high pressure: This option is incorrect as it describes fuel injection, not scavenging.
• D) The technique of lubricating the engine's moving parts to reduce friction: This option is incorrect because it refers to lubrication, which is unrelated to scavenging.

The correct answer is:

A) The period during which both the intake and exhaust valves are open

Explanation:

In diesel engines, "overlap" refers to the period when both the intake and exhaust valves are simultaneously open. This typically occurs near the end of the exhaust stroke and the beginning of the intake stroke.

The purpose of valve overlap is to allow for better scavenging of exhaust gases from the cylinder and to ensure that a fresh charge of air (or air-fuel mixture, in some engines) enters the cylinder. The overlap helps to improve engine efficiency, particularly at higher speeds, by reducing the residual exhaust gases in the cylinder and allowing more air to be drawn in.

The other options are incorrect because:

• B) The overlap between the crankshaft and camshaft rotational cycles: This option is incorrect because overlap does not refer to the relationship between the crankshaft and camshaft rotations.
• C) The overlap in the timing of fuel injection and ignition: This is not the correct definition of overlap, as it refers to valve timing rather than fuel injection or ignition timing.
• D) The overlap between the compression and power strokes: This option is incorrect because compression and power strokes do not overlap; they are sequential events in the engine cycle.

The correct answer is:

B) The volume of air or fuel mixture that is displaced by the piston during one complete stroke

Explanation:

In a diesel engine, the swept volume (also known as displacement volume) refers to the volume of air or fuel mixture that is displaced by the piston as it moves from top dead centre (TDC) to bottom dead centre (BDC) during one complete stroke. It is the volume swept out by the piston in the cylinder as it travels through its stroke. The swept volume is important because it contributes to the total engine displacement, which affects the engine's power output and efficiency.

The other options are incorrect because:

• A) The volume of the combustion chamber at top dead centre (TDC): This describes the clearance volume, not the swept volume.
• C) The total volume of the engine's cylinders when they are filled with air or fuel: This would be the total cylinder volume including both swept and clearance volumes, not just the swept volume.
• D) The volume of exhaust gases expelled from the engine during the exhaust stroke: This describes the exhaust process, not the swept volume.

The correct answer is:

A) To release air or gas trapped inside the heat exchanger to prevent air locking

Explanation:

In a tube-type heat exchanger, a vent cock is used to release any air or gas that may become trapped inside the heat exchanger. Trapped air can cause air locking, which reduces the efficiency of heat transfer because air has a much lower thermal conductivity compared to liquids. By venting the air or gas, the vent cock ensures that the heat exchanger operates efficiently, allowing the fluid to flow freely through the tubes and improving overall heat transfer.

The other options are incorrect because:

• B) To control the flow rate of the cooling fluid through the tubes: This is typically done with valves or flow regulators, not a vent cock.
• C) To monitor the pressure inside the heat exchanger during operation: Pressure monitoring is done with gauges or sensors, not a vent cock.
• D) To drain the fluid from the heat exchanger during maintenance procedures: Draining is usually done through a drain valve, not a vent cock. The vent cock specifically addresses the removal of trapped air or gas.

The correct answer is:

B) The ratio of the volume of the cylinder when the piston is at bottom dead centre (BDC) to the volume when the piston is at top dead centre (TDC)

Explanation:

In a diesel engine, the compression ratio is defined as the ratio of the total cylinder volume when the piston is at bottom dead centre (BDC) to the clearance volume when the piston is at top dead centre (TDC). Mathematically, it is expressed as:

Compression Ratio=Total Cylinder Volume (BDC)/Clearance Volume (TDC)​

This ratio indicates how much the air-fuel mixture (or just air) is compressed before ignition. A higher compression ratio means more compression of the air-fuel mixture, which can lead to greater engine efficiency and power.

The other options are incorrect because:

• A) The ratio of the engine’s power output to its fuel consumption: This describes fuel efficiency or power-to-fuel ratio, not the compression ratio.
• C) The ratio of the swept volume to the clearance volume in the cylinder: This describes a different aspect of engine volume, but not the compression ratio. The compression ratio involves the total volume at BDC and the clearance volume at TDC.
• D) The ratio of the engine speed to the crankshaft speed: This refers to gear ratios or speed ratios, not the compression ratio.

B) To facilitate easy cleaning and maintenance of the heat exchanger

Explanation:

In a tube-type heat exchanger, a sliding tube plate allows for easier access to the tubes for cleaning and maintenance. The sliding feature enables the tube plate to move, which can help in removing the tube bundle for inspection or cleaning without having to dismantle the entire heat exchanger. This design improves the practicality of maintaining the heat exchanger and ensures that the system remains efficient over time.

The other options are incorrect because:

• A) To allow for thermal expansion and contraction of the tubes: While thermal expansion is a consideration in heat exchanger design, the sliding tube plate specifically aids in maintenance, not in managing thermal expansion.
• C) To support the tubes and prevent them from moving during operation: The primary function of the sliding tube plate is not to support the tubes but to facilitate maintenance. Tube support is generally handled by other components of the heat exchanger.
• D) To improve the heat transfer efficiency by increasing turbulence: The sliding tube plate does not affect the heat transfer efficiency or turbulence; its main role is to enable easier maintenance and cleaning.

The correct answer is:

B) The continuation of combustion after the fuel injection has ended

Explanation:

In diesel engines, "afterburning" refers to the undesirable condition where combustion continues in the cylinder even after the fuel injection has ended. This can occur when there is incomplete combustion during the normal combustion phase, causing the remaining fuel to burn later than intended. Afterburning can lead to higher exhaust temperatures, reduced engine efficiency, and potentially harmful effects on engine components due to excessive heat.

The other options are incorrect because:

• A) The process of burning unburned fuel in the exhaust system: This option is incorrect because afterburning occurs within the cylinder, not in the exhaust system. The burning of unburned fuel in the exhaust system is typically related to issues with the exhaust or catalytic converter.
• C) The burning of excess lubricating oil in the combustion chamber: This option is incorrect as it describes oil burning, not afterburning. While burning lubricating oil can lead to smoke and other issues, it is not the same as afterburning.
• D) The use of an auxiliary burner to heat the intake air before combustion: This option is incorrect because it describes a method for improving combustion efficiency by preheating the intake air, not afterburning.

The correct answer is:

B) The volume of the cylinder above the piston when it is at top dead centre (TDC)

Explanation:

In a diesel engine, the clearance volume is the volume of the cylinder that remains above the piston when it is at top dead centre (TDC). This volume is crucial because it represents the space where the air-fuel mixture (or just air, in the case of a diesel engine) is compressed before ignition. The clearance volume, together with the swept volume (the volume displaced by the piston during its stroke), determines the engine's compression ratio, which is a key factor in engine performance and efficiency.

The other options are incorrect because:

• A) The volume of air or fuel mixture displaced by the piston during one complete stroke: This describes the swept volume, not the clearance volume.
• C) The total volume of the engine's cylinders when they are filled with air or fuel: This includes both the clearance volume and the swept volume, not just the clearance volume.
• D) The volume of exhaust gases remaining in the cylinder after the exhaust stroke: This is not related to the clearance volume but rather to the exhaust process and residual gases.

B) To direct the flow of the fluid, increasing turbulence and improving heat transfer efficiency

Explanation:

In a tube-type heat exchanger, baffle plates are used to direct the flow of the fluid inside the shell side of the heat exchanger. They help create turbulence in the fluid flow, which enhances the heat transfer between the fluid and the tubes. By forcing the fluid to follow a specific path and making it flow across the tube bundle multiple times, baffle plates improve the heat exchanger's efficiency in transferring heat.

The other options are incorrect because:

• A) To support and stabilize the tubes within the heat exchanger: While tube support and stabilization are important, baffle plates primarily affect fluid flow and heat transfer rather than supporting the tubes.
• C) To reduce the overall size and weight of the heat exchanger: Baffle plates do not specifically aim to reduce the size or weight of the heat exchanger; their main role is to enhance heat transfer.
• D) To filter out impurities from the fluid passing through the heat exchanger: Baffle plates are not designed to filter impurities; their purpose is to improve the flow and heat transfer efficiency.

C) The measure of the fuel's resistance to flow, affecting its ability to be pumped and atomized

Explanation:

In diesel engine terms, viscosity refers to the measure of a fuel's resistance to flow. It indicates how thick or thin the fuel is and affects how easily it can be pumped and atomized in the fuel injection system. Higher viscosity means the fuel is thicker and more resistant to flow, which can impact fuel injection performance and atomization, potentially affecting combustion efficiency and engine performance.

The other options are incorrect because:

• A) The measure of the fuel's resistance to compression in the engine: This refers to the fuel's cetane number or compression characteristics, not its viscosity.
• B) The rate at which the fuel evaporates at different temperatures: This describes the fuel’s volatility, not its viscosity.
• D) The energy content of the fuel, indicating how much heat is released during combustion: This describes the calorific value of the fuel, not its viscosity.