MDE 8 Q4

B) The point where the piston is at its lowest position in the cylinder

Explanation:

In a diesel engine, bottom dead centre (BDC) refers to the position where the piston reaches its lowest point in the cylinder during its cycle. This is the end of the power stroke and the start of the intake stroke. At BDC, the volume of the cylinder is at its maximum, and the piston is furthest away from the cylinder head. This position is crucial for understanding the engine's cycle, including where the intake and exhaust strokes occur.

The other options are incorrect because:

• A) The point where the piston is at its highest position in the cylinder: This describes top dead centre (TDC), not BDC.
• C) The position of the crankshaft when the engine is at maximum speed: BDC refers to the piston's position, not the crankshaft's position or engine speed.
• D) The moment when the engine reaches its maximum torque output: BDC is related to the piston's position, not the engine's torque output.

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.

The correct answer is:

B) The point where the piston is at its highest position in the cylinder

Explanation:

In the context of a diesel engine, top dead centre (TDC) refers to the position where the piston reaches its highest point in the cylinder during its cycle. This is a critical position in the engine's operation, as it typically marks the point at which the compression stroke ends and the power stroke begins. At TDC, the piston is closest to the cylinder head, and the air-fuel mixture (or just air in a diesel engine) is fully compressed and ready for ignition.

The other options are incorrect because:

• A) The point where the piston is at its lowest position in the cylinder: This describes the bottom dead centre (BDC), not TDC.
• C) The position of the crankshaft when the engine is in a state of maximum power output: TDC refers to the piston's position, not the crankshaft's position related to power output.
• D) The angle at which the fuel injection occurs during the compression stroke: While fuel injection timing is important, TDC specifically refers to the piston's highest position in the cylinder, not the fuel injection angle.

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:

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.

A) The distance the piston moves from the top dead centre (TDC) to the bottom dead centre (BDC)

Explanation:

In a diesel engine, the piston stroke refers to the distance the piston travels within the cylinder between its highest position (top dead centre, TDC) and its lowest position (bottom dead centre, BDC). This stroke is crucial as it determines the engine's displacement and affects the engine's power and efficiency. The length of the piston stroke, along with the bore (cylinder diameter), helps define the engine's total swept volume.

The other options are incorrect because:

• B) The amount of time the piston spends in the combustion chamber during each cycle: This describes the duration of each stroke or the timing of the engine cycle, not the distance traveled.
• C) The force exerted by the piston on the crankshaft during the power stroke: This describes the force or power generated, not the piston stroke.
• D) The angle of the crankshaft when the piston reaches its highest position in the cylinder: This describes the crankshaft angle at TDC, not the distance the piston travels (stroke).