MDE 8 Q1

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 increase in volume of gases after combustion, driving the piston downward

Explanation:

In a diesel engine, "expansion" refers to the phase in the engine cycle where the high-pressure gases generated by combustion expand, increasing in volume and forcing the piston downward. This is known as the power stroke. During this phase, the energy from the combustion is converted into mechanical work, which drives the crankshaft and ultimately powers the engine.

The other options are incorrect because:

• A) The process of air intake into the cylinder before compression: This option is incorrect because it describes the intake stroke, not expansion. Expansion occurs after combustion, not before compression.
• C) The process of releasing exhaust gases through the exhaust valve: This option is incorrect as it describes the exhaust stroke, not the expansion process. Expansion happens during the power stroke when the gases are pushing the piston down.
• D) The method of increasing the engine's displacement by modifying the cylinder size: This option is incorrect because it refers to a physical modification of the engine, not the expansion process within the engine cycle. Expansion relates to the behavior of gases during the power stroke, not changes in engine displacement.

The correct answer is:

B) The reduction in volume of air or air-fuel mixture in the cylinder before ignition

Explanation:

In a diesel engine, "compression" refers to the process of compressing the air (or air-fuel mixture in some engines) in the cylinder by the upward movement of the piston during the compression stroke. This compression significantly increases the temperature and pressure of the air within the cylinder, making it possible to ignite the fuel when it is injected. Diesel engines rely on this high compression to achieve auto-ignition of the fuel, as they do not use spark plugs like gasoline engines.

The other options are incorrect because:

• A) The process of mixing air and fuel before combustion: This option is incorrect because it describes the preparation of the air-fuel mixture, not compression. In a diesel engine, fuel is typically injected directly into the compressed air.
• C) The method of cooling the engine's cylinders using compressed air: This option is incorrect as it confuses compression with a cooling process. Compression is about increasing pressure, not cooling.
• D) The process of exhausting combustion gases from the cylinder through the exhaust valve: This option is incorrect because it describes the exhaust process, which occurs after combustion, not the compression of air before ignition.

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 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.