Understanding the 4-Stroke Diesel Engine Cycle

A 4-stroke diesel engine operates through a cycle involving four distinct piston movements or strokes: Intake, Compression, Power (or Expansion), and Exhaust. Each stroke involves the piston moving from one extreme position to the other (either Top Dead Center - TDC or Bottom Dead Center - BDC) while specific valves are open or closed.

The Compression Stroke in a 4-Stroke Diesel Engine

Let's focus on the compression stroke in a 4-stroke diesel engine. This stroke happens after the intake stroke and before the power stroke.

• What happens: The piston moves upwards, from Bottom Dead Center (BDC) towards Top Dead Center (TDC).
• Valves: Both the intake valve and the exhaust valve are closed during the compression stroke.
• What is compressed: During the intake stroke in a diesel engine, only fresh air is drawn into the combustion chamber. Therefore, during the compression stroke, this trapped volume of air is compressed into a much smaller volume.
• Result of compression: Compressing the air significantly increases its pressure and, crucially, its temperature. Diesel engines achieve very high compression ratios, leading to very high temperatures (often above 700°C or 1300°F). This high temperature is essential because diesel fuel is injected towards the end of the compression stroke and ignites spontaneously upon contact with the hot compressed air (compression ignition).

In summary, during the compression stroke in a 4-stroke diesel engine, the piston moves up with both valves closed, and only air is compressed.

Analyzing the Options

Let's look at the provided options in the context of the 4-stroke diesel engine cycle:

1. Exhaust gases come out.
   • This describes the Exhaust Stroke, where the piston moves up, and the exhaust valve is open to push burnt gases out of the cylinder. This is not the compression stroke.
2. The air-fuel mixture is compressed.
   • This describes the compression stroke in a petrol (gasoline) engine, not a diesel engine. In a petrol engine, an air-fuel mixture is drawn in during intake and then compressed. Ignition is initiated by a spark plug. In a diesel engine, only air is drawn in and compressed; fuel is injected later.
3. Air enters the combustion chamber.
   • This describes the Intake Stroke in a diesel engine, where the piston moves down, and the intake valve is open, allowing fresh air to be drawn into the cylinder. This is not the compression stroke.
4. Only air is compressed in the combustion chamber.
   • As explained above, during the compression stroke of a diesel engine, the piston moves up with both valves closed, compressing the air that was drawn in during the intake stroke. This statement accurately describes the compression stroke in a 4-stroke diesel engine.

Therefore, the correct description of what takes place during the compression stroke in a 4-stroke diesel engine is that only air is compressed in the combustion chamber.

Revision Table: 4-Stroke Diesel Engine Cycle

Additional Information on Diesel Engine Operation

• Compression Ratio: Diesel engines have much higher compression ratios (typically 14:1 to 25:1) compared to petrol engines (typically 8:1 to 12:1). This high compression is necessary to achieve the temperature required for compression ignition of diesel fuel.
• Fuel Injection: Fuel is injected directly into the combustion chamber as a fine spray towards the end of the compression stroke. The timing and amount of fuel injection are precisely controlled.
• Ignition: Ignition in a diesel engine is caused solely by the high temperature of the compressed air, unlike petrol engines which use a spark plug. This is why it's called a compression-ignition engine.
• Differences from Petrol Engines: The main difference in the cycle is what is drawn in and compressed. Petrol engines draw in and compress an air-fuel mixture. Diesel engines draw in and compress only air, injecting fuel separately.