A diesel engine, also known as a compression-ignition engine, operates on a principle distinct from petrol (gasoline) engines, particularly regarding what substance is compressed within the cylinder before combustion.

Diesel Engine Operation

In a diesel engine, the intake stroke draws in only fresh air. This air then undergoes a critical process: compression. Here is a breakdown of why and how air is compressed:

• Air Intake: During the intake stroke, the piston moves downwards, drawing a charge of pure, fresh air into the cylinder through the intake valve.
• Air Compression: As the piston moves upwards during the compression stroke, it compresses only this air to a very high pressure and consequently, a very high temperature. The compression ratio in a diesel engine is typically much higher than in a petrol engine (e.g., 14:1 to 25:1).
• Heat Generation: This intense compression causes the temperature of the air to rise significantly, often reaching $\text{500}^\circ\text{C}$ to $\text{600}^\circ\text{C}$ ($\text{930}^\circ\text{F}$ to $\text{1110}^\circ\text{F}$). This high temperature is crucial for the next step.
• Fuel Injection and Ignition: Once the air is highly compressed and heated, diesel fuel is injected into the combustion chamber as a fine spray. Because the air is already at a temperature above the auto-ignition point of diesel fuel, the fuel spontaneously ignites upon contact with the hot, compressed air. This process is called compression ignition.

The fundamental difference lies in the ignition method. A petrol engine compresses an air-fuel mixture and uses a spark plug for ignition, whereas a diesel engine compresses only air to generate the heat required for the fuel to self-ignite.

Analysis of Options

Let's examine why the other options are incorrect:

• Fuel: Fuel is not compressed on its own. Instead, it is injected into the cylinder after the air has been compressed and heated.
• Air and lubricating oil: While lubricating oil is present in the engine to reduce friction and cool parts, it is not intentionally compressed within the combustion chamber as part of the power cycle. Its purpose is lubrication, not combustion.
• Air and fuel: This mixture is what is compressed in a petrol (gasoline) engine. If an air-fuel mixture were compressed to the high ratios found in a diesel engine, it would likely auto-ignite prematurely (before the piston reaches top dead center), leading to engine knock or damage.
• Air: This is the correct answer. Only air is drawn into the cylinder and then highly compressed in a diesel engine. This compression generates the necessary heat for the subsequent auto-ignition of the injected diesel fuel.

Therefore, the substance that is compressed in a diesel engine is solely air.