Fuel Injection in Compression-Ignition (Diesel) Engines – Why High Injection Pressure? Why must a diesel injector nozzle deliver fuel at sufficiently high pressure during the end of the compression stroke?

Introduction / Context:
Compression-ignition engines rely on direct injection of liquid fuel into very hot, high-pressure air. The injector must create the right spray characteristics so that mixing, vaporization, ignition, and combustion proceed quickly and uniformly, preventing smoke and knock while maximizing efficiency.

Given Data / Assumptions:

• Cylinder pressure is very high near top dead center (TDC) at the end of compression.
• Air temperature is above self-ignition temperature of the fuel.
• Air motion (swirl/squish) exists to enhance mixing.

Concept / Approach:

High injection pressure provides the momentum required to push liquid fuel into a high-pressure environment, producing fine atomization (small droplets) and sufficient penetration so that the spray interacts fully with the air core rather than forming wet walls. Properly designed nozzles balance penetration and dispersion to cover the combustion chamber volume while avoiding excessive wall impingement and soot formation.

Step-by-Step Solution:

Verification / Alternative check:

Modern common-rail systems use 1000–2500 bar precisely to achieve these outcomes, with multiple injections per cycle for noise and emissions control—evidence of the central role of high injection pressure.

Why Other Options Are Wrong:

Option E is incorrect: injection does not cool the air; rather, atomization and mixing prepare fuel for auto-ignition in the hot air.

Common Pitfalls:

Assuming ‘‘more penetration’’ is always better—excessive penetration causes wall wetting and smoke; the goal is adequate, well-distributed penetration and atomization.

Final Answer:

all of the above