Diesel (Compression-Ignition) Intake Process – What Is Drawn In During Suction? During the suction stroke of a conventional diesel engine, which working fluid is inducted through the intake valve into the cylinder?

Introduction / Context:
Compression-ignition engines differ fundamentally from spark-ignition engines in the way fuel and air enter the cylinder. Recognizing what is drawn in during the suction stroke is critical for understanding diesel combustion, emissions formation, and turbocharging strategies.

Given Data / Assumptions:

• Conventional four-stroke diesel engine with direct fuel injection.
• Fuel injection occurs near the end of the compression stroke.
• EGR may be present in small fractions but does not replace the fundamental intake of fresh air.

Concept / Approach:

In diesels, only air is inducted during the intake stroke. The piston then compresses this air to a high temperature and pressure. Near top dead center, injectors spray atomized fuel directly into the hot, compressed air, where it auto-ignites. There is no premixed air–fuel charge drawn in during suction, unlike carburetted petrol engines.

Step-by-Step Solution:

Verification / Alternative check:

Cycle diagrams and timing charts confirm that fuel injection is a compression event, not an intake event. Any EGR is mixed with the incoming air but fresh air remains the primary inducted medium.

Why Other Options Are Wrong:

Diesel only or a diesel–air premix is incorrect; diesels do not intake premixed fuel. Exhaust-only statements do not describe a working induction process.

Common Pitfalls:

Confusing diesel operation with spark-ignition or with two-stroke gasoline port-injection strategies. Also, assuming EGR replaces air rather than being a small, controlled fraction of the intake mixture.

Final Answer:

air only