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Question

Which factor most influences the completeness of combustion in a diesel engine?

This question was previously asked in
RRB ALP 2025 CBT 2 Mechanic Motor Vehicle Question Paper (28-Jul-2026) (Shift 1)
The correct answer is

The mixing of air and fuel

The principle governing diesel combustion is that fuel and air must meet intimately, because diesel fuel is injected into hot compressed air very late in the compression stroke.

This leaves only a very short time for the fuel to find oxygen and burn before the piston moves down.

For every droplet to burn fully, the fuel must be finely atomised and thoroughly mixed with the swirling air; where mixing is poor, fuel is left unburnt and the engine smokes.

So the quality of air and fuel mixing is the factor that most directly governs how complete the combustion is.

Octane number rates a petrol's resistance to knock, whereas diesel is graded by cetane number, so octane does not govern diesel combustion at all.

High-voltage frequency and battery condition matter to a spark-ignition system, but a diesel ignites by compression heat with no spark, so neither of those options applies.

Hence, the answer is The mixing of air and fuel.

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