Typical Colour of Diesel Engine Exhaust – Diagnostic Indication In routine operation, what exhaust colour is most commonly associated with a diesel engine running under heavy load with incomplete combustion (soot formation)?

Difficulty: Easy

Correct Answer: black

Explanation:


Introduction / Context:
Exhaust colour is a quick diagnostic for combustion quality and possible faults. Diesel engines, especially under transient or heavy load, can produce visible smoke whose colour points to root causes like over-fuelling, oil burning, or coolant ingress.


Given Data / Assumptions:

  • Compression-ignition diesel operating with varying loads.
  • No after-treatment (or it is ineffective during the observation).
  • Observation in normal ambient lighting.


Concept / Approach:

Black smoke indicates soot—carbonaceous particulate matter from incomplete combustion, often due to over-fuelling relative to available air, poor atomization, restricted air intake, or inadequate boost. Bluish smoke indicates oil burning (lubricating oil entering the combustion chamber), while persistent white smoke suggests unburned droplets (low temperature misfire) or coolant/steam in the exhaust. Violet is not a recognized diagnostic colour.


Step-by-Step Solution:

Link colour to cause: black → soot from rich or air-starved operation.Identify operating condition: heavy load or transients increase fuelling demand.Infer that air–fuel mixing/time may be insufficient → soot forms.Therefore, black is the typical colour under the stated condition.


Verification / Alternative check:

Opacity meters and particulate measurements correlate visible black smoke with high soot mass concentrations. Modern DPF systems capture soot, reducing visible black smoke under regulated operation.


Why Other Options Are Wrong:

White is more indicative of coolant ingress or cold misfire. Bluish signals lubricating oil burning. Violet is not standard. Colourless exhaust occurs when after-treatment is effective and combustion is complete, but the question asks the 'generally' seen colour linked with soot.


Common Pitfalls:

Assuming all diesel smoke is black; conditions and faults matter. Also, confusing condensation plume (white) on cold mornings with faults.


Final Answer:

black

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