Relative ignitability — diesel versus petrol fuels: Considering volatility and ignition requirements, diesel fuel as compared to petrol (gasoline) is generally ________ to ignite under spark-ignition conditions.

Introduction / Context:
Ignitability depends on volatility, auto-ignition characteristics, and intended ignition method. Petrol is formulated for spark ignition with high volatility and high octane (resistance to auto-ignition), whereas diesel is formulated for compression ignition with lower volatility and high cetane (short ignition delay at high temperature).

Given Data / Assumptions:

• Comparison is under spark-ignition conditions (low-pressure electrical spark in a premixed environment).
• Petrol vaporizes readily at ambient intake conditions; diesel does not vaporize as easily.
• Diesel ignition in practice relies on high temperature from compression, not on a low-energy spark.

Concept / Approach:
For a spark to ignite a mixture, sufficient vapor must be present near the electrodes. Petrol’s high volatility produces ample vapor; diesel’s lower volatility and higher boiling range make it harder to form a combustible vapor cloud at typical SI intake conditions. Thus, diesel is more difficult to ignite via a spark, though it auto-ignites readily in hot compressed air (its intended mode).

Step-by-Step Solution:

Verification / Alternative check:
Misfueling experiences show petrol engines run poorly or not at all on diesel due to poor vaporization and plug fouling, confirming low spark-ignited ignitability for diesel.

Why Other Options Are Wrong:

• Highly/less difficult to ignite: Contradicts volatility and intended ignition mode.
• None: A clear comparative statement exists.

Common Pitfalls:
Confusing auto-ignition ease in a hot diesel cylinder (high) with spark-ignition ease at ambient intake conditions (low for diesel).

Final Answer:
more difficult to ignite