Lean-mixture operability — which engine type tolerates very lean operation? Identify the engine class that can operate satisfactorily with very lean overall air–fuel mixtures due to its combustion mode and excess-air capability.

Introduction / Context:
Lean-burn capability influences fuel economy, emissions, and combustion stability. Engine types differ in how lean they can operate based on how the mixture is prepared and ignited. Understanding this helps in selecting and calibrating engines for efficiency and emissions targets.

Given Data / Assumptions:

• Spark-ignition (SI) engines ignite a premixed, largely homogeneous charge with a spark.
• Compression-ignition (CI, diesel) engines inject fuel into hot air, burning in a diffusion-controlled, heterogeneous manner with high excess air.
• Focus on overall equivalence ratio, not local stratification details.

Concept / Approach:
Diesel engines routinely operate with large excess air (overall lean conditions). Combustion occurs where injected fuel mixes to locally combustible proportions, so the overall mixture can remain very lean without misfire. SI engines require the overall mixture near the plug to be within flammability limits; too-lean mixtures lead to misfire and unstable combustion, constraining lean operation unless advanced stratification and ignition strategies are used.

Step-by-Step Solution:

Verification / Alternative check:
Diesel engines at light load inject small fuel quantities into essentially full cylinders of air (lambda often > 2), demonstrating robust operation at very lean overall equivalence ratios.

Why Other Options Are Wrong:

• Spark ignition: Limited by flame kernel development and misfire at lean extremes without special measures.
• Rotary or gas turbine only: Not a general statement; gas turbines also run lean overall but the question targets reciprocating engine classes; CI is the standard answer.

Common Pitfalls:
Confusing local mixture near the spark with overall mixture; SI stratified-charge systems can be lean overall but still require careful control to avoid misfire.

Final Answer:
compression ignition