Spark-Ignition vs. Diesel Engines — Core Differences Which statement best summarizes a fundamental difference of the Diesel (compression-ignition) engine compared with the spark-ignition engine?

Introduction / Context:
Internal-combustion engines are broadly divided into spark-ignition (SI) and compression-ignition (CI or Diesel). Understanding the thermodynamic and hardware differences is foundational for automotive engineering and service diagnostics.

Given Data / Assumptions:

• Conventional four-stroke SI and DI Diesel architectures.
• Modern fuels used (petrol for SI; diesel for CI).
• Generalized description without brand-specific exceptions.

Concept / Approach:
Diesels operate by compressing air to a high pressure and temperature. Near the end of compression, fuel is injected; atomized droplets mix with the hot air and ignite spontaneously. SI engines instead pre-mix air and fuel upstream and ignite the mixture via a timed electric spark. Therefore, several interrelated statements about air-only compression, autoignition due to temperature, and late fuel injection are simultaneously true for Diesels.

Step-by-Step Solution:
Check statement 1: Diesels compress air only → Correct.Check statement 2: Ignition source is hot compressed air → Correct.Check statement 3: Fuel is injected near T.D.C. on the compression stroke → Correct for typical DI Diesels.Thus all statements are valid → choose “all of the above”.

Verification / Alternative check:
Review of CI cycle shows high compression ratio (often 15–25) raising temperature above fuel autoignition threshold, with injection timing in degrees BTDC to ATDC around T.D.C.

Why Other Options Are Wrong:
Any single statement alone is incomplete; together they define the Diesel principle.“None of the above” contradicts established engine operation.

Common Pitfalls:
Assuming Diesels always use pre-chambers; many modern units are direct-injection.

Final Answer:
all of the above