Combustion Characteristics – Effect of Higher Compression Ratio in Spark-Ignition Engines Increasing the compression ratio in a petrol (spark-ignition) engine primarily tends to cause which of the following effects on combustion behavior?

Introduction / Context:
Compression ratio strongly influences temperature and pressure at the end of compression in spark-ignition (SI) engines. This has multiple, sometimes competing, effects on ignition delay, flame speed, and knock propensity. Understanding these effects drives fuel octane selection and limits for efficiency gains.

Given Data / Assumptions:

• Typical SI engine operating near stoichiometric mixture.
• Higher compression ratio raises end-of-compression temperature and pressure.
• Fuel octane rating and hot spots in the chamber affect abnormal combustion behavior.

Concept / Approach:

As compression ratio increases, the unburned end-gas becomes hotter and more reactive. This can shorten the ignition delay and quicken normal flame propagation, but it also moves the end-gas closer to auto-ignition conditions, raising the risk of detonation. Local overheated parts (deposits, plug electrodes) may trigger pre-ignition when the mixture ignites before the spark, a serious fault exacerbated by higher temperatures associated with high compression ratios.

Step-by-Step Solution:

Verification / Alternative check:

Octane requirement increases with compression ratio. Engines designed for high CR rely on combustion-chamber shaping, charge motion, knock sensors, and cooled EGR to avoid abnormal combustion while reaping efficiency gains.

Why Other Options Are Wrong:

Picking only one effect ignores that all three are plausible outcomes under different conditions. ‘‘None of these’’ is incorrect because the listed effects are well documented.

Common Pitfalls:

Confusing pre-ignition with detonation: pre-ignition occurs before the spark; detonation is end-gas auto-ignition after normal flame starts. Both are encouraged by elevated temperatures caused by high CR.

Final Answer:

any one of these