End-of-compression pressure (petrol engine): The approximate cylinder pressure at the end of compression in a spark-ignition petrol engine is

Introduction / Context:
Estimating the pressure at the end of compression is useful for sanity checks on indicator diagrams and for diagnosing compression-related faults. Typical values depend on compression ratio, charge temperature, and valve timing.

Given Data / Assumptions:

• Conventional petrol engine with compression ratio roughly 8:1–11:1.
• Near-ambient intake conditions and throttled operation typical of SI.
• No boost or abnormal heating.

Concept / Approach:
A simplified adiabatic compression estimate from about 1 bar to end-of-compression yields pressures on the order of 10–12 bar for common SI ratios and polytropic indices between isothermal and adiabatic. Hence, 10 bar is a representative textbook value for “approximately” the end-of-compression pressure in SI engines.

Step-by-Step Solution:

Verification / Alternative check:
Compression testers for petrol engines often report pressures roughly 9–14 bar depending on engine and test conditions, aligning with the 10 bar nominal figure.

Why Other Options Are Wrong:

• 20–35 bar: More typical of higher compression diesels or boosted cases; beyond standard SI expectations.

Common Pitfalls:
Confusing cranking compression gauge readings with true thermodynamic end-of-compression under hot, running conditions; however both are of similar order for healthy SI engines.

Final Answer:
10 bar