Compression ignition (Diesel) engine — typical pressure at the end of compression For a standard automotive Diesel (compression-ignition) engine in healthy condition, the cylinder pressure at the end of the compression stroke is generally of the order of:

Introduction / Context:
The question checks basic engine-thermodynamics awareness for compression-ignition (Diesel) engines. Knowing the typical end-of-compression pressure helps in diagnostics (compression testing) and in understanding why Diesel fuel auto-ignites without a spark plug.

Given Data / Assumptions:

• Light-duty automotive Diesel engine (not a large stationary or marine unit).
• Normal compression ratio in the range of about 16:1 to 22:1.
• Healthy engine with standard cranking speed and minimal leakage.

Concept / Approach:
End-of-compression pressure depends on compression ratio, inlet conditions, heat losses, and polytropic exponent. Typical service values for road Diesels fall in the tens of bar, substantially higher than spark-ignition engines, enabling auto-ignition at the appropriate temperature.

Step-by-Step Solution:
Recognise that CI engines need high pressure and temperature at the end of compression for auto-ignition.Practical workshop data for automotive Diesels commonly show end-of-compression pressures around the mid-30s bar at running conditions.Select the closest realistic figure among the choices: 35 bar.

Verification / Alternative check:
Compression test specifications from OEM service manuals typically report acceptable ranges whose central tendency is near 30–40 bar for light-duty Diesels. This corroborates the 35 bar order-of-magnitude choice.

Why Other Options Are Wrong:

• 15 bar: Too low for reliable CI operation in automotive engines.
• 25 bar: Possible in marginal cases but low as a general “order of magnitude.”
• 45 bar: Seen in some heavy-duty or high-CR cases; higher than the common automotive baseline.

Common Pitfalls:
Confusing peak firing pressure (hundreds of bar) with end-of-compression pressure before combustion; these are very different values.

Final Answer:
35 bar