Diesel engines will have a compression ratio below __:__ to prevent diesel knock

• A. 15:1
• B. 25:1
• C. 30:1
• D. 20:1

Answer: (D)

Diesel engines ignite fuel by compressing the air–fuel mixture until it gets hot enough to ignite when the fuel is injected. The compression ratio directly affects how hot and high the pressure gets before combustion starts. If the compression ratio is too high, the mixture can reach auto-ignition temperatures too early, causing diesel knock—an abrupt, uncontrolled ignition that creates a loud ping and extra stress on the engine. To keep this knocking risk in check while maintaining reasonable efficiency, designers target a compression ratio that is low enough to avoid premature ignition. In this context, keeping the ratio below about 20:1 helps prevent diesel knock, whereas higher ratios (like 25:1 or 30:1) increase the likelihood of knocking. A ratio as low as 15:1 would reduce knock risk even more but at the cost of efficiency, so 20:1 is described as the practical upper bound for preventing knock.