A Review of the Effect of Engine Operating Conditions on Borderline Knock

The effects of engine operating conditions on the octane requirement and the resulting knock-limited output were studied on a single cylinder engine using production cylinder heads. A 4-valve cylinder head with port deactivation was used to study the effect of fuel octane, inlet air temperature, coolant temperature, air/fuel ratio, compression ratio and exhaust back pressure. The effect of the thermal environment was studied in more detail using separate cooling systems for the cylinder head and engine block on a 2-valve cylinder head.

The results of this study compared closely with results found in the literature even though the engine and/or operating conditions were quite different in many cases. Guidelines on the effect of various parameters on the octane requirement of an engine can be summarized as follows:

• Spark Advance: 1 ON/1° SA
• Intake Air Temperature: 1 ON/7 K
• Air/Fuel Ratio: peaks at 5% rich of stoichiometric, decreases by around 2 ON / air/fule ratio around peak
• Intake Pressure: 3-4 ON/10 kPa
• Compression Ratio: 5 ON/CR
• Exhaust Back Pressure: 1 ON/30 kPa
• Coolant Temperature: 1 ON/10 K (Similar effect of head and block temperature., Larger effect at low speed.)