The pressure ratio of an engine, which is the ratio of the highest pressure reached in the cylinder when the piston is at top dead center (TDC) to the pressure when the piston is at bottom dead center (BDC), is one of the main factors that determine whether knocking will occur. Higher pressure ratios increase the temperature and pressure in the combustion chamber, creating conditions that are more conducive to pre-ignition and knocking.
The specific pressure ratio at which knocking starts to occur depends on several factors, including:
1. Fuel Octane Rating: Octane rating is a measure of a fuel's resistance to knocking. Fuels with higher octane ratings are more resistant to knocking.
2. Engine Design: Some engines are more prone to knocking than others due to differences in design features such as combustion chamber shape, spark plug placement, and cooling system effectiveness.
3. Operating Conditions: Knocking is more likely to occur when the engine is operating under conditions of high load, high engine speed, or high ambient temperature.
In general, naturally aspirated (non-turbocharged) petrol engines typically have pressure ratios ranging from around 8:1 to 11:1 without experiencing knocking. Turbocharged engines, which operate at higher boost pressures, may have pressure ratios as high as 14:1 or more and rely on additional features like intercoolers to manage knocking.
However, using fuel with an appropriate octane rating for the specific engine is crucial to prevent knocking and ensure smooth engine operation.
