The Influence of Engine Design on Spark Knock at Low Engine Speeds

Abstract

Although spark knock at low engine speeds is unlikely to cause engine damage it is unacceptable to the car driver and must therefore be avoided. The degree of, or freedom from, knock may be influenced by both gasoline formulation and engine design. An analysis of the octane requirements of some 30 European manual transmission car models revealed that, apart from compression ratio and ignition advance, the features of engine design which affect low speed antiknock requirements significantly are mixture velocity through the carburettor and the type of intake manifold heating employed. When considering mass produced engines of modern multi-cylinder design, factors such as cylinder head, combustion chamber and camshaft design have little influence upon engine anti-knock requirements under low speed, accelerating, conditions. To achieve optimum utilization of currently available gasoline quality, the calculated mean mixture velocity should be at least 75 ft/s through the carburettor venturi and 45 ft/s through the carburettor barrel. That such conditions are already employed in some designs is proof of their practicability.