![]() Knocking should not be confused with pre-ignition-they are two separate events with pre-ignition occurring before the combustion event. Effects of engine knocking range from inconsequential to completely destructive. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. In spark ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. Principles The problem: detonation, aka knocking The octane rating alone did not only determine the performance of the gasoline, but also the versatility of the fuel in handling a range of lean to rich operating conditions. The octane rating of aviation gasoline was extremely important in determining aero engine performance in the aircraft of World War II. Using gasoline with lower octane may cause engine knocking (pre-ignition). Therefore, high compressibility of the fuel matters mainly for gasoline engines. Gasoline engines rely on ignition of air and fuel compressed together as a mixture, which is ignited near the end of the compression stroke by electric spark plugs. ![]() In contrast, fuels with lower octane (but higher cetane numbers) are ideal for diesel engines because diesel engines (also called compression-ignition engines) do not compress the fuel, but rather compress only air and then inject fuel into the air that was heated by compression. Such higher power comes from the fuel's higher compression by the engine design, and not directly from the gasoline. In broad terms, fuels with a higher octane rating are used in higher-compression gasoline engines, which may yield higher power for these engines. Whether or not a higher octane fuel improves or impairs an engine's performance depends on the design of the engine. Octane rating does not relate directly to the power output or the energy content of the fuel per unit mass or volume, but simply indicates gasoline's capability against compression. The higher the octane number, the more compression the fuel can withstand before detonating. For the video game, see Hi-Octane.Īn octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating.
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