19 March 2016

How Much Gasoline Octane Rates

One way to prevent detonation is to use a higher octane fuel. The octane rating of a motor fuel is a measure of its detonation resistance. The octane that's posted on the filling station pump is "pump octane," which is an average of something called "research" and "motor" octane ratings (which are two different laboratory methods of measuring octane). The higher the pump octane number, the better able the fuel is to resist detonation.

A gasoline's octane rating depends on the blend of hydrocarbons in the fuel and other ingredients that are added to it. Tetraethyl lead was long used as an anti-knock additive to improve gasoline octane. In fact, it was the most effective and least expensive octane-boosting additive that could be used for this purpose. But leaded fuel cannot be used in a vehicle with a catalytic converter because the lead fouls the catalyst. So unleaded fuels in the U.S. were phased out back in the early 1980s.

Most regular unleaded gasolines today are rated at 87 octane (or less in higher altitude locales), which is sufficient for engines with compression ratios of up to about 9 to 1, and up to 10 to 1 compression on some newer engines. Higher compression engines, engines with turbochargers or superchargers, or ones used frequently for towing usually require a higher octane premium grade of gasoline (91 octane).

Why octane rate must true for your engine

The term OCTANE refers to a fuel's ability to resist spark knock or detonation. The chemical properties of the fuel determine its combustion characteristics - the temperature and pressure at which it ignites and how quickly it burns. Higher octane fuels can withstand higher temperatures and pressures, and typically burn somewhat slower than lower octane fuels.

The least amount that's necessary to prevent detonation (spark knock). On most vehicles, that would be regular unleaded 87 octane gasoline. But on higher compression engines, or turbocharged or supercharged engines, the engine may require premium grade 91 to 93 octane fuel.

Detonation (also called spark knock) occurs when the octane rating of the fuel isn't high enough to handle the heat and pressure. Detonation is most noticeable when lugging the engine under load or accelerating. It may should like a pinging, clattering or rattling noise. Instead of a single flame front forming when the fuel is ignited, multiple flame front form spontaneously throughout the combustion chamber. These collide and produce shock waves that cause the noise. The hammer-like blows produced by detonation are very hard on the pistons, head gasket and bearings and may damage the engine if the problem is not corrected.

Other common causes of detonation (besides low octane gasoline) include: a faulty Exhaust Gas Recirculation (EGR) system, too much compression due to a build up of carbon deposits in the combustion chambers (use a top cleaner to clean the combustion chambers), a lean fuel condition (dirty fuel injectors), or engine overheating.

Most late model engines have a "knock" sensor to detect engine vibrations caused by detonation. When the sensor detects detonation, it signals the PCM to temporarily retard spark timing. This helps protect the engine from possible detonation damage, but it also reduces engine performance and fuel economy.

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