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Octane numbers measure a fuel’s resistance to detonation, with two primary rating systems (RON and MON) and a third average (AKI). Iso-octane is used as a benchmark, and higher compression engines require anti-knock fuel. Octane ratings do not indicate energy capacity, and high octane can refer to high compression, not necessarily more power.
An octane number is a designation that refers to the resistance to detonation of a particular fuel blend. There are two major octane rating systems, and a third is an average of the first two. Ratings are determined in test engines by first checking the knock characteristics of a fuel mixture and then comparing them to various iso-octane and heptane ratios. A fuel blend with an octane rating of 80 would have similar detonation characteristics to a blend of 80% iso-octane and 20% heptane. In some circumstances, a fuel may have a rating higher than 100, as some substances have better resistance to detonation than pure isooctane.
In a typical internal combustion engine, the fuel/air mixture is bound to ignite and burn in a controlled process known as deflagration. Detonation occurs when the air/fuel mixture ignites uncontrollably outside the flame front starting at the spark plug. The detonation process is sometimes referred to as knocking or pinging and can cause increased cylinder pressure and severe engine damage. Higher compression engines are both more efficient and more likely to cause detonation, so the fuel may need to have anti-knock characteristics.
To create a benchmark against which to measure fuels, iso-octane was chosen for its resistance to detonation. Because heptane increases the likelihood of detonation, this substance is typically mixed during testing. The two primary test methods for determining octane number are Research Octane Number (RON) and Engine Octane Number (MON). The RON of a fuel is determined by burning a fuel under controlled conditions in an engine that has variable compression at 600 revolutions per minute (RPM). Speeds above 900 rpm are used to achieve MON, which also uses a preheated air/fuel mixture and variable ignition timing.
The third type of octane number is the anti-knock index (AKI), which is the average of RON and MON for a particular fuel. Each of these numbers indicates how a fuel compares in resistance to detonation to a blend of iso-octane and heptane and can be useful in predicting how it will react in modern high-compression engines. Many engines have a recommended minimum octane rating for fuel, and anything lower than that can cause detonation and engine damage.
Octane ratings refer to anti-knock qualities only and do not necessarily indicate energy capacity or anything else. Despite its excellent antiknock properties, isooctane has a lower energy content than many other fuels. Horsepower that is often associated with high octane typically refers to high compression. Modern engines with high compression ratios tend to be more powerful and require higher octane fuel to avoid detonation. Even if the fuel itself has a relatively low energy content in relation to its volume, such as gasoline blended with ethanol, a high compression ratio engine can still produce outstanding horsepower.
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