How is octane rating determined

These engines rely on different ratings, known as cetane numbers. The main difference between whether or not one uses high or low octane gasoline is determined by the level of "knocking" ones engine experiences. Inside the engine, an air-fuel mixture is ignited by the spark plug.

While this is occurring, the piston is also moving upwards, compressing the air-fuel mixture and simultaneously increasing its temperature since temperature increases as pressure increases. With lower octane fuels, the air-fuel mixture can also ignite due to this compression. When this flame front collides with the flame front ignited by the spark plug, an audible "knock" is heard. When the air-fuel mixture within an engine ignites due to compression rather than from the spark plug it is known as "pre-ignition" [4] If the fuel combusts and burns before it is ignited by the spark plug, it burns incompletely.

The leftover fuel from this incomplete ignition causes residue to stick to the inside of the fuel chamber, eventually leading to the sounds from ones engine known as engine knock. For a helpful video explaining engine knock, click here. Generally, the owners manual that comes with new vehicles has manufacturers recommendations as to which octane rating a vehicle should use to prevent this harmful pre-ignition and knocking due to build-up.

This uncontrolled secondary combustion causes pressure in the cylinder to spike and causes the knock to occur. The competition between the intended controlled and unintended spontaneous combustion causes the energy from the burning fuel to disperse unevenly, which can cause damage and place high pressure on the engine's piston before it enters the power stroke the part of the cycle when the piston's motion is generating power.

Before electric computerized ignition was widely used, this knocking commonly occurred and could cause significant engine damage. Most modern engines have sensors to detect knocking. When detected, the computer delays the initial spark, which causes the controlled combustion to take place at a point when compression is not at its highest point.

Although this eliminates the knock, it can cause the engine to run less efficiently. A similar undesirable condition is called pre-ignition, when the fuel ignites on its own before the spark ignites it.

Modern engine computers minimize this condition by controlling the timing of valves and fuel injection; however, this control mechanism can also come with a fuel-efficiency or emissions penalty.

The standard means of testing octane is with an octane testing engine. This test is similar to the way the mass of an object can be determined by comparing it to objects references of known mass on a balance scale.

Primary Reference Fuels PRF of precisely known octane are formed by combining iso-octane, heptane, and other well-known standards such as toluene. These PRFs are used to bracket a given fuel sample to determine the pressure at which similar knock intensities are observed. To determine the RON, the fuel is tested under engine idle conditions with a low air temperature and slow engine speed.

To determine the MON the fuel is tested under the more stressful conditions of higher air temperature and engine speed. Current designs see image below allow the same engine to perform both tests. Despite this flexibility, many testers still prefer to use more than one machine with each specifically set up and calibrated to perform either RON or MON tests. Gasoline explained. What is energy? Units and calculators. Use of energy.

Energy and the environment. Also in What is energy? Forms of energy Sources of energy Laws of energy. Also in Units and calculators explained Units and calculators Energy conversion calculators British thermal units Btu Degree days. But because compression creates immense heat, a fuel must be able to withstand extreme temperatures, otherwise it can misfire prematurely.

An octane rating is simply a measure of how heat resistant a fuel is in order to prevent knocking. If you hear knocking, you could be using the wrong octane for your ride. Which octane rating should you use? You should always use at least the minimum octane rating recommended by your vehicle manufacturer.

Using a lower-octane fuel than required can cause knocking and will prevent your vehicle from meeting its stated fuel economy.