Description-
The antiknock value of fuel is measured in octane number rating. The fuel iso-octane is highly resistant to knock. It is given an octane rating of 100. Another fuel normal heptane nocks very easily. It is given a rating of Zero.
Iso-octane and heptane are actually the reference fuels to test the ONR of unknown fuels. The fuel to be tested is used in an engine and the tolerance to knocking is noted. Now, the two reference fuels are mixed in varying proportions and used to run the engine under the same conditions. The mixture which gives the same knocking characteristics would identify the ONR of the fuel. There are two basic methods for testing the fuels-
1. Laboratory method-
In this method, the test is carried out on a special engine that has an adjustable head. In this engine, the compression ratio can be changed with the adjustable head. The fuel to be tested is used to operate the engine and the compression ratio is increased until a certain intensity of knocking is obtained. Now, the engine is operated by a mixture of iso-octane and heptane without changing the compression ratio. The proportion of iso-octane is decreased until the same intensity of knocking is noted.
2. Road test method-
One method, known as the Corporative Fuel Recharge modified union town road test, rates fuels for knocking intensity at wide-open throttle at various speeds. Octane is assigned by comparing the fuel being tested to the reference fuels of known octane values.
Another method is known as the modified Border line method rates. The fuel at very speeds and is considered to give more information on fuel performance. The test is carried out by running a car on the road at various speeds and then determining the amount of ignition spark advance, the fuel can tolerate at each speed without knocking. If the spark is advanced too much at any particular speed, the knock characteristics of the fuel are tested.