Octane number is a measure of a gasoline's ability to resist knocking as it burns in the combustion chamber. There are two laboratory test methods to measure the octane number of gasoline. One yields the Research octane number (RON); the other, the Motor octane number (MON). RON correlates best with low speed, mild temperature knocking conditions; MON correlates best with high speed and high temperature knocking conditions.
For a given gasoline, RON is always greater than MON. The difference between the two is called the sensitivity of the gasoline. Actually, sensitivity is affected by the gasoline chemical composition chemistry. Typically, paraffins possess near zero sensitivity. On the other hand, olefins, naphthenes and aromatics show sensitivity ranging between 8 and 20. This can be explained by the autoignition chemistry of each hydrocarbon group present in gasoline. MON and RON for different hydrocarbon groups in gasoline can be seen in Table 1.
Table 1: MON and RON for different hydrocarbon groups in gasoline
Hydrocarbon Group
Chemical Compound
RON
MON
Sensitivity
Paraffins
n-Pentane
62
62
0
2-MethylButane
92
90
2
Olefins
1-Pentene
91
77
14
2-Methyl-2-Butane
97
85
12
Naphthenes
Cyclopentane
101
85
16
MethylCyclopentane
91
80
11
Aromatics
Benzene
99
91
8
Toluene
124
112
12
Since vehicle testing is more involved than laboratory testing (single cylinder), there have been a number of attempts to predict antiknock performance in multi-cylinder engines. The resulting value is called Antiknock Index (AKI) which is equal to [(RON+MON)/2]. AKI is commonly used in the US and Canada.
European standards for gasoline octane are 95 RON and 85 MON. This translates into an AKI of 90. In the US the industry standards for AKI are 87 (regular), 89 (mid), 91+ (premium). The World Wide Fuel Charter, which is represented by the global automobile manufacturers, specifies more than one antiknock rating. The recommended RON/MON for the three grades is 91/82.5, 95/85, and 98/88.
Knocking is the sound of abnormal combustion. It is also known as spark knock. Normal combustion in a spark ignition internal combustion engine is initiated by a spark. The flame front fans out from the spark plug and travels across the combustion chamber rapidly and smoothly until almost all the fuel is consumed. When combustion is abnormal, the last part of the unburned mixture ignites spontaneously (auto-ignites) and burns very quickly, causing the pressure in the cylinder to rise rapidly. This creates the characteristic knocking sound. In fact, knock occurs because the gasoline's octane number is below the antiknock requirement of the engine at that moment.
Generally, brand new engines will not knock when operated on a gasoline with the RON recommended by the automobile manufacturer. However, an engine's octane number requirement can increase during the first several thousand kilometers. This effect, called Octane Requirement Increase (ORI), is caused by the buildup of carbonaceous deposits in the fuel injector, intake valve, and the engine's combustion chamber.
