Calculation method for octane number of gasoline

Standard gasoline is composed of isooctane (2,2,4-trimethylpentane) and n-hexane. Isooctane has good blast resistance, with a specified blast resistance of 100; The anti explosion performance of n-heptane is poor, and its octane rating is set to 0. Mix isooctane and n-hexane in different volume ratios to form standard fuels with different octane values. If the fuel contains 50% iso octane, its octane number is 50% x 100+50% x 0=50.

Determination of octane number of unknown gasoline

When determining the octane number of a gasoline to be tested, the gasoline can be tested under specified conditions using a standard engine. By comparing it with a standard fuel, a standard fuel with the same anti knock properties as the gasoline can be found. The volume percentage of iso octane in the standard fuel (i.e. octane number) is the octane number of the gasoline to be tested.

Basis for selecting gasoline grades

The octane rating of gasoline, also known as the octane number of gasoline, is an evaluation index for the anti knock performance of gasoline. The higher the compression ratio of the engine, the higher the grade of gasoline selected, and the slower the combustion speed of gasoline, the lower the possibility of detonation.

Low grade gasoline has a fast combustion rate and a delayed ignition angle; High grade fuel burns slowly and the ignition angle should be advanced. High compression ratio engines use low-grade gasoline and need to adjust the ignition time, otherwise the engine will produce detonation, but it will lead to increased fuel consumption and power loss. According to driving tests, using gasoline with lower grades in the engine will increase fuel consumption by about 3%, and also cause an increase in carbon buildup in the engine cylinder and fuel injectors, leading to an increase in the failure rate of the car. Meanwhile, due to insufficient combustion of gasoline in the engine, exhaust emissions deteriorate.

In modern car engine computer programs, adaptive programs for micro adjustment are set for gasoline with poor explosiveness, while there is no corresponding program for high-grade gasoline. So if a low compression ratio engine uses high octane gasoline, it not only causes economic waste, but also leads to slow ignition and long combustion time, resulting in insufficient conversion of combustion heat energy into power and weak acceleration during driving. Its high anti knock advantage cannot be fully utilized, and the high-temperature exhaust gas may burn out the exhaust valve due to the high temperature of the combustion gas.

The relationship between gasoline grade and purity

The high or low octane rating of gasoline only indicates its octane value, reflecting its anti knock performance, which is related to its composition. High branched isomers of alkanes, alkenes, and aromatic hydrocarbons have higher octane numbers, but they are not directly related to the purity of gasoline.

Research octane number (RON)

The anti knock performance of gasoline is measured under the following operating conditions: intake temperature of 51.7 ℃, cooling water temperature of 100 ℃, engine speed of 600r/min, and ignition advance angle of 13 °. The experiment is conducted at a lower temperature of the mixture (usually without heating). The octane number measured by research methods represents the anti knock performance of gasoline when driving at low speeds with frequent acceleration. Research octane numbers are commonly used in the United States and Western European countries, with high-quality gasoline generally having a research octane number of 96-100 and regular gasoline having a octane number of 90-95.

Motor octane number (MON)

It is the anti knock performance of gasoline when the engine operates under the following conditions: intake temperature of 149 ℃, cooling water temperature of 100 ℃, engine speed of 900r/min, and ignition advance angle of 14 °~26 ° before top dead center. The measurement is carried out at a higher temperature of the mixed gas. Representing the anti knock performance of gasoline during high-speed or long-distance driving under heavy load conditions. The conditions stipulated by the motor method are stricter than those of the research method, so the measured octane number is relatively low. When the octane number of the same fuel oil measured by the motor method is 85, it is equivalent to a research octane number of 92; When the motor method is 90, the research method is 97. The current gas stations in our country also use research octane ratings.

Road octane number

Also known as driving octane rating, it is measured using a car or simulated on a full power test bench to determine the driving conditions of the car on the road. The road octane number can also be calculated using empirical formulas for motor octane number and research octane number.

During the use of gasoline engines, the operation is very complex due to various reasons such as road conditions, and any method of measuring octane number is a highly conditional simplified test that does not represent the actual usage of the car. The severity of conditions represented by different standards varies, resulting in different measured values.

Anti explosion index of gasoline

Both motor octane number and research octane number cannot fully reflect the anti knock performance of combustion during vehicle operation. Therefore, an empirical formula for calculating the anti knock performance of vehicles during operation is proposed:

Explosion resistance index=K1 • RON+K2 • MON+K3

K1, K2, and K3 are coefficients with different values for different vehicles, which are related to the operating characteristics and conditions of the engine. They are all determined through typical road tests. The general simplified formula is based on the average anti explosion performance of the total number of vehicles. Generally, K1＝0.5，K2＝0.5，K3＝0。 The formula for the blast resistance index is:

Explosion resistance index=(RON+MON)/2