These days, when the cost of repairing and replacing car parts has increased a lot, we can avoid injuries and imposing exorbitant costs by raising the level of our technical knowledge. In this article, we want to talk more about the octane number of gasoline and the harmful effects of knocking the engine and how to prevent it.
One of the cycles in the operation of four-stroke gasoline engines is called compression, which means the combination of gasoline and air by the piston, which is compressed to a certain extent, and after the piston reaches the highest possible point (dead point), this mixture is ignited by the spark plug and the piston is pressed. It drives down a lot. The amount of compression done is measured by a number called compression ratio, which is 5 to 1 in old engines and 12 to 1 in very advanced engines. When the fuel mixture, which is a highly flammable substance, is ignited earlier than sparking by the spark plug and due to compression, the engine will knock or knock, which is very harmful to the engine.
What is Knock?
When the fuel mixture explodes due to compression and high temperature, before sparking the spark plug and reaching the highest level of the pistons, the engine experiences a knock, which is very harmful to the engine. In this situation, the piston tries to rise, but due to the premature explosion of the fuel mixture, a strong force hits it from above, and as a result of this impact, the temperature of the car’s combustion chamber rises and puts double pressure on the valves, crankshaft and other engine components. will be This phenomenon is usually associated with a rattling sound, especially in the low speed and dead gears.
What is octane number?
The amount of fuel’s ability to condense before spontaneous combustion (spontaneous combustion) is determined by the octane number, which is a measure of fuel quality. The higher the number, the better the fuel.
When crude oil is refined in refineries, we will have hydrocarbon chains of different lengths, and these separate chains are combined together to produce different fuels such as gasoline, diesel, kerosene, etc. Meanwhile, heptane with 7 carbon atoms and octane with 8 carbon atoms are the main hydrocarbons that make up gasoline. Various experiments have shown that heptane has a very weak condensation property; But octane has a very high condensation ability. Therefore, if a gasoline has a very high amount of octane, it has a very good quality in terms of resistance to self-ignition. The nature of heptane is such that it will burn very quickly due to low density. Based on this, 2 hydrocarbons, which have almost similar boiling and evaporation points, have been selected as the basis for the self-combustion resistance of the fuel against compression. By giving the number 100 to octane and zero to heptane, a ratio called octane number is obtained. For example, gasoline with an octane number of 87 has a ratio of 87 octane to 13 heptane. But the production of fuel with this amount of octane will have very high costs. Currently, these two substances are not the only constituents of gasoline, and other substances and additives are responsible for raising the octane number, i.e. the resistance against auto-ignition or knocking of the engine. The heptane-octane ratio is used only to provide a comparative basis and as an ideal gasoline to determine the octane number.
RON (Research Of Octane Number) d
Research octane number is the most common type of parameter measurement in the world. In this method, the octane number is determined in a test engine with a variable compression ratio, which is measured with an engine speed of 600 RPM, and the compression ratio is variable during the test. This test checks the anti-knock property of the fuel and a higher number indicates more knocking.
The higher the compression ratio of the engine, the more likely the knocking phenomenon will occur, and one of the ways to increase the power and efficiency of car engines is to increase their compression ratio. The only way to increase the engine density is to increase the resistance of the fuel against self-combustion and prevent knocking. This issue is much more important in cars equipped with turbo and supercharger systems due to the density of air entering the engine and its higher temperature.
The octane number for each type of gasoline is checked at sea level. For example, first they test a car with 82 octane gasoline in an area at sea level and then in a place at an altitude of 915 meters above sea level with the same fuel that has the same conditions in terms of temperature and humidity.
In this case, the engine of this car will self-combust at an altitude of 915 meters, much later than at sea level conditions, and since the octane number declared by the manufacturer is always announced based on sea level, at an altitude of 915 meters, gasoline with octane 82 is similar to gasoline with 87 octane works and it self-combusts under higher pressure. The approximate formula for calculating the increase in altitude from sea level to 1800 meters is done with this formula: for every 300 meters of altitude increase, 1.4 octane number of gasoline is added, and at an altitude above 1800 meters, this amount is 2.5 numbers for every 300 meters. (The above numbers are based on RON octane number). Altitude is not the only atmospheric factor affecting octane, and humidity and air temperature are also effective factors in the change of gasoline self-combustion in the engine.
In winter, a lower octane is required than in summer, and a fuel in similar conditions in terms of humidity, temperature, and test location will self-combust later than in the hot season. Therefore, raising the octane number in summer and lowering it in winter is considered a good solution for using gasoline in the country.
Of course, the effect of altitude on octane is very high