An indicator showing the ratio of various components in the product in question is the knock resistance of gasoline. This is covered in this article.
The concept of detonation
The latter occurs when the gasoline-air mixture ignites spontaneously in the part that is furthest away from the spark plug. Its burning is explosive.
Optimal conditions for its flow are formed in the part of the combustion chamber, in which there is an increased temperature and a large exposure of the mixture.
Knock can be identified by the characteristic metallic knocks that are formed due to the reflection of shock waves from the walls of the combustion chamber and the resulting vibration of the cylinders.
Knock combustion of gasoline can occur withmore likely if there is carbon deposits in the combustion chamber, as well as when the engine condition deteriorates. This phenomenon leads to a decrease in its power, a decrease in economic indicators, as well as toxicological indicators of exhaust gases.
Properties of gasolines that cause detonation
These include: fractional composition, sulfur content, stability from the physical and chemical points of view, the structure of hydrocarbons, etc.
The highest detonation resistance is typical for aromatic hydrocarbons, and the lowest - for normal paraffinic ones. Others, which are part of gasoline, occupy an intermediate position.
Evaluate the knock resistance of gasoline by octane number.
Ways to prevent detonation
It must be prevented at the time of engine operation, when the vehicle is moving, and therefore it becomes necessary to take urgent measures in order to prevent damage to the engine to the greatest extent. In addition, the efforts of designers should be directed to the development of the latter with a comprehensive counteraction to the phenomenon under consideration.
One of the main ways to prevent potential detonation is to produce gasoline with a sufficiently high knock resistance.
Determination of octane number
Above, we decided on what number determines the knock resistance of gasoline. Octane number (OC) is determined using a single-cylinderequipment with a dynamic compression ratio, using research or motor methods. When it is determined, the combustion of the studied gasoline and reference fuel with a known desired value is carried out. The composition of the latter includes heptane with RON=0 and isooctane with RON=100.
When testing, gasoline is poured into this equipment. When carrying out research, the compression ratio is gradually increased until detonation appears, after which the engine is refueled with a reference fuel with a preliminary measurement of detonation and fixing the compression ratio that led to it. The volume content of isooctane in the mixture determines the OC.
The name of the brand of gasoline may contain the letter "I". This indicates that the OC was determined by the research method. In case of its absence, the motor method was used. The SP obtained by different methods differ somewhat in their values. Therefore, the octane number for the knock resistance of gasoline must be accompanied by an indication of the method by which its value was determined.
The last value is determined with the motor method at nominal loads, and with the research method - at unsteady modes.
Besides these two methods, the road method can be used to determine the ROI. Mixtures containing normal heptane and isooctane are fed into the heated engine. The car is accelerated to the maximum possible speed in direct transmission and the ignition timing is adjusted until knocking disappears. After that, according to the same method, the ignition setting is determined,at which detonation starts. A base curve is built depending on the degree of the angle of rotation of the crankshaft, according to which the OC is determined.
In order to increase the OC of straight-run gasolines, they are subjected to catalytic reforming. How much they increase is determined by the rigidity of these regimes.
Thermal process gasolines are superior in knock resistance to straight-run ones.
The concept of increasing knock resistance
The above indicates that the latter must be increased in order to extend the life of the engine.
To increase the knock resistance of gasoline, special anti-knock additives are used. The octane number increases with an increase in the molar mass of hydrocarbons and the degree of branching of the carbon chain, as well as with the transformation of alkanes into alkenes, naphthenes and aromatic hydrocarbons having the same number of carbon atoms.
Ways to increase the indicator in question. Characteristics of ethyl gasolines
There are the following ways to improve the knock resistance of gasolines:
- Introduction of high-octane components;
- selection of raw materials and processing technology;
- Introduction of antiknocks.
Until recently, the main of the latter was tetraethyl lead (TEP), which is a poison in the form of a liquid, insoluble in water, but easily soluble in petroleum products.
However, lead as a productcombustion builds up in the combustion chamber, which increases engine compression. Therefore, together with TPPs, scavengers of this element are added to gasoline, which form volatile substances during combustion, which are removed with exhaust gases.
As the last substances, those containing halogens such as bromine or chlorine can be used. The mixture of scavenger with TES is called ethyl liquid. Gasolines in which it is used are called leaded. They are highly poisonous and their use must be accompanied by the use of enhanced security measures.
Over time, new requirements for the environmental friendliness of engines began to be introduced, which led to the transition to unleaded gasoline.
Characterization of safer anti-knock additives
Unleaded gasoline required a change in the production technology of this product and the use of anti-knock additives that would be distinguished by reduced toxicity.
The knock resistance of gasoline is evaluated, among other things, by the use of non-toxic anti-knock agents in the latter. Efficiency at the TPP level is shown by manganese substances, which are non-toxic liquids. However, they have found limited use as they reduce engine durability.
Methyl tert-butyl ether (MTBE) additive with physical and chemical properties similar to gasoline is considered promising. When it is added in an amount of 10% to the fuel, the octane number increases by 5-6 units.
For high-octane gasolinesuse an organic substance called cumene.
In addition, high-octane additives based on monohydric alcohols and isobutylene are used.
Ethers have found the greatest distribution in the production of clean gasoline.
Organic iron compounds, manganese based additives based on N-methyl-aniline, dewaxed raffinate are also used
In addition, tetramethyl lead (TMS) can be used instead of TPP in gasoline, which evaporates better and is more evenly distributed over the cylinders.
From the practice of using thermal power plants
Motorists with significant driving experience are familiar with "red candles". The color of candles in this color occurred when a pure anti-knock agent was added to low-octane gasoline instead of TPP with scavengers. This led to the lead of these devices. After that, it is no longer possible to repair and restore candles. Thus, the knock resistance of gasoline is characterized not by thoughtless, but by the correct use of antiknock agents specially designed for this purpose.
Leaded gasolines contribute to less wear on the camshaft cams compared to non-CHP gasolines. It is assumed that the products formed as a result of combustion fell through the oil to the surface, which protected it from wear. The latter also decreased in relation to other engine parts when using leaded gasolines.
Other fuel additives
To inhibit oxidative reactions, antioxidants are added to gasolineadditives, which can be wood-tar, which is a mixture of phenols with oils, paraoxyphenylamine and PF-16, which is a mixture of phenols.
To prevent carburetor icing, anti-icing additives are used. They are used as compounds that dissolve water and form low-freezing mixtures with it, as well as forming a shell on ice particles, preventing their growth and settling on the carburetor walls.
Various detergent additives can be used to remove deposits.
Factors influencing the indicator under consideration
Knock resistance of gasoline is evaluated not only by octane number. It is influenced by various factors.
Knock increases with increasing engine compression, increasing cylinder diameter, using cast iron pistons and heads. These factors are constructive.
Knock-enhancing performance features include an increase in engine load at a constant crankshaft speed, or a decrease in engine speed at a constant load with an increase in the ignition timing, a decrease in air humidity, an increase in the layer of soot in the combustion chamber and the combustion temperature of the coolant.
Besides this, detonation is caused by the influence of physical and chemical factors. The latter are due to the fact that the fuel is capable of forming peroxide compounds, which, when a certain concentration is reached, contribute to the formationof this phenomenon. The decomposition of these compounds proceeds quite quickly, while heat is released and a “cold” flame is formed, which, when propagated, saturates the mixture with peroxide decay products. They contain active centers, due to which a hot flame front appears.
The main physical factor is the compression ratio of the engine. It is directly proportional to the pressure and temperature in the combustion chamber. When critical values are reached, a portion of the working mixture ignites and burns out at an explosive speed.
Knock resistance of various engine types
High knock resistance of motor gasoline is typical for light-fuel engines. It ensures the normal combustion of these types of fuel in various engine operating modes. The process of detonation in this case was discussed above.
To ensure a normal duty cycle in diesel engines that operate by self-ignition from the compression of the working mixture, the knock resistance of the fuel must be low. For these engines, a characteristic such as "cetane number" is used, which shows the period of time from fuel entering the cylinder to the start of its combustion. The higher it is, the shorter the delay, the more smoothly the combustion of the fuel mixture is carried out.
Gasoline grade
In addition to the knock resistance of gasoline for aviation types of this fuel, the concept of grade is used. She isdemonstrates how much the power changes when a single-cylinder engine is running on a rich mixture on the studied fuel, compared with the power developed by the same engine on isooctane, the power of which is taken as 100 grade units or 100%.
In closing
The knock resistance of gasoline is a parameter that characterizes the ability of a given type of fuel to resist self-ignition during compression. It refers to the most important characteristics of any fuel, including for the type in question. For light fuel engines, it is determined through the octane number. In order to increase this indicator, high-octane additives are used, antiknock agents are introduced, raw materials are selected and technologies for its processing are developed.
