An indicator showing the ratio of the various components in the product in question is the detonation resistance of gasoline. This is described in this article.
The concept of detonation
The latter occurs during self-ignition of a benzene-air mixture in the part that is most distant from the spark plug. Its burning is explosive.
The 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.
Detonation can be determined by the characteristic metal knocks, which are formed due to the reflection of shock waves from the walls of the combustion chamber and the resulting vibration of the cylinders.
Detonation combustion of gasoline may occur more likely in the presence of carbon deposits in the combustion chamber, as well as in the deterioration of the engine. This phenomenon leads to a decrease in its power, a decrease in economic indicators, as well as toxicological indicators of exhaust gases.
The properties of gasoline, causing the occurrence of detonation
These include: fractional composition, sulfur content, stability from a physical and chemical point of view, the structure of hydrocarbons, etc.
The highest detonation resistance is characteristic of aromatic hydrocarbons, and the lowest - for normal paraffinic. Other of them, which are part of gasoline, occupy an intermediate position.
Estimate the detonation resistance of gasoline with an octane rating.
Knock Prevention Methods
It should be prevented at the time of operation of the engine, then when the car is moving, and therefore there is a need for urgent measures to prevent damage to the engine to the greatest extent. In addition, the efforts of designers should be aimed at developing the latter with a comprehensive opposition to the phenomenon under consideration.
One of the main ways to prevent potential detonation is the release of gasoline with a detonation resistance quite high.
Octane Number Definition
Above, we decided on what number determines the detonation resistance of gasoline. The octane number (OC) is determined using single-cylinder equipment with a dynamic compression ratio, using research or motor methods. When determining it, the test gasoline and reference fuel with a known desired value are burned. The latter consists of heptane with OR = 0 and isooctane with OR = 100.
When tested, gasoline is poured into this equipment. During research, the degree of compression is gradually increased until detonation appears, after which the engine is refueled with standard fuel with a preliminary measurement of detonation and fixation of the degree of compression that led to it. The volumetric content of isooctane in the mixture is determined by OCh.
The name of the gasoline brand may contain the letter “I”. This indicates that the OR was determined by the research method. In case of its absence, the motor method was used. The HF obtained by different methods differ somewhat in value. Therefore, the octane number for the detonation resistance of gasoline must be accompanied by an indication of the method by which its value was determined.
The latter value is determined by the motor method at rated loads, and by the research method under unsteady conditions.
In addition to these two methods, the road method can be used to determine PF. Mixtures containing normal heptane and isooctane are fed into a heated engine. The car is accelerated to the maximum possible speed in direct transmission and the ignition timing is adjusted until detonation disappears. Then, 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 crankshaft rotation angle, from which the OR is determined.
In order to increase the pure-run gasoline pure-base gasoline, they undergo catalytic reforming. How much they grow is determined by the rigidity of these modes.
Gasoline thermal processes in detonation resistance superior straight run.
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 detonation resistance of gasoline, special antiknock additives are used. The octane number increases with increasing molar mass of hydrocarbons and the degree of branching of the carbon chain, as well as with the conversion of alkanes to 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 increase the detonation resistance of gasoline:
- input of high octane components;
- selection of raw materials and processing technologies;
- the introduction of antiknock.
Until recently, the main of the latter was tetraethyl lead (TPP), which is a poison in the form of a liquid, insoluble in water, but easily soluble in petroleum products.
However, lead as a combustion product accumulates in the combustion chamber, which increases engine compression. Therefore, together with TPPs gasoline is added to the scavengers of this element, which form volatile substances during combustion, which are removed with exhaust gases.
As the latter substances, those containing halogens such as bromine or chlorine can be used. The mixture of scavenger with TPP is called ethyl fluid. The gasoline in which it is used is called leaded. They are very toxic, 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 gasolines.
Description of safer antiknock additives
Unleaded gasolines demanded to change the production technology of this product and apply antiknock additives that would be distinguished by reduced toxicity.
The detonation resistance of gasoline is evaluated, including the use of non-toxic antiknock in the latter. Efficiency at the TPP level is shown by manganese substances, which are non-toxic liquids. However, they have found limited use, since they reduce the durability of the engine.
The perspective is considered to be methyl tert-butyl ether (MTBE) with physicochemical properties similar to gasoline. When it is added in an amount of 10% to the fuel, the octane number increases by 5-6 units.
For high-octane gasolines, an organic substance called cumene is used.
In addition, high-octane additives based on monohydric alcohols and isobutylene are used.
The most widespread in the production of pure gasoline are esters.
Also used are iron-containing organic compounds, additives based on manganese organics, based on N-methyl-aniline, dewaxed raffinate
In addition, tetramethyl lead (TMS) can be used in gasolines instead of thermal power plants, which evaporates better and is more evenly distributed over the cylinders.
From the practice of using TPPs
Motorists with significant driving experience are familiar with the “red candles”. Candles were painted in this color when pure antiknock was added to low-octane gasoline instead of thermal power plants with scavengers. This led to the lead of these devices. After that, it is already impossible to repair and restore the candles. Thus, the detonation resistance of gasoline is characterized not by thoughtless, but by the correct use of antiknock agents specially designed for this.
Leaded gasolines contribute to less cam wear on camshafts compared to using gasoline without a thermal power plant. It is believed that the products resulting from combustion fell through the oil to the surface, which protected it from wear. The latter was reduced in relation to other engine parts when using leaded gasolines.
Other fuel additives
To inhibit oxidative reactions, antioxidant additives are introduced into gasolines, which can be wood-tar, which is a mixture of phenols with oils, paraoxyphenylamine and FC-16, which is a mixture of phenols.
To prevent carburetor icing, anti-icing additives are used. Compounds that dissolve water and form low-freezing mixtures with it, as well as form a shell on ice particles that impede their growth and sedimentation on carburetor walls, are used as them.
Various detergents can be used to remove deposits.
Factors affecting the indicator in question
The detonation resistance of gasoline is estimated not only by the octane number. It is influenced by various factors.
Detonation is enhanced by increasing the compression ratio of the engine, increasing the diameter of the cylinder, using pistons and cast iron heads. These factors are constructive.
The operational properties that enhance detonation include an increase in engine load at a constant speed of the crankshaft, or a decrease in speed at a constant load with an increase in the ignition timing, a decrease in air humidity, an increase in the carbon layer in the combustion chamber and the temperature of combustion of the coolant.
In addition , detonation is due to the influence of physical and chemical factors. The latter are due to the fact that the fuel is able to form peroxide compounds, which, when a certain concentration is reached, contribute to the formation of 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 the products of decay of peroxides. They contain active centers, due to which a hot flame front arises.
The main physical factor is the compression ratio of the engine. Pressure and temperature in the combustion chamber are directly proportional to it. Upon reaching critical values, a portion of the working mixture ignites and burns with the speed of the explosion.
Knock resistance of various types of engines
High detonation resistance of gasoline is characteristic for light-fuel engines. It provides the normal combustion of these types of fuel in various modes of engine operation. The process of the occurrence of detonation in this case was considered above.
To ensure a normal duty cycle in diesel engines that operate due to self-ignition from compression of the working mixture, the detonation resistance of the fuel should be low. For these engines, a characteristic such as "cetane number" is used, which shows the period of time from the ingress of fuel into the cylinder before it begins to burn. The higher it is, the less the delay, the more calmly the combustion of the fuel mixture.
Gasoline grade
In addition to the detonation resistance of gasoline, the concept of grade is used for aviation types of this fuel. It demonstrates how much the power changes when a single-cylinder engine runs on an enriched mixture of the test fuel, compared with the power developed by the same engine on isooctane, whose power is taken as 100 units of grade or 100%.
Finally
The detonation resistance of gasoline is a parameter by which a characteristic of the ability of a given type of fuel to withstand self-ignition under compression occurs. It refers to the most important characteristics of any fuel, including for the species 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.