Automotive definitions and terms

Octane number

Octane number - an indicator characterizing the detonation resistance of fuels for internal combustion engines. The octane number is numerically equal to the content (in% by volume) of isooctane (the octane number of which is taken as 100) in its mixture with n-heptane (the octane number is 0), at which this mixture is equivalent in detonation resistance to the fuel in question. The higher the octane number, the higher the detonation resistance of the fuel. The octane number of the most common domestic brands of gasoline 76-93 (determined by various methods), aviation gasoline 91-95.

In the CIS countries, motor gasolines are designated by the letter "A" and figures corresponding to the octane number. For high-octane gasoline it is determined by the research method, as evidenced by the letter "I" in the designation. Abroad, the results of the measurement by the motor method are designated as MON, and the research - RON. The methods differ only in the test conditions: in motor temperature and speed higher, therefore the octane number determined by this method is lower than that obtained by the research method. For example, according to the motor method, the ОЧ Аи-98 would be equal to 88.

Gasoline, obtained from oil by simple distillation (such gasoline is called straight-run), has a low octane number - in the range of 41-56, so now such gasoline is not used. To improve the octane number, more modern methods of oil refining are used. Also, to raise the octane number, high-octane components are also introduced into gasoline.

Among car owners widespread misconception that "pour liters of the twenty-98th" - a thing is periodically very useful. In fact, the use of fuel with OCH above the recommended does not make any practical sense. On models with relatively simple engines (without a detonation sensor and an automatic correction of the ignition), this does not give any advantages. Many modern cars are designed for various types of fuel with an OCH from 91 to 95 or even up to 98, and on high-octane gasoline their power and economy increase by a maximum of 5%. Alas, the consumer practically has no opportunity to check the quality of the fuel being poured, so that there is no sense in giving any advice on the choice. Here, the only criterion can be only the reputation of a particular gas station.

Compression ratio

The degree of compression is the ratio of the volume of the piston cylinder over-piston space at the position of the piston at the bottom dead center to the volume of the cylinder over-piston space when the piston is at the top dead center (TDC), ie. to the volume of the combustion chamber. Increasing the compression ratio requires the use of fuel with a higher octane number to avoid detonation.


Power - is the ratio of work to the time interval of its commission. Usually measured in the "Horsepower" (1 hp = 736 watts).


MAF сенсор или ДМРВ

MAF sensor or DMRV Mass air flow sensor at the inlet to the engine. Designed to determine the amount of air entering the engine.

The air flow sensor readings are one of the basic parameters used by the engine control unit to calculate the required amount of fuel and the optimum ignition timing. The sensor is installed after the air filter in front of the throttle in the flow of air consumed by the engine.

The signal from the DMRB can be emulated. In other words, if it is difficult for you to purchase a DMR vehicle, then you can use the MAF emulator.


The sensor of absolute pressure in the intake manifold (MAP - Sensor)

  MAP sensor or absolute pressure sensor Almost all engine systems that do not use an air flow sensor are equipped with an absolute pressure sensor in the intake manifold (or vacuum sensor).

  MAP sensor

In such systems, based on data on the pressure and temperature of the air in the intake manifold, the engine control unit calculates the air mass contained in each cubic centimeter of the intake manifold. In accordance with the calculated mass of air consumed by the engine, the ECU generates control pulses of fuel injectors of the appropriate duration, achieving an optimum fuel mixture by these preparations.

Oxygen sensor

 oxygen sensor The oxygen sensor is a lambda probe. The Lambda probe is an oxygen sensor. Designed to determine and regulate the concentration of oxygen in the exhaust gases, the composition of which depends on the ratio of fuel and air in the mixture supplied to the cylinders of the engine. Information, which the sensor gives out, is used by the electronic injection control unit to adjust the amount of fuel supplied. For the complete combustion of 1 kg of fuel, 14.7 kg of air is needed. Such a composition of the fuel-air mixture is called stoichiometric, it ensures the lowest content of toxic substances in the exhaust gases and, accordingly, their effective "afterburning" in the catalytic converter.

Composition of exhaust gases depending on lambda

To evaluate the composition of the fuel-air mixture, the excess air factor is used-the ratio of the amount of air entering the cylinders to the amount of air theoretically necessary for complete combustion of the fuel. In world practice, this coefficient is called lambda.


With a stoichiometric mixture of Lambda = 1, if lambda <1 (lack of air), the mixture is called rich, with a lambda> 1 (excess air), the mixture is called poor. The greatest economy with a fully open throttle valve of the gasoline engine is achieved with a lambda = 1.1-1.3. The maximum power is provided when the lambda = 0.85-0.9.


Fuel mixture

The composition of the mixture depends on the completeness, speed and temperature of combustion of fuel, as well as many other factors that determine the power of the motor. The composition of the mixture is characterized by the coefficient of excess air "lambda".

To achieve the most tangible results when tuning the engine, you can use the graph of the dependence of power and fuel consumption on the composition of the mixture. The graph below is only the key to understanding the process, since the characteristics of the various motors can differ significantly.

The dependence of engine power (P) and fuel consumption (b) on the excess air factor

 Dependence of engine power and fuel consumption on the excess air factor

Examples of the fuel mixture under various engine operating conditions:


Зависимость мощности двигателя и расхода топлива от коэффициента избытка воздуха

 - Cold start. With a cold start and to facilitate the start-up of a cold engine, additional fuel is required at the time of starting (a <1);
- warming up the engine. Due to the fact that at low temperatures the mixture is deteriorated, a fuel film is formed in the intake pipe, which evaporates only when high temperatures are reached. Therefore, at low temperatures, the fuel-air mixture must be enriched (a <1). In engines equipped with a catalytic converter, in the temperature range from + 15 to + 40 ° C the fuel-air mixture is depleted (a> 1). This is done specifically to quickly warm the catalyst to operating temperatures;
- partial loads. For engines equipped with a catalytic converter, at partial loads, the stoichiometric composition of the air-fuel mixture must be exactly maintained (a = 1). For engines without a catalyst, the main criterion for optimizing the fuel-air mixture is the minimum fuel consumption (a = 1.05-1.2);
Full load. With the throttle completely open, the engine must reach its highest torque or maximum power. For this, the fuel-air mixture should be enriched to a = 0.8-0.9;
Acceleration. With the rapid opening of the throttle valve, the composition of the air-fuel mixture is briefly impoverished due to the limited ability of the fuel to evaporate as the pressure in the intake pipe rises. Therefore, to prevent this phenomenon and to achieve good overclocking characteristics of the car, the air-fuel mixture must be enriched (a <1);
- Forced idling. In this mode, the car slows down, moving by inertia. In order to save fuel in a certain range of engine speed, the fuel supply can be completely stopped.


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