Today we will talk about such a phenomenon rarely discussed by ordinary people as octane and cetane numbers in fuel. What is it compared to one another? What is the difference? Where is octane used, and where is cetane? And do I need to know?

Octane number

Let's start with the basics, the simplest and most common indicator, the octane number. It is used as a gasoline rating scale. Octane number, as most likely heard, is an indicator that characterizes the knock resistance of gasoline, that is, the ability of the fuel to resist spontaneous ignition during compression .

What does this mean? Gasoline with a higher octane rating can be compressed to higher atmospheres ( high pressure), while the air-fuel mixture does not ignite ahead of time. In other words, you can draw such a conclusion under one of those numbers that we usually see at gas stations: 92, 95, 98 - this is an indicator of how much you can compress the air-fuel mixture before the gasoline in it catches fire.

C₈H₁₈ Octane layout

If premature ignition of the fuel occurs, before an electric spark slips through in a given period of time, we will get the so-called detonation, an extremely harmful phenomenon for any motor, which engineers have long learned to deal with. As you can see, including with the help of fuel.

By the way, did you know that the scale of alkanes, which includes octane, extends from 0, which corresponds to heptane (C₇H₁₆) to 100 units, which corresponds to octane (C₈H₁₈)? Now you know, or at least remembered. Here is such a small digression into the chemical jungle of the school curriculum.

What else is useful high octane gasoline, other than the absence or minimal presence of the possibility of detonation? With a high compression ratio, the engine is able to develop more torque with the same amount of fuel consumed. That is, the engine becomes efficient. At the same time, the engine allows you to set the phases in advance, which will give a temporary margin when the piston moves to top dead center and optimally ignite the mixture.

And finally, thanks to the above advantages, high octane fuel works better in engines with turbines and superchargers.

A few words about low-octane gasoline. All those indicators, only with a minus sign, are observed in low-octane fuels. This is due to the use of AI-80, 92 gasolines in relatively low-power atmospheric engines.

cetane number

Cetane is used for a similar rating scale, only diesel fuel. Flammability characteristic of diesel fuel, which determines the combustion delay period of the working mixture.

What does it mean? This is a measurement of the time interval from when fuel is injected into a cylinder to when that fuel starts to burn. By science - the period of time elapsed from the injection of fuel into the cylinder until its ignition .

The high cetane number of diesel fuel will be a measure of how quickly the fuel ignites after being injected into the cylinders. Conversely, a low cetane number means that it will take some time to ignite the fuel.

C 16 H 34 Cetane layout

The cetane index is also measured on the hydrocarbon scale, the leftmost one in which will be 1-methylnaphthalene, (C 11 H 10) - 0, 100 - cetane (C 16 H 34) is displayed.

The advantage of faster ignition of diesel fuel is that the engine running on it can develop more torque, the ignition delay will be minimal, the speed of acceleration will be higher. Phase synchronization by running the engine on high cetane fuel can be carried out more accurately by igniting air-fuel mixture when the maximum torque is required.

Final plus high rate cetane can be called low emissions of hydrocarbons into the atmosphere, since such fuel burns more completely. But, at the same time, as studies show, solid particles from exhaust pipe emitted more than low cetane diesel fuel.

Be that as it may, the higher the cetane number, the better, this is an indicator of a better diesel fuel.

We hope there will be no more questions.

Video taken from YouTube channel: "Engineering Explained"

Most car owners have no idea what octane number. As a rule, their knowledge is limited to the understanding that the higher this indicator, the better. Alas, this interpretation is far from the truth. And since novice drivers are taught so many things, but OC is not included in the training course, a bit of information on this topic will not hurt anyone.

Appointment of the octane number of gasoline and how to change it.

This incomprehensible term is understood as the degree of resistance of fuel in liquid form to detonation. In other words, what is the ignition threshold of gasoline under pressure, that is, under compression. A mixture of two hydrocarbons is taken as a reference: isooctane and n-heptane. However, methods for determining OC may vary, so there is a classification of this indicator depending on the method of determination:

• ROI - research ROI;
• OCHM - motor OCH.

Since the instrumental mechanisms of these methods are not the same, there is a difference between the RON and RON values, which is called the sensitivity of the liquid fuel. Both methods are laboratory. And if it's about real indicator obtained on a working power unit, they talk about the actual octane number. It is determined using a special stand on a running engine. But the closest indicator to reality is road octane, which is measured on a moving vehicle.

Isooctane is a substance that is practically non-flammable under compression, so a mixture consisting of only it has a maximum possible octane equal to 100. Conversely, a mixture consisting of only n-heptane has an octane number of zero. In this case, the combustion of n-heptane as a result of exposure to even minimal pressure is accompanied by characteristic knocks in the CPG, which are called detonation. It is they who arise if you use fuel with an octane number that is not suitable for a given power unit. Metallic ringing is formed as a result of the propagation of sound waves that appear as a result of too rapid combustion of fuel assemblies. Repeatedly reflected from the surface of the pistons / cylinders, they become clearly audible and recognizable experienced drivers. Thus, the octane number indicates how fast is happening in the engine's cylinders.

The influence of OC on the characteristics of the power unit

The relationship between the combustion rate of gasoline and OS has a linear form. The lower the octane number, the less time it takes to ignite the fuel assemblies, which directly affects the fuel consumption - if it burns faster than expected, then it enters the combustion chamber at a speed increased by the corresponding value. But this does not mean that by simply increasing the RH, we can save: if the combustion is slower than it should be, this is also bad, since the efficiency of the motor decreases, which leads to a loss of engine response and deterioration dynamic characteristics. By filling in an engine running on 95 gasoline, fuel with an OC equal to 92, you will get. If the situation is the opposite (instead of the working 92nd, we fill in the 95th), the flow rate will remain the same, and the engine power will decrease. So it is possible to use unsuitable gasoline, but it is undesirable. It is recommended to do this only under force majeure circumstances, but not on a regular basis.

How to determine the octane number of gasoline

TCHM, according to GOST 511/82, is measured using a special UIT65M installation, consisting of the following components:

• a device that measures detonation;
• a single-cylinder engine characterized by a dynamic compression ratio and a degree of detonation;
• devices designed to control the likelihood of detonation processes.

The measurement algorithm itself is as follows:

• the test fuel is poured into the engine and, by manipulating the compression ratio, detonation phenomena of a specific magnitude are achieved;
• make up a reference mixture characterized by the same amount of detonation, and the ratio of n-heptane to isooctane determines the OC of the original fuel.

GOST 8226/82 describes the procedure for measuring OC by the research method. In this case, the mode of operation of the motor is characterized by lower loads, as a result of which the RON is always obtained at par with a slightly larger MON. If it becomes necessary to independently measure the octane number, this can be done using devices that are commercially available. An example is an octane meter, which uses a measurement method based on the dielectric constant of liquid fuel (this value varies in proportion to the octane number).

The peculiarity of the method lies in the compilation of a calibration scale that allows one to determine the SP with acceptable accuracy. To build such a scale, you will need some amount of n-heptane and gasoline with a precisely set octane number. A similar procedure can be used to determine the cetane number for diesel fuel. The possibility of applying this method is based on the fact that currently gasoline is produced without using the direct distillation method, but through the technology of camping, or mixing of components. However, the determination of the octane number of gasoline by this method also has disadvantages:

• to perform research, it is necessary to have already identified fuel;
• the influence of external factors can greatly distort the measurement results;
• method is unacceptable for measuring gasolines different types and produced in different ways;
• each instrument must be calibrated using reference devices;
• measurements must be carried out with care temperature regime given in the device specification.

All devices that measure SP use a similar measurement principle, and therefore their advantages and disadvantages are generally identical. Note that the octane number can be more than 100 due to the inclusion of certain additives in gasoline, however, none of these devices is able to work with such liquids.

One of the most affordable octane meters is domestic development OKTIS, which can be purchased for approximately 3,500 rubles. The German device Digatron is more accurate and reliable, however, it also costs several times higher, about 600 euros. However, it is he who is considered the most in demand on Russian market. For its operation, a reference fuel is needed, which is compared with the one under study, and on the basis of this, conclusions are drawn about the octane number of the latter. The disadvantage of this octane meter, as well as other analogues, is that for each measured gasoline from different manufacturers an appropriate reference sample will be required, since in this case the calibration relationship between SP and permittivity will be different. In addition, each measurement must be accompanied by the implementation of calibration procedures, which also affects the accuracy of measurements and the reliability of the results obtained.

Quite expensive, but effective device Russian production is the octanometer OKTAN-IM (45-50 thousand rubles). Among its features is the presence of a built-in memory that allows you to store up to 10 calibrations. The accuracy of the device is not satisfactory.

The PE-7300 M device is included in the same price category, and its "chip" is the proprietary software, which allows you to interface with a computer / laptop. The octane meter is able to take into account the temperature factor, which increases the accuracy of measurements (the dielectric constant has a weakly pronounced dependence on temperature environment).

Approximately similar functionality is incorporated into the SHATOX SX-100M device, which costs about $1,800. It is equipped temperature sensor and therefore measures this indicator more accurately than the PE-7300, which does this in a purely software way. Whether you are measuring 92 octane or 95 octane gasoline, with a reference fuel sample, the results should match to the required accuracy, but within the same batch of gasoline. Other shipments, even from the same manufacturer, may require additional instrument calibration.

Compression ratio and oct

All possible varieties of gasoline are characterized by a very specific compression ratio, which is regulated by GOST. You can get acquainted with the correspondence of the octane number of the compression ratio from the following table:

Gasoline name	OC		GOST	Compression ratio
	EYES	HMO
A72		72	208477	7.00
A76		76	208477	7.50
AI80	80	76	5110597	8.00
AI91	91	82.5	FS 5110597	9.00
AI92	92	83	TU 38001168/97	9.20
AI93	93	85	208477	9.30
AI95	95	85	FS 5110597	9.50
AI96	96	85	TU 38001168/97	9.60
AI98	98	87	FS 5110597	10.00

Currently, in the network of domestic gas stations, gasoline grades 76/80 can no longer be found. However, equipment designed specifically for these grades of gasoline is still available and, as we noted above, it cannot work on gasoline with a higher octane number. So the need for low-octane gasoline remains at a fairly stable, albeit relatively low level. There is a way out of this situation - an artificial decrease in the octane number of gasoline to the required levels. At home, this can also be done, but approximately enough. If you, for example, have a walk-behind tractor manufactured more than 10 years ago, you will need to lower the octane number from 95 to 80.

The easiest way is to just open . When interacting with air, the OC will decrease by about 0.5 units daily, so the procedure will take about a month. The obvious disadvantages of the method are the duration. The second, more common method is the dilution of gasoline with kerosene. And although it is considered the most common in relation to cars running on low-octane gasolines, its main drawback lies in the difficulty of selecting the required proportion, that is, the accuracy of the method is very low. However, the same can be said about the first method, so in any case it will be necessary to measure the RH using an octane meter.

How to increase the octane number of gasoline

There are also opposite situations, when only 92nd gasoline is available at the gas station, but 95/98 is required. In this case, you can try to independently raise the octane number of gasoline, for which a special substance, collectively referred to as an antiknock, is added to the fuel. Consider all the main additives used as an antiknock agent. Ordinary ethyl (methyl) alcohol may well raise the OCh by the required amount. By adding a liter of alcohol to 10 liters of 92 gasoline, you can get fuel with an OC of 95. By the way, such a mixture will be characterized by a less toxic exhaust composition, however this way also has disadvantages. One of them is considered to be an increase in the vapor pressure of fuel assemblies, which leads to an increase in the likelihood of traffic jams in the fuel line of the vehicle's power system. The second drawback of the alcohol mixture is an increase in hygroscopicity, which excludes the possibility of storing fuel for any long time in open form due to the possibility of accumulating a critical mass of water in gasoline.

Tetraethyl lead has physical characteristics making it one of the most effective antiknock agents. This and high viscosity, and a very high boiling point (about two thousand degrees). It was first used as an OC booster nearly a century ago in 1921. Allows you to increase the SP by 15-17 units. It is this substance that is most often used to increase the octane number of gasoline with your own hands. Unfortunately, tetraethyl lead also has disadvantages that limit its use. This is the formation of lead oxide during the combustion of dilute fuel. This substance forms a precipitate that settles on internal surfaces pistons, valves and other components of the CPG.

To minimize the formation of such deposits, special substances (diromethane, bromoethyl, dibromopropane) are added to tetraethyl lead, which bind the combustion products of lead, facilitating their removal to the outside through the exhaust tract.

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In order to understand what an octane number is, it is necessary to have at least a small understanding of the principle of engine operation. internal combustion. In fact, this concept refers to the chemical resistance of fuel to ignition. The higher this indicator, the lower this probability.

Detonation

At one of the stages of its work, the piston of any car engine starts to compress the air-fuel mixture. As a result of the action of high pressure, it is likely that it will spontaneously ignite before the spark from the spark plug appears. This phenomenon is called detonation and becomes a serious problem for many cars. Its main threat is damage to the connecting rods and melting of the piston bores. Repair of these breakdowns is quite expensive. Be that as it may, such cases are relatively rare in our time, since most manufacturers equip the power units of machines with computerized units capable of detecting frequencies that are characteristic of detonation.

Compression ratio

A high compression ratio allows the engine to deliver more power while using less fuel. in motors modern models machines, it is 10:1. At the same time, if we are talking about direct injection units, it increases significantly. AT this case especially important role plays the octane number of the fuel. It should be noted that this indicator does not affect the consumption in any way. As a rule, sports cars have the highest compression ratio. In this regard, they most of all need high-octane fuel.

Methods for calculating the indicator

In order to calculate what is the octane number of a particular type of fuel, you must first select a mixture of reference hydrocarbons. These are isooctane and normal n-heptane. Their indicators are respectively equal to 100 and 0. The value is then determined by the motor or research method, by using a special installation that provides variable degree compression. Using motor way a high engine load is simulated when the temperature fuel mixture reaches 150 degrees. In this case, the rotational speed is in the same value, which is 900 revolutions per minute. When using the second of these methods, the fuel mixture does not heat up, and the rotational speed is 600 revolutions per minute.

Determination of octane number

You can accurately calculate the value of the indicator as follows. This is done using a test bench, which is a motor with one cylinder and a carburetor. In this case, the level of detonation is fixed by special sensors. First, the engine is started with the test fuel, after which a reference mixture is selected. The mode of operation does not change. Expressed in percentage the amount of isooctane in the reference mixture that will result and will show the stability of the gasoline. In other words, if the mixture is 75 percent isooctane, this means that the octane number of the tested fuel is 75 units. When using the motor method, the detonation features of the machine are determined when driving at low speed, frequent starting and regular stops of the power unit. As for the research method, it provides an opportunity to examine in detail the process of fuel combustion while driving along the highway in the same mode. As practice shows, in the second case, the value of the indicator will always be slightly larger.

Octane Boost

The more pronounced the smell of fuel, the higher its octane number. This is due to the fact that paraffinic and aromatic hydrocarbons, which have a branched structure, increase the value of the indicator. In this regard, it becomes clear why it is recommended to store gasoline in closed containers. By using special additives improvement in fuel performance. It should be noted that each of their types is designed to perform a separate task. Some of them are designed to increase the octane number, while others are designed to reduce the emission of harmful elements into the atmosphere. Currently, in our country at gas stations you can see such brands of gasoline as AI-95, AI-92, AI-96, A-76 and A-80. It should be noted that the letter "I" in the name is evidence that the indicator was determined by the research method. At the same time, the number in the name is the octane number.

Gas

Gas has become a very convenient and relatively cheap type of fuel, which is becoming more and more popular among motorists from year to year. In this case, we are talking about a propane-butane mixture. It is produced from oil and associated concentrated petroleum gases. In order for it to always remain in a liquid state, it is stored and transported under a pressure of 16 atmospheres. It should be noted that in the mixture, butane acts as fuel, while propane provides pressure. This type of fuel, compared with others, has many significant differences, starting with chemical composition and ending with cost. There is an opinion among many car owners that gas harms the engine, and its performance is much worse than gasoline. In fact, this assertion is erroneous. Bright to that proof can be called the octane number of the gas, which for a propane-butane mixture is 110 units. As noted above, the maximum value of this indicator for gasoline is 98 units.

Anyone who has ever had a chance to refuel fuel tank or a spare can of gasoline, knows that it comes in many varieties, from regular unleaded to expensive premium gasoline. Each "bouquet" has its own number (80, 92, 95), which corresponds to its octane number, but what does this mean? What is octane number and what is its function? To find answers to these questions, we turned to fuel experts.

Recently, we published an article about. In it, we touched on the topic fuel system car, and decided not to dwell on it, but to go even deeper, finding out what types of fuel poured into our tanks exist, and also by what criteria they qualify. It turned out that the octane number is a qualifying feature in the fuel we are used to. What it is and “with what it is eaten”, we did not know, so decide to look into this issue in more detail. To understand the function of octane, which is essentially the chemical resistance of gasoline to self-ignition - the higher the number, the less likely it is to explode under high pressure - you first need to know the basics of operation. . If you are already familiar with pistons, valves, and spark plugs, feel free to skip the next three paragraphs, if not, please continue reading for a quick introduction to the internal combustion engine. Without this, it will not be possible to understand what an octane number is.

How does a car engine work?

The engines you can find in almost every modern vehicle operate on a principle known as the four-stroke cycle. Every engine has pistons that move up and down inside the cylinder. The pistons are attached to connecting rods that rotate on crankshaft. The force applied to these parts by the transmission is what ultimately drives your car's wheels to get you to work on time.

What are these four measures in this critical process? If we talk about them in the order of their occurrence, then we are talking about the intake stroke, compression, working stroke and exhaust stroke.

The cycle begins when the piston begins to move down the cylinder. This process is called the intake stroke. As it descends, a precisely controlled mixture of air and fuel vapor is sucked into the cylinder through the open intake valve. Next, the intake valve closes and the piston begins its upward movement, the compression stroke, when the air-fuel mixture is compressed. Once the piston reaches the top of its travel, the spark plug releases a spark that ignites the air/fuel mixture in a massive explosion. At this moment, the engine cycle occurs. The force generated by the explosion again pushes the piston down and provides driving force which makes the engine run and the car move. As soon as the piston reaches the bottom, it's time for it to rise again. The exhaust stroke comes into play, during which all the hot, remaining gases from the combustion process are pushed through the already open exhaust valves. As soon as the piston reaches its top point, open intake valves and the whole process starts over. Imagine how fast all this happens when you are speeding down the highway!

We hope that we have explained everything intelligibly; if you like to get to the bottom of the matter (like Pasternak), then we recommend that you familiarize yourself with our more detailed material on this topic, called "". Now we got to the main thing, to the reason why the writing of this article was started - to find out what role does the octane number play in a car engine, or simply put, what is an octane number?

So, what is this concept - octane number?

Steven Russ, technical head of engine development at the company Ford motor, is a fuel expert. According to him, “The octane number is simply a measure of the chemical resistance of gasoline to automatic ignition". He also said that "gasoline with a higher octane rating is more resistant to spontaneous combustion."

So what, in essence, does this octane rating do? Remember how earlier we explained how the piston compresses the air/fuel mixture during the compression stroke? If this mixture is pressurized too high, it can spontaneously ignite, which is a serious problem if the mixture ignites before the spark plug has sparked.

Self-ignition, better known as "detonation" ( detonation combustion fuel mixture) can result in a distinctly audible noise. This potentially devastating rumble sounds like the sound of coins being tossed into an empty piggy bank.

According to Bill Stadzinski, a fuel specialist from General Motors, self-ignition can lead to "high-pressure waves that collide with each other, and that is what causes the knocking and ringing that you hear."

In addition to noise, detonation can cause damage internal components engine. Imagine, self-ignition can melt the holes in the pistons and even bend the connecting rods, leading to inevitable engine failure. Luckily, Russ assured us that "this rarely happens these days" thanks to advanced computer blocks engine control.

“Our cars have knock sensors,” Stadzinski assured us. These are small electronic transducers attached to the engine block that listen for specific sound frequencies that are characteristic of detonation. If the sensors detect such frequencies, the powertrain control module performs a series of actions to regain control of the combustion of the air/fuel mixture. The unit can reduce boost levels in supercharged and turbocharged engines, delay spark plug sparking, or richen the air/fuel mixture to prevent internal engine damage.

The high compression ratio allows the engine to create more power by burning less fuel. The compression ratio is essentially a measure of how tightly the air-fuel mixture inside the cylinder. Compression ratio of the majority modern engines is approximately 10 to 1, but it can be higher, for example 12 to 1 or more, if we are talking about a direct injection engine. And in supercharged engines, the compression ratio may, on the contrary, be slightly lower.

Automakers must always be aware of fine line which will not lead to self-ignition. This is where the octane number comes into the equation. Higher compression engines - which are commonly found in high performance sports cars- almost always require fuel with a higher octane rating, which reduces the likelihood of self-ignition. According to Russ, higher octane gasoline "does not affect fuel efficiency in any way".

Recall that higher pressure inside the cylinder requires fuel with a higher octane rating to prevent damage to the engine, which can be caused by self-ignition, which in turn will lead to. But no one is immune from mistakes. What happens if you fill your car with the wrong grade of fuel?

“If you own a car that requires premium fuel, and you poured 87 octane gasoline into it and start to hear distinct knocks while driving, then you will need to treat the car like a child until you get to the nearest gas station,” Stadzinski said, adding that “even if you don’t hear a distinct knock, the reaction in the car is still going on.” There is a decrease in productivity, an increase in fuel consumption, a large amount of heat is sent to exhaust catalyst which reduces its strength. If you read these lines and waved your hand, deciding to continue to “feed your iron horse” with gasoline with an octane rating lower than that recommended by the automaker, car repairs cannot be avoided.

So, how is the octane number calculated?

Sadzinski says there are two main separate tests to determine the numbers you see at gas stations. You may have already noticed this formula (R+M)/2. It is usually written in filling stations. "R" means "research octane number" or "octane number of gasoline according to the research method", and "M" means "motor octane number" or "octane number of gasoline according to motor method". Each of these methods is one of the tests we mentioned above. The average is designed to provide each driver with an acceptable grade of gasoline rating. The fuel requirements of a minivan that is still towing a trailer over a mountain pass in the desert are very different from those of a small passenger car, which predominantly rides at sea level.

The science-octane test favors larger, older vehicles -- cars and small trucks with large displacement, naturally aspirated engines, Sadzinski said. These engines are characterized by more heat in the cylinder than for engines whose octane number is calculated by the motor method. Such a test is more focused on small, efficient engines turbocharged.

But since our expert is an American, he told us about the "American" octane number, which is calculated using the average of the two methods. But Sadzinski drew our attention to the fact that “there is no one agreed standard for calculating the octane number in the world, therefore the rest of the world, including Russia, uses only the octane number of gasoline according to the research method.”

If you have ever been to Europe, you may have noticed the sign "95 RON" on their gas stations. RON stands for "research octane number" of course.

According to Sadzinski, it is interesting that in Europe the most common gasoline is 95 octane, which, in terms of American system gives 90. And this means that the European vehicles must be calibrated to run on the most common American 87 octane gasoline, and vice versa.

There are not many car owners in our country American manufacturers, and they don't have to worry about engine calibration. But if you are wondering what kind of gasoline is best for your car, then it is best to follow the recommendations that the manufacturer displayed in the car's owner's manual. The recommended grade of gasoline will help your iron horse show the best performance.

We hope we were able to answer your question, what is the octane number, and why is this indicator needed in fuel. If you still have any questions on this topic, feel free to leave them in the comments under this article, we will try to answer all of them.

With a great deal of confidence, we can say that absolutely all car owners have heard about the octane number of gasoline, however, few of them know what this number actually represents, what it depends on, and what depends on it. It is about him that will be discussed in this article, but first let's look at such a phenomenon as detonation.

What is detonation, and what is its detriment to the engine

One of the main requirements for motor gasoline is its resistance to detonation. In no case should the combustible mixture in the cylinder ignite until it is ignited by a spark from the spark plug electrodes. In the event of self-ignition of the fuel mixture under high pressure, a detonation effect will inevitably occur in the cylinder - explosive ignition of the fuel, accompanied by a corresponding sound.

This phenomenon has a detrimental, and sometimes even destructive effect on the details. piston group. The point is that the speed complete combustion of the fuel mixture in the cylinder, provided it is ignited by a spark, is 15-60 m/s, and when a detonation effect occurs, it burns out at a speed of 2000-2500 m/s. And this is no longer combustion, but a real explosion, repeating with each cycle, causing resonance. The harmful effect of the latter leads to the destruction of the piston itself, the piston pin, connecting rod and other engine parts.

Good, modern cars are equipped with knock sensors that are able to capture its slightest signs and transmit the appropriate signal to the electronic unit control (ECU) of the engine, which, in turn, either reduces the amount of fuel in the mixture, or corrects the ignition timing. However, the ECU cannot always cope with similar problem, especially if there is low-quality or inappropriate gasoline in the tank.

The concept of octane number

Each of the drivers, refueling his car at the gas station, orders the operator right amount fuel, indicating its usual name (80, 92, 95, 98). In fact, this is not a name, not a brand, not a degree of flammability, and not even a measure of detonation, as some "experts" explain. The numbers in the name of gasoline indicate its octane number, which determines its knock resistance. It determines the percentage of the mixture of isooctane with n-heptane in gasoline. Why exactly these substances? Everything is simple. The fact is that isooctane is practically non-explosive, which is why it does not lend itself to detonation, and its detonation resistance is 100. In turn, n-heptane explodes at the slightest increase in pressure, therefore its resistance to detonation processes is equated to zero.

By mixing these substances in the right proportions, and adding them to the fuel, we are able to adjust the value of its octane number, thereby adapting gasoline to different engines.

How is the octane number determined?

There are two generally accepted ways to calculate the octane number: research and motor. The first method involves checking gasoline for its resistance to detonation processes with a moderate load on power unit. Tests are carried out on a special stand using a single-cylinder gasoline engine at variable load, speed 600 rpm, air temperature in the fuel mixture +52 0 C and ignition timing equal to 13 0 . The motor is first run on the test fuel until detonation occurs. After fixing it, the engine at the same load is transferred to the reference fuel from a mixture of isooctane and n-heptane in different concentrations. Having fixed the moment of occurrence of the detonation effect, the tests are stopped. The amount of isooctane in gasoline at which the detonation process began is the so-called research octane number. And if the letter “I” (AI) is present in the labeling of gasoline, this means that it was determined by the research method described above.

The motor method involves determining the resistance of the fuel to detonation phenomena in real driving conditions with an increased load on the engine (900 rpm at a fuel mixture temperature of +149 0 C and a variable ignition timing). The process for determining the octane number is similar to the one described above.

There is another method for determining the value of the octane number. Its essence lies in measuring the amount of isooctane with a special device - a digital octane meter. It is quite simple and easy to use. The principle of operation of the octane meter is to compare the composition of the studied gasoline with reference fuel samples, and is based on its dielectric features. This method to date, it has not yet been certified in Russia, so the octane meter cannot be an official research tool.

The value of the octane number with different methods of its determination may differ slightly. Below is a table of the main brands of gasoline with an indication of their octane number

How does the use of gasoline with a high or low octane rating affect engine performance

For each brand and model of car, the manufacturer provides gasoline with a certain octane rating. You can find it in the owner's manual of the car. But what happens if you do not follow the recommendations?

The use of fuel with a lower octane rating, as we already know, leads to detonation. In addition, consumption increases, engine power decreases, and with a prolonged load on it, valves can burn out, the engine overheats, and parts of the piston group fail. When using gasoline with a high octane number, nothing terrible will happen, except that the dynamics will decrease slightly due to the longer combustion time of the combustible mixture.

Below is a table from which you can find out which fuel better fit for engines with different compression ratios.

How to increase octane

Until recently, fuel manufacturers used tetraethyl lead, a substance with high anti-knock characteristics, to increase its resistance to detonation. But, since it also turned out to be super-poisonous, and also quickly disabled catalysts and oxygen sensors in exhaust system, he quickly found an alternative.

Today, to increase the octane number, various aromatic (have a high octane number) and paraffinic hydrocarbons (have the lowest octane number), called additives, are used. Many of them have high volatility, which often leads to the fact that the gasoline to which they were added, with a leaky container, can quickly become, for example, 92 or 80 from 95.

You can also increase the octane number yourself. To do this, you need to purchase one of the additives and add it to the fuel. One such agent is methyl tert-butyl ether. This additive is considered practically harmless to the environment and engine elements, which cannot be said about ferrocene, which contains ordinary iron, which settles with a strong reddish coating on the candle electrodes.