Compression ratio and octane number of gasoline

2024 Author: Erin Ralphs | [email protected]. Last modified: 2024-02-19 11:42

Very often, novice drivers wonder what kind of gasoline is better to fill in a car. At the moment, there are several varieties of fuel, with different octane ratings. Which type is better to use so as not to "sentence" the motor? What is the octane number and compression ratio of an engine? Let's try to understand our today's article.

About compression ratio

So, first, let's deal with this definition. The compression ratio is a geometric dimensionless quantity, which is calculated according to the following principle. The total volume of the cylinder is divided by the volume of the combustion chamber. The result is the compression ratio. On the engines of old VAZs, this value was about 8 units. And the engines of the old ZILs and GAZons had a compression ratio of 6. Now the picture has changed. Modern foreign cars have these indicators of 12 or more units. The most technologically advanced now is the SkyActive engine from the Mazda company. Its compression ratio has been increased to 14 units.

What determines this number? The higher it is, the more likely it is that the mixture inside the chamber will self-ignite from high pressure. Also note that the compression ratio determines the power and fuel consumption. Accordingly, the higher this number, the more powerful and economical the motor, and vice versa (ZIL engines are just a direct confirmation). And in order for the fuel to resist self-ignition, it must have certain characteristics. This is where the concept of "octane number" came from. We will talk about this later.

Octane number - what is it?

This characteristic determines the detonation resistance of the fuel. That is, the octane number is the value of the ability of gasoline to resist spontaneous combustion during compression. In other words, the higher the RH, the less likely the fuel is to ignite from pressure. At the moment, you can buy gasoline with different OCH. Usually this fuel is A-92 and A-95. However, there is also the 98th and "hundredth" gasoline, but they are much less common. There are also sports engines with a compression ratio of 16 units. They require gasoline with an octane rating of at least 102.

As you can see, compression ratio and octane number are very important parameters. It should be noted that during the production of gasoline during hydrocracking, its OC will not exceed 85 units. But how then do gas stations sell fuel with a much higher octane? It's simple - additives are added to the fuel before sale. It is they who bringgasoline to the desired octane number. Alcohols and esters are used as additives.

There is also a distinction between naturally aspirated and turbocharged engines. In the latter case, under any circumstances, gasoline with an RON of at least 95 should be used.

Is it possible to fill in fuel with a lower OC?

We continue to study the dependence of the octane number on the compression ratio. Let's take this example. We have a car in which the manufacturer recommends pouring 95th gasoline. What happens if you use A-92 fuel? In this case, there is a high probability of detonation. What it is? This is the process of explosive ignition of fuel in the combustion chamber of an engine. during detonation, the flame can spread at a speed of up to 2 thousand m / s (the norm is no more than 45). The shock wave negatively affects all parts of the engine with which it comes into contact. These are the cylinder head, intake and exhaust valves, and the crank mechanism.

The mixture ignites during detonation long before the piston reaches top dead center. In view of this, the piston is under enormous stress. Also note that the mixture will ignite not from a candle, but from pressure, as in a diesel engine. With such work, the engine resource is reduced tenfold. Therefore, it is so important to know what compression ratio the car has and the octane number of gasoline that the manufacturer recommends using.

Modern engines have knock sensors. They are in the case of using low-octanefuel correct the ignition angle. Thus, the risk of detonation is reduced several times. However, intentionally using 92nd gasoline where 95th is prescribed is not recommended.

Compression ratio and octane number: can higher octane fuel be filled in?

Let's consider another example. The car is filled with 95th gasoline, but the manufacturer recommends A-92. What will happen in such a case? There is a myth that in this situation the head gasket burns out. But as practice shows, this happens only on old carburetor engines. In the case of injection cars, of which the vast majority are now, nothing tragic will happen. The electronics will automatically correct the ignition angle. There will also be a slight 2 percent increase in power. But it makes no sense to pour expensive 98th gasoline into a foreign car of the 2000s. Therefore, it is better to use the variety that is prescribed by the manufacturer.

Engine compression ratio and gasoline octane number: what is better to fill in?

Determine what kind of fuel can be poured into the car, you can and the degree of compression. So, if the last indicator is no more than 8.5, then the engine can run on A-76 fuel. If the octane rating is between 8.5 and 9 units, A-80 fuel is recommended. The 92nd gasoline is poured into cars where the engine compression ratio is from 10 to 10.5. These are most foreign cars of the 90s and early 2000s. If you have a newer car, where the engine compression ratio is from 10.5 to 12, you need to use gasoline with RON 95. Which cars are suitable for the 98th grade? This fuel is recommended to be poured intoengines with a compression ratio of 12 to 14 units. And if this is a sports motor, it is worth using the “hundredth” here. This applies to engines with a compression ratio of more than 14 points.

Are savings possible?

Regarding compression ratio and octane number, it is worth noting the fact that when using fuel with a higher octane, engine consumption is slightly reduced. But it will not be possible to achieve significant savings, drivers say. The difference is only within the margin of error - no more than four percent. At the same time, it should be understood that the price of high-octane gasoline is always higher, and therefore the savings are reduced to nothing.

Conclusion

Now we know what the compression ratio and octane number of gasoline are. As you can see, these two concepts are closely related. Knowing the compression ratio of the power unit, you can know exactly what fuel is shown to it by the manufacturer.