Compression and compression ratio: difference, principle of operation, similarities and differences

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

Many novice motorists who recently purchased their vehicle are trying to understand the features of its device. In particular, it is useful to understand what is under the hood. And the engine is of particular interest in this regard. This is an extremely complex mechanism, consisting of various parts. Therefore, it is worth understanding this matter at least in order to independently eliminate a number of malfunctions. At the same time, inexperienced motorists are not able to fully understand the difference between compression and compression ratio. But there is a difference, because each of these terms corresponds to its purpose.

Compression ratio

To begin with, let's consider what should be understood by this term. The compression ratio is a geometric value that does not have units. This is due to the fact that fordefinitions, the parameters of the power unit are used. In other words, the compression ratio is the ratio of the total volume of the cylinder to the volume of the combustion chamber.

With regard to engines running on gasoline, this value can vary dramatically - in the range from 8 to 12. As for diesel power units, they have this characteristic even more - 14-18 units. This is largely dictated by design features.

In search of an answer to the question, what is the difference between compression ratio and compression ratio, it is worth considering another point regarding gasoline engines. The point is this. The higher the compression ratio, the higher the power density. At the same time, a strong increase in this parameter will inevitably lead to a noticeable decrease in the motor resource. And on top of that, serious problems can arise if you fill the car with poor quality fuel.

Compression ratio calculation

For any internal combustion engine, it is important that this parameter has the maximum possible value. However, if you need to force the motor, you should know how this characteristic can be calculated. This is necessary in order to avoid detonation, due to which the motor may simply fail.

The formula used to calculate is as follows:

CR=(V+C)/C, where CR is the compression ratio, V is the working volume of the cylinder, C is the volume of the chambercombustion.

Those motorists who want to know what is the difference between compression and compression ratio will be interested in such calculations. Perhaps this will be useful to him in practice.

To determine this parameter for only one cylinder, the total engine displacement should be divided by the number of "glasses". As a result, we get the value of V from the formula above.

But it is much more difficult to determine the indicator C, but it is also possible. For this, experienced motorists and mechanics involved in engine repair have the right tool in mind - a buret. It is graduated in cubic centimeters. The easiest way is to pour gasoline into the combustion chamber, and then measure its volume with a burette. It remains to enter the received data into the formula.

Compression

Now let's get acquainted with this characteristic. Unlike the compression ratio, compression is the pressure in the cylinder at the end of the cycle. And this characteristic is a physical quantity, so it can already be measured. For this, special equipment is used - a compression tester.

From a theoretical point of view, this parameter should be equal to the compression ratio. But this is only in theory, in reality everything is different. The compression is almost always greater than the compression ratio. This is due to several reasons, which will be discussed below.

Explanation of theory and practice

Both characteristics will be equal only if an infinitely longisometric gas compression. As a result, the released energy will be absorbed by the piston, cylinder walls, block head and other parts of the engine, and completely. Due to this, the heat balance will not change. Compressed gas gives off heat, but does not press on the pressure gauge with more force than the calculated value.

In practice, everything is different - there is a difference between compression and compression ratio in the readings. The process is adiabatic. The compression of the gas is accompanied by a significant increase in temperature.

Not all of the heat generated by the compressed gas is absorbed by the cylinder walls, and for this reason, pressure is generated from the remainder.

Old and new engines

In motors that have already worked out a decent amount of time, compression rates will be noticeably lower than in recently released power units. This is due to tightness. New car engines are largely gas-tight. Therefore, much heat will not be released through the locks of the rings and other places in the cylinders. Accordingly, the compression will not drop. The difference between compression and compression ratio will be minimal.

With old engines, everything is clear - the service life does its job. And as a result of long-term use of the vehicle, including exposure to high temperatures, the elements lose their original properties. Of course, this happens over a long period of time, but one way or another, the characteristics of the engines change anyway.

Methods for changing the compression ratio

With modern power units, this characteristic can be adjusted both up and down. If you need to increase the parameter, then for this the cylinders are bored and pistons with a large diameter are installed. Anyone who is interested in understanding the difference in compression and compression ratio of a combustion engine will benefit from this information. Indeed, among motorists there are supporters of various kinds of tuning.

Another, no less effective way to change the compression ratio is to reduce the combustion chamber. In this case, a layer of metal is removed from the junction of the cylinder head with the engine block. Such an operation is carried out using a planer or milling machine.

If, for some reason, it becomes necessary to lower the compression ratio, then, on the contrary, it is worth placing an duralumin gasket between the cylinder block and the cylinder head. Another way is to remove the metal layer from the piston bottom. However, it is more difficult to implement, since it will require certain efforts, skills and abilities. In addition, this procedure requires a lathe.

Comparison results

Ultimately what is the difference between compression ratio and compression ratio? Analyzing these two terms, one can notice a significant difference. The compression ratio is a dimensionless quantity. You can change it, but only by interfering with the design of the engine.

Compression is able to vary during the operation of the vehicle. In addition, this parameter largely depends on the degree of compression. After all, the pressure in a smaller volumewill always be big.

In other words, if the compression ratio increases, then the compression also increases.

How does the impact happen?

So what does the compression ratio affect? Here it is worth considering the amount of work that the power unit produces. And the higher this parameter, the more energy will be released during the combustion of the air-fuel mixture. Accordingly, the engine power also increases.

For this reason, most manufacturers are trying to increase the power performance of the motor through one effective technique. They began to resort to it since the end of the last century. Instead of moving towards increasing the volume of cylinders and the combustion chamber, specialists, and they certainly know what the difference is between compression and compression ratio, are striving to increase the latter figure.

However, there are limitations. The working mixture cannot be compressed indefinitely - upon reaching a certain value, it detonates, that is, it explodes. At the same time, this applies only to engines running on gasoline. Diesel power units are free from the risk of detonation. Actually, this explains their higher compression ratio.

And in order to avoid such a devastating effect, because detonation is detrimental to the engine, the octane number of gasoline rises. And this, in turn, increases the cost of fuel. In addition, those additives that serve this purpose lead to a deterioration in the environmental parameters of the motor.