What is the compression ratio of an internal combustion engine

One of the important design characteristics of a piston internal combustion engine is the compression ratio. This parameter affects the power of the internal combustion engine, its efficiency, and also fuel consumption. Meanwhile, few people have a true idea of ​​​​what is meant by the degree of compression. Many people think that this is just a synonym for compression. Although the latter is related to the degree of compression, however, these are completely different things.

To understand the terminology, you need to understand how the cylinder of the power unit is arranged, and understand the principle of operation of the internal combustion engine. The combustible mixture is injected into the cylinders, then it is compressed by a piston moving from bottom dead center (BDC) to top dead center (TDC). The compressed mixture at some point near the TDC ignites and burns out. The expanding gas performs mechanical work, pushing the piston in the opposite direction - to the BDC. Connected to the piston, the connecting rod acts on the crankshaft, causing it to rotate.

The space bounded by the inner walls of the cylinder from BDC to TDC is the working volume of the cylinder. The mathematical formula for the displacement of one cylinder is as follows:

Vₐ = πr²s

where r is the radius of the inner section of the cylinder;

s is the distance from TDC to BDC (length of the piston stroke).

When the piston reaches TDC, there is still some space above it. This is the combustion chamber. The shape of the upper part of the cylinder is complex and depends on the specific design. Therefore, it is impossible to express the volume Vₑ of the combustion chamber with any one formula.

Obviously, the total volume of the cylinder Vₒ is equal to the sum of the working volume and the volume of the combustion chamber:

Vₒ = Vₐ+Vₑ

And the compression ratio is the ratio of the total volume of the cylinder to the volume of the combustion chamber:

ε = (Vₐ+Vₑ)/Vₑ

This value is dimensionless, and in fact it characterizes the relative change in pressure from the moment the mixture is injected into the cylinder until the moment of ignition.

It can be seen from the formula that it is possible to increase the compression ratio either by increasing the working volume of the cylinder, or by reducing the volume of the combustion chamber.

For various internal combustion engines, this parameter may differ and be determined by the type of unit and the features of its design. The compression ratio of modern gasoline internal combustion engines is in the range from 8 to 12, in some cases it can reach up to 13 ... 14. For diesel engines, it is higher and reaches 14 ... 18, this is due to the peculiarities of the ignition process of the diesel mixture.

And as for compression, this is the maximum pressure that occurs in the cylinder as the piston moves from BDC to TDC. The international SI unit for pressure is the pascal (Pa/Pa). Units of measure such as bar (bar) and atmosphere (at / at) are also widely used. The unit ratio is:

1 at = 0,98 bar;

1 bar = 100 Pa

In addition to the degree of compression, the composition of the combustible mixture and the technical condition of the internal combustion engine, especially the degree of wear of the parts of the cylinder-piston group, affect the compression.

With an increase in the compression ratio, the pressure of the gases on the piston increases, which means that, ultimately, the power increases and the efficiency of the internal combustion engine increases. More complete combustion of the mixture leads to improved environmental performance and contributes to more economical fuel consumption.

However, the possibility of increasing the compression ratio is limited by the risk of detonation. In this process, the air-fuel mixture does not burn, but explodes. Useful work is not done, but the pistons, cylinders and parts of the crank mechanism experience serious impacts, leading to their rapid wear. The high temperature during detonation can cause burnout of the valves and the working surface of the pistons. To a certain extent, gasoline with a higher octane rating helps to cope with detonation.

In a diesel engine, detonation is also possible, but there it is caused by incorrect injection adjustment, soot on the inner surface of the cylinders, and other reasons not related to an increased compression ratio.

It is possible to force the existing unit by increasing the working volume of the cylinders or the compression ratio. But here it is important not to overdo it and carefully calculate everything before headlong rushing into battle. Errors can lead to such an imbalance in the operation of the unit and detonations that neither high-octane gasoline nor adjustment of the ignition timing will help.

There is hardly any point in forcing an engine that initially has a high compression ratio. The cost of effort and money will be quite large, and the increase in power is likely to be insignificant.

The desired goal can be achieved in two ways - by boring the cylinders, which will make the working volume of the internal combustion engine larger, or by milling the lower surface (cylinder head).

Cylinder boring

The best moment for this is when you have to bore the cylinders anyway.

Before performing this operation, you need to select the pistons and rings for the new size. It will probably not be difficult to find parts for the repair dimensions for this internal combustion engine, but this will not give a noticeable increase in the working volume and power of the engine, since the difference in size is very small. It is better to look for larger diameter pistons and rings for other units.

You should not try to bore the cylinders yourself, because this requires not only skill, but also special equipment.

Finalization of the cylinder head

Milling the bottom surface of the cylinder head will reduce the length of the cylinder. The combustion chamber, partially or completely located in the head, will become shorter, which means that the compression ratio will increase.

For approximate calculations, it can be assumed that removing a layer of a quarter of a millimeter will increase the compression ratio by about one tenth. A finer setting will give the same effect. You can also combine one with the other.

Do not forget that the finalization of the head requires an accurate calculation. This will avoid excessive compression ratio and uncontrolled detonation.

Forcing an internal combustion engine in this way is fraught with another potential problem - shortening the cylinder increases the risk that the pistons will meet the valves.

Among other things, it will also be necessary to re-adjust the valve timing.

Combustion chamber volume measurement

To calculate the compression ratio, you need to know the volume of the combustion chamber. The complex internal shape makes it impossible to mathematically calculate its volume. But there is a fairly simple way to measure it. To do this, the piston must be set to top dead center and, using a syringe with a volume of approximately 20 cm³, pour oil or other suitable liquid through the spark plug hole until completely filled. Count how many cubes you poured. This will be the volume of the combustion chamber.

The working volume of one cylinder is determined by dividing the volume of the internal combustion engine by the number of cylinders. Knowing both values, you can calculate the compression ratio using the above formula.

Such an operation may be necessary, for example, to switch to cheaper gasoline. Or you need to rollback in case of an unsuccessful engine forcing. Then, to return to their original positions, a thickened cylinder head gasket or a new head is required. As an option, use two ordinary spacers, between which an aluminum insert can be placed. As a result, the combustion chamber will increase, and the compression ratio will decrease.

Another way is to remove a layer of metal from the working surface of the pistons. But such a method will be problematic if the working surface (bottom) has a convex or concave shape. The complex shape of the piston crown is often made to optimize the combustion process of the mixture.

On older carburetor ICEs, deforcing does not cause problems. But the electronic control of modern injection internal combustion engines after such a procedure may be mistaken in adjusting the ignition timing, and then detonation may occur when using low-octane gasoline.