Compression Ratio

Compression leads to power, and the belief that “more is better” should be applied to some circumstances does have some relevance.

However, before we start arbitrarily raising compression ratios, it would be good to have a better grasp of how to get there first. This would be preferable because:

The compression ratio of an engine as a whole or of an individual cylinder is defined as the ratio between the swept volume of the cylinder  with the piston at bottom dead centre (BDC) and the volume with the piston at top dead centre.

 This ratio can alternatively be stated as the ratio between the swept volume of the cylinder 

when the piston is at the top dead centre and the volume when the piston is at the bottom dead centre (TDC).  For instance, if the ratio of the volume at BDC to the volume at TDC is thirteen times greater than one another, then the compression ratio is thirteen to one. 

Types of Compression

It is a fundamental requirement for such engines and can be measured in two distinct ways:

the static compression ratio, which is calculated based on the relative volumes of the combustion chamber and the cylinder when the piston is at the bottom of its stroke,

and the volume of the combustion chamber when the piston is at the top of its stroke. 

These two measurements are collected with the piston at its lowest point of travel in the cylinder. The dynamic compression ratio is a more complex computation that takes into account gases entering and exiting the cylinder while the compression phase is occurring. 

This ratio is calculated by taking the dynamic compression ratio. Additionally referred to as the dynamic compression ratio, this ratio has a few different names.

Variable Compression Ratio of Engine

The compression ratio of the vast majority of engines is fixed.

But, certain engines, known as variable compression ratio engines, are able to vary their compression ratio even while the engine is running. 2019 marked the debut of the industry’s first production engine equipped with a variable compression ratio. the term “variable compression ratio” refers to a technology that allows the compression ratio of an internal combustion engine to be adjusted even while the engine is running. This is done in order to improve the efficiency of the fuel used while the load is being changed. 

The volume that is above the piston when it is at its top dead centre can be adjusted in engines with variable compression.

What Factors go into Determining the Compression Ratio?

Reviewing the method by which compression ratios are determined could prove to be quite beneficial. This can be accomplished by dividing the space into a number of smaller regions, each of which needs to have its volume determined individually.

 After that, the total volume that will be swept can be calculated by adding all of these smaller volumes together. For instance, the area on top of the piston at TDC needs to take into consideration the different volumes of the chamber, the top of the piston (the crown design), the head gasket, and the height of the piston relative to whether it is above or below the deck of the cylinder.

Compression Ratio of Diesel Engine

Because diesel engines do not have spark plugs, their compression ratios are higher than those of gasoline engines. This is because the air temperature in the cylinder needs to be raised sufficiently for the diesel to be ignited through the process of compression ignition, which requires a higher compression ratio. Compression ratios for direct injection diesel engines typically range between 14:1 and 23:1, while compression ratios for indirect injection diesel engines typically range between 18:1 and 23:1.

Other Types of Fuel

Because liquefied petroleum gas (also known as “propane autogas”) and compressed natural gas both have a higher octane rating than conventional gasoline, engines that run only on these fuels may have a higher compression ratio than conventional gasoline engines.In most cases, a compression ratio of 6.5 or lower is utilised in kerosene engines. The compression ratio of the petrol-paraffin engine variant of the  Ferguson TE20 tractor was 4.5:1, and it was designed to run on tractor vaporising oil with an octane rating of between 55 and 70.

Conclusion

The ratio of the swept volume of the cylinder when the piston is at the bottom dead centre to the swept volume of the cylinder when the piston is at the top dead centre is the compression ratio. Compression ratio is an essential component in the generation of power since it is the squeezing together of the air and fuel mixture that both improves the combustion process and generates power. It is essential to keep in mind that increased compression results in a higher octane demand for the fuel. This is something that must be taken into consideration. As an example, we’ll look at single-cylinder engines used in dirt bikes. The compression ratios of production dirt bike engines have increased over the years, and they are now in the range of 12.5:1 to 13.5:1. Despite this increase, they are still able to burn pump premium gasoline with 91 or 93 octane. This is made possible through improvements in combustion space design, as well as enhancements in air-fuel ratio control made possible with electronic fuel injection (EFI).