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What is Compression Ratio and How is It Calculated

What makes compression ratio important? To find out, including understanding how it is exactly calculated, follow the helpful guide below.

What is Compression Ratio and How is It Calculated - KB Delta

Here’s all you need to know about compression ratio and how to calculate this ratio in a diesel or gas engine.

Your ability to determine this ratio means it’s not left only to an engineer with professional knowledge to work it out. And for the uninitiated, compression ration relates to the engine of cars and how it aids in their movement.

On the other hand, we’ve outlined the impacts of a high compression ratio on a gas engine and diesel engine.

Where one of the engines would require a high ratio, the other does not due to the fact that it could lead to problems. This includes knocking and further damage to the engine.

And that being the case, it can be said that a high compression ratio has its advantages and disadvantages based on the engine in question.


What is the Compression Ratio?

Compression ratio denotes how much air and fuel mixture that the combustion chamber found in a combustion engine can hold.

Specifically, this is the amount of mixture held by the chamber when it is empty and at its largest size. This amount is compared with the volume the chamber can contain when the air and fuel mixture is compressed to its smallest size.

Accordingly, compression ratio can be defined as a calculation that is based on the relative volumes of the cylinder and combustion chamber.

On the other hand, the ratio is applicable to internal combustion engines (including those present in today’s vehicles) and external combustion engines.

Coupled with that, the compression ratio is evident in diesel- and gas-powered engines even though the diesel engine’s design encourages a higher compression ratio.

It’s useful for an engine to have higher compression ratios given that these engines are believed to be better. The reason is, these engines are able to generate more power and at the same time maintain efficiency.


How to Calculate Compression Ratio

It’s worth knowing how to calculate the compression ratio of an engine. The steps outlined below will help you out.

1. Calculate the Volume Supported by the Cylinder

The first step to determining an engine’s compression ratio is to calculate the volume that can be contained in the engine’s cylinder. This is the volume that can be supported when the piston is at the cylinder’s bottom.

And at each stroke of the engine, there is a movement of the piston from the bottom towards the top. This leads to a compression of the air-fuel mixture.

Therefore, this is a calculation of the volume of the cylinder when the piston is down is taken. Here, the mixture is not compressed.

2. Calculate the Volume When the Piston is Up

The next step is to determine the volume when the piston is up. In this case, the air-fuel mixture has been compressed.

If you have a ratio of 13:1, it translates into the engine being able to accommodate 13 times more volume. This amount of support occurs when the piston is down unlike when the air-fuel mixture has been compressed (in a case where the piston is up).

On the other hand, it is worth noting that the air-fuel mixture remains the same. However, it is compressed into a small space in a bid to create a large explosion.


High Compression Ratio

Diesel engines rely on compression in a bid to create the temperature that the diesel will ignite the air-fuel mixture. This mixture will create the power that is required to move the vehicle forward.

Despite this, high compression ratios could cause a problem. For instance, this high ratio could lead to engine knocking or pinging.

Therefore, this is a disadvantage of a higher compression ratio in a gas engine.

Knocking or pinging could occur as a result of larger explosion than what is required occurs. This explosion leads to a quick upward or downward movement of the piston.

The result is a loud knocking noise that needs to be fixed.

And if this noise is not fixed, it could lead to a continuous knock of the engine and possibly lead to permanent damage.

Over and above that, higher compression ratios can be used by cars that rely on gas with a knocking sensor or a higher octane rating – but are unable to match the high ratio evident in a diesel engine.


Effect of High Compression Ratios

As already stated, diesel engines rely on high compression to enable their functionality. A high ratio in a gas engine would be 13:1, whereas that of a diesel engine is around 14:1 to 23:1 based on the type of engine.

The high compression ratio offers more power given that the air and fuel are compressed tighter than average. And this level of compression creates a more powerful explosion.

Also, the tight packing of the mixture enables the air and fuel combination to blend better. And in cases where an explosion occurs, the mixture could evaporate.

When more evaporation occurs, it signals higher thermal efficiency. This translates to an engine that performs better even without relying on high extra energy in a bid to gain power.


What is Compression Ratio and How is It Calculated - KB Delta



Now that you know what compression ratio is and what makes it important, you can hold the compression ratio of your engine in high esteem.

The steps outlined above will enable you to calculate the compression ratio and know what’s ideal for your engine.

Therefore, take the extra time to implement these steps and see how well it works for you.

And remember, you don’t have to be an engineer to get your vehicle in good shape. Given that an understanding of what this ratio entails is what is required to help you begin.

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