You've probably heard the term compression ratio in conversations revolving around car engines. However, not many people understand what the term means. Generally, people roughly have an idea about what it is but find it difficult to explain when someone asks them pointblank, ‘What is compression ratio?’

More often than not, people pretend to understand the term when used in conversations, even though only a few know the actual meaning. It is a ratio, yes, but it’s more complicated than that. The most important thing to know, however, is that high compression ratio is desirable, whereas low compression is not.

You don’t need to go through the highly technical aspects about compression in engines to know the answers to your practical questions. This article will delve into what compression ratio is and many related questions that car owners ask. Learn more from this article. 

What is compression ratio?

Compression ratio (CR) in an engine refers to the ratio of the total cylinder volume when the piston is at the bottom dead centre (BDC) to the clearance cylinder volume when the piston is at the top dead centre (TDC). CR is the degree to which the fuel-air mixture is compressed before being ignited.

Imagine an engine cylinder which has, say, a 7-cc total volume when the piston is at BDC (note that at this point, the volume of the uncompressed gas mixture is also 7 cc). The piston goes up, compressing the gas mixture as it does, until it reaches the TDC. At this point, the air is compressed to 1 cc, which is the smallest volume (called clearance) of the cylinder. This engine has a compression ratio of 7:1, meaning, it can compress the fuel-air mixture to one-seventh of its original volume.

Diesel engines generally have a higher compression ratio than petrol engines, and there’s a reason for that. Read from our article on the difference between the compression ratio of diesel and petrol engines. High-performance sports cars have a high compression ratio. For instance, the Ferrari 458 has a 14:1 compression ratio. Formula 1 cars typically have a 17:1 compression ratio.

Why does the compression ratio matter in cars?

Understanding compression ratio is but the first step. More important is understanding why it matters and why carmakers care about it. As mentioned before, a high compression ratio is desirable in engines. You are probably asking why so. 

Well, a high CR gives an engine the ability to generate more energy from combustion because of better thermal efficiency than a low-CR engine using the same amount of fuel. This results in a longer expansion cycle, i.e. lower exhaust temperatures and more mechanical power output.

With increasing compression ratio, the piston moves higher in the cylinder. The higher the piston is, the greater force is exerted during expansion to push the piston down, resulting in much greater power. In other words, an engine that compresses the gas mixture to one-seventeenth of its original volume will produce greater power during expansion than another engine capable of compressing gas to only one-seventh. 

Static vs dynamic compression ratio

Compression ratio can be considered static or dynamic. 

Static compression ratio in a combustion engine is computed based on the volumes of the cylinder and the combustion chamber. 

The dynamic compression ratio, on the other hand, is a little bit advanced and takes into consideration the gases entering and leaving the cylinder during the compression phase. 

What happens if compression ratio is too high?

You’re probably thinking that since a higher CR is desirable, then why not produce all engines with a CR of, say, 50:1 or higher. Unfortunately, this is not feasible because metal can’t withstand the level of stress from such an engine. 

It's a case of too much of a good thing is bad. While a high compression ratio increases efficiency, it can also lead to engine knocking. 

When it comes to engine fuel, you want one that ignites at the precise moment you want it to, not at any other moment. The uncontrolled combustion action is referred to as knocking. Knocking reduces torque in a vehicle and can result in serious damages to the engine.

The higher the compression ratio, the higher chance of the engine knocking. 

What octane rating is needed for high compression ratio?

High compression ratio requires high-octane fuel. Octane ratings are used as a standard of the gasoline’s ability to resist auto-ignition.When compressed, gases heat up. Since low-octane fuel is more likely to combust under pressure spontaneously, it is not recommended in vehicles with high compression. Instead, a higher-octane fuel is preferred; or simply:

Higher CR = Higher Octane

An octane rating of 93 is ideal for engines with compression ratio of less than 8.5:1. Octane 95 and 98 are more suitable for vehicles with CR ranging from 8.5:1 to 10:1. Racing cars make use of racing fuels with much higher octane rating.

Conclusion

So, what is a good compression ratio? To sum it up, a higher CR means that the engine generates more power using the same amount of fuel than an engine with lower CR. This is desirable in vehicles since it is good for fuel economy. However, a very high compression ratio is dangerous as it can cause knocking and damage the engine.

By Sam O.