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Did You Know?: Compression Ratio

What is compression ratio?

Every engine has a specific compression ratio. The air-fuel mixture is compressed in the cylinder to create an ignition, the force of which depends on the compression ratio: the volume of the cylinder when the piston is at the bottom of its stroke versus the volume of the cylinder when the piston is at the top of its stroke. Incidentally, you should know that engine displacement refers to the total capacity of all the pistons during a complete cycle.

The ignition occurs when the piston is at the top of its stroke, therefore at the top of the cylinder (aka the cylinder head) which forms the combustion chamber. The remaining volume of air-fuel mixture inside the combustion chamber makes it possible to determine the compression ratio proportionally.

Compression ratios usually range from 8:1 to 10:1. A higher compression ratio -- say, from 12:1 to 14:1 -- means higher combustion efficiency.

2013 Ford Shelby GT500 engine
Photo: Sébastien D'Amour

The benefits
Higher compression ratios and combustion efficiency mean more power with less fuel, and fewer exhaust gases. On the other hand, the more violent ignitions intensify heat, friction, and wear, making it tough on the engine's internal components. Automakers have to find the right compromise.

Consider Mazda's SKYACTIV technology, for example. Engineers redesigned the internal components to increase the piston's stroke to allow a higher compression ratio. That being said, drivers who want to take advantage of it absolutely need to use premium gasoline (gasoline with a higher octane rating).

Boosted engines and diesel engines
Naturally aspirated engines can have a higher compression ratio than boosted engines (supercharged or turbocharged). In a turbocharged engine, for instance, the air entering the combustion chamber is already pressurized, so the compression ratio must be slightly lower to avoid excessive stress on the components. Supercharged engines usually have a compression ratio between 8:1 and 8.5:1.

However, when it comes to diesel engines, the absence of spark plugs requires a higher compression ratio -- from about 14:1 to as much as 22:1. They use hot air to vaporize and then ignite the fuel.

Fuel grades
The more compression and heat the fuel can withstand before igniting, the higher the octane number (87, 91, 94, etc.) and the higher the fuel grade (regular, premium, etc.).

Like I said; a higher compression ratio means more heat inside the engine. A fuel with a higher octane rating can withstand a greater rise in temperature and is less prone to premature ignition or pre-ignition, also known as engine knock. That phenomenon alters the piston's stroke and can lead to serious engine damage.