What are compression ratios and why are we varying them?

If you google “Compression Ratio”, two things are going to happen. hopefully, you are going to misspell definition just like I did, and Google will tell you that a compression ratio is “the ratio of the maximum to minimum volume in the cylinder of an internal combustion engine”.

Now that may seem simple for some, but lets break it down even further. Imagine a piston in a cylinder moving up and down, at the top of the cycle there is a small gap between the top of the piston and top of the cylinder. If the total amount of max space inside the cylinder is 10 times what the minimum space is, you got yourself a compression ratio of 10:1. (Most Gasoline Engines run close to 10:1)

Most engines compression ratios are constant and can only be changed by swapping engine components.

The exception to the rule is the Variable Compression Turbo (VC-T) engine Nissan introduced to production in 2015 in the Infintit QX50. The VC-T can change its ratio by using a lever to move the pistons pivot point up or down, increasing or decreasing the total amount of space it travels in the cylinder.

Why?

Because even though high compression ratios are typically more powerful and efficient, but you can have too much of a good thing. High ratios increase pressure in the cylinders, pressure=heat, heat + gas = early combustion (knock). Which is just as bad as it sounds.

The Nissan VC-T engine can alter its compression ratio depending on what is required from the scenario at that moment.  Or to quote Nissan directly, “The sophisticated engine control logic automatically applies the optimum ratio, depending on what the driving situation demands.”

Nissan has said that this engine make comparable power to larger displacement V6s, with 27 % better fuel economy. Will the complexity make it unreliable, or will the technology allow for car makers to start making cars that are fun and efficient at the same time? Tell us what you think.