How does the new Infiniti VC-T variable compression ratio engine work?

An engine with a variable compression ratio is a longtime dream among engineers. It would prevent knock easily, while still making a very efficient piece of machinery. But early attempts have never worked. Even when electronics came to give a hand, no one has managed to make a variable compression ratio engine reliable enough to reach the production lines. Until today, when Infiniti announced the VC-T (Variable Compression – Turbocharged) engine. Something that took them more than 20 years to develop.

infiniti_vc-t_2

The new engine is a 2.0 4-cylinder unit that would generate, according to early reports, from magazines that have already got in touch with it, the same power of a V6, such as the 3.5 V6 from Nissan (around 200 kW and 350 Nm). But how does it work? Here is a diagram that may help us figure it out.

infiniti_vc-t_1

Instead of having a fixed point of connection to the crank shaft, each connecting rod has what Infiniti calls a multi-link. Imagine a connecting rod with no need of a bearing cap, only of a pin a little bigger than the one used at its small end. The multi-link makes the role of the bearing. In one extreme, it is connected to the connecting rod. In the other, to a shorter connecting rod attached to a control shaft.

Infiniti_VC-T_patent_2

This control shaft is moved by a single actuator arm that is controlled by an electric engine. Infiniti calls it “harmonic drive”. It can vary the compression ratio from 8:1 up to 14:1. And to any compression ratio inbetween these 2. You can have a better idea on the way it works with this video. Check it out.

Considering the way it works poses no more mystery, some questions remain that the patents do not answer. The first and more pressing one is: will this engine resist the high strengths a common engine endures after many thousands of kilometers? Won’t they make the control shaft adopt higher or lower compression ratios than the originally intended?  How much more will this engine cost over common ones? Which will be the first car to use it? Infiniti promises to answer some of these questions at the Paris Motor Show. Anyway, the most amazing thing about the VC-T is not that it took so long for Infiniti to develop it. It is that the Japanese premium manufacturer has actually managed to get it done.

Gustavo Henrique Ruffo

I have been an automotive journalist since 1998 and have worked for many important Brazilian newspapers and magazines, such as the local edition of Car and Driver and Quatro Rodas, Brazilian's biggest car magazine. I have also worked for foreign websites, such as World Car Fans and won a few journalism prizes, among them three SAE Journalism Awards and the 2017 IAM RoadSmart Safety Award. I am the author of "The Traffic Cholesterol", a book about bad drivers that you can buy at Hotmart, Google Play, Amazon and Kobo.