Infiniti's Variable Compression Engine - Why It's Important And How It Works

Variable compression engines have existed as concepts for a while, but none of them reached production. The new unit from Infiniti is close to series production, but we prefer to remain skeptical and wait for it to reach showrooms before calling the VC-T a production power plant, because you cannot buy it yet. Next year might be a different matter for the new engine, as it is confirmed to feature in the next QX50.

Infiniti was skimpy on details when it unveiled this engine, but they still explained how it works and what are its features. Thanks to the advancement in technology when comparing the VC-T to the engines of yesteryear, we expect the Japanese corporation to possess every means necessary to build this unit on a large scale.

The previous variable compression engines also followed similar ideas, but none of them ended up in production. Saab’s SCV was scrapped by General Motors because of high costs, while Mercedes-Benz’s so-called DiesOtto did not make it to production.

The only benefit of Dieselgate is that these units might come back on the tables of automakers, and they have a new chance of production.

Why variable compression?

Fitting a smaller engine is not always the best solution. Engineers have worked with compression to change the characteristics of a power plant, but they have not made a production car that could change it.

This move, once it is done, will be comparable to the ability to adjust propeller angles in aircraft, which brought a significant increase in performance and economy in the golden age of piston engined aircraft.

In the case of cars, employing a variable compression ratio will allow automakers to switch from an engine focused on performance to one that can be extremely economical, all without making noticeable changes to the vehicle.

The concept is not unique at all, as it was first imagined and described by Mr. Harry Ricardo (the man that devised the octane rating system) in the 1920s, but the time’s technology was not as capable as it is now, making those units unfeasible.

Automakers have experimented with various compression ratings, but once a unit can switch between two, it should operate with a level of efficiency that cannot be met by a conventional equivalent. In the case of Infiniti’s unit, it can work with a compression ratio that ranges between 8:1 and 14:1.

How does it work?

Infiniti’s solution involves something that was not done by its predecessors at Saab — altering the link between the piston and the crankshaft. Instead of conventional con-rods and a traditional crankshaft, a multi-link pivoting arm with connections at each end is employed.

Each piston is connected to an end of the pivoting arm, while another end is linked to the main shaft. However, the part that is joined to the shaft has another arm, which is operated by a system called "Harmonic Drive." That component does all the magic, while the control shaft that holds the lower link of the actuator arm acts as a balancing shaft.

With the movement of the Harmonic Drive, the maximum height reached by the piston in the combustion chamber can be changed, which leads to altering the compression ratio.

The higher the piston goes in the cylinder, the greater the compression ratio, and vice versa. Instead of compromising between one compression ratio and another, Infiniti offers the best of both worlds.

What are the setbacks?

First of all, a unit like this has not been implemented in production cars. No motorsport prototypes have ever used anything like it, which means that the automaker will have to spend a considerable number of hours to develop it for production automobiles.

We are mentioning this because various race-derived technologies exist today because of the advances done for racing cars, which the VC-T only received in the field of materials technology. Because everything will be done almost from scratch, it will lead to increased costs, which might be trickled down to the consumers.

Infiniti will start selling this new engine in 2017, so we will have a price on the car that comes with it by the end of next year, when the QX50 should be in showrooms across the world.

Its price might be a strong deterrent for some clients, but Infiniti is fortunate enough to be a premium brand so that it can get away with something a bit more expensive than the norm.

Other setbacks include developing ancillary components for this engine type, which will probably be a headache for engineers. Everything, from the water pump and oil pump, to the alternator and even the most simple bolts will have to be able to operate in the intricate manner dictated by this engine type. Because of this pressure, many eyes will be on this type of powerplant, which will have high expectations ahead of it.

What can go wrong?

Infiniti’s solution is somewhat modular, which means that it can be scaled for four-cylinder engines. Unfortunately, it does not add up with V6 or V8s, so this might not save all internal combustion units in a few years time.

Infiniti might begin to offer the engine in a single displacement configuration, with a single transmission option, and other limitations. These problems could deter sales from some models, while some customers will be upset they cannot experience the latest technology in the automotive industry.

The worst thing that could go wrong with this engine is a design flaw or a problem with a supplier. If anything that makes up this power plant will fail, consumers will not look twice at an Infiniti after an unfortunate experience with the revolutionary engine, while other automakers could ditch their similar plans out of fear or making a comparable error.

For the moment, we can only hope for the best in the case of this engine, as it has to go smoothly and impress the audience with all its traits. If Infiniti pulls this off, we are about to witness a new age in the automotive industry, where electric vehicles and internal combustion engined cars will share the same roads.