A look at Nissan's variable compression system

Aug. 1, 2020
Nissan is the first manufacturer to utilize a truly variable compression system that changes the stroke of the engine through electromechanical technology. This article will look at the technology as well as considerations for service, diagnosis, and repair.

Nissan is the first manufacturer to utilize a truly variable compression system that changes the stroke of the engine through electromechanical technology. This article will look at the technology as well as considerations for service, diagnosis, and repair.

It's hard to imagine the internal combustion engine is over 150 years old. What's even more surprising is that most of the advancements in engine technology that we are utilizing on today's engines have been developed in the last twenty years. For the longest time, engines were mechanically interconnected via timing chains or gears with little ability to make adjustments. Fast forward to today and we have engines capable of varying valve timing and simulating varying compression ratios. With the introduction of the 2019 Altima 2.0L, dubbed the VC Turbo, Nissan has taken the variable compression idea a step further by creating an engine that mechanically changes the compression ratio by altering the stroke of the piston.  A look at the technology as well as the progression in technology leading up to it will help to put this in context and prepare the technician for seeing this vehicle in their bays.

The Nissan VC Turbo sports a single-scroll turbo induction system. (image courtesy of Nissan)

A Stroke Of Genius

A look at some concepts of engine operation helps understand the importance of the development of Nissan's technology. While the actual operation of the engine on an engineering level is much more extensive, the definitions you see below are provided to give context to the reader.

Compression RatioCompression ratio can be defined by comparing the volume of the combined cylinder and combustion chamber at Bottom Dead Center (BDC), or lowest point of piston travel, against the volume of the cylinder and combustion chamber at top dead center (TDC), the highest point of piston travel.

A Unique motor and eccentric system allow the engine’s compression ratio to be changed by altering the TDC location of the piston. (image courtesy of Nissan)

This concept of compression ratio has existed since the beginning of internal combustion engine design. In terms of performance, engine builders have long desired high compression ratios which take a given volume of air and fuel and squeeze it into a high-temperature event that results in a highly excited power stroke. While high compression ratios are quite desirable for performance, they are not ideal or desirable when attempting to force induction. More induction typically means more pressure and temperature. For example, a compression ratio of 12:1 would result in a catastrophic failure, should it be enhanced by the boost from a supercharger or turbo on a production engine.

This image provides a visual of the movement within the VC system’s inner workings. (image courtesy of Nissan)

With many manufacturers seeking to downsize their engines for weight reduction, many have begun to adopt turbos to compensate for the loss in displacement. In this case, manufacturers desire to utilize the full ability of high compression ratio while also utilizing forced induction to provide peak volumetric efficiency.

Otto Cycle The Otto cycle, named for German scientist Nicholaus Otto, is the traditional engine cycle that we know as automotive technicians. The four-stroke, five event cycle that consists of Intake, compression, combustion, power, and exhaust, has been utilized across internal combustion engines for a long, long time. In this cycle, we typically think of the four strokes, or movements up or down of the piston, to be equal in length. This cycle is flawed in the aspect that some of the movement is essentially a wasted opportunity to create efficiency and power. For example, the engine's displacement and ability to take in air and fuel are dictated by the length of the intake stroke and the physical limitations of the induction system. The same can be said for the compression, power, and exhaust stroke. The equality of the strokes is prohibitive.

Now imagine altering the length of the strokes to our advantage. If we could utilize less fuel, less physical loss during compression, and ultimately utilize a long power stroke to fully harness the power generated during combustion, we would have an engine capable of performing much more efficiently. Segue to the Atkinson Cycle.

A visual of the shaft utilized to alter the height of piston travel via an electric motor and ECM input. (image courtesy of Nissan)

When Toyota first released its Prius, it boasted the ability to gain efficiency through the use of an Atkinson cycle. This raised many eyebrows across the automotive community as an Atkinson cycle engine typically has four unequal strokes of the piston. The Atkinson cycle can often be found on old farm machinery.

Atkinson Cycle –  The Atkinson Cycle typically had a mechanism to allow for an adjusted stroke of the piston. This enabled the engine to utilize a short intake stroke, followed by a short compression stroke then resulting in a full-length power and exhaust stroke. In a nutshell: a small amount of air and fuel and work with an output that is proportionally greater. In one word: “efficiency.”

So how did Toyota achieve this without altering the stroke of the piston? They utilized the variable valve timing to alter the effective “bottom” of the cylinder. By changing the closing-point of the intake valve, the effective length of the intake stroke is shortened, as is the compression stroke. This results in the effect of the Atkinson cycle without the fancy mechanical gear in the crankcase to change the compression ratio. This system is ultimately limited by the valves' ability to open and close, which is usually dependent on the camshaft and cam phasing technology. These fixed constants do not allow for an infinitely variable compression ratio for a variety of different circumstances, which is where Nissan has picked up the ball with the 2.0L VC Turbo engine.

The VC Turbo is a unique combination of variable compression, turbo, direct injection and other technologies.

Nissan 2.0L VC Turbo

This Nissan 2.0L turbo, named by Nissan the “VC-Turbo”, is a limited production engine that made its debut in the 2019 Nissan Altima as a replacement for the long-standing 3.5L V-6 which has reached its end. The VC Turbo also appears on the INFINITI QX50 crossover. The majority of the Altima line will receive the 2.5-liter cousin of the VC-turbo, which indicates that the VC-Turbo is being "piloted" rather than fully releasing it on one of the most popular car platforms. The VC-Turbo has been twenty years in development but the limited release causes one to wonder if Nissan wants to further vet this technology.

The Nissan VC Turbo is the first of its kind with truly variable compression ratios. (image courtesy of Nissan)

Unlike the previously claimed Atkinson cycle engines, Nissan’s VC-Turbo is a true variable compression engine and Nissan claims it to be the World’s first. According to Nissan “VC-Turbo changes its compression ratio seamlessly through an advanced multi-link system, continuously raising or lowering the pistons' reach to transform compression ratio – offering both power and efficiency on demand. A high compression ratio gives greater efficiency, but in certain applications poses the risk of premature combustion (knocking). A low compression ratio allows for greater power and torque and avoids knocking.”.

The multi-link system is controlled through the use of a motor mounted below/adjacent to the engine oil pan. The system utilizes typical powertrain inputs as well as inputs from a somewhat-sophisticated variable valve timing and cam sensor setup (that could be a Motor Age article in itself). The multi-link motor moves an eccentric-like mechanism that alters the high and low point of travel of the pistons, ultimately, changing the compression ratio. This change in compression is applied to all-four cylinders equally. This alteration allows Nissan the ability to utilize high compression ratios for greater efficiency. The downside to higher compression in some cases is the risk of premature combustion known as knocking. In these cases, Nissan utilizes a lower compression ratio for greater power and torque while avoiding knocking. The range of compression ratios on this engine varies between 8:1, an ideal ratio for forced induction, to 14:1, a ratio which was once inconceivable to utilize on a forced-induction engine. The overall variability of the compression ratios in conjunction with variable valve timing and forced induction allows for a great performing engine with excellent fuel economy, low emissions, and an increase of 8 horsepower (248@5600RPM) over its 3.5L predecessor while arriving in a lighter 4 cylinder package. Nissan claims that this engine utilizes two engine cycles: Atkinson and a "regular" cycle (see Otto). The Atkinson cycle in this case is achieved through a change in the piston and a change in variable valve timing.

An underside view of the VC turbo with actuating motor and oil pan assembly removed

Observations and Considerations for the future – Service and Diagnostic considerations

As in all science, the details are in the data. With this engine being so new, it is difficult to provide concrete detail of case-studies and failure analysis. However, there are a few simple observations we can make as technicians that will certainly weigh-in to how this engine stands the test of time. Having had one of these engines apart on a stand allowed for some observations. First, it seems like the eccentric mechanism that controls the travel of the pistons is quite robust. According to Nissan, these components are made from a high-carbon steel alloy. It is highly unlikely that these will be subject to component failure providing the engine is serviced and oil is replaced at recommended (or better) service intervals. Service and regular oil changes are critical. Second, the motor that controls the multi-link mechanism also looks to be very robust. My thoughts on the system were: “What happens if the motor fails?"

As with most other technologies, it seems there is a fail-safe strategy that would allow the engine to operate while setting a DTC and illuminating the MIL. The driver could still operate the vehicle somewhat-normally until It can be repaired.

This motor is responsible for the actuation of the change in piston TDC height, resulting in variable compression ratios.

It would be helpful for the technician working on this vehicle to have access to accurate service information and capable scan tools. Factory access for Nissan is available on a subscription basis starting at $19.99 per day. While it is somewhat-cumbersome to navigate, it will provide the technician with the most complete package of service information for the Nissan platform.

In summary, the combination of technologies utilized in this engine is impressive and the first of its kind, according to Nissan. While many other manufacturers have claimed to utilize the Atkinson cycle with smoke and mirrors (VVT), Nissan is doing it through changes in the stroke of the piston. How this plays out over time remains to be seen but hopefully, when you find one of these in your bays you will quickly recognize this engine as unique.

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