Infiniti QX50 and the Nissan VC-T variable compression ratio engine

Decades ago, variable valve cam timing became a thing and is now present on most gasoline engines. Sometime after that, we went away from the distributor and adopted direct ignition systems that allow for multiple sparks during one power stroke. Direct injection was another big thing, allowing a precise amount of fuel injected directly into the combustion chamber at just the right time.

The idea behind all of those advancements is to generate power from the engine more efficiently. Combined, the above systems allow for a flatter torque curve in a smaller engine while retaining, and likely raising peak power output. Better fuel economy and lower emissions came along with it.

Concurrently, turbo technology has also been advancing. The lifespan of turbochargers units has been extended. They no longer cooked themselves, spewing oil all over the back of the car, or blow gaskets. Physical changes, such as twin-scroll setup significantly reduced turbo lag without sacrificing peak power. With turbos, there were always fears of detonation. Pre-ignition, caused by high cylinder pressures and temperatures could have catastrophic results. Early on, this issue was solved by adding intercoolers, using a lower cylinder compression ratio, typically around 8.5:1, retarded ignition timing, and higher octane gasoline.

The above-mentioned improvements in fuel, spark, and valve timing control have pretty much solved that problem. Generally, the compression ratio of turbocharged engines is still slightly lower than those of naturally aspired engines.

With all those technologies, and advanced turbocharging already being mainstream, what is the next new thing for the internal combustion engine?

An engine with a variable compression ratio may be one such thing. It isn’t exactly a new idea, but it is, as Nissan calls it “world’s first production-ready variable compression turbo inline 4-cylinder.” and currently has three vehicles available with it: the Infiniti QX50, QX55, and the new Nissan Altima. If you know of another vehicle with a variable compression production engine, please tell me about it.

Variable compression ratio makes the most sense when applied to turbocharged gasoline engines. Nissan’s 2.0-liter VC-Turbo varies the compression ratio between 8.0: 1 and 14.0:1. That is a huge range. I can’t think of a modern automotive gasoline engine that has either such a low or such high compression ratio.

In power mode, with the turbo dumping all the air it can into the combustion chambers, the compression ratio lowers. In efficiency mode, with the engine working at an even speed and load, the turbocharger is resting, and the engine breathing on its own, the compression ratio raises. At the higher rate of compression, aided by the earlier mention advancements, fuel burns more efficiently, creating more power (vs. lower compression ratio, non-turbo applications) and fewer emissions.

So, does a sum of all of those fuel, ignition, engine timing, turbocharging, and variable compression ratio engines produce amazing results? And more importantly, is this something we can notice in real-world driving?

I drove the Infiniti QX50 AWD around for a few days in various conditions: city, highway, chilly rainy day, and a hot and humid day. Whatever is going on under the hood is really transparent to the driver. The engine does not really sound any different than a conventional engine, perhaps because like most engines today, its sound-maker is muted by the turbo. The one odd noise is what can be described as a knock at low engine speeds. I imagine that that’s the noise of the compression varying itself.

The 2.0-liter VC-Turbo in the QX50 makes 268 horsepower at 5,600 rpm and 280 lb-ft (380 Nm) of torque between 1,600 and 4,800 rpm. The Altima, which I did not drive, makes 248 hp and 273 lb-ft in front-wheel-drive format only. Those numbers are certainly are not bad but at the same time, they are not anything revolutionary. 

Similarly, the fuel economy is good but not necessarily significantly improved, if at all, over the competition. The FWD QX50 gets 24 MPG in the city and 31 on the highway. The AWD version gets the same 24 MPG in the city but drops to 30MPG on the highway. Like the power out of this four-cylinder, the fuel economy of this CUV would have been most impressive a decade or so ago.

In all cases, the power gets from the engine to the wheels via Nissan’s continuously variable transmission (CVT). While it seems like Nissan has been continuously attempting to improve it and make it feel more like a conventional transmission, most enthusiasts will know right away what they’re dealing with. In theory, the CVT works great. Imagine, at WOT keeping the engine steady at its peak power while accelerating the vehicle by reducing the drive gear.

My issue with this transmission was that I felt like I couldn’t really explore this engine. I couldn’t feel its power band, torque curve, and reaction to throttle inputs because it was all masked by the transmission that had a mind of its own. I want to say that it is doing quite a disservice to the 20 years of research, development, and engineering that took to create this engine.

And that is the other issue. It took Nissan a significant amount of time and money to develop this variable compression ratio engine. Unfortunately, the results are not in any way revolutionary, even if the engine is. While it may still be in its production infancy, the days of the internal combustion engine as we know it are coming to an end. All that effort would have been better spent on hybrid or other electrification systems that work in conjunction with existing engines.

The real shame here is that this isn’t a gizmo. It is amazing technology, the next step in evolution, with a solid potential of making the internal combustion engine significantly better. Unfortunately, it seems to be a decade or two too late.

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5 responses to “Infiniti QX50 and the Nissan VC-T variable compression ratio engine”

  1. Alexander Moore Avatar

    Nissan: why don’t we make every component continuously variable! it’ll be great

  2. outback_ute Avatar

    I’ve seen a variable compression ratio engine running before, but it was a single cylinder laboratory engine and not turbocharged. It was used to explore pre-ignition and different fuels. I can’t remember exactly how the compression was varied as it was 25 years ago but I am pretty sure it wasn’t anything like how this is done.

    As you say it is almost a wonder that all that effort and innovation didn’t produce more impressive results.

  3. bhtooefr Avatar

    Not a complete vehicle, but a retrofit bicycle engine with friction drive: https://onlinebicyclemuseum.co.uk/1951-lohmann-18cc-diesel-engine-new-old-stock-unused/

    Compression-ignition, with variable compression accomplished by sliding the cylinder towards/away from the crankshaft

    1. bhtooefr Avatar

      Oh, also, if you’re fine with reducing effective displacement as well, you can reduce effective compression through either early or late intake valve closing.

      …combine that with variable valve timing, and you can now vary effective compression (and displacement).

      That adds the following engines to the list of variable compression engines:

      Wide-authority VVT being used to implement part-time LIVC:
      Mazda SkyActiv-G family: P3 (1.3), P5 (1.5), PE (2.0), PY (2.5, 2.5T)
      Toyota Dynamic Force family in non-hybrid form (not sure about the turbo ones though): M15 (1.5), M20 (2.0), A25 (2.5) (not sure about the T24 or V35)
      (I might be missing some older VVTi-W Toyota engines as well.)

      Camless (at least on the intake side) engines being used to implement part-time LIVC:
      FIAT FIRE MultiAir (1.4)
      FIAT SGE TwinAir (0.9T, 1.0 NA)
      FCA GSE: T3/T200 (1.0T), T4/T270 (1.3)
      FCA GME (2.0T)
      Chrysler Tigershark (2.4)
      Koenigsegg TFG (2.0)

      Cam-switching being used to implement part-time EIVC:
      Volkswagen EA888 Gen 3B (2.0T)

  4. Salguod Avatar

    Saab showed a variable compression engine over 20 years ago, but it was shelved when GM took over.

    https://en.m.wikipedia.org/wiki/Saab_Variable_Compression_engine

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