By on November 29, 2017

Image: 2019 Infiniti QX50 VC Turbo Engine

At this week’s L.A. Auto Show and Traffic Negotiation Event, Infiniti will reveal the next generation QX50 — an overdue replacement for the aged model formerly known as the EX35. While the introduction of a crossover that’s losing its V6 and rear-wheel-drive platform wouldn’t normally interest me, the model’s new engine does.

Today we’re going to discuss variable displacement and the future of internal combustion engines. Fly or flop, what say you?

A little background is necessary, in case not all of you are avid readers of TTAC (like you should be). The upcoming QX50, set to appear in showrooms next summer, will come equipped with the first-ever variable displacement engine. Earlier in November, Matthew Guy did a nice job summarizing how the engine works.

Basically, the engine is able to alter the reach of the pistons, adjusting the compression ratio between 8:1 (for power) and 14:1 (for efficiency). This effectively alters the displacement of the engine on the fly. It’s never been done before. This leads me to today’s inquiry, and a few of the things I’ve been considering recently:

  • This is a major change for internal combustion engines, and the largest shift in piston engine function in the past hundred years. Will other manufacturers quickly follow Nissan’s lead, or shy away from this sort of bold engineering move?
  • Generally speaking, major tech advances in engine technology have often been accompanied by spotty reliability for a period of time after introduction. Eventually, the wrinkles are ironed out by the manufacturer, but by that time the majority of the public has rejected the technology. Does reliability in such a complex engine concern you? Or is Nissan, builder of VQ V6 engines, competent enough in that area to assuage your fears?
  • There’s always a risk that the market will not accept a “new way” of propelling a car. But perhaps the fuel economy promised by the technology will override any other concerns. Do you think this is the sort of engine the public is brave enough to purchase, assuming the idea of variable compression is something they can comprehend?

One more thing. Infiniti and Nissan are betting big on this technology’s success to ensure a return on their investment. After a development timeline of over 20 years, the company has decided against employing the engine on some high-end, limited-production halo car, installing it instead in a mid-market crossover which will theoretically sell many thousands of units. If it fails, it’s going to fail spectacularly and publicly. If it succeeds, it’s going to change the future of the Infiniti lineup at the very least, and perhaps passenger cars as a whole.

Use your crystal ball, B&B. What do you see in the future of the variable displacement engine?

[Image: Infiniti]

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53 Comments on “QOTD: Variable Displacement – New Hotness or Inevitable Blowout?...”

  • avatar
    bumpy ii

    Variable displacement /= variable compression. To change the displacement, you’d have to vary the stroke length.

    • 0 avatar

      I was going by what Matthew Guy said in his article on this subject.

    • 0 avatar

      Ah ha, see link below. The stroke is varied.

      • 0 avatar
        bumpy ii

        Not according to the press release. The length of the rod from the piston to the multi-link, and the multi-link to the center of the crankshaft remains the same. To quote the diagram, “The multi-link adjusts the height the piston can reach within the cylinder, thus changing the compression ratio.”

        To make up some numbers here, a 100mm stroke in a conventional engine would have the piston travel from the top of the block (zero mm) to 100 mm below the top of the block. Infiniti’s gadget lets the piston move from 20mm below the top of the block to 120mm below the top of the block (or 10mm to 110mm, or 5mm to 105mm, etc.)

      • 0 avatar

        Addendum: I took a very quick look at the link below. It does say, “The ingenuity of VC-Turbo engine technology lies in its ability to transform itself and seamlessly raise or lower the height the pistons reach. As a consequence, the displacement of the engine changes . . . .”

        It’s a little unclear without an animation. It looks like:
        – Swept volume could be described as fixed from the standpoint of the distance between the piston’s positions at “power” TDC and “power” BDC or its positions at “efficiency” TDC and “efficiency” BDC.
        – Swept volume could be described as variable from the standpoint of (for example) the distance between the piston’s positions at “power” TDC and “power BDC” vs “power” TDC and “efficiency” BDC.

        Since the system may adjust on the fly, during a single stroke, variable displacement probably is a true description. I’d argue that variable compression is a more straightforward description.

      • 0 avatar

        If the stroke changes at all, it is by a very small amount. The point of this technology is to vary the compression ratio. I don’t know why Corey keeps calling it “variable displacement”.

        • 0 avatar

          A very small amount is to be disregarded then?

          “The ingenuity of VC-Turbo engine technology lies in its ability to transform itself and seamlessly raise or lower the height the pistons reach. As a consequence, the displacement of the engine changes . . . .”

          I didn’t say it, Infiniti did. Take it up with them.

          • 0 avatar

            I think the point is that variable compression is a better name, not that variable displacement is untrue. Yes, Infiniti mentions variable displacement in its August 2016 press release, but the subject of the release is “INFINITI VC-Turbo: The world’s first production-ready variable compression ratio engine.”

        • 0 avatar

          @ chris724
          Exactly (I made the same point in the previous article on this engine).

          As far as I can tell there’s two scenarios that might be described as a displacement change:
          One, when the compression ratio changes in the midst of the piston stroke. To expand on the example above, BDC starts out at 110mm, but due to the in-process compression change, stops at 0 instead of 10mm, effectively increasing displacement *for one combustion event*. But a) I doubt the compression ratio change happens within one piston stroke, and b) if it did, it would only affect cylinders on that part of their otto cycle, not then entire engine’s displacement.

          And second, due to lateral movement of the big end of the conrod through the various compression ratio changes, the angle of the conrod will change slightly, and thus cause a very small change in the effective stroke (probably 1mm or less).

          Note that both of those scenarios are incidental to changing the compression ratio (and the first is very temporary). “Variable displacement” implies (to me, at least) the ability to change the size of an engine as a goal unto itself, as a steady state change, and of a degree that is not insubstantial. To call this engine variable displacement is really stretching things.

    • 0 avatar

      Interesting point, bumpy ii. As I understand it, this technology varies combustion chamber size, not the stroke length, in which case you’re right.

      This is mostly a pretext to link a cool animation, but I believe an Atkinson “Cycle Engine” would have a better claim to being called “variable displacement”:

    • 0 avatar

      “By moving the axis of the stroke up and down, compression ratio and by extension displacement can be varied continuously. In this case, the displacement itself ranges from 1997 to 1970 cc and as said the compression ratio from 8:1 to 14:1.”

      In other words, there is a variation of displacement too, but it is negligible compared to the huge variation of CR.

  • avatar
    Landau Calrissian

    Half the commenters here still don’t trust anything with overhead cams or more than 4 gears in an automatic transmission. I can predict an upcoming consensus from the church of the GM 3800.

  • avatar

    I’m hoping to see comments born from well-reasoned thought, not passionate feelings. Leave your biases at the door, please.

    It seems to me that a large portion of the buyers are adopting Turbos in this second wave without much hesitation despite the general unreliability of the first wave (80s) and teething problems this time around (Ford’s incendiary 1.5 Ecoboost for example). I don’t see this going much differently.

    Will it be reliable? Anyone’s guess. But, honestly, the fact that they have spent so long working on variable compression before bringing it to market makes me think that they feel confident that they have got it right. We shall see.

  • avatar

    Corey, given that the bland FWD turd pictured is the replacement for the EX, I’d say the EX shoulda kept on trucking.

    • 0 avatar

      Well I knew YOU would say that. Ha.

      • 0 avatar

        So, here is my general take on it. And I sincerely hope that this comes across as a reasonable evaluation on new car tech such as this. The comment above was “I’m hoping to see comments born from well-reasoned thought, not passionate feelings. Leave your biases at the door, please.” This is my distilled take.

        So, about me, so y’all can understand where my take comes from. I love technology. I love building PCs, I enjoy my smart phone. But I also believe humans are analog, and making things shiny and fancy for the sake of it doesn’t fly with me. So, (and acknowledging that this is just the way we ask the computer to do things for us) I am a card carrying member of the knobs and buttons over touchscreens camp. Heck, if 28CL is the Pope of the Church of 3800, then I am probably Inspector General of Analog Interfaces. So while I acknowledge that the fan speed knob in my car simply asks the CPU for more airflow, rather than being the actual variable resistor, its ok. I just prefer proper, physical feedback when I execute a function. I also hate these digital instrument clusters, but that’s another rant. There’s nothing better than watching the sweep of a needle. I know that the needle is being driven by the ECU, but that’s not the point. A well crafted and laid out gauge cluster is a joy to me. Sadly, these are becoming more and more rare.

        When it comes to powertrain tech, I like to think I am “pragmatic”. Some of the biggest advances in the last… two decades let’s say are “variable”. Variable valve timing, variable cylinder count, and now variable stroke. So when it comes down to reliability/long-term viability, it becomes to me a question of, how robust is the mechanism, and is it optimally positioned for success.

        VVT seems, overall to be reliable, though there have been some notable issues. Early VANOS systems, and I believe some Coyote engines. Overall though, cam phasers are a stationary item, typically hydraulically actuated, and typically reasonably accessible for service, so, this is a good starting point for success, and overall, VVT has become ubiquitous, and the issues not so amplified.

        Displacement on demand is all the rage, and with modern engine management, seems easy enough to pull off. Nothing in the system is prone to failure, BUT: I believe the early Gen 4 smallblock issues with MDS came actually from the engine being cold around the shut off. So, this one is still one a lot of people balk at, but the good news here is the actual physical mechanism, and electronic controls, aren’t overly complicated. So it’s not as if they added all this crap to the engine thats suspect now. Disable it and you have a good ol’ SBC. So this one again, maybe has some quirks but nothing crazy.

        So, both of these, on the surface, could be prone to issues, but also, have stationary mounted hardware that should be relatively robust and serviceable.

        This is where I have my worries about this VC/VD/VC technology. The “variable” hardware in this instance is not a cam phaser bolted to the block, but the con-rod itself. The technical challenges of this are much greater, in my opinion.
        1) If even one of these mechanisms fails, that’s a rebuild.
        2) Connecting rods are under a ton of load.
        3) How do you even get engine management to them?

        Now I’m not saying these guys haven’t considered this all very carefully, I’m just saying, this is a pretty big step of putting potentially fragile components into a very high stress part. And I don’t want any part of it.

        An aside, as some people have mentioned turbos. I personally have no issues with turbocharging except the following items. I have no issues with extracting decent power from a smaller turbo engine than a comparably bigger NA engine. I do think, however, there ought to be a limit. A 1.5T hauling around a Terrain? Heck no. That’s a 2.0T minimum in my books. Tiny, overworked engines are never a recipe for success, regardless of whether they are NA or forced induction. The FI engine though will have more parts and likely be a bit more overstressed. I’m not one of those who thinks a turbo will always be a bad idea, just that proper specification really matters.

        Anyways, I hope this is all cogent, and may spur further intelligent yet impassioned discourse.

  • avatar
    30-mile fetch

    I don’t think the public generally knows or will care what is under the hood. CVTs and hybrids are widely accepted now and their peculiar characteristics are far more noticeable than the variable compression engine will be.

    As for reliability? Not getting into that one again, but even Toyotas can have issues with newly launched technology, such as the carbon buildup on the direct injection IS250s.

    I’d prefer to take risks on a fun or interesting vehicle. This isn’t either.

    • 0 avatar

      I agree, customers will have no clue, its a CUV so they’ll buy tons of them. Normally a new technology comes out on a niche vehicle as a safe path. So this is a bold move… and thus it could back fire in a massive way.

      I know people fear all these “new” turbos but boost is a really an old technology and is very proven. It may not be the best answer for all applications but it works.

      So to me the question here is two fold: #1 does it work? and #2 is this the right application for it? Time will tell. I’m not buying one because it comes wrapped in a package I have no desire to own at this time.

    • 0 avatar

      “I’d prefer to take risks on a fun or interesting vehicle. ”

      You realize what you’ve done here, right? You just convinced me to go get the GTI I’ve been looking at.

  • avatar

    Audi Q5: 25 sticker.
    BMW X3 : 25 sticker.
    Benz GLC : 24 sticker.
    This: 26 sticker.

    And most importantly, the Exxon sticker down the street: 2.499.

  • avatar

    Never buy 1st model year of a new engine. Unfortunately somebody has to, otherwise the bugs never get worked out.

    Let’s hope Nissan does better than Mazda did with the Renesis- 5 model years worth of engine failures before they sorted it out.

  • avatar

    Its variable compression, not displacement, I don’t believe the terms are interchangeable.

    • 0 avatar

      It’s both, by dint of simple geometry.

      Each cylinder is nominally 500 cc or 1/4 of 2.0 liters – when the CR (compression ratio) is at 14 to 1, the default position.

      The bore is likely to be about 80 mm, so let’s assume this round number.

      When the mechanism lowers the crank point, the piston is about 6mm (1/4 inch) lower at top dead center, to give a CR of 8 to 1.

      The increased cylinder volume in cc is thus [pi*(80/2)squared*6}/1000, (the cylinder volume equation from 9th grade or earlier) which is near enough 30 cc. So the cylinder volume, to air rushing in, has gone from 500 to 530 cc, and the engine is 2120 cc at the low 8 to 1 CR, not 2000cc as at 14 to 1 CR.

      At 8 to 1 CR it also now has a lousy combustion chamber shape, because squish, the effect you get when the piston comes close to the cylinder head at Top Dead Center, squishing air/fuel mixture usually towards the spark plug – is ruined, losing capability to control knock and 70 years of work on combustion chamber shape by all engine designers, beginning with the 1949 Cadillac V8.

      Presumably, the engine only spends time at 8 to 1 CR during high power requests, when it can be overfueled to give a rich air/fuel mixture to generate power and avoid knock. Not pretty, and the biggest drawback this monkey motion engine has as I see it.

      No, Nissan didn’t mention this point; it wouldn’t be in their best interests to reveal the shortcoming, they’re proud of themselves for what they did manage to accomplish.

      The Mazda SPPCI engine is really far more clever because it effectively varies the compression ratio “virtually”. At low CR of 14 to 1 where it makes maximunm power, it acts just like the existing SkyActiv engine plus a bit more due to the low pressure supercharger adding extra air needed to make the system work in the other “diesel” mode. About 20 extra horsepower apparently on the 2.5l engine from the current 185.

      When the Mazda goes into “diesel” mode, the CR has to be increased to get auto-ignition of a lean mixture. So they direct a tiny amount of extra fuel right to the spark plug area. When the plug fires, this creates an explosion and rapid expansion of the rich fuel area. Which immediately raises the compression pressure on the remaining main air/fuel mixture which is set deliberately lean. Knock occurs, but that is exactly what is wanted, just like in a diesel, as the lean air fuel mixture essentially explodes all at once. The engine could be called a controlled knock engine, and if you read the C/D review of it, yes, it does knock – on purpose!

      I wouldn’t expect the Infiniti engine to spawn many imitators, once the competitor engineers realize the poor combustion chamber shape resulting at low CR. No, they’ll be off copying the Mazda, where the power CR is at the same CR as the Infiniti is when it is at its most efficient, 14 to 1. The Mazda will be much more fuel efficient when it runs in diesel mode with a lean (under-fueled) mixture.

      Famous last words. Mea culpa. I could be wrong even though it all seems straightforward enough to understand if you spend some time at it.

    • 0 avatar

      JohnTuarus – – –

      JT: “Its variable compression, not displacement,..”

      Actually, all ICE’s vary their compression during their 4 phase (or 2 phase) Otto cycle operation.
      But not all ICE’s can vary their maximum displacement.
      To my understanding, the correct term (and that used in the original French patent) is “Variable Displacement”.


      • 0 avatar

        A normal ICE can’t very the compression _ratio_, “variable compression” is short-hand for variable compression ratio which is unique to this engine (from a production point of view, variable compression ratio engines have been used for years in experimental applications.

        Looking at the mechanism I had a bit of a lightbulb moment. The stroke does change, due to the adjusting mechanism being on the opposite side of the crankshaft from the conrod.

        From an article on Ward’s:

        “Engine bore is 84 mm, but the stroke varies from 90.1 mm at 8:1 compression ratio to 88.9 at 14:1, effectively changing volume from 1,997 cc to 1,970 cc.”

  • avatar

    Two new technologies; which one is best?
    Mazda’s Skyactive-X or Nissan’s variable compression ratio engine?
    I’m betting on Mazda’s technology. 30% fuel efficiency gains with less mechanical complexity.

  • avatar

    I’d rather see cyl deactivation more universally applied than variable compression ratios. Although even that can be problematic depending on how it is applied.

    • 0 avatar

      PrincipalDan – – –

      I’m afraid the jury is still out for me on this mechanical Rube-Goldberg approach.
      Can it be made robustly enough for long endurance? (I.e., 200K miles?)
      Can it handle high torque and high RPMs and hIgh HP?
      How much more money will such an engine cost?
      If it’s great why haven’t the innovative Germans pursued this? (I.e., What do they know that Infiniti doesn’t?)

      Yes, cylinder deactivation is one way to change engine displacement, but are their bad consequences for that too? (I/e., carbonization; engine wear by imbalance; etc)

      Now that direct injection ALONE has been shown to cause problems, I am suspicious of that, and even turbocharging!


  • avatar

    I don’t know.

    I think that a HCCI (GM’s or Mazda’s) engine may net the same results with far less Rube Goldberg-ian busyness going on inside. The other 98% who don’t care what propels the car will probably never take the time to find out what variable displacement or compression means.

    Whither hybrid or electric cars? While the take rate is still tiny, there’s nowhere to go but up. I think that Hybrids and PHEVs have a pretty decent mind share right now. This will come off as another complex gasoline engine that has a bunch of acronyms no one can explain.

  • avatar

    Here is the information from Infiniti. I’m no engineer and Infiniti never writes, “variable displacement,” but their description of how this technology is achieved is by adjusting the stroke to create additional compression on demand.

  • avatar

    I have been itching to point this out….

    How on earth has Toyota gotten the Dynamic Force 2.5 4 cylinder in the new Camry so damn efficient? Its rated at 28/39/32 in most trims and 29/41/34 in basic trims. These are some pretty lofty fuel economy numbers for a mid-size naturally aspirated car sporting a regular geared transmission.

    Some tidbits
    – 13:1 compression
    – Direct and port injection
    – Variable control oil and cooling
    – wide range electrically actuated valve timing
    – 8 Speed auto

    I just dont see the need for all the complexity when Toyota is showing producing the results above with relatively straightforward engine design and obsession with incremental improvements.

    • 0 avatar

      Good point. Slow and steady wins the race in the automobile industry – there are few advantages to a “moonshot” engine program and plenty of risks and downsides to it. All joking aside, look at how long it took Volkswagen to get the DSG transmission and the turbocharged 4 cylinder (in 1.8T guise originally, then 2.0T) up to acceptable levels of reliability? A solid 5 years, and some would argue 10. The current EA888 engine variants are very good, but it took them a long time to get here.

      VW’s move to modular systems for everything has helped them do the incremental improvement thing much better without throwing the baby out with the bathwater. This applies to infotainment, HVAC, steering racks, suspension kits and engines. Audi started doing this with the previous generation A4, making incremental improvements over the span of the vehicle’s life.

      The Japanese incremental improvement philosophy is a far better bet in the auto industry.

    • 0 avatar

      Sounds like Toyota used the same engineering that Mazda used with the original Skyactive engine design plus 2 more gears in the trans. Maybe that’s why they recently hooked up…

    • 0 avatar

      @ suspekt – Some of those tidbits are pretty darn complex to those of us of a certain age. But I definitely agree that Toyota deserves praise for incorporating so many proven technologies (dual injection that’s been used successfully in various Lexus engines, pseudo-Atkinson valve timing that’s been used successfully in the Prius’ engine, and so forth). I especially like the use of dual injection. I woudn’t call DI-only engines half-baked, but I don’t like that industry took steps backwards from port-injected engines in terms of particulate emissions and maintenance/reliability concerns.

  • avatar

    This is incremental stuff that may stick around for a while but I don’t see it going across the gas engine spectrum. HCCI is more likely to become mainstream. But good for the Nissan folks for trying something interesting. I remember the variable compression Continental diesel we had in GM’s competitor for the main battle tank contract back in the 70s. That one bumped up the piston crown via oil pressure, of course they didn’t have today’s wonderful ability of mechatronics to operate it.

  • avatar

    All I want is an iron block 2v port-injected 7.0L V8 with an active exhaust and limited-slip rear end installed in a RWD sedan that looks like the 2012 Maserati Quattroporte and has the interior from a 2017 Silverado WT. For under $50k.

    Is that so much to ask?

  • avatar

    This is going to be a reliability disaster.

    I can see the resale values on these cars being worthless . . .

  • avatar

    Based on the graphs from Matt’s article it is about 4 extra links and 5 bearings to make the piston travel height adjustable (yes guys, go through the pictures, it is variable displacement not variable compression). I don’t think crank and bearings are reliability weak points but I do think they will rob quite a bit of power (maybe 2% extra?) and you may do better for economy with variable cam profile to do pseudo Atkinson like the Prius, Tula’s Dynamic Skip Fire (not firing all the time, fire only based on power need and to cancel the vibration), or Fiat’s multi-air.

    The problem is if you want BIG TURBO you will need low compression, and if you get low compression with variable air intake you will leave HP on the table. It won’t get mainstream because buyers don’t really care about big turbo.

  • avatar
    SCE to AUX

    Most buyers won’t care, and won’t even lift the hood on the vehicle. They’ll see the MPG ratings, noting that gas is $2.75/gallon, and make sure their stuff fits inside.

    Mainly, they’ll make sure the 4-square provides a payment they can afford.

    But as for this technology, I think we’re nearing Peak Complexity when it comes to the ICE, and new technology is called for – but this isn’t it. This happened during the transition from carburetors to fuel injection.

    Remember the infamous Honda vacuum line diagram from 1985?×900.jpg?itok=hcgOx7Cx

    Even EV detractors can agree that electric drivetrains offer a simpler, more reliable product. But EVs have other issues that detract from that fact. I always liked this ad from Nissan, showing the parts missing from the Leaf:

    • 0 avatar

      “I think we’re nearing Peak Complexity when it comes to the ICE, and new technology is called for – but this isn’t it.”

      Well said and I completely agree.

      “This happened during the transition from carburetors to fuel injection.”

      And how, some of the early fuel injection systems are amazing in what they were. The Roadkill Garage Ford Muscle Truck came with an early aftermarket fuel injection, it was amazingly bad. They slapped a carb on and it immediately drove better.

  • avatar

    There’s a lot of engineering talk here that I don’t understand. I’d probably have to see ho it worked vs a traditional engine design to get it.

    That being said, I’m interested enough in what it promises to keep an eye on the reliability numbers on models with this engine going forward.

    Much like Mazda’s SkyActiv – X, if it gives the models its used in a noticeable difference in fuel consumption, and it turns out to be otherwise an engine with no extreme reliability issues (no one is expecting perfection), then it’s going to be a plus for shoppers who look at fuel efficiency and a non-issue for almost anyone else.

    If it works as they say it will, the question becomes will other automakers try to develop similar technology?

  • avatar

    Good discussion here.

    It’s not a vehicle that interests me (I doubt that Infiniti ever will sell another vehicle that interests me) but if I were asked for my opinion I’d simply recommend giving the technology some time to prove itself.

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