By on October 27, 2016

Infiniti four-cylinder turbocharged gasoline VC-T engine, Image: Infiniti

It was 1998 and my friend Tom had just picked up a 1991 Eagle Talon TSI AWD.

“Ok, go ahead and floor it, but don’t rev it past 5,000 rpm,” Tom said.

I mashed the throttle and … nothing happened.

We were moving, but it was at the pace of a Toyota Corolla and nowhere near the rate of acceleration promised by the 2.0-liter turbo’s claimed 195 horsepower.

Disappointed, I left my foot on the throttle for a few seconds. Suddenly, I heard the whistle of the spooling turbo and a sudden shove of boost kicked in.

Four-cylinder turbo engines from the ‘90s were all similar to this. While they generated relatively big power at the top end, they also suffered from massive turbo lag and had fuel economy similar to a much larger V8.

Nissan’s new Variable Compression Turbo engine promises big power, minimal turbo lag, and decent fuel economy using some new trickery.

Is this the holy grail of turbocharged motors?

Why it matters

Infiniti’s introduction of its Variable Compression Turbo (VC-T) engine at the Paris Auto show was the big engine technology news from last month. Infiniti claims its new engine generates superior power and fuel economy compared to similar four-cylinder turbocharged motors.

The perfect turbo motor would have a low compression ratio to prevent knock and enable big power when the turbo was feeding it copious amounts of atmosphere, then switch to a high-compression ratio when off-boost to promote fuel economy and responsiveness.

We’ve been talking about variable compression engine designs for decades. The ideal pairing for a variable compression engine is with a turbocharged motor as it can take advantage of the low compression ratio to generate power without compromising fuel economy.

The closest OEM to production previously was Saab, which had a working prototype turbocharged 1.6-liter five-cylinder engine in 2000 known as Saab Variable Compression (SVC). The SVC engine was fairly complicated and had the entire head pivot on the crankcase to vary compression. Saab had a long history with turbocharged four-cylinder engines and this looked like the next development. Unfortunately for Saab, GM took over the company in 2000 and, as a result of shared engines with GM, the SVC engine was never put into production.

Nissan’s VC-T is a completely different design.

Originally filed as a patent in 2001, Nissan has worked on the technology for over a decade. The variable compression system works by varying the cylinder stroke. A complex, multi-link system changes the length of the stroke depending on the angle of a diamond-shaped center link, which is rotated by an actuator controlled by an electric motor. Using this system, the compression ratio can be changed between 8:1 to 14:1 in less than 1.5 seconds. The linkage does add some weight to the design; Nissan’s engineers have admitted that it’s probably 25-pounds heavier than a comparable fixed compression engine.

Besides variable compression, the engine — codenamed MR20 DDT — is a fairly advanced design with port and direct fuel injection, individual cylinder ignition timing, variable valve timing, electronic boost control and an all-aluminum construction. Infiniti claims the MR20 DDT, which will first find a home in the 2018 Infiniti QX50, has 27-percent lower fuel consumption than its 3.5-liter V6.

Regular engines use the Otto cycle, which has four stages — intake, compression, expansion, and exhaust — in the combustion cycle. Nissan’s VC-T engine uses a modified version of the Atkinson cycle, known as the Miller cycle. The Atkinson cycle differs from the Otto cycle in that the expansion phase lasts longer, decreasing the compression ratio and increasing the volume. Because of its longer expansion phase, additional energy can be captured, resulting in greater fuel economy. Toyota has used the Atkinson cycle extensively on Prius hybrids. However, the drawback is the lack of power compared to an Otto cycle engine.

The Miller cycle adds forced induction to the Atkinson cycle to address this lack of power. Mazda used a Miller-cycle engine in the ’90s in the supercharged Millenia, but the engine was eventually dropped because, in the real world, the engine was no more fuel efficient than turbocharged engines of the day while costing more and generating the same amount of power.

The new VC-T engine solves this issue. In low compression mode, the turbo kicks in to provide power. In high compression mode, it runs like an Atkinson-cycle engine with enhanced fuel economy and throttle response. Imagine an Eagle Talon turbo with more power and minimal lag that returns 25 miles per gallon.

How does it compare?

The below chart compares the upcoming 2018 QX50 with its rivals in the compact luxury crossover market. For the sake of level comparison, I selected only 2.0-liter turbocharged two-wheel-drive models when possible.

2018 Infiniti QX50 (projected) 2016 Infiniti QX50 2016 BMW X3 sDrive 28i 2016 Audi Q5 2016 Mercedes-Benz GLC 300 2016 Volvo XC60
Engine, transmission, Drive
2.0L, 4-cyl turbo, Automatic (S7), FWD 3.7L, 6-cyl, Automatic (S7), RWD 2.0L, 4-cyl turbo, Automatic (S8), RWD 2.0L, 4-cyl turbo, Automatic (S8), AWD 2.0L, 4-cyl turbo, Automatic (S9), FWD 2.0L, 4-cyl turbo, Automatic (S8), FWD
Fuel Economy, City/Hwy (MPG)
22/30 17/24 21/28 20/27 22/28 23/30
Power (hp), Torque (lb-ft)
268, 288 325, 267 240, 258 220, 258 241, 273 240, 258
Weight (lbs)
4,020 4,030 4,200 3,892 4,200

Sources: fueleconomy.gov;  caranddriver.com

The comparison shows the VC-T engine stats are impressive. It puts out more horsepower and torque than any of its rivals while having the same or better fuel economy.

Obviously, there is some theory crafting at work here. The cars have slightly different transmissions, weights, aerodynamics, and tuning, which can all lead to differences in fuel economy. In addition, it’s unclear how much of the other technologies in the VC-T engine (e.g. dual injection) contribute to the improved fuel economy.

Nevertheless, this exciting new technology has potential. Future development may lead to advances such as Homogenous Charge Compression Ignition (HCCI) — basically ignition by compression instead of spark, similar to diesel engines — and automatic adjustment of compression based on the fuel octane.

In the past decade, Nissan has not been known for leading the four-cylinder engine technology arena. In fact, the only four-cylinder turbo engine used by Infiniti is a Mercedes-Benz unit. That said, Nissan is no stranger to turbocharged drivetrains (e.g. Juke, GT-R) and it will be exciting to see further development of VC-T in the near future.

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43 Comments on “Is Infiniti’s Variable Compression Turbo the Holy Grail of Power and Efficiency?...”


  • avatar
    JimZ

    “Unfortunately for Saab, GM took over the company in 2000 and, as a result of shared engines with GM, the SVC engine was never put into production.”

    or maybe- just maybe- it wasn’t something that could work in the real world. but yeah, keep banging the “GM killed SAAB” drum.

    • 0 avatar
      Henry Leung

      If you watch the SVC videos on youtube, it’s an impressive (the entire crankcase pivots!) engine. The fact that they had working prototypes means they were probably pretty close to production. Saab was a pioneer of turbocharged four cylinder engines so they had the technical knowhow.

      ‘GM killed SAAB.’ I don’t think that’s up for debate. By 2010, Saab had gone into administration (i.e. bankruptcy) and the brand was sold to Spyker soon after.

      • 0 avatar
        JimZ

        I don’t need to go to Youtube, I remember reading about it when they first disclosed it. Working prototypes in the lab are one thing, but mass producing engines which will last for 200,000 miles is another thing entirely. And I look at the SAAB design and I see a ton of challenges with durability of the VC mechanism, not to mention how much of a b**** sealing would be. History is littered with engine designs (Rand Cam, Kauertz Rotary Vane, etc.) which never make it out of the lab due to fundamental design problems.

        SAAB’s VC engine never passed “proof-of-concept” stage.

        “‘GM killed SAAB.’ I don’t think that’s up for debate. By 2010, Saab had gone into administration (i.e. bankruptcy) and the brand was sold to Spyker soon after.”

        Post hoc ergo propter hoc. SAAB was always doomed. GM just delayed the inevitable by about a decade.

        • 0 avatar
          heavy handle

          “GM just delayed the inevitable by about a decade.”

          Not that old tripe again…

          Yeah, Saab’s demise was inevitable in 1990, and so was Audi’s, and Jaguar’s, and Land Rover’s, and Volvo’s, and Porsche’s. Just draw a Venn diagram of which ones were owned by GM, and which ones met their demise…

          I understand GM company men toeing the company line, but the fact is that Saab was in no worse shape than any of those other brands at the time. Jaguar, LR and Audi were in worse shape.

          • 0 avatar
            JimZ

            Why didn’t Scania keep them then?

          • 0 avatar
            heavy handle

            Saab and Scania merged in the late 1960s, so it’s not a matter of “keep.” This must have made some sense at the time, but the two companies were always separate divisions of a common group.

            Why were they spun-off? For the same reasons HP spun-off Agilent. Different companies, different markets, different priorities, different capital requirements.

            Saab the defense contractor (maker of fighter jets, among other products) was also in a different business. As you may know, there are two Rolls Royce companies, one in automotive, one in aerospace.

      • 0 avatar
        Scoutdude

        Having working prototypes does not in any way mean that they were pretty close to production.

        If it was such a revolutionary engine and it showed promise of being commercially viable and durable then GM wouldn’t have killed it they would have either kept working on it or sold the patents.

    • 0 avatar
      heavy handle

      It was never meant for production. The Saab variable compression engine was used to aid in engine and engine management development.

      No need to go all conspiracy, this was a widely known fact when Saab was part of GM.

    • 0 avatar
      Detroit-Iron

      Whether or not GM killed SAAB, they definitely prevented a bunch of elbow-patch wearing professors from getting saddled with an insane engine. Assuming that SAAB ever had any intention of bringing it to market.

    • 0 avatar
      MrIcky

      GM owns all of SAABs intellectual property and they’ve been under the same or more pressure to improve fuel economy. If SAABs variable compression system was feasible, don’t you think it’s reasonable to assume that GM would have still been working on it? Nissan’s is complicated, the SAAB version sounds a nightmare.

      Now pair this with Christian Von K’s freevalve engine and it will basically be a perpetual motion machine.

  • avatar
    psarhjinian

    “…they also suffered from massive turbo lag and had fuel economy similar to a much larger V8”

    This is, frankly, intentional. The idea is that you had economy or power: stay off-boost and you have the power and economy of a naturally-aspirated four; put your foot to it, and you got the power and economy of a V8. Turbocharging is just displacement-on-demand, but with forced induction instead of cylinder deactivation.

    We’ve actually made turbo fuel economy and/or driveability worse by eliminating lag: you go on-boost more quickly and/or the transmission has to play games to keep fuel economy up.

    Variable compression seems like a good way to gradually build boost and thusly avoid transmission trickery and full-boost-from-low-RPM fuel consumption, but you’ll still see journalists whining about poor fuel economy for a small engine when they drive it hard. The idea that power comes at a price seems to escape people.

    • 0 avatar
      DenverMike

      Exactly! When did “turbo lag” become a bad thing?? I test drove an SVO and it went from Pinto to Mustang GT in 300 RPM, with the nose lifting and slamming you deep into the seat back all at once. Pinto MPG Is possible though, but why?

      • 0 avatar
        otaku

        I still have fond memories of my ’86 T-Bird Turbo Coupe, which used a slightly less powerful, non-intercooled version of the 2.3L from the SVO you mentioned. It was no sports car, but it was still a fun ride that attracted a lot less attention from the local police compared to the Mustang.

        My brother drove an ’87 Mustang GT and, even when driven conservatively, it struggled to achieve upper teens for fuel economy. My T-Bird was nowhere near as powerful, but it could easily average in the low to mid twenties (unless you were flooring it all the time).

        I guess it all came down to your individual driving style. Most of the time I would just cruise through the city around 2,000 RPM or so, but it still had more than enough power in reserve to pass slower moving traffic or for the occasional quick burst of acceleration when merging onto the highway.

        I don’t think it was a legitimate rival of any V-8, but I would argue that it got the job done as well if not better than most V-6’s of it’s day.

    • 0 avatar
      yamahog

      “stay off-boost and you have the power and economy of a naturally-aspirated four; put your foot to it, and you got the power and economy of a V8. Turbocharging is just displacement-on-demand”

      Close but no cigar.

      A 4 cylinder turbo off-boost has some trade-offs a N/A motor doesn’t have to make – chiefly, N/A motors can run higher static compression. A Mazda 6 runs a 13:1 compression, a Toyota Corolla runs 10:1 compression, and a WRX STI runs 8.2:1 compression. Higher compression ratios generally make for more favorable brake specific fuel consumption curves.

      Additionally, the N/A 4 cylinders can run well on lower octane gas, the turbo 4 cylinders require high octane gas (even when they’re cruising down the road).

      When they’re on the boost, emissions-complaint cars need to run rich air fuel ratios to keep the combustion temperatures down. This means a 4 cylinder turbo would consume more fuel to make a given amount of energy (like 100 horsepower for 1 hour) than we’d expect from an N/A V8.

      Turbos have their own trade-offs and it’s disingenuous to portray them as the peers of N/A motors.

      • 0 avatar
        heavy handle

        yamahog,

        Modern turbos run 10:1 or higher on regular gas. You’ll get better performance and economy with higher octane fuel, but you can use regular if that’s your thing.

    • 0 avatar
      JMII

      I had an ’95 Eclipse GS-T and always liked how the engine switched personalities between on and off boost. As mentioned: stay off boost and I had economical 4 banger, get on the boost and V8 Mustangs disappeared behind me. It got good gas mileage which offset the cost of the premium fuel. The torque really threw you back into the seat… I LOVED that car.

  • avatar
    ScarecrowRepair

    I must be missing something. Why could you not just raise and lower the crankshaft? Why all that complicated linkage?

    • 0 avatar
      JimZ

      If it was “just” that easy, that’s how it would have been done. Go design a mechanism which can do that and still endure the stress that the engine’s bottom end has to handle.

      • 0 avatar
        ScarecrowRepair

        I realize I don’t know the answer, that’s why I asked. If your answer is “too much stress”, then my next question is how this complicated linkage mechanism can handle the stress better.

    • 0 avatar
      jmo

      How would you attach the crankshaft to the transmission?

    • 0 avatar
      SP

      You don’t want to raise and lower the crankshaft because modern engines rely a great deal on what is called “quench” for efficient combustion. You want the piston crown to almost kiss the flat part of the cylinder head on each revolution. The optimum clearance is small, around 0.04″ (just over 1/32″).

      To back the piston away from the cylinder head would drop the compression ratio, but would also rob the engine of “quench”. This would hurt power and efficiency when operating in that mode.

      I think the greater amount of cylinder wall exposed to combustion might also hurt thermal efficiency.

      Anyway, it would pretty much defeat the purpose of the whole thing, which was to increase efficiency.

  • avatar
    TR4

    Variable compression has been around since 1929 with the Co-operative Fuel Research engines used to measure octane.

  • avatar
    dividebytube

    Just a side note – Autoblog had a dyno run showing that the BMW 2.0L turbocharged engine is actually closer to 265hp. BMW apparently underrates them.

    • 0 avatar
      PJmacgee

      And [in the X1 anyway, ~3700lbs] does 33-35 real-world mpg hwy, and 30+ easily in mixed driving. Sounds like a tractor though…

      • 0 avatar
        Nick_515

        Holy crap… 265 is yuge for 2 liter. my 3.1 liter audi DIv6 only makes 255, and you have to wind it up. It’s very enjoyable to mash it, wait for the downshifts and go, but there’s not turbo pushback to the seat. And lifetime it’s only 24 mpg.

        Does this come with a stick in the 328i?

    • 0 avatar
      Henry Leung

      Autoblog also rated Audi’s 2.0T engine as running closer to 260HP.

      It would be impossible to know the exact engine output without putting all the cars on the exact same Dyno and even then, the drivetrain losses would be different in each car.

  • avatar
    turbo_awd

    Lag may have given “4-cyl power” before boost, and 8-cyl power afterwards, but 1) it caused some bad situations, cf: early 911 turbo rear-end spin-out accidents 2) 80s 8-cyls had maybe 150-200 hp. Nowadays, an 8-cyl has 400+ HP. To get 400+ HP out of a 2.0 turbo, you can’t just have boost kick in at 5000 rpm – well, you could, but it wouldn’t be that exciting. That big v8 has tons of low torque, and will get better MPG than you AND outrun out. What’s the point, then?

    • 0 avatar
      heavy handle

      Early turbos turned on like a firecracker, but that behavior was under control by the mid-1980s. Advanced engine management allowed auto makers to maximize low-end torque (which is where detonation risks are highest).

      If you want to compare big V8s and turbo 4s, just look at Volvo’s Yamaha-sourced V8 compared to their current “Drive-E” 2.0 4 cylinder. The turbo 4 has more power and torque everywhere in the rev range, and gets 10 more miles per gallon. It’s not even close in terms of performance or efficiency.

      • 0 avatar
        yamahog

        But the Yamaha V8 was not Yamaha’s gem. The V8 was a derivative of a V6 from the 80s. And the V6 never even got to stretch its legs – the Ford accessories couldn’t keep up with the Yamaha motor. A lot of people threw better accessories on the engine and aftermarket engine management and spun the motors to 9k. Others would supercharge them to 500 hp with some new cams.

        I don’t dispute V8 guzzled gas compared to the inline-4. I’m just saying that there was maybe 30 years between the V8’s design and the inline-4s design.

        On the track, the Ecoboost ‘stangs seem to guzzle gas at a clip similar to the GT ‘stangs.

        • 0 avatar
          heavy handle

          You can’t compare mileage on the street and on the track.

          Your Ecoboost ‘stang drinks like a V8 on the on-ramp, for the 7 seconds it takes to get to 85 mph. Then it sips like a 4 for the next 200 miles to Grandma’s house. Track racing is the equivalent of doing 100 on-ramps in a row.

          • 0 avatar
            yamahog

            The Ecoboost ‘stang outdrinks the V8 on the on-ramp and then it gets worse milage than a normally aspirated engine would on the 200 miles to Grandma’s house. If turbos were free power devoid of any trade-off, don’t you think they’d be more prolific? Cost isn’t the reason we don’t see more of them.

            But we’re in agreement that the Volvo’s 2.0 is 30 years removed from the bones of the V8, right?

  • avatar
    sportyaccordy

    It’s a shame that this will kill off the V6 across most of Nissan’s lineup, though they had to do something drastic… the VQ has always had abysmal gas mileage. My 350Z got about the same gas mileage and straight line performance figures as an F-150 Ecoboost, despite making 30% less power, lugging around nearly half as much weight and probably pushing through less than half as much drag. Where the hell did the fuel go?

    • 0 avatar
      JimZ

      how was it geared?

      • 0 avatar
        sportyaccordy

        Decently. I had the 6 speed manual and it did 80 at less than 3000 in top gear from what I remember. On a regular loop of just about 50/50 city/highway I was lucky to do better than ~21 MPG. The best mileage I got was 26 MPG on a road trip.

        For reference I had a C4 Cactus and Focus Wagon in Europe recently. Over about 1500 miles combined, with mixed driving up to highway speeds in the 90s, I got no worse than 45 MPG with both cars. VQ gas mileage is abysmal but I am pretty sure my next car will be a G37

  • avatar
    Vulpine

    The concept of variable compression is an interesting one but at the same time it adds complexity that raises the question of reliability over time. Honestly, I wouldn’t trust one until it has proven itself with at least 100,000 miles of trouble-free operation and maybe a cost factor where replacement is cheaper than repair, since it would take so much more labor to repair a ‘bottom end’ problem than even a current engine overhaul requires.

    • 0 avatar
      indi500fan

      Given the complexity and compactness of current engine bays, I think any engine removal for repair is gonna be big bucks if you’re paying somebody to do it. One of the issues will be “rebuild-ability”. Will the only repair parts be available from Nissan (similar to a lot of CVT transmissions currently) at very high prices?

      • 0 avatar
        Scoutdude

        The rebuilding business isn’t what it used to be, they pretty much supply fleets and fanbois. There is a reason that the number #1 selling reman engine is the old SBC and #2 is the supposedly bullet proof everlasting 22r. In the rare instances when an engine actually does need replacement and the vehicle is worth spending the money to replace it a junkyard/low mile imported from Japan engine is was goes in.

  • avatar
    mikein541

    Way too mechanically complex. Even a conventional turbo engine is
    too complex for my tastes. Are as 7 or more speed transmissions.
    And infotainment systems that do not work properly (are you
    listening, FoMoCo?). You would be wise to follow the old KISS
    principle when buying a vehicle, unless you enjoy lots of repair
    bills and aggrevation.

  • avatar
    jalop1991

    That sound you hear is the cackling of the Cadillac V8-6-4 engineers predicting the future.

  • avatar

    So… 25% better economy than their gas hog V6. Really swinging for the fences at Nissan, aren’t they?

    Maybe I’m a fool but the numbers just didn’t seem that impressive. A little more HP & torque, basically the same MPG, doesn’t really scream “a decade’s worth of innovation”.

    You guys probably know by now I’m always gonna pine for a diesel, emissions be damned, but… in an SUV they really do make more sense. They have the torque to pull those square boxes along nicely and still get ~25-30 mpg average, which seems to be better than the units in the comparison box with this article.

  • avatar
    amca

    I have yet to see anything about what the top end of the engine is. I’m betting that it’s low. All that reciprocating mass. I’m also wondering whether with all that reciprocating mass also makes for a rough running unit.

    We haven’t heard anything yet from a journalist who’s driven one. I’m waiting.

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