When Nissan unveiled the first mass-produced variable compression ratio engine back around 2016, there was a lot of buzz about what this engine could do. Cut to seven years later, and a few VC-Turbo engines are under NHTSA investigation for catastrophic mechanical failure. How did we get here?
For those familiar with the four-cycle internal combustion engine, the sequence of intake stroke, compression stroke, combustion stroke, and exhaust stroke should ring a bell. Today we’re honing in on that second stroke to give you a little more context on the how and why behind Nissan’s Rube Goldberg machine. An engine’s compression ratio is the ratio between combustion chamber volume when a piston’s at the bottom of its stroke versus at the top of its stroke. It’s also often one of the major keys to efficiency, power, and reliability.
A higher compression ratio typically increases thermal efficiency, though if compression ratio is too high, you can start to run into detonation issues when the fuel-air mixture ignites due to sheer compression before the spark plug lights it off. Needless to say, this is bad for your engine’s health.
When an engine is turbocharged, it’s not uncommon to see manufacturers lower compression to fight detonation, since a turbocharger is an air compressor itself. The downside? While at a steady cruise, turbocharged engines are typically out of boost, meaning a higher compression ratio under those conditions could theoretically boost efficiency.
Cue Nissan working on a variable compression ratio engine for decades, and finally getting it into production in 2018. The KR20DDET two-liter turbocharged four-cylinder engine and its little brother, the KR15DDT 1.5-liter three-banger, are currently the only engines you can buy with a variable compression ratio ranging from 8:1 under full boost to 14:1 under exceptionally light load. How do these engines physically achieve that feat? Well, let’s take a look.
Sure, the bottom end of this variable compression ratio engine may look massively different than what you’re used to seeing, but don’t panic. Think of what Nissan calls the “multi-link” as just a really fancy standalone big connecting rod end. After all, each one still sits on a crankshaft journal, and each one still uses traditional rod bearings. Because of this, each upper connecting rod, and therefore each piston, moves in the fairly normal reciprocating manner you’d expect from any engine with cylinders and pistons.
Sure, it might be visually confusing since the lower pivot point of the upper connecting rod isn’t centered above a bearing journal, but it technically doesn’t have to be, so long as it can still have a path of movement. When a VC-Turbo engine changes its compression ratio, it doesn’t physically move the crankshaft, it just rotates this diamond-shaped link by a few degrees.
So, how is this link rotated? Well, it all starts in the bowels of the engine. Deep, deep down sits an electric motor with a gear reduction drive that pushes or pulls an arm. This arm rides on a shaft beneath the crankshaft, and when the arm moves, the shaft and all the lower connecting rods on it move as well. Lower connecting rods? Ah, yes, those connect up top to the aforementioned diamond-shaped links around the crankshaft, and that rocking motion changes the height of the pistons relative to the crankshaft, altering the compression ratio.
If this all sounds hideously complex, you aren’t wrong. There is a considerable quantity of moving parts involved in shifting a VC-Turbo engine from malaise-era-crank-up-the-boost compression to incredibly high compression, and in a brief on a safety investigation into engine failures, NHTSA claims that several technical changes have occurred throughout production due to potential failures of the variable compression ratio KR15DDT and KR20DDET engines:.
Based on an analysis of the VOQ and FR data, ODI has identified three (3) vehicle models with elevated variable compression engine failure rates. These includes 2021-2023 Nissan Rogue equipped with KR15DDT engine; 2019-2021 Nissan Altima; and 2019-2021 Infinity [sic] QX50, both equipped with the KR20DDET engine. During discussions with Nissan, ODI learned that they have attempted to address main bearing and L-link damage/seizures on the KR15DDT and KR20DDET engines by introducing multiple manufacturing processes changes over time.
Well, the Rogue is Nissan’s best-selling vehicle in America by a longshot, with almost 400,000 sold between the start of 2022 and the end of this September, when Nissan’s most recent quarterly sales report came out. The Altima is a hot seller too, although it came with two engines during the affected model years. As we previously wrote in The Morning Dump, a massive Nissan recall could be coming for these engines, and something of this scope certainly wouldn’t be cheap.
Adding insult to injury, the fuel economy of the two-liter VC-Turbo four-banger doesn’t seem that good. Despite the advanced tech, an all-wheel-drive Infiniti QX50 only scores an extra single MPG combined over an all-wheel-drive four-cylinder BMW X3 or Lexus NX 350, and scores one MPG worse combined than an Audi Q5. I’ve personally seen disappointing figures, and Car And Driver took a much deeper dive into dissecting the middling fuel economy.
One unexpected standout in the data: good old-fashioned rounding. To arrive at the integer values that are reported on the window stickers, both the Audi and the BMW round up to 25 mpg while the Infiniti rounds down to 26. The other main factor limiting the EPA rating of the Infiniti is the QX50’s lack of stop-start functionality.
Yeah, it seems like Infiniti could’ve reached desired EPA fuel economy targets with just automatic stop-start and some other vehicle-level or powertrain-level tweaks.
In the end, Nissan’s VC-Turbo variable compression ratio engine technology is interesting, but it might be more headache than it’s worth. Although it’s not radically different from a standard piston-type internal combustion engine, it has just enough extra moving parts that failure, it seems, is an option.
(Photo credits: Nissan, Infiniti)
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I don’t think anyone was really excited about this. Nissan is a company that excels at taking some of the best designs and making them massively overcomplicated for limited gain.
Okay, you slightly boost efficiency – but you’ve made your rotating assembly way weaker, added more points that need to be lubricated and wear, and add more points of failure. Take into account the lower boost due to the weaker rotating assembly (negating the point of having a lower CR), and the extra friction and mass (negating the point of a high CR) and congratulations, you’re negative square one.
These engines don’t even get better mileage than the comparable Honda and Toyota equivalents, and certainly aren’t more powerful.
Also don’t forget the massive increase in inertia for all that crap that now has to reciprocate too. Imagine the parasitic losses from having to arest that much mass and reverse its direction every time a piston moves up and down.
As an engineering thought experiment this is neat, but as a practical production vehicle, I think I’d rather stick with my r53. Its not the easiest to work on, but at least it’s not quite ww2 german 88 overcomplicated.
Owning a Nissan Rogue with both a ticking time bomb CVT and ticking time bomb triple-hamster wheel VC Turbo engine is like waiting for Bruce Willis to ask you “head or gut?” right before he punches your lights out.
Look at those diagrams: adding so much complexity, in the waning days of ICE — seems like a fools errand
They unveiled it in 2016, so they would have started the design (and the commercial justification) in 2012, well before the EVageddon. Back then ICE research was all about reducing emissions and increasing efficiency.
Hybrids were definitely a big thing in 2012. I always thought that even a mild hybrid system would be drastically superior to this VC crap.
This is sad. There is no reason why this couldn’t be robust and reliable, other than insufficient development/validation and poor quality manufacturing. You have to choose to not validate the design with sufficient testing, and choose to not bother about quality.
Oh, Nissan.