| The Disappointment of Nissan's Variable-Compression Engine | | | Nissan achieved something undoubtedly huge bringing the first, and only, variable-compression engine to market. Its future isn't bright. | | — Chris Perkins, Senior Editor | | Complexity in cars is a funny thing. It's tempting to say that it's all bad, that more is always less, but complexity has given us the wonderful cars of today. It's more that complexity must be done right, rather than avoided. Nissan's variable-compression engines are fascinating and complex. I remember when they debuted with a sense that Nissan had pulled off something extraordinary. Frankly, it did. But increasingly, it doesn't look like the savior of internal-combustion once hoped. | | Welcome to the Rabbit Hole, our new newsletter where we explore the curiosities and technology behind how cars work. This full article is exclusive to our newsletter subscribers for 24 hours before it goes live on Motor1.com. Consider it a sneak peak at the future. Enjoy! - Travis Okulski, Automotive Editorial Director | | The first variable-compression ratio engine was completed in 1919 by Sir Harry Ricardo to determine the detonation point of certain fuels. Ricardo built a large, 2.0-liter single-cylinder engine with a mechanism that raised or lowered the cylinder head relative to the crankshaft. The project, funded by Shell, laid the groundwork for how we measure octane to this day. Obviously, Ricardo's variable-compression engine was not practical for use outside the lab, but in the years after its development, many engineers tried to make variable-compression practical. Nissan evidently spent 20 years working on its own version, finally debuting it in 2016 and putting it into production with the 2019 Infiniti QX50. The possible benefits of variable compression are huge. Engines run more efficiently at higher compression ratios, but the higher you go, the more you introduce the chance for detonation, when the air/fuel mixture is compressed so much it explodes before the spark plug fires. This is especially true with a turbocharged engine. | | An SAE paper from 1964, A Variable Compression Ratio Engine Development, details a handful of approaches to achieving variable compression. One idea was to have a sort of piston within a piston, with the inner part fixed to a connecting rod, and an outer part free to move within certain boundaries; another was to use a variable-displacement auxiliary combustion chamber; what's most interesting to us, though, is using a mechanism to change the position of the piston relative to the head. That's what Nissan uses. A 2003 SAE paper authored by Nissan engineers describes it as a "multi-link" mechanism. Starting from the bottom, there is what Nissan calls a "control shaft," which has a single arm connected to an electric motor. The motor rotates the shaft between two positions for low and high compression. For each cylinder, there's a link that looks like a connecting rod and bolts at the bottom to the shaft. This is what Nissan calls the "control link." | | "The possible benefits of variable compression are huge... But the higher you go, the more you introduce the chance for detonation." | | The control link pins at the top with a piece that looks like the bottom half of the parallelogram. Nissan calls this a "lower link." The lower link bolts around the crankshaft with a mirrored piece to form the full parallelogram, which can rotate about the crank. The top half of the parallelogram and the pinned rod that connects to the piston form what Nissan calls the "upper link." When the control shaft rotates, the parallelogram-shaped piece rotates in turn, changing the position of the piston relative to the cylinder head. (The piston stroke also changes slightly between high- and low-compression settings.) Nissan varies the compression ratio between 8.0:1, a fairly low number—though typical for a turbocharged engine—and up to 14.0:1, very high for any engine, let alone one with boost. When the driver wants maximum power, the multi-link mechanism shifts and the engine runs with its lowest compression. But at low loads, the mechanism shifts again to its high-compression setting, since the likelihood of detonation is lower as there's less air and fuel going into the engine. The VC-Turbo comes in two forms, a 1.5-liter three-cylinder used in the Rogue and the (not sold in the US) Quashqai, and a 2.0-liter four-cylinder used in various Infiniti crossovers, the Altima sedan until this year, and the new Murano. | | Actually Building the Thing | | While initially the VC-Turbo engine received a lot of positive attention, and deservedly so for the accomplishment it represented, things quickly started to go south. A November 2018 Car and Driver story noted that the new Infiniti QX50 posted a 6.5-mpg combined improvement over its V-6 predecessor, and estimated that the variable-compression system accounted for 2.6 of those. But for all the extra complexity, the QX50 only scored 1 mpg better on C/D's 75-mph highway test than an Audi Q5, 27 mpg vs 26 mpg, and trailed the 31-mpg BMW X3. The magazine's test of a 2022 Rogue noted disappointing highway fuel-economy figures compared to its naturally aspirated four-cylinder predecessor, though the switch to VC-Turbo power did bring acceleration improvements. The Altima VC-Turbo fared better in terms of delivering segment-topping performance and fuel economy, but sales were evidently so slow, Nissan discontinued the model. Still, the problems for the VC-Turbo go much deeper. In December 2023, the National Highway Traffic Safety Administration (NHTSA) opened an investigation into reports of VC-Turbo engine failures in the 2019-2021 Altima and QX50, and 2021-2023 Rogue. In a response to NHTSA's questions, Nissan said bearing failures were the cause. In the four-cylinder, Nissan found failures of both lower link and control link bearings, and in the three-cylinder, the crankshaft bearings proved problematic. Failure rates are evidently low. Nissan told NHTSA that through February 14, 2024 it found 889 warranty/goodwill service claims for new short blocks on the 454,840 vehicles covered by the investigation. The automaker also pointed out that it found that bearings did not fail spontaneously, and that "bearing failures progress over time and provide drivers with multiple forms of audible and visible warnings including abnormal noise, rough running, malfunction indicator lights (MIL) and warning messages in the instrument cluster, depending on progression and design logic." Nissan reported that "loss of motive power" for the four-cylinder is no more than 0.39% of engines produced and 0.15% for the three-cylinder. Plus, the automaker notes that it has made manufacturing improvements to these engines since their launch, which has decreased incidence rates. And finally, that the company isn't aware of accidents as a result of these failures. Of course, bearing failures are not unique to these engines. But it must sting for Nissan that on top of the disappointing performance of the VC-Turbo, its failures are traced back to the very set of components that make it unique. It would be another thing if it was, say, piston-ring failure. Still, the variable-compression system is very complicated—there's just a lot of moving parts, and thus, more bearings, and more failure points. Variable valve timing and lift has long been ubiquitous in internal-combustion engines, but these systems aren't really all that complicated from a hardware perspective. Just some sort of phaser to shift the camshafts back and forth to use different cam profiles, really. | | All of this comes as Nissan is in an existential crisis. According to a December 4, 2024 Reuters report, Nissan CEO Makoto Uchida blamed its dire situation on the automaker's lack of hybrid offerings, especially in North America. One imagines that the VC-Turbo cost huge, huge sums to develop given its decades-long gestation. If Nissan had put some of that money toward further developing hybrid powertrain offerings, perhaps it could compete with rivals rather than most likely ending up being subsumed into one. And while variable compression does bring performance and fuel-economy improvements over fixed-compression engines, the benefits are nothing like as big as going hybrid. Let's take the 2021 Rogue AWD as an example, as it was available with both a 2.5-liter four-cylinder, and a 1.5-liter VC-Turbo three-cylinder. The switch from conventional engine to VC-Turbo brought performance gains—Car and Driver noted a 0.4-second 0-60 mph improvement—and bumped combined fuel economy from 29 MPG to 33 MPG. Impressive, but let's compare that to hybrid and non-hybrid versions of the 2021 RAV4. Going from base model to hybrid net a 0.7-second improvement in 0-60 time and raised combined fuel economy from 28 mpg to 40 mpg. And arguably, the hybrid RAV4 is simpler from a hardware perspective, given the Toyota Hybrid System uses a pretty simple battery, two electric motors, a very basic internal-combustion engine, and ditches a conventional gearbox. It's all well-proven, too, unlike the VC-Turbo. Now, with Nissan's future very unclear as it looks to be rescued from its current predicament, it's unclear if variable-compression has much life left in it. A ton of focus will be on hybrids and fully electric cars. For engineering enthusiasts, it's bittersweet. Putting variable compression into production is a mighty achievement. Yet, it doesn't seem like its benefits outweigh its drawbacks, at least for now. And in today's world, Nissan being where it is, it doesn't seem worthwhile to continue down this path. | | No longer want to receive these emails? Unsubscribe. Motor1.com 650 Madison Avenue New York, NY 10022 | | | | |
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