PHOENIX, Arizona — This might be what Tomorrowland looked like to the eyes there to see it when it opened in 1955. Rooted firmly in the understandable, the things we’ve built on for decades, but at the same time new and different in delightfully unexpected ways. This is the future of the combustion engine, if it has one. This is the Infiniti VC-Turbo, the world’s first production variable compression automotive engine.
On the other hand, this might also be what Tomorrowland looks like today, to the eyes of us tomorrow-dwellers: a quaint but flawed conceptualization of an idealized but incomplete future that simply never materialized.
To me, it’s simply brilliant, regardless of whether it takes the industry by storm or sidles off into a corner of automotive anorakia. For the mechanically minded, it’s a shockingly simple and elegant solution to a complex problem—but one that relies on some incredibly complex engineering to achieve the production result. For the less mechanically minded, maybe start by staring at the video of the cutaway in operation for an hour or so.
You might be wondering why a variable compression engine is even desirable in the first place. The answer to that is simple: bandwidth. A high-compression engine can be efficient, or it can be powerful, but it can’t really be both. A low-compression turbo engine can also be very powerful, though it’s typically also rather inefficient. But an engine that can be both, or anything in between, can be as efficient or as powerful as the driver needs. According to Infiniti, the new engine is 27 percent more efficient than its current 3.5-liter V-6 engine, which could work out to around 25 mpg city, 34 mpg highway, and 28 mpg combined—outstanding figures for a luxury crossover like the Infiniti QX50, and about 25 percent better than the best of the conventionally powered competition.
Reducing the VC-Turbo’s three-dimensional mechanical symphony to linear language is difficult, but the simplest explanation is this: A lever adjusts the length of the piston’s stroke to increase or decrease the engine’s compression ratio in response to throttle inputs. Taking in a little more detail, a motor-actuated linkage advances or retards the orientation of a two-point link that rides on the crank pin, lifting or lowering the attachment point of the connecting rod and thereby adjusting the effective throw of the piston, and hence the compression ratio, which can be adjusted infinitely and continuously between the system’s extremes of 8:1 and 14:1.
Despite all of this high-tech mojo, Infiniti says the new VC-Turbo engine is slightly less expensive to manufacture than the current 3.7-liter VQ-series engine with VVEL. The VC-Turbo is also more efficient from a friction point of view despite the additional hardware added to control the compression ratio. How? By straightening the run of the connecting rod through use of a smaller diameter crankshaft, there’s less sideways force acting on the piston, the cylinder walls, and the bushings connecting all of the linkages, which translates to substantially less friction. There’s also no longer any need for balance shafts. And because the hardware that controls the variable ratio is reciprocating rather than rotating, there’s much less slinging of oil in the crankcase. Despite the complexity, the new VC-Turbo engine is also about 40 lb lighter than the VQ-series V-6.
Another fun result of the variable compression ratio is that the engine’s displacement is constantly changing, too, varying by 27 cubic centimeters from 1,970 cc to 1,997 cc, just as the stroke increases by 1.2-mm from its minimum of 88.9 mm at 14:1. That means the engine actually gets bigger when you mat the throttle. It feels like it, too.
Not that you actually feel anything about the VC-Turbo’s special-sauce functionality. It’s completely transparent in actual use, invisible except for the instrument-cluster display that graphs the engine’s movement between compression extremes. The graph moves quickly and smoothly, jagging up or down as you press or release the throttle, the compression ratio reaching its target just before the turbo spools fully.
Infiniti says the full adjustment from 8:1 to 14:1 (or vice versa) takes 1.5 seconds in total, but in practice, you’ll more likely be moving from 12:1 or 13:1 to 9:1 or 10:1, meaning the transition is completed in well under a second. And because the transition is continuous, sweeping through every intermediary compression ratio along the way, the engine is constantly moving toward the target, meaning it’s getting more and more powerful as you accelerate, building boost and reducing the compression ratio as the port- and direct-injection fuel system races to keep up with demand.
But back to the driving: At Nissan/Infiniti’s Arizona Testing Center (ATC) an hour outside Phoenix sat a prototype of the upcoming Infiniti QX50 outfitted as a powertrain mule—the car’s suspension, interior, and camouflaged exterior weren’t yet ready for evaluation, but VC-Turbo-equipped models won’t bear any exterior badging marking the fact anyway. Under the hood, the 2.0-liter VC-Turbo, codenamed KR20—it is derived from the current Nissan/Infiniti MR20 2.0-liter turbo engine and the “K” stands for kahen, the Japanese word for “variable.” Out the windshield, the ATC’s enormous oval and some simulated country roads.
From a stop, matting the throttle on the VC-Turbo yields a not-quite-instant but still quick swing from the 14:1 compression ratio of idle to the wide-open, max-boost (1.6 bar) 8:1 ratio. As boost builds, the QX50 prototype dashes forward. How quick? I can’t say for sure, but Infiniti claims it’s a second quicker to 60 mph than the fastest of its close competitive set, defined as Audi, BMW, and Lexus with the Q5, X3, and NX. It definitely doesn’t feel underpowered at any rate.
Acceleration on the move—from, say, 30 mph to 50 mph—is a bit tougher to gauge when you’re the only vehicle on the road, but even with the aging CVT transmission Infiniti saddled the VC-Turbo prototype with, the new engine feels peppy. And that’s without the ability to deliver full torque in the lower gears—the transmission simply isn’t robust enough to handle the engine’s full output of 268 hp and 288 lb-ft of torque. Infiniti is already at work fitting a traditional automatic transmission for production, though the CVT will also be offered.
Engaging manual mode on the paddle-shifted CVT and taking off from 2,000 rpm in the simulated first, second, or third “gears” yields close to the worst-case lag scenario: not much acceleration until the engine has spun up to 3,500 rpm, then a surge of power to redline. Counter-intuitively, doing the same thing in fourth gear results in markedly quicker acceleration—but that makes sense when you remember the torque limiting being done by the computer in lower gears.
My overall takeaway from a day with the prototype was this: the VC-Turbo is a very, very special engine, significant in a way no other combustion engine has been in decades. But it’s also completely normal, with no rough edges, hairy temperament, or jagged transitions between modes or functions. It’s a fully baked, ready-to-rock answer to a question the industry has been asking for close to a century. It’s simply brilliant.
This story was originally published in automobilemag Entry posted by Nelson Ireson