Infiniti claims that its new VC-Turbo engine delivers the power of a six-cylinder engine and the fuel economy of a four-cylinder. This is a claim many other manufacturers have made over the years, but no other manufacturer has had such groundbreaking technology to back it up. Multi Link Variable Compression Ratio system (VCR) is the name of the technology developed by parent company Nissan, and it’s a beautifully complex array of mechanical linkages connected to a fancy actuator and combined with the most current thinking in turbocharging, variable valve timing, and fuel injection.
First let’s level set; if you understand the basic operation of a four-stroke internal combustion engine, the underlying concept behind Nissan’s variable compression ratio technology is easy to grasp. For as long as car engines have been mass produced, their compression ratios—the volume of the cylinder when the piston is at the very bottom of the stroke compared to the volume at the very top of the piston’s stroke—has remained fixed. Traditionally, changing the compression ratio requires tearing apart the engine and swapping out pistons, connecting rods, and even the crankshaft. The holy grail for automotive engineers around the world for the last few decades has been to figure out a way to manipulate this volume on the fly, with an eye toward increasing power and fuel efficiency, on demand.
My esteemed colleague Frank Markus, Motor Trend’s resident automotive engineer and technical director, has deftly explained the inner workings of the multi-link VCR system several times, and I encourage you to read his first look at Infiniti’s new engine here and then go here for a deeper dive on the topic.
To summarize, I pulled Frank’s recap of VCR from his third piece on the topic:
“Compression is altered by moving the range of motion of each piston up and down within the cylinder. In the upper position the ratio is 14:1, and in the lower position it’s 8:1. This is accomplished by an elaborate set of intermediary links between the crankshaft and the piston connecting rods, the positions of which can be altered by a control shaft mounted below the crankshaft and another set of control rods. A glance at the cutaway photos should help demystify this mechanism. Low compression is desirable during periods of peak boost and engine output, and the high ratio (along with Atkinson cycle operation that closes the intake valves partway up the compression stroke) greatly improves efficiency during low-load steady-state operation. The engine also employs direct, multipoint, or a combination of both fuel injection strategies as best befits the operating conditions.”
To provide a better idea of what is going on, Nissan has given names to all the links and rods in its multilink system, and the easiest way to remember the sequence is, oddly, ACLU, starting outside the engine, at the VCR actuator. This high-tech softball-sized motor precisely controls the compression ratio by first shifting the position of a dogleg-shaped actuator-link (A-link). At the VC-Turbo’s highest compression ratio of 14:1, the A-link sits at 9 o’clock, if you imagine the actuator shaft to be the hour hand. To lower compression, the actuator shifts the A-link clockwise to about 2 o’clock, which pushes up on the control-link (C-link) and the corner of the lower link (L-link) to which it is attached. This trapezoid-shaped L-link oscillates about the crankshaft, so as one corner goes up, the opposite corner comes down. That corner is connected to the upper-link (U-link), which you’ll recognize as the piston connecting rod. As the piston’s top-dead-center location falls to its lowest point, more volume is created in the combustion chamber, and the lowest compression ratio of 8:1 is achieved. (The difference in piston heights required to drop compression from 14:1 to 8:1 is approximately 6mm.) As Frank notes, a lower ratio creates nearly ideal conditions for the wonders of turbocharging and direct fuel injection, and a high compression condition maximizes fuel efficiency under light loads, thanks to variable valve timing trickery and a balance of port and direct injection.
So how does it drive? Stunningly … seamless. All the beautifully complex engineering described above is almost entirely undetectable during our brief laps in a preproduction QX50, Infiniti’s all-new premium midsize SUV in which the Multi-Link Variable Compression Ratio system will make its debut. Because the settings of many of the vehicle’s other systems were not finalized, Infiniti asked us to reserve comments about dynamic attributes, aside from the powertrain performance. So, all we’ll say is that it steers, handles, and rides just as you’d expect from a camouflaged, late-stage prototype, premium SUV.
But that new VC-Turbo 2.0-liter 4-cylinder engine certainly does hum. Infiniti’s claims of 268 horsepower and 288 lb-ft of torque seem in-line with our inner-ear and seat-of-the pants dynamometers (though torque will be slightly less in the production QX50, roughly 280 lb-ft). We performed several launches from a stop and simulated merging and passing situations, with the throttle floored from various engine speeds. Acceleration in every instance could be described as brisk but not breathtaking or neck-snapping.
Part of the issue is the transmission, which is the carryover CVT currently used in the Infiniti QX60. A CVT’s reason for being is better fuel consumption and smoothness, and for the purposes of trying to evaluate a new engine’s performance, its relaxed response has a dampening effect. This is especially evident when toggling through the Sport, Standard, and Eco drive modes; in Eco mode, the throttle response and power delivery felt particularly languid. To be clear, these modes only influence throttle response and transmission parameters, not the engine or any part of the VCR system.
So the fundamental takeaway is that the VC-Turbo delivers smooth, seamless acceleration in all the limited conditions we drove the QX50 prototype. This is not like other engine tweaks or power adders that produce a noticeable boost or have a cam-lobe change over point at a proscribed rpm. You might be able to feel Honda’s VTEC kick in (yo), but VC-Turbo technology is largely hushed and refined.
The only clue that anything is going on is a little vertical gauge set between the instrument cluster’s main dials, labeled Compression Ratio. (Infiniti representatives told us the details on this gauge were yet to be finalized.) During our on-ramp merging simulation, with the throttle firmly planted on the floor, the bar on this gauge pegs the highest level, indicated by the word POWER. In this situation, the VC-Turbo should be operating at 8:1 compression, allowing the turbocharger to cram in charge air (at a peak boost pressure of 23.2 psi) without fear of detonation, while the direct-injection fueling system fires in optimally timed pulses of fuel into the combustion chamber.
Once we reach cruising speed and ease off the throttle, both bars on the turbocharger and compression ratio gauges drop precipitously, the latter to the lowest level labeled ECO. Because the VCR system is continuously variable (not just on/off like a light switch), how much the compression changes depends on the lightness of your foot. Infiniti engineers tell us rapidly lifting off the throttle typically sends the VCR system swinging from Power/8:1 to Eco/14:1 in about 1.5 seconds and vice versa. At the eco side of things, the turbo is quietly spinning, sending little to no boost to the combustion chambers because, hey, it’s not needed—we’re cruising to the beat of the Miller & Atkinson (cycle) bands.
So do Infiniti’s opening claims ring true? Yes, and we’ll see. Generally speaking, the 2.0-liter VC-Turbo’s performance does feel on par with current naturally aspirated V-6 engines displacing up to, say, 3.5-liters. That’s convenient because Infiniti reps are eager to compare the VC-Turbo’s output and fuel consumption to the family’s long-running 3.5-liter V-6. At 268 horsepower and up to 288 lb-ft of torque, in a smaller, significantly lighter (up to 55 pounds) package, the VC-Turbo makes only 2 less horsepower and up to 73 lb-ft more torque than that old 3.5 V-6 while returning a claimed 27 percent better fuel economy. But that engine is no longer used in the Infiniti product line and only sprinkled about the Nissan family. The comparison is a little more challenging if you put the VC-Turbo up against Infiniti’s 3.7-liter V-6, which makes 325 horsepower and 267 lb-ft of torque in the current-generation QX50 while delivering and EPA-estimated 17 mpg city and 24 mpg highway.
Against four-cylinder rivals, the early read is that Infiniti has a winner on its hands. Infiniti claims the its VC-Turbo returns 10 percent better fuel economy over four-cylinder rivals from Germany and Japan and should deliver class-leading acceleration. Infiniti’s benchmark testing shows its new QX50 delivers as much as 3 mpg better fuel economy and 1-second faster acceleration to 60 mph over comparably equipped rivals from Audi and BMW.
But that’s all bench racing. When we’re able to perform our instrumented performance and EquaAir RealMPG fuel economy tests, you’ll be the first to see the results. Stay tuned.
