Simanaitis Says

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THIS TWO-PARTER is a road test of sorts. A 2019 Infiniti QX50 recently resided at our place for a week. It bristles with high technology, especially when put in perspective with another crossover vehicle with which I’m familiar, our 2012 Honda Crosstour. Today in Part 1, I focus on the most innovative aspect of the 2019 Infiniti QX50, its variable-compression turbocharged engine. Tomorrow in Part 2, I’ll share my impressions of the car.

2019 Infiniti QX50

Some Theory, Compressed. An internal-combustion engine compresses its fuel-air mixture before igniting it. This is the “squeeze” portion of “suck, squeeze, pop, pooey” of “The Otto Four-Stroke—Self Taught”.

The amount of squeeze is characterized by an engine’s compression ratio. For instance, an 8:1 c.r. compresses the charge to one-eighth of its original volume; a 14:1 c.r., to a much more compressive one-fourteenth. Not coincidentally, these are the two c.r extremes of Infiniti’s and the world’s first production VC-Turbo engine, of which more anon.

Why Squeeze More? The higher the c.r., the more efficient an engine’s combustion. But there’s a catch: For a given c.r., fuel octane, and engine load, there’s a point of detonation, of uncontrolled ignition at the wrong place and time in the combustion chamber. Modern engines have knock sensors monitoring this and adjusting ignition timing to avoid such detrimental phenomenon.

Also, traditional turbocharging achieves enhanced power by having its turbo squeeze more fuel and air into the combustion chamber. However, for a given fuel octane and boost, there’s also a danger of detonation. To counter this, some turbo engines have a c.r. in the 8.5:1 range. Even with this compromised c.r., a turbo engine’s knock sensor still adjusts ignition timing and boost to avoid detonation.

Variable Compression Ratio Designs. Note that a traditional engine’s c.r. has a fixed value set in its design stage. However, there are ways of varying c.r. in real time: “Variable Compression Ratios—Nissan, Honda, and the CFR” gave details here at SimanaitisSays back in November 2016. The 2019 Infiniti QX50 brings the Nissan concept into production.

The Infiniti VC-Turbo. Infiniti’s 2.0-liter inline-four achieves its variable compression by varying its stroke. It has what Infiniti calls Harmonic Drive articulating an auxiliary Multi-link, the pivoting of which controls the piston’s stroke.

For high efficiency, in periods of light load, for example, the engine’s c.r. can be as high as 14:1. In power mode, it may reduce to 8:1. Indeed, the computer-controlled Harmonic Drive varies c.r. continuously between these two extremes. It can switch from 8:1 to 14:1 in as quickly as 1.5 seconds.

There’s a real-time interplay of variable c.r., turbo boost, and engine load, all optimized for best overall performance. The Infiniti 2.0-liter inline-four produces 268 hp and 288 lb-ft of torque.

As an additional benefit, Infiniti claims that its VC-Turbo engine produces just a third of a typical inline-four’s vibration and thus requires no traditional counter-rotating balance shafts. According to Shinichi Kiga, Infiniti chief engineer, and reported in, this increase in smoothness comes from the VC-Turbo’s connecting rods remaining more upright throughout their stroke. What’s more, elimination of balance shaft hardware more than compensates for the added friction of the VC-Turbo’s variable c.r. hardware.

Here’s one insight I gained during my week with the Infiniti QX50: It’s the smoothest, quietest inline-four in my experience. Tomorrow in Part 2, I’ll comment more on the car’s VC-Turbo, others of its 2019 features, and its performance in my all-important Simanaitis Head Scrape Test. ds

© Dennis Simanaitis,, 2018

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