How Infiniti's Variable-Compression Engine Works

At last, the best of both worlds.

Infiniti QX50
Infiniti

For more than a century now, automotive engineers have struggled with an unavoidable balancing act when it comes to engine compression. Now, thanks to an innovation from Infiniti, they may get to enjoy the best of both worlds.

Cars have a single engine compression ratio, which is expressed in a form like 10:1. That ratio compares the maximum to minimum value of cylinder volume as the piston travels throughout its complete range. The more pressure you put on the engine’s air-fuel mixture, the higher the number.

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How it works
Infiniti

The problem is that different compression ratios are good at different times. But the limits of engine technology have forced designers to settle on one ratio for each engine, which has massive consequences on a car’s longevity, emissions, fuel economy, high-RPM power, and low-RPM power. The big prize for engineers would be for a way to let an engine change its compression ratio on the fly, specifying low compression when it's advantageous and high compression at other times. But theories and hope have always outpaced the technology.

Infiniti spent the last 20 years working on the dream of variable compression. In the process, it developed more than 100 engine prototypes, covered more than 1.8 million miles during road testing, and spent more than 30,000 hours working on test beds. The fruit of their labor is the VC-Turbo engine, and the 2019 Infiniti QX50 SUV is the first vehicle to feature the world’s first production variable compression engine ever.

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How Variable Compression Works

Infiniti

The “VC” stands for variable compression, and that means the VC-Turbo can change its compression ratio on a continuum from a pretty low 8:1 to a pretty high 14:1. Here’s why it matters so much.

A high compression ratio means you're squeezing tightly on the air-fuel mixture in the engine's combustion chambers, which in turn means more power and fuel efficiency. The catch is that forced-induction engines—those with superchargers, turbochargers, and twinchargers—don't like high compression ratios. Their job is to collect extra air and jam it into the engine. If the air is already tightly compacted and the turbo forces in even more, then the air (once mixed with fuel vapor) can combust unpredictably. That’s how you get engine knock, and engine knock is not good. So engineers are caught in a tug of war. You want a compression ratio that’s high for the sake of power and efficiency, but not so high that engine knock drives the motor haywire.

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2019 Infiniti QX50.
Infiniti

Infiniti's VC-Turbo is a turbocharged 2.0-liter inline-four that can operate at a very high compression ratio without knocking, thanks to the variable compression. During times of high turbo boost, the VC-Turbo's engine management computer signals an electric motor to move an actuator arm that shortens the reach of the pistons inside the engine, which lowers the compression ratio to avoid knock. When the motor isn't calling upon the turbocharger as much, the actuator arm lengthens the pistons' reach, which ups the compression ratio.

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Infiniti invented this multi-link system in 1998, says Christopher Day, Infiniti’s powertrain performance senior manager. But that was the just the first big breakthrough that made variable compression possible.

Another requirement was to make the engine run so smoothly by ditching the two balance shafts that conventional inline-fours have to balance out vibrations. Infiniti says the QX50 packs the first production engine mount that actively dampens vibrations. Sensors integrated in the upper engine mounts detect vibrations from the VC-Turbo, then creates opposing vibrations to cancel them out. All together, Day says, the design knocks nine decibels off the previous QX50's engine noise, making this four-banger almost as quiet as a V6 design.

Seamless Switching

Infiniti
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The VC-Turbo can also run in the Atkinson cycle in certain situations to boost fuel efficiency. The Atkinson cycle works by creating a small window of time in which the engine's intake valves open slightly, drawing extra air into the combustion chambers right as the pistons begin to compress the fuel-air mixture. By reducing the engine's displacement (volume), it lets the engine behave like a smaller, more efficient engine during intake.

Once ignition is fully under way, the VC-Turbo generates power like the bigger engine it truly is. Under high compression, the VC-Turbo switches seamlessly to the Atkinson cycle because the engine is making enough power that it can suffer the small power drop for the sake of efficiency. Atkinson-cycle engines are common in hybrid gasoline-electric cars that emphasize fuel efficiency and in which electric motors compensate for the reduced power, but it's rare on straight-fuel engines.

Under the hood of the QX50
Infiniti
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The 2019 Infiniti QX50 is a heavy SUV at 3,800 to 4,000 pounds, but Day says the VC-Turbo makes it 35 percent more fuel efficient compared to the previous QX50's V6 in front-wheel-drive layout, and 30 percent more fuel efficient in all-wheel-drive layout. Combined fuel economy is 27 MPG and 26 MPG, respectively. The VC-Turbo gives up horsepower compared to the outgoing V6, but not torque. You get 268 hp and 280 lb-ft of torque for the 2019 model vs. 325 hp and 267 lb-ft of torque for the 2017 model. (There was no 2018, as Infiniti chose to run the 2017 model year longer into the 2018 calendar year.)

The VC-Turbo is on sale now in the 2019 QX50. Toughening fuel economy and emissions standards are squeezing internal combustion engines from one side, and hybrids and pure-electric vehicles are squeezing them from the other. Car companies expect gas-only motors to stick around for a while. They still make up the vast majority of sale, so expect to see automakers pull out all the stops like Infiniti did with the VC-Turbo, all in the name of making tinier engines perform like the big power mills of recent years

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