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.
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.
How Variable Compression Works
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.
