Ever since the oil crisis of the 1970s, automakers have been faced with a dilemma: How can they satisfy both the industry’s need to improve fuel economy and their customers’ need for speed? Over the years, the companies have responded with engine technologies like electronic fuel injection, variable valve timing, direct injection, turbocharging and more. All can deliver benefits for both efficiency and performance. But none do the job like the newest breakthrough from Infiniti. The brand’s 2019 QX50 SUV, is the first production vehicle to offer a variable compression engine.
What Is Engine Compression?
To understand why this latest technology is so important, it helps to understand the basics of a typical internal combustion engine. The action takes place inside the engine’s cylinders following a four-step process. First, fuel and air are drawn into the cylinder as the piston moves down. The mixture then gets squeezed together after the bottom of the piston rotates around the crankshaft, so that piston starts rising again.
When the piston is at the top of its path, a spark plug ignites the mixture of fuel and air in the cylinder. The resulting explosion creates the engine power and drives the piston downward. It rotates around the crankshaft again and rises, pushing out the exhaust gases. After the piston moves back down, the cycle repeats.
That squeezing part of the process is known as compression.
What Is Compression Ratio?
In talking about engine compression, folks also refer to the compression ratio. For this, you start by measuring how much empty space is left in the engine cylinder when the piston is at the bottom of its cycle. Next, you compare that with how much empty space is left when the piston is at the top of its cycle. So, if a cylinder can hold 1000 cc of fuel and air when the piston is all the way down, and that gets squeezed into a 100-cc space when the cylinder rises, the compression ratio is 10:1.
Generally speaking, engines with higher compression ratios are more efficient and powerful. This has to do with physics, but you can imagine what happens if the fuel-air mixture is ignited in two different engines. In the one with a higher compression ratio, the mixture is ignited with the top of the piston closer to the center of the explosion. That way it’s more affected by the force.
To put some real-world numbers on this, a popular compact SUV like the 2018 Nissan Rogue has a compression ratio of 10:1. The standard engine in the 2018 Ford F-150 pickup has a compression ratio of 12:1. In the 2018 Mazda3, from a brand that pioneered high-compression modern-day engines, the compression ratio is 13:1.
Yet there are definite limits to how much you can compress the fuel and air. At some point, the combination will combust just from the pressure, not from the spark plug. This is called knocking or detonation, and it can lead to massive engine damage.
What Makes Infiniti’s New Variable Compression Engine so Special?
Still, you might be wondering why anyone would want a variable compression engine. Since there are no performance or efficiency advantages to running at a lower ratio in a traditional gas engine, why not focus on increasing engine compression ratios as high as possible? Why even have the option of operating at a lower ratio?
The answer is simple: The new Infiniti engine isn’t a traditional unit. It’s turbocharged. With a turbocharged engine, or a supercharged one, the extra burst of performance comes from forcing more air into the engine cylinders during the intake cycle. So, in a high-compression engine, the piston is trying to squeeze that extra air into a smaller-than-normal space in the cylinder. The results include more power and more efficiency, but more stress on the engine, too.
The Infiniti variable compression engine offers the best of both worlds. In normal driving situations, the engine features a high compression ratio. That allows drivers to enjoy notably better fuel economy and performance than a similar engine with a lower ratio. If drivers want more power, the engine shifts to a lower compression ratio. However, the turbocharger kicks in at the same time, and the increase in turbo power more than makes up for the change in compression ratios. Also, the engine actually optimizes the compression ratio depending on the driver’s requirements. It can vary from 14:1 to 8:1 and be automatically adjusted to any ratio in between.
To make the system work, engineers have eliminated the conventional crankshaft from the engine. In its place is a “multi-link” arrangement that can change how far up the pistons travel in the engine cylinders. They move farther in the high-compression mode, which leads to a smaller space between the piston heads and the cylinder tops.
What Are the Specs for Infiniti’s New Variable Compression Engine?
The 2019 Infiniti QX50 is available with a 2.0-liter four-cylinder engine with variable compression and turbocharging. It delivers 268 horsepower and 280 pound-feet of torque, as well as EPA ratings that include a grade of 27 mpg in combined driving. The all-wheel-drive QX50 is estimated at 26 mpg combined.
For comparison’s sake, the 2018 BMW X3 entry model is about the same size as the QX50 and features a conventional 2.0-liter four-cylinder turbocharged engine. The X3’s engine specs include 248 horsepower, 258 pound-feet of torque and a 25 mpg EPA rating in combined driving (with all-wheel drive).
A Saab Story
Although Infiniti may be the first automaker to offer a variable compression engine to the public, gearheads with a long memory may remember that Saab got pretty close. The now-defunct Swedish brand introduced its own concept for the technology back in 2000. The Saab system relied on a movable cylinder head. Raised slightly, the head would be farther away from the top of the piston and the compression ratio would be lower. And vice versa. Unfortunately, the Saab “monoblock” design was shelved after the automaker was purchased by General Motors.