Infiniti's KR20 Variable Compression Turbocharged Engine

Every new electric-car announcement is touted as another nail in the coffin of the internal-combustion engine. But the internal-combustion engine keeps fighting back. The latest counterpunch comes by way of the new 2019 Infiniti QX50, expected to hit the market in early 2018. It will be powered by a technically fascinating engine that promises to improve its fuel efficiency by about 10 to 15 percent compared with the best of its competitors, while also delivering better performance.

The engine is the new KR20 variable-compression-ratio inline-four, dubbed VC-Turbo (or VC-T) by Infiniti, for Variable Compression Turbo. A development of the MR20 2.0-liter four-cylinder, it is fitted with a complex mechanism that can vary the volume of the combustion chamber by changing the relative position of the piston over its stroke to adjust the compression ratio between 14.0:1 and 8.0:1.

A high compression ratio enhances the output and efficiency of any internal-combustion engine for two reasons: During combustion, it increases the internal pressure that pushes on the piston to turn the crankshaft. And it also increases the expansion ratio so that this higher pressure can motivate the piston over a longer stroke, extracting more useful work from each combustion cycle.

The downside is that the higher temperature and pressure also promotes detonation, which can quickly damage an engine. Therefore, it must be limited by retarding ignition timing, which reduces efficiency. This problem is exacerbated in a turbocharged engine, where the combustion pressures are even higher and usually additional fuel must be injected to cool the intake charge at high power settings. This is why so many modern cars with downsized turbocharged engines provide excellent mileage on the sedate EPA test cycles but turn downright thirsty when driven enthusiastically.

This VC-Turbo engine, developed over 20 years and embodying some 300 patents, solves these problems by employing a 14.0:1 compression ratio at light throttle to squeeze the maximum bang from each drop of gasoline. However, when you step into it to muster full boost and peak output, the magic mechanism lowers the piston’s stroke by six millimeters, reducing the compression ratio to 8:1. That allows the engine to continue efficient operation without needing to dump fuel to avoid engine meltdown.

The VC-T engine performs this neat trick by splicing several new components in between the piston and the crankshaft. The connecting rod is now called the upper link, and its lower end is connected to one end of a parallelogram-shaped control link that rides on the crankpin of the crankshaft. The other end of this control link is connected to a lower link that looks much like another connecting rod. This one is anchored to an eccentric pivot below the crankshaft on the control shaft. An actuator link connected to the control shaft can rotate this pivot when it is moved by an electric motor via the actuator shaft, and this motion ultimately effects the variable compression ratio.

It sounds like a lot of monkey motion, and you really need to study the accompanying video to see how it works. But the theory is sound, and the added components—an additional control link and lower link per cylinder, plus the eccentric pivot shaft, its actuating link, the control shaft, and the operating motor—seem solid. Nissan is taking no chances with these pieces breaking, so they are all beefy, steel parts.

However, it’s not all extra pieces. The modified motion of the piston produces a more symmetrical piston motion than a conventional rod and crank assembly, so the twin balance shafts required by typical four-cylinder engines are no longer needed. Furthermore, the VC-T’s piston motion—coupled with a crankshaft that is offset from the cylinder centerline by 15 to 20 millimeters—produces less side thrust on the piston, offsetting some of the friction added by the additional bearings in the complex mechanism. Still, the variable-compression KR20 engine weighs 302 pounds, about 22 pounds more than the conventional MR20 engine—including the weight saved by removing the balance shafts. However, that’s still 40 pounds less than Nissan’s 3.5-liter V-6.

With a peak boost pressure of 23.2 psi, the new engine develops a maximum torque of 288 lb-ft from 1600 through 4800 rpm and peak horsepower of 268 at 5600 rpm. Shinichi Kiga, Nissan’s chief engineer of gasoline engines, points out that this about the same power and over 10 percent more torque than the company’s naturally aspirated 3.5-liter V-6. But he says that the VC-T engine delivers 27 percent better fuel economy. Compared with the QX50’s competitors powered by conventional turbocharged 2.0-liter four-cylinder engines—the Audi Q5, BMW X3, and Mercedes-Benz GLC—the VC-T engine coupled to a continuously variable automatic transmission is expected to deliver 3 to 4 mpg better fuel economy.

We got a chance to drive a preproduction QX50 at Nissan’s Arizona proving grounds, and the experience was anticlimactic. The engine, mated to the CVT, felt very conventional. Acceleration was strong and smooth, the engine felt responsive, and it didn’t generate any funky sounds. Although the variable-compression mechanism takes about one and a half seconds to cycle between its ratio extremes, this interval is not apparent when you are cruising at a steady speed and call for more acceleration. The only indication you have that the compression ratio is changing is from an indicator in the instrument cluster next to the boost gauge.

As in all turbocharged engines, response is not instantaneous, but between the time it takes for the turbo to spool and the CVT to shift ratios, the QX50 responds quite normally. Mat the throttle from rest and there’s a bit of hesitation, just as there is in all turbo engines when you don’t resort to brake-torquing to build boost in advance.

As promised by Kiga, the balance shafts are not missed, as the engine revs smoothly to its 6000-rpm redline. However, there is a slight sense of complex mechanisms whirring rapidly at that engine speed. When we pointed out that competitive engines redline at 6500 or even 7000 rpm, Kiga explained that there was no performance benefit to revving that high with the VC-T engine, and he also admitted that friction rises more rapidly at high rpm than in conventional four-cylinders.

All of this sounds promising, but the proof will be in the mileage. Based on the claims, we expect the QX50 to deliver about 26 mpg on the EPA city cycle and low 30s on the highway. If those figures are achieved with commensurate real-world performance, this engine will represent major progress—and could add a few more years to the life of the internal-combustion engine.