Infiniti Develops a Variable Compression Ratio Engine

It is the best of both worlds—performance and economy— from an engine going into production for Infiniti 2018 vehicles.

It’s taken the development and testing of more than 100 engine prototypes, over 30,000 hours on a test bed, and 1.8-million miles of equivalent road testing. And in 2018, Infiniti is going to be offering what it says is the world’s first production-ready variable compression ratio engine, an engine that can offer any compression ratio between 8:1 and 14:1.

As you know, the compression ratio is the ratio of the combustion chamber’s volume from its greatest capacity to its smallest capacity, or from when the piston is at bottom-dead-center within the cylinder to when the piston is at top-dead-center. Typically, an 8:1 compression ratio is used for high-performance engines, for power and torque, while the 14:1 is ideal for high-efficiency engines, for lower fuel consumption.

But Infiniti wanted both performance and efficiency so they developed the 2.0-liter, four-cylinder VC-Turbo engine. The engine packages smaller than an equivalent six-cylinder and the company says that it provides the same type of performance more characteristic of a four-cylinder diesel. They’re targeting a power output of about 268 hp and 287 lb-ft of torque.

So how is this accomplished?

Infiniti engineers developed a multi-link system. And there is an electric motor actuator that uses a Harmonic Drive reduction gear with a connecting control arm.

In action, the Harmonic Drive rotates based on the required compression ratio such that the control shaft at the base of the engine rotates, which, in turn, moves the multi-link mechanism. By changing the multi-link angle, the height of the top-dead-center of the pistons is adjusted, thereby varying the compression ratio. It should be noted that the piston stroke position for all four cylinders is adjusted at the same time through an eccentric control shaft.

The engine is able to operate as required in either an Atkinson-cycle or regular-cycle. In the Atkinson-cycle, the air intake overlaps with the compression cycle in the cylinder. This allows the compression gas to expand to a larger volume, which results in greater efficiency. The Atkinson cycle is used under higher compression ratios; the stroke of the pistons is longer.

Under lower compression ratios, when greater engine performance is the goal, the engine runs under the regular combustion cycle.

During the combustion cycle the connecting rods are nearly vertical rather than having a lateral motion as is the case in conventional crankshaft rotation. This helps make the engine smoother than the typical in-line engine. And even though the lateral motion of con rods contributes to piston friction within the cylinders, and even though this isn’t an issue with the multi-link design, the cylinder walls of the VC-Turbo are plasma sprayed, hardened and honed, such that whatever cylinder friction there is is reduced by 44 percent.

What’s more, the multi-link design, compared with a connecting rod/crankshaft system, results in a better reciprocating motion. One consequence is that there are fewer vibrations so that there are no balance shafts required to reduce second-order vibrations. Infiniti engineers benchmarked the engine-vibration noise of the VC-Turbo against other four-cylinder turbos. The benchmark average was approximately 30 dB. The new engine comes in at 10 dB.

One development challenge that was addressed by both Infiniti engineers and their colleagues at the Renault Sport Formula 1 team was related to bearings. The multi-link system results in an engine with three times the number of bearings of a conventional engine. Under high speeds and certain conditions there were small bearing vibrations. As the Formula 1 engineers are familiar with working with dynamic motion analysis of engines at high rotational speeds—as in up to 20,000 rpm—they were called in to help address the vibration issue.

The block and head are aluminum. (Thanks to the plasma sprayed coating, no cylinder liners are required.) The multi-link system components are produced with a high-carbon steel alloy.

The engine features include a single-scroll turbocharger, an exhaust manifold built into the cylinder head, a catalytic converter positioned next to the turbo to reduce the length of the flow path for exhaust gases for both improved emissions and improved turbo response, an electronically controlled wastegate actuator, a high-capacity intercooler, and a two-stage variable displacement oil pump.