Infiniti Preparing World’s First Variable-Compression Engine for 2018

The ability to vary an engine’s compression ratio has been a “holy grail” quest of engine engineers for decades, because “one size” of compression ratio cannot possibly fit all the operating conditions required of a typical internal combustion-powered vehicle. Saab tried hinging the upper half of the engine in 2000, various other inventors have envisioned racks and pinions to vary the piston position relative to stroke, Lotus suggested a movable piston in the cylinder head to vary the clearance volume, and much more recently FEV demonstrated an eccentric piston-pin that provided two distinct connecting rod lengths (and hence compression ratios) while an outfit called Lugo Developments showed a crazy cammed-crankshaft concept. Today these ideas are all conspicuous by their absence from the production car landscape, but now Infiniti appears ready to introduce the world’s first variable-compression engine in 2018.

Dubbed VC-T for “variable-compression, turbocharged,” it features a mechanical connecting-rod modification that was first described in a U.S. patent assigned to Nissan on June 14, 2003 (6,505,582 B2 which can be downloaded here). In fact, there were rumors years later that that the technology might be poised for imminent production in a 1.6-liter turbo destined for the 2011 Silvia (we knew it as the 240SX). Well, perhaps the kinks proved harder than expected to work out, or more likely, the onboard processing power required to make everything work just didn’t exist until now.

We’ll have to wait until the engine’s official Paris auto show debut to get every nitty-gritty detail, but here’s what we know now: an advanced multi-link system seamlessly raises and lowers the height the pistons reach within the cylinders, varying the clearance area above the piston and thereby altering the compression ratio between 8.0:1 (at the low point of the mechanism’s travel) to 14.0:1. The low numerical compression ratio is used during periods of high performance and turbocharger boost to prevent knock; the high ratio optimizes efficiency during low-load operation. We know that by employing variable valve timing on both cams with a high range of authority (using faster-acting electric control of the intake cam and hydraulic control of the exhaust), the engine can operate in Atkinson-cycle modes during those low-load periods (by dramatically delaying the intake-valve closure, the effective compression ratio is reduced relative to that high 14.0:1 expansion ratio to squeeze out even more efficiency from the cycle). The engine will utilizes both port and direct injection, switching between them as appropriate for peak efficiency. We expect port injection to be used at start up, direct under full load, a combo of the two at other times.

Based on the patent drawings, here’s how we interpret the multi-link system: Located below and to one side of the crankshaft is another shaft with eccentric lobes that move a set of secondary connecting rods up and down. These rods and the ones connecting to the pistons are connected by triangular links that mount to the crankshaft. Rotating that eccentric shaft raises or lowers the pivot point of these triangular links, which in turn shifts the range of motion of the pistons up or down, varying the compression ratio infinitely between the 8.0:1 and 14.0:1. There’s another huge benefit: At each piston’s top-dead-center position, when combustion forces are huge, rather than wasting power by momentarily pushing straight down on the crankshaft bearings, this linkage provides an offset, allowing that force to provide leverage on the crankshaft.

That feature of this design is claimed (along with “mirror-bore” coatings and other measures) to dramatically reduce friction and even vibration, to the point that no counter-rotating balance shafts are needed (just as well, as they’d be crowding the eccentric shaft, and anyway, balance shafts really become necessary above 2.0 liters). But considerable friction is added back in by quadrupling the number of journal bearings involved, even though most do not experience anywhere near the rotation and loading of the main crank bearings.

This engine will be even costlier to produce than the typical direct-injected turbocharged 2.0-liters that are proliferating through luxury-brand cars today, which explains its roll-out in Infiniti products that will likely demand premium fuel. Expect V-6 levels of peak power and torque with 25-30 percent greater efficiency (diesel efficiency with gas performance and emissions cost is the usual claim for variable-compression technology). But if/when it proliferates to other Renault-Nissan alliance vehicles, don’t be surprised to see regular gas and flex-fuel functionality added in certain markets (ethanol loves high compression).

We’ll learn more about Infiniti’s variable-compression engine on September 29 at the Paris auto show.

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