Infiniti Variable Compression Turbo Breaks Cover

Engine revealed ahead of its Paris Motor Show debut

Engine revealed ahead of its Paris Motor Show debut

Back in August Infiniti tantalized us with a bit of info on a radical new 2.0-liter turbocharged gasoline engine that promises V-6 gas-engine performance with four-cylinder diesel fuel economy and four-cylinder gas engine emissions, mass, and packaging benefits. The engine is slated for production in 2018. Everything we learned and surmised in August has been confirmed by press releases issued in advance of the engine’s Paris debut, along with considerably more information.

The nutshell recap: Compression is altered by moving the range of motion of each piston up and down within the cylinder. In the upper position the ratio is 14:1, and in the lower position it’s 8:1. This is accomplished by an elaborate set of intermediary links between the crankshaft and the piston connecting rods, the positions of which can be altered by a control shaft mounted below the crankshaft and another set of control rods. A glance at the cutaway photos should help demystify this mechanism. Low compression is desirable during periods of peak boost and engine output, and the high ratio (along with Atkinson cycle operation that closes the intake valves partway up the compression stroke) greatly improves efficiency during low-load steady-state operation. The engine also employs direct, multipoint, or a combination of both fuel injection strategies as best befits the operating conditions.

Infiniti Variable Compression Turbo engine 13

Perhaps the biggest revelation was official output, which Infiniti pegs at 268 hp and 288 lb-ft of torque (rpm levels have yet to be disclosed). The target improvement in fuel efficiency is established at 27 percent relative to a V-6 of equivalent output, so think miles per gallon, not liters per 100 km when you do that math. Infiniti claims to have built and tested more than 100 engine prototypes, subjecting them to almost 2 million miles of equivalent road testing during 30,000 hours of test-bed development. The engine is currently undergoing real-world testing. The 20-year development program has resulted in more than 300 patents, including one for the multilink system and its electric motor actuator, which incorporates a Harmonic Drive reduction gear. Renault Sport Formula 1 even assisted with the development testing in the later stages of the program and is credited with helping resolve a high-speed bearing vibration problem.

Other details: The turbocharger is a single-scroll unit that bolts directly to a cylinder head–integrated exhaust manifold. The bore is listed as 84.0mm, and the official stroke is published at 94.1, but that appears to be a total stroke distance from top dead center during 14:1 operation to bottom dead center at 8:1 operation, as the displacement is listed at 1,997cc during 8:1 compression and 1,970cc at 14:1. Yes, the peculiar geometry of this multilink system actually alters the piston stroke infinitesimally between 90.1mm and 88.9mm. Infiniti also claims that this same monkey motion of the various rods and links dramatically reduces vibration, which it quantifies at 10 decibels—one-third that of a typical benchmark 2.0-liter turbo-four and within spitting distance of the 3 dB claimed for the 3.5-liter VQ V-6 this engine will end up replacing. (All of this is without balance shafts.) In addition to quelling some inherent vibration, this motion also reduces the lateral movement of the connecting rods, which naturally reduces the natural piston side-skirt friction. This does much to offset the added friction induced by the multilink crank/rod system (which triples the number of bearings in the engine), and the plasma-arc-sprayed ultra-smooth cylinder bore linings reduce friction by a further 44 percent.

Infiniti Variable Compression Turbo engine 02

That sprayed-in liner negates the need for iron liners and contributes to an overall weight savings of 55 pounds (24.9 kg) relative to the reference VQ V-6. A two-stage variable displacement oil pump enables lower pressures at engine speeds below 3,000 rpm and higher oil-flow pressure as speeds or loads increase. Similarly, a multiway flow-control valve manages engine thermal loads, routing coolant as warranted between the radiator, cabin heater, and oil cooler depending on conditions.

After decades of following numerous attempts to crack the variable compression code, we eagerly await our first drive of this world-first technology. Oh, and we hear there’s yet another variable-compression story brewing in Paris from another source, so stay tuned.

Questions we’ll be asking after the press conference:

  • How does the cost compare with the reference VQ engine?
  • Variable compression has been identified as an enabling technology for homogeneous-charge compression ignition. Do studies suggest any further efficiency improvements could be wrought from incorporating HCCI here?
  • Variable compression can also unlock the performance potential of gasoline/alcohol flex-fuel vehicles. Might there be global applications in places such as Brazil for this technology?
  • What does that Harmonic Drive reduction gear do to improve the customer experience over a chain or normal gear drive?
  • Does your integrated exhaust manifold include any volumetric efficiency tricks such as those we’ve seen from Mazda (twinning cylinder outputs to leverage exhaust pulse energy scavenging benefits) or lag-reducing measures such as small and large passages to accelerate exhaust flow onto the turbine wheel?

If you have further questions, send them my way, @MT_Markus on Twitter.