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The variable compression ratio engine has arrived

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Car & Driver  /  November 1, 2016

For ages, automakers have been searching for a practical means of varying an engine’s compression ratio on the fly. How hard the air—and sometimes the fuel—is squeezed before ignition plays a crucial role in overall efficiency: Compressing the mixture as much as possible without detonation yields a longer expansion ratio and more power for every increment of fuel consumed. Since the compression ratio is determined by basic engine geometry (the cylinder volume with the piston at the bottom of its travel versus the top), that isn’t easily changed.

One expedient now in wide use is combining the Atkinson cycle with variable valve timing.

But Infiniti (Nissan) has made a ­significant stride with its Variable Compression-Turbo (VC-T). This boosted 2.0-liter four-cylinder, the product of two decades of research and 300 patents, will power unspecified Infiniti models beginning in 2018. Here’s how it works (see below):

The key enabler is a piece Infiniti calls the “multi-link,” (001) a diamond-shaped component that replaces the connecting rod’s big end and allows computer control over each cylinder’s compression ratio.

When the electronically controlled harmonic drive mechanism (002) rotates, the actuator arm (003), eccentric control shaft (004), lower link (005), and, finally, the multi-link all move to vary the piston’s stroke.

Infiniti claims VC-T is capable of providing any desired compression ratio between 8:1 and 14:1. Bumping the ratio to 14:1 during light-throttle cruising maximizes fuel efficiency. Then, when the driver dips into the throttle, the compression ratio can drop as turbo boost rises, avoiding detonation. Infiniti hasn’t yet revealed any power, torque, or mileage ratings but claims that VC-T will combine the power of a 2.0-liter turbo gasoline engine with the torque and efficiency of a similarly sized diesel. Many auto­makers have experimented with compound-connecting-­rod and moving-cylinder mechanisms, but Infiniti appears to be the first to solve the durability issues of a crankcase stuffed with links and levers.

Stroker Ace

Honda has filed a patent in Japan for an engine in which each cylinder has a different stroke, their altering displacements translating to varied power potential and fuel consumption. The goal is to take fuller advantage of cylinder deactivation. When, for the purposes of saving fuel, the engine-control computer deactivates cylinders, it will be able to pick and choose which cylinders to drop to best match power and efficiency with the demands placed on the engine. If it moves beyond the developmental phase, it will still be years out. But probably not the 20 years Infiniti has into its VC-T.

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Infiniti [Nissan] rolls out industry-first variable compression engine

Richard Truett, Automotive News  /  November 16, 2017

PHOENIX -- When the next-generation Infiniti QX50 crossover arrives next year as a 2019 model, it will be powered by a 2.0-liter turbocharged four-cylinder engine introducing a technology never seen before in a regular production vehicle: variable compression.

Under development at Nissan for 20 years, the Variable Compression – Turbo -- VC-T engine -- has a device that changes the distance the pistons travel in their cylinders by as much as 6 mm, or about a quarter of an inch.

It's an idea that automakers, such as Saab, and engine developers, including AVL, have tried to perfect over the years. The reason: Varying the compression ratio has the potential to offer dramatic improvements in power and efficiency.

Shinichi Kiga, Nissan's chief powertrain engineer, says Infiniti's VC-T engine is expected to deliver a combined city-highway EPA-rated fuel economy gain of 27 percent over the QX50s's outgoing 3.7-liter V-6. The VC-T engine is rated at 268 hp and 288 pound-feet of torque. Kiga says the 2019 QX50 will reach 60 mph almost one second faster than its four-cylinder competitors.

Infiniti last week allowed a few reporters to drive early pilot-build 2019 QX50s at the company's test trackhere. The VC-T's power and refinement were impressive. The technology works in such a manner to make it invisible to the driver. There are no buttons to press, no switches to turn and no strange noises, vibrations or sounds -- just smooth, abundant power and a pleasing, performance-tinged growl from under the hood.

How it works

The pistons in all of today's automotive internal combustion engines travel the same distance up and down in the cylinder, regardless of speed and vehicle load. The pistons are connected to rods that are mounted to the crankshaft. The engine's compression ratio is fixed and is determined by the amount of space above the pistons when they are at the top of their travel. The smaller the space above the pistons, the higher the compression ratio and the greater the engine's power output.

New layout

The Infiniti engine's layout is the first major change to the system in more than a century. The pistons in the VC-T engine are connected to rods that are attached to one end of an elliptic device Nissan calls a multilink. The multilink is mounted on the crankshaft where the connecting rods normally go.

The other end of the multilink has a short rod that is connected to a shaft. An electrically controlled actuator twists the shaft, causing the multilink to tilt up or down slightly. When the end of the multilink closest to the piston is tilted up, the piston travels to the top of the cylinder. That is high-compression mode, 14.1.

When the multilink tilts down, the piston's travel in the cylinder stops before it reaches the top.

This lowers the compression ratio to 8.1, but because the turbocharger kicks in at the same time, the engine's power output increases. The turbocharger pressurizes each cylinder with a denser mixture of fuel and air, which increases power.

"Variable compression solves the age-old problem of compromise," says analyst Dave Sullivan of AutoPacific, a consulting firm. "Automakers have the task of meeting increasing emissions regulations and tightening fuel economy standards while consumers want more power. They have to dial down the compression to make everyone happy.

"Nissan's variable compression engine should appease the enthusiasts as well as those concerned with fuel economy."

Robust target

Kiga said Nissan has long known that its multilink mechanical system of varying piston travel would not just work but also be robust enough for a production engine. The reason for the 20-year gestation is that engine-control technology was not yet advanced enough to manage the complex system.

"Control technologies helped us accelerate the development," Kiga said.

The engine uses nearly all the advanced technologies found in today's production engines. In addition to the turbocharger, the VC-T has both port and direct fuel injection, variable valve timing and a cooling system with four circuits. All of these systems provide inputs to the engine's computer, which runs a fast-spinning electric motor that operates the actuator arm that changes the piston travel.

The VC-T adds cost but is less expensive than a diesel engine.

The VC-T engine, Kiga says, is about 10 percent more expensive to produce than a regular 2.0-liter turbo four-cylinder engine. And although the VC-T has more moving parts than other turbocharged, four-cylinder engines, Kiga says it produces less friction. One reason is because the piston travels straight up and down in the cylinder, eliminating side forces that cause friction.

The current QX50, with its 3.7-liter V-6 engine, has a EPA fuel economy rating of 17 mpg city, 24 highway and 20 combined. When the new QX50 goes on sale, the combined fuel economy should be around 26 mpg, about what a diesel-powered vehicle of the same size would achieve, but without a lot of the emissions-system complexity.

The VC-T engine will be the only one offered in the QX50, and it will be available with just one transmission, a continuously variable automatic.

'Longer legs'

Although the Infiniti VC-T engine appears to be a significant advancement of the internal combustion engine, it may not have a huge impact, as governments around the world crack down on internal combustion engines and as automakers gear up to sell electrified vehicles.

"The 2.0-liter turbo has essentially replaced the 3.5-liter V-6. It's a very crowded space in which to set yourself apart," said Sullivan. "Combined with the continually variable transmission, the VC-T engine could yield some impressive fuel economy numbers. But with the relative stability of low fuel prices, it may not mean much to QX50 buyers. The immediate impact may not be felt with the QX50. But this technology only gives the internal combustion engine longer legs."

While the VC-T's performance and refinement in the test QX50s were outstanding, perhaps a downside for potential buyers is that the engine requires premium gasoline, which was averaging $3.09 per gallon, according to AAA, 52 cents more per gallon than regular.

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Mark, my favorite car is a 2-door Saab 99 Turbo with a 5-speed manual. Also always liked the fiberglass-bodied 1970-74 Saab Sonett sports car with the 1.7-litre Ford Taunus V4.

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One of my favorites were SAAB-96. There was a guy here in Moscow in late 90's who collected 4-5 of them.

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2 hours ago, Dirtymilkman said:

Is this going to be like the Variable Venturi carburetor experiment?

At least the carb was easy to replace.

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