MOTOR Magazine

A MOTOR Magazine Newsletter
November 3, 2016

Contributed by Bob Chabot
Meet Infiniti's Variable Compression Ratio Engine

New VC-Turbo engine sets power, efficiency and emissions benchmarks

Infiniti Motor Company Ltd. unveiled its variable compression ratio four-cylinder 2.0L MR20 DDT gasoline engine (the VC-Turbo) at the September 2016 Paris Auto Show. The VC-Turbo's technology is potentially an industry game-changer, similar in impact to previous powertrain innovations such as liquid cooling, fuel injection, catalytic converters and turbocharging. In short, the new powerplant delivers the power of a larger, high-performance six-cylinder gasoline engine, with the torque and efficiency of an advanced diesel powertrain, but without the equivalent emissions (notably particulate matter and NOx). But there's much more to the story.

"Fixed compression ratio engines have been in existence throughout the 130-year history of the automobile," stated Infiniti President Roland Krueger. "Throughout this time, automotive engineers have believed and sought a breakthrough in internal combustion technology would come from mastering the variable compression ratio. While other manufacturers have taken on this challenge, Infiniti will be the first to bring the technology to market."

Infiniti's new turbocharged four-cylinder VC-Turbo engine claims the power of a six-cylinder gasoline engine with the fuel economy and torque of a diesel powerplant. (Images — Infiniti)

An Engine That Continuously Transforms According to Driver Behavior
Krueger explained the automaker has worked on this technology for more than 20 years. "The first major breakthrough was our invention of an advanced multi-link technology in 1998, which held the key to a true variable compression ratio. Since then, to refine and perfect the technology, Infiniti has tested and developed more than 100 engine prototypes, covered over two million miles of on-road testing and spent more than 30,000 hours on the test bed (the equivalent of an additional two million miles of road testing). This R&D has resulted in our multi-link system, which can seamlessly raise or lower the stroke of the pistons, detect and integrate the vehicle's driving condition and driver inputs, then instantly select the most suitable compression ratio."

Krueger cited R&D done with the Renault Sport Formula 1 team as being instrumental in accelerating the VC-Turbo's prototype testing during the latter stages of its development and refinement. One such instance revolved around the VC-Turbo engine's bearings. Infiniti engineers had identified small bearing vibration and noise that manifested at the highest engine speeds under certain operating conditions. To identify and isolate the root causes of this issue, the automaker relied on the race team's expertise in dynamic motion analysis in conditions of up to 20,000 rpm – far higher than any series-production engine.

The team's experience in movement behavior, and competence in bearing technology for Formula 1 applications provided valuable insight into addressing, resolving, testing and confirming the durability of the MR20 DDT engine under particularly challenging operating conditions. As a result of this collaboration, the VC-Turbo engine features nearly three times as many bearings and produces 33 percent less vibration and 44 percent less cylinder friction than that generated by a conventional gasoline 4-cylinder engine, allowing smoother and quieter operation.

The VC-Turbo engine features a multi-link system that automatically alters piston height to vary the compression ratio on demand between 8:1 (for high performance) and 14:1 (for high efficiency). Piston height is altered by rotating the lower shaft which changes the position of the multi-link, connected to the piston connecting rod. Note that the multi-link system keeps the piston connecting rods more vertical during the combustion cycle, rather than moving wider laterally as in a traditional crankshaft rotation. This reduces piston friction.

Dynamic Variable Compression Ratios
The compression ratio is the ratio of the combustion chamber's volume from its greatest capacity (when the piston is at bottom-dead-center within the cylinder) to its smallest capacity (when the piston is at top-dead-center). Engines with a higher compression ratio are able to achieve greater efficiency and improved fuel consumption. In contrast, engines with lower compression ratios offer greater power and torque, particularly in turbocharged engines, but are known for reduced fuel efficiency.

"Until now, in every production engine ever built, this ratio has been fixed as a result of the dimensions, components and engine layout," Krueger explained. "The VC-Turbo engine is able to offer any compression ratio between 8:1 [typical for high performance engines] and 14:1 [used by many manufacturers for high efficiency]. The VC-Turbo engine will provide performance and efficiency comparable to, or better than, similar- or larger-sized six-cylinder gasoline powertrains."

"As well as offering more flexibility, other VC-Turbo engine technology negates the downsides posed by fixed compression to deliver high performance along with high efficiency," added Shinichi Kiga, chief powertrain engineer for Infiniti's Gasoline Engine Project Group. "For instance, the MR20 DDT employs a combination of both multi-point injection (MPI) and direct injection (DI), to balance efficiency and power in all driving conditions."

The DI system improves combustion efficiency and performance due to the heat-absorbing effect of fuel vaporization, which aids cylinder cooling, and allows the engine to avoid 'knocking' at higher compression ratios — an inefficient process that can occur in higher compression ratio engines when the air-fuel mixture combusts prematurely in the cylinder, potentially resulting in damage. In contrast, MPI allows for earlier mixing of air and fuel to increase engine efficiency at low loads.

"Like the compression ratio, the engine can switch instantly between DI or MPI at regular engine speeds, while both sets of injectors work in conjunction under a combination of high engine speeds and load," Krueger noted. "This latter characteristic ensures greater efficiency than many high performance engines with a fixed, low compression ratio."

Infiniti's short video highlights some of the key features of the VC-Turbo variable compression engine.

High Performance, With High Efficiency
Kiga shared that the advantages of the engine's variable compression ratio and other innovations applied to the MR20 DDT engine has his engineering team expecting a 27 percent improvement in fuel efficiency over V6 engines with a similar power output, depending on its vehicle application. In addition, the production-series VC-Turbo engine will deliver power output of around 200 kW (268 bhp, 272 ps) and approximately 390 Nm torque —higher than many four-cylinder turbocharged gasoline engines currently in vehicles today.

"The VC-Turbo engine represents a new breed of engine which can transform continuously, and will establish new benchmarks for future internal combustion engines," noted Kiga. "Previous VCR engines altered the compression ratio by raising or lowering the height of the piston at top dead center, but the Infiniti engine achieves this in a different way. Our engineers adopted a multi-link system with an electric motor actuator, which they describe as the Harmonic Drive reduction gear."

"Together, multi-link system and an electric motor actuator enable the engine to transform its compression ratio continuously," he continued. "Infiniti created the electric motor, which is connected to the Harmonic Drive reduction gear with a connecting control arm. The Harmonic Drive spins according to the compression ratio required, and then rotates the control shaft at the base of the engine and, in turn, moves the multi-link mechanism's piston connecting rods. Changing the multi-link angle adjusts the height of the top-dead-center (TDC) of all four pistons at the same time, which varies the compression ratio."

In more detail:

  • The electric motor drives the reduction gear, which moves an angled actuator arm. The arm in turn rotates a control shaft with four aligned eccentric cams, one for each cylinder. An intermediate link with bearings at each end connects the eccentric cam at the bottom end to the multi-link at the top end. The center of the multi-link runs in a bearing around the crankshaft journal.
  • A second bearing on the multilink, positioned 180° degrees from that, connects to the intermediate link to effectively serve as the piston connecting rod big-end bearing. This arrangement produces a 17° offset of the connecting rod from the crankshaft journal center point.
  • A dedicated electronic control unit manages the Harmonic Drive. For instance, a low compression ratio is preferable when power is required, while a high compression ratio facilitates efficiency. The ECU gathers data from engine sensors to determine the compression ratio required for given driving conditions, and continually adjusts the piston height at TDC by rotating the Harmonic Drive. This action determines the position of the multilink system and subsequent height of the piston in the cylinder bore.

VCR allows the VC-Turbo engine to switch between both Atkinson and regular combustion cycles, without interruption — another major advantage cited by Kiga. Each cycle enables greater combustion efficiency and optimal engine performance as the combustion ratio transforms. Under the Atkinson combustion cycle, air intake overlaps with the compression cycle in the cylinder, allowing combustion gas to expand to a larger volume for greater efficiency. The VC-Turbo engine employs the Atkinson cycle under higher compression ratios, where the stroke of the pistons is longer. The modern Atkinson cycle – employed in many advanced hybrid engines – allows the intake valves to be open for a short time as the compression stroke starts. As the compression ratio lowers, to enable greater engine performance, the engine is able to operate a regular combustion cycle: intake, compression, combustion, and exhaust, each stage taking place in distinct, separate phases, resulting in greater performance.

Infiniti shared these technical specifications for the MR20 DDT 2.0-L VC-Turbo gasoline engine.

According to Kiga, a wide-range, single-scroll turbocharger provides seamless access to the engine's power and torque reserves and enables immediate acceleration. As the engine shifts to allow greater performance or efficiency, the turbocharger works in conjunction with the VC-Turbo system to maximize acceleration response at any engine speed or compression ratio. The engine moves seamlessly between compression ratios, detecting the car's driving condition and driver inputs, and instantly selecting the most suitable compression ratio.

"For instance, if the driver is cruising at a steady speed but needs to accelerate to overtake, the turbocharger reacts instantly to boost acceleration as the engine shifts seamlessly to a lower compression ratio. The turbocharger enables the four-cylinder VC-Turbo engine to benefit from greater efficiency than a larger, naturally aspirated six-cylinder engine, yet ensures equivalent performance. The single-scroll turbocharger also improves the loss of thermal energy and exhaust pressure, and keeps weight and cost low."

When under hard acceleration or heavier engine loads, such as when overtaking another vehicle or driving up a hill, Kiga said the engine shifts to a lower compression ratio. This results in higher performance from the engine, offering a compression ratio that can be compared with some of the highest-performing turbocharged 2.0-liter four-cylinder engines on the roads today. Conversely, when driving at steady highway speeds, under slowing driving conditions, at idle, and at low speeds in stop-start traffic, the engine defaults to a higher compression ratio to maximize efficiency.

"Variable compression ratio technology will provide drivers with an engine that makes an intelligent choice between performance and efficiency," Krueger emphasized. "Together with other advances that helped create the VC-Turbo, Infiniti will be able to provide motorists with an engine that adapts to conditions, and empowers drivers with the engine they want, when they want it — the world's first truly driver-focused powertrain. We are now confident the VC-Turbo will be ready for series-production and sales beginning with the all-new 2018 QX50 and then certain other models."

[Editor's note: Visit MOTOR for the latest diagnostic and service insights.]

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