MOTOR Magazine

A MOTOR Magazine Newsletter
May 2, 2017

Contributed by Bob Chabot
Will Variable Compression Drive Opportunity Your Way?

Nissan’s Chief Engineer Shares More Details Behind the Technology

Nearly 100 years ago, Sir Harry Ricardo described the performance advantages of variable compression ratio (VCR) engines. Despite many attempts to design and implement VCR since then, it wasn’t until six months ago that Nissan-Infiniti introduced its new VC-Turbo engine. “Variable compression is poised to power future gasoline engines,” says Shinichi Kiga, Nissan’s chief powertrain engineer for its Gasoline Engine Project Group. He then shared details behind his belief.

Multi-link Technology was the VCR Engine Code-Breaker
“Nissan had been working on VCR engine technology for over 20 years,” Kiga said. “Our first VCR-related patent was filed in 1932; since then, Nissan filed more than 400 patents. But our big breakthrough — multi-link technology — came in 1998; it enabled us to design, develop and operate a VCR engine. Since then, our R&D program has featured more than 100 prototype engines, 30,000 hours on test beds and the equivalent of three million miles of on-road, real-world testing. In particular, the development of our electrically powered reduction gear, which we call Harmonic Drive, allowed Nissan to master the VCR engine. It will debut first in the 2018 Infiniti QX50 premium crossover.”

According to Kiga, the 2.0-liter VC-Turbo engine is equal to, and in many aspects better than, the conventional 3.5-liter V6 engine currently used. In addition to delivering a 27 percent improvement in fuel efficiency as well as a dramatic reduction in NOx and particulate emissions, the new engine will output 200 kW and 390 Nm of torque, which is impressive for a 2.0-liter. In addition, despite the complexity of the VCR linkages, the engine is 25 kg. lighter than the V6 thanks to the use of high carbon steel alloys for the linkages and linerless aluminum block and cylinder heads.

“We achieved a 44 percent reduction in cylinder friction by specifying plasma-jet coated bores that are hardened and honed to deliver a low friction surface,” explained Kiga. “This eliminated the need for cylinder liners. As a result of reduced side forces, there’s also lower friction between the rings and bore. Admittedly, there’s an increase in pressure on some of the bearings, but it’s no more than you’d find in a similar sized diesel engine, so it is easily containable.”

Nissan VC-T engine multi-link technology rotates to alter the height pistons rise to. See how high and low compression ratios in the VC-Turbo engine compare. (All images — Nissan-Infiniti)

Electrically Powered Harmonic Drive Reduction Gearing Refined Efficiency
The Harmonic Drive reduction gear rotates according to the compression ratio required at any given moment. It drives the Connecting Control Shaft at the base of the engine, which in turn allows the multi-link mechanism to move the connecting rods to vary the piston stroke position for all four cylinders at the same time. Changing the angle of the multi-link adjusts the height of the top-dead-center of the pistons, thereby varying the compression ratio. For example, the pistons’ connecting rods are almost vertical during the combustion cycle, which is quite different from the wider lateral movement of rods per a conventional crankshaft rotation.

The VCR engine is able to shift seamlessly between compression ratios by sensing the car’s driving condition and driver inputs, then instantly selecting the most suitable compression ratio. For instance, under hard acceleration or at heavier engine loads (e.g. when overtaking another vehicle or driving uphill), the engine shifts to a lower compression ratio. “This results in optimized performance, offering a compression ratio comparable to that of powerful turbocharged 2.0-litre four-cylinder engines,” Kiga noted. “At steady highway speeds, under slowing driving conditions, at idle, or at low speeds in stop-start traffic, the engine defaults to a higher compression ratio to maximize efficiency.”

“The exhaust manifold and cylinder head have been amalgamated into one assembly, which improves packaging and enables a more intelligent integration with the turbocharger and catalytic converter,” Kiga shared. “The VCR system also reduces vibrations by a third, which allows us to avoid secondary harmonic vibrations. This eliminated the need for a balancer shaft while still delivering a really smooth running and quiet engine. For instance, the vibration noise level in the VC-Turbo engine is just 10 decibels, compared to the benchmark average of approximately 30 dB in the V6.”

Watch the video to learn how Nissan-Infiniti’s variable compression ratio engine technology delivers efficiency and performance.

VCR Fuel Strategy Enhances the Future Prospects of the Gasoline Engine
There are alternative solutions to improving fuel economy. “VCR engines are a more conventional solution and less expensive, when compared to electrification solutions, such as a plug-in hybrid or 48 volts,” Kiga noted. “The VC-Turbo engine is 25 kg. lighter, achieves good fuel efficiency at a lower price by using a four-cylinder engine equipped with VCR technology that delivers the equivalent or better performance of a V6 engine. We also believe our VCR solution offers better value for the motorist’s money than some of the other VCR technologies currently under development elsewhere. For example, our technology is truly variable from 8:1 to 14:1, rather than being a simple two-stage system other automakers are working toward. The VC-Turbo engine’s fuel strategy employs a combination of both multi-point injection (MPI) and direct injection (DIG), further improving the engine’s ability to balance efficiency and power in all driving conditions.”

Kiga explained that, like the compression ratio, the engine could switch instantly between DIG or MPI at regular engine speeds, while both sets of injectors work in conjunction under a combination of high engine speeds and load. The DIG 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. The MPI system allows for earlier mixing of air and fuel to increase engine efficiency at low loads.

The engine can also switch seamlessly between Atkinson and regular combustion cycles. 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. In the VC-Turbo engine, the Atkinson cycle is activated under higher compression ratios, where the piston stroke is longer. Then, as the compression ratio lowers, the engine reverts to a regular combustion cycle to enable greater engine performance.

“VCR is not just a ‘one-shot’ technology for a single model,” Kiga emphasized. “While the 2.0-liter VC-Turbo will initially replace the 3.5-liter V6 in the QX50, it will soon be the basis of a new engine family that will be available to power all brands within the Renault Nissan Alliance. It’s also possible to develop other VCR engine applications, where the cost and market capacity match up well.”

These engines are headed your way. That’s opportunity knocking.

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

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