Nissan unveils 2018 production variable-compression-ratio ICE

  • 03-Oct-2016 11:13 EDT
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The 200-kW Nissan VC-Turbo engine is designed to replace the company's 3.5-L V6, first in Infiniti's QX50, while reducing weight by 25 kg and targeting fuel consumption reductions of 27% vs. the V6

Nissan broke new ground for gasoline engines at the 2016 Paris Motor Show when it unveiled the first production-intent variable-compression-ratio gasoline engine. The VC-Turbo, a 2.0-L inline four cylinder designed in house, features a novel cranktrain and control system that enables the effective compression ratio to be varied between 8.0:1 and 14:1, depending on load.

Making its debut under the automaker's Infiniti brand, the VC-Turbo (VC-T) has been designed to replace the company's 3.5-L V6, with a target output of 268 hp (200 kW) and 288 lb·ft (390 N·m). Infiniti has set a target for fuel consumption reduction of 27% compared with the V6. The VC-Turbo uses two fuel injection systems. The engine is being readied for series production in early 2018. The initial first vehicle application is the new QX50.

Click this link to see Nissan's U.S. patent application: http://pdfaiw.uspto.gov/.aiw?PageNum=0&docid=20130327302

'Harmonic Drive' system

Typically VCR engines alter 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. Nissan engineers have adopted a multi-link system with an electric motor actuator, with what they describe as "Harmonic Drive reduction gear."

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 connecting the intermediate link effectively serves as the piston connecting rod big-end bearing. This arrangement produces a 17º offset of the conrod from the crankshaft journal center point.

The Harmonic Drive is controlled by a dedicated ECU which gathers data from engine sensors to determine the compression ratio required for given driving (load) conditions. Since a low compression ratio is desirable when power is required and a high compression ratio when efficiency is preferable, the piston height at TDC can be continuously varied as required by rotating the Harmonic Drive, which will determine the position of the multilink and hence the height of the piston in the cylinder bore.

See this Nissan video of the VC-T's operation: https://www.youtube.com/watch?v=3rn-gZNJBIU

Outperforms the 3.5-L V6

As with other VCR engines, complexity, mass and cost are greater than in a conventional 4-cylinder gasoline engine. "Instead of one conrod, we have three," noted Alain Raposo, Global Vice-President Powertrain and EV Engineering for the Renault-Nissan Alliance at the engine's Paris debut.

Raposo admitted that while the VC-T is more expensive to build than a conventional 4-cylinder turbocharged gasoline engine, it is also "cheaper, lighter and performance is also better” than the 3.5-L V6 it is designed to replace. The VC-T engine is around 25 kg (55 lb) lighter than the V6, according to Infiniti. The linerless cylinder block and head are aluminum and the multi-link cranktrain components are high-carbon steel alloy.

The motion described by the piston conrod big end is not circular as in a conventional engine's big-end bearing, it is more elliptical with the conrod not passing through the vertical axis between the big and small end bearings. During the power stroke, the conrod remains more or less vertical. This reduces the side force on the piston and helps to reduce the vibration.

Raposo claims that the engine produces 33% less vibration than that generated by a conventional gasoline I4 and no balancer shaft is required. There is more pressure on some bearings, he admits, but the pressure is similar to the pressures acting on diesel engine bearings, he explained.

Reduced side force on the reciprocating components in the cylinder during operation results in lower friction between ring and bore, Raposo claims. Nissan's development team specified plasma-jet-coated bores that are hardened and honed to produce a low friction surface. The company claims a 44% reduction in cylinder friction as a result. No cylinder liners are required.

Two fuel injection systems

The development team addressed the fuelling needs generated by the range of compression ratios by fitting both multipoint injection (MPI) for low compression and direct injection (GDI) for high-compression operation. Since GDI engines inherently generate high particulate emissions, the continual phasing between GDI and MPI helps to contain particulate emissions, Raposo noted. Both sets of injectors are brought into use under high load and engine speed conditions.

Both the inlet and exhaust camshafts are fitted with variable valve timing--electronically controlled on the inlet side and hydraulically actuated on the exhaust side.

Forced induction is provided by a single scroll turbocharger, equipped with an electronic wastegate actuator. The variable compression system results in a variable displacement for the engine, which varies between 1.97-L and 1.997-L.

The VC-T engine development process was highly iterative. Engineers tested more than 100 prototype engines, covering the equivalent of over 3 million km (nearly 1.9 m miles) of road testing and over 30,000 h on test beds.

The new engine is currently in final development and on-road testing.

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