Infiniti’s Variable Compression Turbo Engine Explained in Detail – Drive Safe and Fast

Automotive

Published on December 15th, 2017 | by Subhash Nair

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Infiniti’s Variable Compression Turbo Engine Explained in Detail

INFINITI’s VC-Turbo engine is the world’s first production-ready variable compression ratio engine – and it makes its production debut on the new QX50. This unique variable compression technology represents a breakthrough in combustion engine design – the QX50’s 2.0-liter VC-Turbo continually transforms, adjusting its compression ratio to optimize power and fuel efficiency. It combines the power of a 2.0-liter turbocharged gasoline engine with the torque and efficiency of an advanced four-cylinder diesel engine.

The VC-Turbo changes its compression ratio seamlessly with an advanced multi-link system, continuously raising or lowering the pistons’ reach to transform compression ratio – offering both power and efficiency, on demand.

A high compression ratio gives greater efficiency, but in certain applications poses the risk of premature combustion (‘knocking’). A low compression ratio allows for greater power and torque, and avoids knocking. In operation, the QX50’s VC-Turbo engine offers any compression ratio between 8:1 (for high performance) and 14:1 (for high efficiency). Deploying smart power for greater control, the empowering engine exemplifies INFINITI’s driver-centric approach.

The blend of performance and efficiency represents a compelling alternative to diesel, challenging the notion that only hybrid and diesel powertrains are capable of delivering high torque and efficiency. The engine delivers 268 hp (200 kW) @ 5,600 rpm and 280 lb ft (380 Nm) @ 4,400 rpm. The VC-Turbo’s specific power output is higher than many competing turbocharged gasoline engines, and comes close to the performance of some V6 gasoline engines. The unit’s single-scroll turbo ensures immediate accelerator responses, on demand.

Equipped with the VC-Turbo engine, the QX50 is competively efficient, delivering gasoline fuel economy of 27 mpg (US combined, front-wheel drive; 26 mpg all-wheel drive). In front-wheel drive specification, this offers a 35% improvement in fuel efficiency over the V6 gasoline engine in the previous QX50, while the new all-wheel drive model’s 26 mpg represents a 30% improvement.

Other highlights include compact packaging and weight-saving technologies. The engine block and cylinder head are cast in lightweight aluminum alloy, while the transformative multi-link components are manufactured in high-carbon steel alloy. Compared to INFINITI’s 3.5-liter VQ V6 engine, the 2.0-liter VC-Turbo weighs 18kg less and requires less space in the engine bay.

The engine employs the world’s first multi-link system and an electric motor with a unique Harmonic Drive reduction gear to transform its compression ratio. The electric motor is connected to the Harmonic Drive with a control arm; as the Harmonic Drive rotates, the control shaft at the base of the engine rotates, moving the multi-link system within the engine. As the angle of the multi-link arms changes, it adjusts the top-dead-center position of the pistons – and the compression ratio with them. An eccentric control shaft changes the compression ratio of all the cylinders at the same time. As a result, the engine capacity varies between 1,997 cc (for a low 8:1 ratio) and 1,970 cc (high 14:1 ratio).

The VC-Turbo can switch between both Atkinson and regular combustion cycles without interruption, enabling greater efficiency and performance as it transforms.

Under the Atkinson cycle, air and fuel intakes overlap, allowing the fuel in the combustion chamber to expand to larger volumes for greater efficiency. The INFINITI engine operates the Atkinson cycle under higher compression ratios, with longer piston strokes allowing the intake valves to open for a short time as the compression stroke starts. The Atkinson cycle is commonly used in hybrid engines to maximize efficiency.

As the compression ratio drops, the engine reverts to a regular combustion cycle – intake, compression, combustion, exhaust – in separate phases to enable greater performance.

The VC-Turbo engine combines a number of existing INFINITI technologies to realize its variable nature. The engine employs both MPI (multi-point injection) and GDI (gasoline direct injection) to balance efficiency and power in all conditions:

  • GDI improves combustion efficiency and performance, and enables the engine to avoid knocking at higher compression ratios
  • MPI mixes fuel and air earlier, enabling complete combustion in the chamber for greater efficiency at low engine loads




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