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Paris show: Infiniti reveals VC-T engine

Eccentric behaviour: Infiniti’s new VC-T uses a clever articulated crankshaft to alter the effective con-rod length for a variable compression ratio.

Infiniti VC-T engine introduces the world’s first variable compression ratio


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15 Aug 2016

WHILE many of the world’s car-makers plan which vehicles to roll out at the Paris motor show in September, Infiniti will introduce an engine that has been 20 years in development to function with a variable compression ratio.

Until now, all vehicle internal combustion engines have had a fixed compression ratio dictated by the piston stroke, but Infiniti’s innovative VC-T four-cylinder petrol engine has a unique arrangement that allows that dimension to change.

Some rival brands have previously introduced ingenious valve timing technology which can simulate the effect of a variable displacement engine, but Infiniti’s solution is the first example that actually alters the operation of the crankshaft and connecting rods.

Depending on the driving style and conditions, the turbocharged 2.0-litre engine can operate anywhere between the minimum ratio of 8:1 and 14:1 for optimising either power and torque or efficiency.

In addition to significantly improved fuel economy, Infiniti says the VC-T is smoother than conventional engines with reduced vibration and noise levels, and is more compact.

Performance figures are yet to be released, but Infiniti says the clever arrangement allows the VC-T engine to operate with the power characteristics of a spark-ignition engine but the torque and economy of a diesel.

“The VC-T engine will be comparable to certain six-cylinder gasoline powertrains for performance, while significantly outperforming them in efficiency,” it says in an official announcement.

Infiniti has not confirmed which models will be the first to be powered by the new engine but says it will arrive in 2018, which places the forthcoming QX50 mid-size SUV or a mid-life updated QX30 as prime candidates for the new engine, although several versions are likely to be in the pipeline for a number of applications.

Specially calibrated engine management electronics constantly monitor vehicle conditions and driver behaviour and alters the compression ratio according to whether maximum efficiency or power is required, or a balance of both.

The key to the system lies in a Harmonic Drive control arm which is connected to an unusual crankshaft that is articulated with a multi-link. As the control arm rotates, so does the multi-link, which connects the con-rod and piston to the crank journal.

Altering the angle of the multi-link changes the top-dead-centre point of the piston. When a long stroke is allowed, the piston face gets closer to the cylinder head reducing the combustion chamber volume for a high compression ratio.

With a shorter stroke, the combustion chamber volume is larger for a lower compression ratio, which allows the turbocharger to force more air and fuel into the cylinder per cycle for greater power, without the danger of pre-ignition that high compression can cause in turbo engines.

Infiniti does not explain how the system compensates for the inherent lack of ‘squish’ caused by preventing the piston getting close to the cylinder head when in the lowest compression state.

Squish is an important part of compression process that creates turbulence to promote better mixing of the air and fuel.

Two-stage fuel injection is applied with one direct injector and one in the inlet manifold of each cylinder, which may partly compensate for the reduced squish effect.

In addition to the most significant design innovation, the new engine also employs a number of other cutting-edge technologies for overall efficiency and power gains, including low-friction ‘mirror bore’ cylinder coating and integrated exhaust manifold into the cylinder head, while variable valve timing and lift allows the engine to run using the more efficient Atkinson cycle when necessary.

As a by-product of the more complex crankshaft, the VC-T engine does not need additional balancer shaft to eliminate the vibrations inherent to in-line engines.

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