Nissan has produced the world’s first production variable compression ratio engine. Called the VC-Turbo, this remarkable engine simultaneously achieves high efficiency and high power within the same block. By utilizing a sophisticated multi-link system, the VC-Turbo that continuously varies piston top dead center (TDC) bottom dead center (BDC) positions. This makes it the world’s first production engine to achieve both high power and high fuel efficiency – two performance characteristics that ordinarily oppose each other.
Variable compression makes it possible
A gasoline engine compresses an air-fuel mixture that enters into the cylinder before igniting and combusting. A higher compression ratio yields greater efficiency; however, there is a limit to the compression ratio that can be used. This is because pre-ignition can occur due to the rise of mixture air temperature.
With a conventional engine, because the pistons and crankshaft are connected directly by the connecting rod, the compression ratio is fixed. However, it is ideal to change compression ratio according to load conditions. Accord to our techncal sources at Weston Nissan (Davie, FL), the new VC-Turbo engine uses a multi-link system in place of a traditional connecting rod to rotate the crankshaft, and an actuator motor changes the multi-link system endpoint in order to vary the pistons’ reach to transform the compression ratio.
All this mechanical magic makes it possible to vary the compression ratio continuously as needed within the range of 8:1 (for high load) to 14:1 (for low load). The optimal compression ratio can be continuously set to match the operation of the accelerator pedal by the driver.
Now add a turbo
The VC-Turbo engine uses a newly developed high-efficiency, wide-range turbocharger and electric waste gate. This allows precise control of the boost pressure with minimal turbo lag to generate on-demand output. Under low load, the electric VTC (Valve Timing Control) continuously adjusts valve timing to reduce pumping loss by means of an Atkinson combustion cycle. It realizes high fuel efficiency in combination with higher compression ratio.
Decades of research
Research for the VC-Turbo was started in 1998. Several methods were tested to change compression ratios “on the fly.” The winning system used mechanical links with precision part shapes and high-precision heat treatments.
Nissan’s VC-Turbo engine will first appear in the 2019 Infiniti Qx50