Variable Compression Ratio — Nissan VC-Turbo

Every other engine in production has a fixed compression ratio — the pistons always travel the same distance. Nissan's VC-Turbo engine (KR20DDET) is the only production engine in the world that can physically change its compression ratio while running. It goes from 8:1 under heavy load and boost to 14:1 during light cruising. That is a massive range.

Instead of connecting the piston directly to the crankshaft with a conventional connecting rod, Nissan uses a multi-link system. There is a U-shaped lower link on the crankshaft, an upper link connecting to the piston, and a control shaft driven by an electric motor through a harmonic drive gear. When the electric motor rotates the control shaft, it changes the geometry of the multi-link system and physically moves the piston's top dead center position up or down. Higher TDC means higher compression. Lower TDC means lower compression.

High compression ratio gives you better fuel efficiency during light load driving — the engine squeezes more energy out of each combustion event. But high compression under boost causes detonation (knock). A traditional turbocharged engine has to run low compression all the time to survive boost pressure, which wastes efficiency during cruising. The VC-Turbo gets the best of both — high compression when cruising, low compression when the turbo is building boost. Nissan claims it combines the fuel economy of a four-cylinder diesel with the power of a V6 gas engine.

The VC-Turbo is used in the Nissan Altima (2.0 VC-Turbo option), Infiniti QX50, and Infiniti QX55. Nissan is the only manufacturer with this technology in production as of now. The engine makes about 268 horsepower in the Infiniti application and around 236 horsepower in the current Altima (earlier models were rated at 248).

The multi-link mechanism adds complexity. The harmonic drive motor, control shaft, and additional bearings are all components that do not exist on a conventional engine. Listen for unusual noises from the lower end that could indicate wear in the multi-link system. The ECM monitors compression ratio position through a sensor on the control shaft — if the actual position does not match the commanded position, you will see DTCs related to the variable compression system. Oil quality is critical because the multi-link system has additional bearing surfaces that need proper lubrication.