Infiniti changes the way an engine works

Variable compression ratios maximize peak power and efficiency as needed

Published: July 24, 2018, 9:30 PM
Updated: November 21, 2021, 2:59 PM

Infiniti QX50 VC-Turbo engine cover

Infiniti is hoping to improve its recognition in the auto industry as an innovation leader with a whole new take on engine design — variable compression.

Under the 2019 QX50’s clamshell hood is a turbocharged 2.0-litre 4-cylinder, an engine choice that’s becoming the standard in this segment. Inside that engine, however, is a radical new system that varies the compression ratio in the combustion chamber, switching between ratios that enhance fuel efficiency to all-out peak performance.

This very complex system is most simply explained by saying that Infiniti engineers have added a link between the piston’s connecting rod and the crankshaft, unlike the conventional engine setup with the connecting rod linked directly to the crankshaft.

In conventional engines, the space at the top of the piston where combustion occurs, creating the power that ultimately drives the wheels, is fixed. The piston is driven up to the top of the cylinder bore, then thanks to a spark from the spark plug boom, combustion occurs, driving the piston back down, which in turn rotates the crankshaft. The components are all fixed and robust in their construction to handle the intense power generated in the cylinder

Infiniti’s innovative design, some 20 years in development, allows the length of the stroke (the distance the piston moves up and down) to vary from 90.1 mm to 88.9 by slightly rotating the link between the connecting rod and the crankshaft. It also slightly varies the engine’s displacement from 1,970 cc to 1,997 cc.

The amount of rotation is controlled by a lever connected to a computer controlled electric motor. For ultimate efficiency, the compression ratio can be squeezed to 14:1; when peak power is required, the ratio can be reduced to 8:1 and the full boost of the turbocharger can be engaged. The vehicle’s computer responds to the driver’s input on the accelerator in a totally seamless manner. You can’t sense what’s going on inside that engine – it reacts like any other conventional combustion engine.

The variable compression engine design is just the latest in industry-firsts from Infiniti, which also pioneered lane departure warnings (in 2004), blind sport warning and intervention (2011), hydraulic body motion control (2011) and direct adaptive steering (2013), among others.