Lexus Adds Variable Combustion

The 5.0-liter, 450-hp V-8 engine in the new Lexus RC F sport coupe uses variable valve timing to switch between Atkinson and Otto combustion cycles.

The system combines the efficiency of the former cycle with the higher power of the latter, depending upon demand. Lexus says the new design makes RC F more fuel efficient than the IS F coupe it replaces.

Atkinson cycle engines lengthen the power stroke and shorten compression by altering intake valve timing. The design enhances low-speed and steady-state efficiency. But it reduces the volume of incoming air, thus limiting power output at high rpm.

The Atkinson cycle is popular in hybrid powertrains, which rely on an electric motor to prove additional power at higher speeds. In the naturally aspirated RC F, intake valve timing shifts to the Otto cycle under high power/high rpm conditions.

The RC F's V-8 mates to an 8-speed automatic transmission. The system delivers power to the rear wheels through a torque-vectoring differential that proportions torque to each rear wheel independently. The differential has three driver-selectable modes: standard, slalom (for nimble steering response) and track (which helps the car hold a cornering line).

The chassis for the new RC F is reinforced with large cross-section rocker panels and additional bracing. The car also gets a new grille, enhanced aerodynamics and an active rear spoiler built into the trunk lid. Front fender ducts help cool the brakes.

The RC F, which will go on sale this autumn, has an electronically limited top speed of 168 mph. Pricing has not been disclosed. The car will compete with the high-performance Audi A5 and BMW M4 coupes.