Timing Changes: How Honda‘s VTEC Variable-Timing System Works
From the August 2015 issue
Trademark erosion may not impact the environment, but marketers have noted the lexicographical slide of some brand names into generic labels. Kleenex, Band-Aid, Xerox, and Zamboni (yes, other companies make ice-resurfacing equipment) all underwent the transformation, and Honda’s VTEC, or Variable Timing and lift Electronic Control, teeters on the edge of name interchangeability with other variable-valve-timing systems—including some of its own variations on the theme.
When Honda started tinkering with VTEC in the early 1980s, it sparked an internal-combustion revolution that spread to nearly every brand. The simplest of the resulting variable-timing systems employ cam phasers, or mechanisms that change the relationship between the crankshaft and the camshaft(s). Phasers provide a range of adjustment, but only for valve timing. VTEC, on the other hand, works in a stair-step fashion, shuttling between two or more distinct cam profiles to change three variables: valve timing, duration, and lift.
Honda merged the worlds in 2001, adding those relatively simple cam phasers to the already complex VTEC system. Initiating a sort of controlled trademark erosion, Honda has evolved VTEC into a family of separate systems encompassing various combinations of a core set of technologies. Here is how they work:
As revs climb, the computer directs oil flow (A) through the rocker shaft. This slides a pin (B) that locks the low-rpm rocker arms (C) acting on each cylinder’s two intake valves to a high-rpm rocker (D). That third rocker follows a different cam profile optimized for higher engine speeds. Early VTEC operated nearly identically to today’s basic system.
Variable Timing Control is by far the most common form of variable valve timing on the market. Everything from 1.0-liter Fords to many-liter Ferraris use similar camshaft phasers. These devices use oil pressure (A) or an electric motor to advance or retard cam timing (B) relative to crank position.
This controls half the intake valves. At low rpm, just one of the two intake valves operates to promote swirl in the cylinder for better combustion efficiency and lower cold-start emissions. As load and rpm increase, the second valve (A) is engaged via oil pressure (B) against a pin (C).
Variable Cylinder Management deactivates some of an engine’s cylinders when maximum torque isn’t required. It decouples a cylinder’s rockers by disengaging a pin (A) similar to a VTEC pin, so that the rockers (B) no longer open the valves, effectively sealing the cylinder. Not only does it cut mixture flow to the dead cylinders, it also reduces pumping and friction losses when activated.
Engineers have long fantasized about eliminating camshafts altogether. Solenoid-controlled valves could, in theory, have infinite variability without mechanical limitations. A camless engine could run the Atkinson cycle as easily as it could the Otto, have little valve overlap or lots, and run on any or all of its cylinders. But cost, reliability, noise, and other concerns have kept the concept from production. Koenigsegg has a system in the works, and we expect it to be the first to bring it to market—well, the one-percenter’s market, anyway.