Variable Valve Timing... for Power and Fuel Economy

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The structure of Honda's VTEC engine, the ultimate expression of an original concept.



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In order to develop its next generation of engines for the mainstream market, Honda's NCE (New Concept Engine) program was launched in March 1984. Specific targets identified through the program included high torque in both the low- and high-rpm ranges and dramatic increases in horsepower per liter. The program was a success, resulting in a series that included the DOHC engine found in the 1985 Civic and Integra, and the SOHC center-plug engine in the 1987 City.

Ikuo Kajitani, who was employed in the First Design Dept at Honda's Tochigi R&D Center, was involved in the development of these four-valve engines. Through his experience in engine design, Kajitani had become convinced that Honda's next engine should offer a mechanism that could alter the timing of the valves.

"Characteristically," Kajitani said, "four-valve engines are known as high-revving, high-output machines. And for that reason we knew it would be quite difficult to achieve low-end performance if the engine's displacement were too small."

Problems certainly arose during the process of development. A reduction in the valve's interior angle, attempted in order to increase low-end torque, resulted in a broken timing belt and valve spring as the unit reached the upper range of revolutions. To address the problem, the development staff put in uncounted hours studying how to balance these two critical areas of engine performance. They knew they had already succeeded with their DOHC and SOHC powerplants, but to develop a new unit that would outperform its predecessors they would have to bridge the gap between the low end and the upper limit.

One group already had examined the idea of switchable valve timing. In January 1983, a year before the NCE program began, a research team was formed to study the mechanism as a means of enhancing fuel economy. Even though by the end of 1982 Honda engines were already capable of a world-beating 50 miles per gallon (mpg), there would be an effort to improve.

A possibility was thus identified through the study of a new valve mechanism. Specifically, it was believed that the installation of a new set of cam followers and rocker arms for high-speed operation on the intake and exhaust sides would help, along with the switching of cam hills according to engine speed.This was to be their solution to higher engine efficiency.

This was the so-called "valve stopping + variable valve timing" mechanism employed in the NCE program. As a core technology for Honda's proposed new line of engines, the mechanism then underwent a program of study and refinement under the careful supervision of Honda's research staff. And eventually the mechanism evolved into Honda's VTEC (Variable Valve Timing & Lift Electronic Control System) engine. Launched via the 1989 Integra, this innovative technology surprised the world with a new level of performance from a compact, fuel-efficient engine.
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