Clemson camshaft boosts auto fuel economy.

Clemson Camshaft Boosts Auto Fuel Economy

Engineers at Clemson University (Clemson, S.C.) have developed a variable valve timing system for automobile engines that improves fuel economy by 20 percent in lab tests. The patented design employs two concentric camshafts and features adjustable camlobe phase positioning to provide continuous variation in valve timing in response to changing operating conditions, said one of its inventors, Alvon Elrod, a professor emeritus of mechanical engineering at Clemson.

The Clemson camshaft system, which can be added to existing engine designs with no major changes to other components, is applicable to engines employing pushrod single-overhead-camshaft and dual-overhead camshaft designs. There is a slight difference in the camshaft configurations for use with tappet-type cam followers compared to those for use with roller-type cam followers.

Conventional solid (one-piece) camshafts, Elrod explained, are timed to operate valves at peak efficiency at a single set of running conditions, usually highway cruising. At other times, the engine runs less efficiently due to the inertial flow characteristics (lag) of the fuel/air charge as it moves into the cylinder. As a result, some of the fuel fails to burn and is lost as waste through the tailpipe. These effects are more pronounced at higher engine speeds because of the shorter time period available for the intake and exhaust of cylinder gases.

One way to reduce the effects of inertial lag is to use variable valve timing. At very low speeds, the gas inertial effects are small so that a reduced valve dwell (the time during which the valve is open) provides better engine volumetric efficiency and reduces the problems of exhaust backflow. On the other hand, an extended valve dwell is needed at very high speeds to achieve higher volumetric efficiency. A variable valve timing system can be programmed to provide changing opening and closing times for the valves to assure the proper dwell for the various operating conditions.

Elrod began working on the Clemson camshaft in 1984 with Tim Nelson, at the time an undergraduate student and now an employee of Lucas Automotive (Greenville, S.C.). The Clemson camshaft incorporates a hollow tubular shaft, with openings, or windows, cut into it, and a solid shaft set concentrically within. The inner shaft positions a set of movable camlobes relative to a set of fixed camlobes attached to the outer tubular shaft to vary valve timing.

A complete variable valve timing system incorporating the Clemson adjustable camshaft would also include a camshaft control device (phaser), several sensors to supply data defining engine operating conditions, and an electronic control unit (the engine management computer).

According to Elrod, several U.S. and European automakers who do not wish to be named are now evaluating the Clemson apparatus. Initial discussions with Japanese car manufacturers that are considering taking a look at the camshaft have also begun. (Some of these companies are also developing their own variable valve timing systems.)