Super-efficient light-truck engines are coming, but don't expect to notice.

That's the impression we came away with after driving General Motors' latest light truck concepts. The reason: something engineers call transparency, an effort to make new technology do its thing without anyone noticing.




In a prototype Sierra pickup, we tested a V8 engine that keeps half of its cylinders closed until more power is needed. The key: a tricky lifter that closes valves when prompted by an engine computer. At that point, the air trapped inside the cylinders acts like a spring, neither absorbing nor creating energy. The fuel economy gains come from running the remaining cylinders
at improved efficiency levels.




A light on the instrument panel is the only hint that any of this is happening. During our tests on the highway and in stop-and-go traffic, the engine ran on four cylinders a full 40 percent of the time. GM plans to introduce the system this fall as an option in
certain heavy-duty trucks, hoping for mileage gains in the 8 percent range.




We also tested a so-called mild hybrid in a prototype Denali sport-utility. With this setup, an innovative starter-alternator cuts the engine at a stop and stores energy from braking in a secondary battery system. During our short drive, the engine action was barely noticeable. The best part: At one point, we stopped to make popcorn, plugging a microwave into the alternator's auxiliary power output.




Mild hybrids offer fuel economy gains of around 15 percent, and should hit the market in some SUVs by 2003.