Hypermileage fanatics have used manual stop/start procedures for ages. In 1999, Volkswagen used this trick to boost its European Lupo over the 75-mpg mark. Shutting down half the cylinders in V-8s and V-6s during cruising has been a popular strategy at Chrysler, GM, and even Honda. Most hybrids exploit engine stop/start to curb consumption and emissions during urban driving.
With a bit of powertrain-control-module reprogramming, automatic stop/start is a slam dunk no matter what type of transmission backs up the engine. The preferred trigger is brake-pedal pressure held for more than a few seconds after the vehicle has stopped. Moving a foot off the brake and onto the throttle signals the need to revive the slumbering engine. One hitch is climate control: when the engine is off, the air-conditioning compressor also goes on hiatus unless it's powered by an electric motor.
Mazda invented the most ingenious automatic stop/start method we've seen so far, called Smart Idle Stop System (SISS). In full accord with the well-known acronym KISS (Keep it Simple, Stupid), SISS does not use the electric starter to crank the engine back to life after shutdown.
SISS's trick is stopping the engine with all of the pistons near the midpoint of their strokes. This is achieved by manipulating the load on the crankshaft via the alternator during shutdown. Holding the throttle wide open allows one cylinder to fill with air.
To restart the engine, fuel is sprayed into the cylinder on its compression stroke and the corresponding spark plug is fired. Combustion drives the piston down, temporarily spinning the crankshaft backward. This reverse rotation compresses the fuel and air in an adjacent cylinder. Firing the spark plug in that cylinder initiates a second combustion event that halts the reverse rotation and restores normal engine operation.
Mazda claims that SISS is both quicker and quieter than using the starter. A brief test drive in a SISS-equipped Mazda Demio in Japan confirmed that this is a smooth, sure path to painless stop/start operation. While SISS should be ready for implementation next year, Mazda as yet has divulged no plans to fit it to U.S. models.
As always, your mileage will vary, depending on the size of your engine and how much time you spend in heavy traffic. Plan on automatic stop/start yielding a 1- or 2-mpg improvement around town in small cars, a bit more in large V-8-powered behemoths.