Remember when Uncle Bill used to insist with absolute certainty that starting an engine burns more gaso­line than idling for an hour? Yeah, well, he was as wrong about that as he was about Plymouth. Yes, a cold engine runs rich, consuming more fuel than normal. For an engine already at optimum temperature, though, stopping and starting does not use an excessive amount of fuel. This is why manufacturers have introduced systems that automatically limit idling in a motionless vehicle. Volkswagen debuted its first production stop/start system in 1983 with the Europe-only Polo Formel E.

All these systems work similarly and are ubiqui­tous on hybrids. When the car comes to a stop, the engine computer cuts spark and fuel. When the driver lifts his foot off the brake, or engages the clutch, the engine fires back up.

Of course, the starter has to be stout enough to withstand the dramatically increased on-off cycling. A heartier battery is also required to ­satisfy the car’s entire electrical needs with its engine off. Otherwise, stop/start is about as easy a fuel-saving technology as driving downhill.

Mazda tried to complicate things in 2008 when it teased engine geeks with its theoretical Smart Idle Stop System (SISS), which attempted the operation without relying on the starter motor. SISS would have manipulated the alternator load to stop all four pistons exactly halfway through their stroke. To restart, the cylinder in its compression stroke would get a small squirt of fuel that, when burned, would spin the internals backward just enough to produce compression in the neighboring cylinder. That cylinder would then receive a normal supply of fuel, and its ­combustion would spin the crank in the proper direction. Unfortunately, this elegant idea never materialized, and Mazda went the more traditional route with its i-stop, available on 2.0-liter gasoline ­Mazdas in Japan and Europe. The i-stop system, though, does use an initial combustion blast to aid firing, easing starter stress and reducing the restart time by half, to a claimed 0.35 second.

Ford recently announced that its stop/start system, available on some 2012 cars and SUVs, has the real-world potential to boost fuel economy by “as much as 10 percent.” Mazda, more concerned with EPA results, says a 3 with i-stop gains only one-tenth of one percent on the EPA city cycle. Therefore, U.S. Mazdas will do without stop/start until it can be part of a worthwhile package of efficiency improvements. (The Japanese city-cycle test has 40 seconds of idling, so the cost of i-stop is justified there.)

Canada is one market in which stop/start should succeed. Toronto has a law that makes idling in boats, cars, and even buses for more than one minute per hour punishable by a fine of up to $5000. It would take a Mountie on every corner and maybe the ghost of Lord Stanley himself to enforce it, but we admire the attempt to promote this simple way of saving fuel.

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