I have been wondering this very same thing.
My main concerns are fuel consumption during idle, extra fuel to start and run the car for the first few seconds, extra pollution upon restart and wear to the battery and starter. I add the battery in because I am mostly driving at night and must have the lights on during this coast and sitting at a light for safety.
Searching for this information I found this from the
Car Talk Guys. They (sort of in their joking manner) recommend only shutting off for much longer idling (3-5 min), but not for the short 30-60 second stops due to wear and tear on the starter and battery.
DanQ I would like to see when/if open-loop occurs upon restart of a warm engine. Of course every car would vary.
In a
discussion there was a good explaination about the open vs closed loop which was informative for those of us (me) that didn't know exactly what that meant.
"When a car starts up (and it's cold), the engine management software operates in whats called open-loop. In this mode, the software uses pre-programmed values to determine how much fuel is needed, how much spark advance to use, what the idle speed is, and more. These values are designed to insure that your car starts reliably, minimizes damage to the engine and allows it to be drivable immediately. They are not chosen to be fuel efficient.
Once the engine has warmed up, the engine moves to closed-loop operation. By now, the O2 sensor is working and the car adjusts the fuel/air ratio based on how much oxygen remains in the exhaust after combustion. The knock sensors also come into play, allowing the engine to advance the spark to the bleeding edge of knocking, meaning more power and efficiency.
If the car is already warm when it is started, it may move very quickly to closed-loop. However, some open-loop operation still happens, the length of which can vary."
So their conclusion is that this really depends on the car and it's software.
My thought is that "
some open-loop operation still happens" after start up in most cars. This could be small, but considering the amount of fuel you are saving there is a trade off. 7s= $0.00124444 gal or even 20 sec = 0.003556 could be more than that small amount of extra fuel savings during restart/first seconds of running rich. Especially in older model cars. A new hybrid car that shuts down may have better programing to compensate for this since it is designed to shut off and restart more often.
Another comment puts that time of fuel savings between 30 sec and 2 minutes and indicates that there is more emissions upon start up.
"From what I understand, 30 seconds to 2 minutes is the cross-over point for fuel consumption. Older cars take more gas to start than newer ones, so turn off your late-model cars earlier.
The trade-off is for increased tailpipe emissions. After 30 s to a minute the catalytic converter cools off. On restart, the car produces higher levels of NOX and CO until the converter reheats.
To summarize: turning off saves fuel but creates more smog. Idling burns more fuel, but keeps the air cleaner."
So my thought is that it you can save fuel if you are going to idle for around a minute and you might make more smog. Depending upon how often that occurs in your daily driving may require more replacement of the starter and battery.
I have been shutting off only at longer lights or coasts with lights at the end of them.