Auto question: where's the idling/shutoff crossover point?

I believe that starting a car requires an extra ‘jolt’ of gasoline than regular idling does. If that’s true (and it may vary on whether it’s fuel injected or carbeuretor, I don’t know), there would be some crossover point where one is smarter than the other.

In other words, when you come to a stop in a car, and you’re stopped for less than X time, you should continue idling to use less gas. But if you’re pretty sure you’ll be stopped for at least X, you’re better off killing the engine and restarting.

I’m talking only about gas engines, not hybrids, because I understand they shut down on their own all the time, and I’m guessing they’re specially made for that.

I’m sure it depends slightly on the car, but around a minute. Really, it’s a lot less time than you think.

Engines with carburetors were much less effecient at idle than injected, computer controlled cars. There was a relatively crude bypass called the idle circuit that only worked at high vacuum, and it dumped fuel into the carb throat just under the throttle plate. Carb engines ran really rich at idle, so your crossover point would be pretty short.

Yep. There is a significant diff between carbureted engines and computer-controlled fuel injection engines.

This isn’t a spec that is talked about often, so concrete info is kind of scarce.
But I can tell you that some fuel injection systems go to double injection during crank. So if you crank for 4 seconds you inject about 8 seconds worth of fuel.
Some other systems will go from sequential to group injection if the RPM is below a set point (say 500 RPM) this causes each injector to squirt twice as often, but they do shorten (somewhat) the duration of each squirt.
You also have to realize that the engine will be in open loop, that is the ECM is not looking at the oxygen sensor, right at restart. The base injection program might be slightly rich, compared to how it runs in closed loop.
Also the alternator will have to work to replace the electrical energy that was taken out of the battery to do the restart.
Best answer I can give is yes it takes a little extra fuel to restart the engine, but not very much on a modern car. I would put the crossover point at well less than a minute 30 seconds maybe. I will be having dinner with some colleagues that might have a better feel for this question, I will ask them.

Well I am back from dinner with the guys. I asked our department genius what he thought. He agreed that the crossover point would not take too long. 3 minutes for sure. After listening to my thoughts, he revised his opinion to maybe a minute.
he did bring up a point that I had not thought of. If you were to be very anal about shutting off and restarting your car every time you were likely to be stopped for more than 30 seconds you would subject your starter and ring gear to several lifetimes of normal wear in a fairly short period of time. You can buy a whole lot of gas for what one ring gear replacement would cost.

Thanks. I’m not talking about stoplights - but when I’m at the drive-up at the bank, and I know the transaction will take a while, or if I’m waiting at a train crossing. Maybe once or twice a week.

And mostly I was just curious.

This is also very dependent on the temperature.

Up here in Minnesota in the winter, when the temp gets very cold (like -20ºF or so), it can be difficult to start an engine, and will put a lot of wear & tear on it. In such a case, you let it idle for a lot longer time rather than turn it off & restart it.

It’s not uncommon for people here to carry a second set of keys, so that when running into a store for a quick errand, they can leave the engine running while locking the car doors.

This site takes the opposite slant:

Fuel Efficiency Tip #3 Turn Off Your Car

If you are going to be stopping for more than 10 minutes you should switch off your car to keep fuel efficiency at its peak and not burn more gas than is necessary.

Fuel Efficiency Tip #4 Don’t Turn Off Your Car If you are only stopping for a brief moment or less than 5 minutes then don’t turn off your car because the amount of gas used to crank up is more than you would burn in that short period of time.

You can buy a hybrid and have the computer decide for itself when to shut off. That took some getting used to.

Of course, my car is powered mostly by my smug sense of self-satisfaction. :wink: