When does using a car's start-stop system become more economical?

Question

In this question it was asked how economic it is to turn your car's engine off when waiting for a traffic light. In the accepted answer it is said that it becomes more economical to turn it off if you are waiting for more than 10-15 seconds (in a comment someone said it's more likely 9-12 seconds, but still). If I understood correctly, this number applies to turning your engine off yourself manually.

My question is: what is the break-even point when using a built-in start-stop system? How can I calculate this threshold for my own car? More specifically, I drive a Skoda Fabia Greenline and want to know how long I should keep my engine turned off before it is more economical than leaving it running when stopped.

I know that start-stop systems for non-hybrid engines only work when the motor is already warm and that such engines are usually able to restart within half a second, so if you only consider fuel a start-stop restart is more efficiently than a manual restart. But start-stop restarting still costs extra battery power. I suspect that if I only stop the car for 1 second and then restart I will have used more power than if I kept it running, but I don't know if that's really the case and if so where the break-even point is.

Answer 1

I think that you can assume, worst case, that your car will be no worse than the 10-15 seconds break-even given in the linked answer. Since your car is engineered for start-stop, if there are easy improvements to be made to minimize the cost of restarting then the manufacturer may have made them, but in any case it should be no worse than average.

The electrical energy used to re-start is miniscule compared to the gasoline energy wasted by idling. In this link on the physics SE it was estimated to be 1 / 4000 of a gallon of gasoline to re-start the motor. The number given there for idling of .3 gal / hour is consistent with my observation of .24 gal / hour in my Prius. Given those numbers they calculated a break-even point of only 3 seconds.

Although I don't have hard data to provide an exact break-even point, my conclusion would be to take advantage of your vehicle's start-stop at every opportunity where you don't actually know that you will be moving off immediately - and don't worry if you guess wrong, the cost of one restart is much smaller than what you are saving from having the system for the much longer stops.

Answer 2

Built in start-stop systems work differently than manually shutting off and turning back on. I am more familiar with American vehicles. For example, the Toyota Prius has this auto start-stop feature but you can't hear the engine start or stop. Not being a engine technical person, I still assume that this sort of on/off is more efficient than manual on/off and thus a shorter time is ok.

I drive a regular sedan and turn off my car as soon as I stop and if I think the wait will be over 1 minute. I only do this in areas where I am familiar with the traffic patterns or when it is very obvious that there will be a long wait, i.e. traffic accident.