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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.

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    Can you clarify what you mean by "if I stop the car" and "if I kept it running", since most start-stop systems are not under the direct control of the driver? There is often indirect control based on turning off the environmental controls, whether the clutch is in or out, etc. But, to first order you can assume that the system has been designed to choose the best times to stop and that it has been engineered for an easier restart than a traditional engine. – half-integer fan Nov 15 '13 at 19:56
  • @half-integerfan In our car, the start-stop system is (more or less) under our control. The system stops the engine if the car is not moving, the gear lever is in neutral and I release the clutch, but only if the engine's temperature is warm enough (and possibly some other conditions such as battery charge level). When I move the clutch down again, the engine restarts and is running within 1 second. However, restarting does cost energy from the battery so activating the engine again after I've stopped it only briefly is probably not the most efficient choice. – THelper Nov 16 '13 at 18:39
  • I don't know of the Fabia Greenline is exactly the same, but my Citigo Greentech also has a dashboard switch to disable the stop-start. So choosing to keep the engine running even without just keeping your foot on the clutch is under the direct control of the driver. Wondering when to disable it in stop-start traffic was why I found this question in a search.... – armb Sep 30 '14 at 8:11
  • The fuel consumed isn't the only consideration: autocar.co.uk/car-news/new-cars/… (Mazda have a system that supposed uses no extra fuel compared with idling for even a very short time - home.bt.com/lifestyle/motoring/motoringfeatures/… mazda.com/csr/environment/special_features/2009_02_01.html ) – armb Sep 30 '14 at 8:20
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+50

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.

  • Thanks for those numbers! My car usually idles at 0.4 or 0.5 liter/hour (at least that what it says in the display when I'm standing still), so that would be 0.13 gallon/hour. But since I can restart within 1 second (and not 3 secs as assumed in the calculation), the wasted gasoline should be much less than 1/4000 gallon If I take a rough estimate of 1/(4000*3) wasted gallons then the break-even point would be somewhere between 2-3 seconds. – THelper Nov 18 '13 at 12:28
  • That seems about right. The Prius only idles when the engine is cold so it is the equivalent of a "high idle" - there is no "low idle" as it just shuts off. So I can believe that you achieve 1/8 gal/hr for low idle. Note that the number given is just the gasoline to replace the battery energy - there is also the initial charge in the engine itself, but I would expect that to be optimized in a start-stop engine, and as you say it cranks for much less time, so I think all these factors are still within the rough estimates we are making here. – half-integer fan Nov 18 '13 at 14:15
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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.

  • The Prius is a hybrid vehicle. While it certainly does automatically stop and start its engine, its efficiency characteristics will be different to the auto stop/start systems that are fitted to conventional petrol/diesel cars. – Flyto Nov 16 '13 at 10:43
  • I basically said that. "this sort of on/off is more efficient than manual on/off". The engine is much smaller in prius so turning it on and off is more efficient – grayQuant Nov 16 '13 at 18:36

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