There are many new cars now that automatically turn off their engines when stopped at traffic lights. They then automatically turn them on when moving off. These systems have many names along the lines of active or eco .

The manufacturers claim that these systems are to save fuel, reduce running costs and save polar bears. At the same time there are no published figures detailing the economies involved. Is this just marketing hyperbole? It seems to me that switching your engine off when it's at a warmed up idle is absolutely the last place that would generate any fuel saving worth printing. Most cars idle at ~ 0.3 gallons per hour anyway which is nil in the context of a typical trip. These systems also require a larger alternator, battery, and starter motor. It's a con init?

  • $\begingroup$ were there a demonstrable benefit they would all have it $\endgroup$
    – agentp
    Apr 17 '17 at 0:11
  • $\begingroup$ @agentp that's silly and wrong as well. Manufacturers only care about fuel economy as a selling point. Automatic start/stop hardware & software costs money, and they really hate increasing the BoM cost. $\endgroup$ Apr 17 '17 at 15:23
  • $\begingroup$ The cost of auto stop start is actually pretty tame. Spec a beefier starter motor and slightly higher capacity battery. The control systems are already in place. If there was a significant fuel savings it would buy its way on on a cost basis. $\endgroup$
    – agentp
    Apr 17 '17 at 15:51
  • $\begingroup$ @agentp That's not the main reason for suspicion though. I don't believe a word of it because it's not written in large letters on the car brochure. If it saved any money, the amount would be writ large as in "saves 25% fuel" or "go another 50 Km per tank". It doesn't say anything at all. Competitive market forces would make it prominent if it were real. Isn't it really an appeasement to the eco terrorists like F1 KERS? $\endgroup$
    – Paul Uszak
    Apr 17 '17 at 16:29

From a study conducted by Oak Ridge National Labs:

Light-Duty Idling Summary:
• Idling beyond 30 seconds is excessive and should be avoided
• Extended idling wastes fuel and causes reduction in fuel economy
• Excessive idling can create engine wear and carbon soot buildup in the engine and components
• Excessive idling can affect the life of engine oil
• At start-up: idle for 0-25 seconds and drive gently to warm the engine (as recommended in manual)

The full report is here: https://sustainabilityornl.org/documents/ORNL%20Idle%20Reduction%20Guide.pdf

  • $\begingroup$ So ~1 tank of gas /year for my car? $\endgroup$
    – Paul Uszak
    Apr 13 '17 at 23:08
  • $\begingroup$ It's really tough to put a number on the savings without knowing where you drive. Assuming you live in greater Los Angeles, much more than a single tank. I agree that the fuel savings aren't always significant, but if the technology extends the life of the engine and oil, and decreases emissions, this starts to sound like it's not a scam. I don't know if it's over-pitched or not, maybe that's a question for marketing stack exchange? $\endgroup$
    – EMiller
    Apr 14 '17 at 15:10
  • $\begingroup$ No, that's the summary figure from your report. $\endgroup$
    – Paul Uszak
    Apr 14 '17 at 20:42
  • $\begingroup$ @PaulUszak pretend one tank is 15 gallons. Multiply that by 100 million vehicles and it's not a small savings. $\endgroup$ Apr 17 '17 at 15:25

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