19

A start-stop system stops the engine when the car comes to a halt, for example, in front of a traffic light or during a traffic jam. The engine automatically restarts when pressing the clutch or the accelerator. Obviously, this saves fuel and reduces emissions.

Claim: This system saves money. Here is an example from the Volkswagen homepage:

We think that sitting in traffic is bad enough without knowing that you have to pay for it too. That's why we created Start/Stop Technology, a really simple way of helping you to save money whenever you come to a stop.

Reason to doubt: Cars consume little fuel when halting. Starting the engine more frequently might wear out starter motors or batteries faster, increasing service cost.

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    there is a minimal time before start-stop is more fuel efficient (because startup requires more fuel than idling and the battery needs to be recharged) but that is typically measured in seconds – ratchet freak Jan 28 '14 at 10:03
  • @ratchetfreak: If you have references, that's an answer... – Oddthinking Jan 28 '14 at 11:54
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    @ratchetfreak: doesn't start take more fuel when engine is cold, but not so much if it's already warmed up? – vartec Jan 28 '14 at 13:52
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    This thread of speculation should be happening in Skeptics Chat. If somebody has an answer, write it as an answer. – user5582 Jan 28 '14 at 15:43
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    @vartec as someone who programs traffic lights for Flanders (Belgium) I'll say that a cycle of between 60 and 120 seconds is striven for with a minimum of 7 seconds of green for each light. This depends on the actual situation road of course. – ratchet freak Jan 28 '14 at 16:44
12

TCS, a Swiss automobile club, has performed such tests in 2013. The study itself has not been published, but I have received a copy on request and the permission to show below table extracted from it. Other references to this study (in German) can be found in an article in their monthly magazine, a campaign to encourage switching off the engine, and similar articles.

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Translations: The left column shows the tested car model, its year of construction, and the type of fuel (Benzin for gasoline). The second column shows the measured consumption in grams per warm start, the third column the presence of a start-stop-system (ja=yes, nein=no). The right column indicates after how many seconds switching the engine off saves fuel.

In conclusion, you should switch off your engine when you expect that your stop lasts for 5-10 seconds or more.

This test focuses only on fuel-efficiency. Comparing the fuel savings with potential service cost is unrealistic in my view, as fuel prices vary widely (in time due to the oil price and by location due to taxes and subsidies) and should also include external costs related to emissions as well as fines (60 CHF in Switzerland for not switching off, though rarely enforced).

  • My Mitsubishi that had this also had a long life starter motor that, while much more expensive than a normal one, was designed for this kind of constant use. The theory was that it would last the lifetime of the car, so should not increase service costs. – user18902 Oct 7 '15 at 15:50

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