Start-Stop-System: Minimum Meaningful Stoppage Time?
 

Love my 2015 Q5 TDI, and have gotten used to the Start-Stop feature, but wonder if anyone has seen any research on the minimum time the engine needs to remain stopped before any fuel is actually saved. I assume that restarting the diesel requires a momentary injection of fuel greater than that necessary to maintain the idle state for that split second or so, but I don't know if this is really true. For example, I'm sure that stopping the engine while waiting two minutes at a train crossing will save fuel, but will a five second stop in slow & go traffic? In other words, if I anticipate, e.g., no more than a 15 second stop in traffic, should I disable Start-Stop? For the sake of this question, I'm not concerned with the amount of electrical energy required to restart the engine, nor with any additional wear & tear on the starter, etc. It would also be interesting to know whether there's any difference between gassers and diesels. I've searched the various forums, and have been unable find anything on this particular issue.

I haven't seen any published specific time specs from Audi(Despite looking a lot), but I did see Mazda state a few years ago that in order for their i-stop systems to see 1/10 of 1% fuel savings of the estimated mpg, the vehicle would have to be at idle(shutdown) for at least 40 seconds. So I naturally assume Audi's system would be significantly better. However, the temperature has to be rather normal(not cold and not too hot) outside in order for the system to function properly. So, the engine can't become cold during it's time off otherwise it requires more energy to restart.

Article: Engine Stop-Start Systems Explained - Tech. Dept. - Car and Driver

At least from what I remember of the manual's wording (or maybe it was start-stop advertising), it was talking about how you should turn the engine off if you'll be stopped more than 10-15 seconds. I don't know if that denotes the anticipated break-even point or even if it's really talking about the efficiency of the start-stop system at all, however.

For me, I'll let it engage anytime I know I'm not going to be having to move within a few seconds (so coming up to a stop sign I'll likely try to not let it engage, but a stop light I will unless it's already green and I'd be moving again very shortly). Anytime I'm not positive, I'll defer to letting it engage if it wants to, since usually if you think a light will turn green in a few seconds you're wrong. ;)

If Mazda's system requires 40 seconds to get 0.1% fuel savings then they seem like they really need to work on that. Idle fuel usage is better than I'm sure it used to be, though, but of course many people prefer to be moving slowly up to stops instead of letting the engine actually go into idle mode. I've seen "5%" thrown around, but I think that's been a general figure for start-stop and have no idea where it actually comes from.

I don't think Audi's given any real data on how much they estimate to be saved on average, but the system will only engage when other systems don't need the engine running and can re-engage the engine as necessary if its conditions stop being met (typically that'll be for A/C or heating needs, or on the TDI it'll happen when the car starts a regen cycle). So for some unlucky people it might almost never be able to engage and so you wouldn't see real savings (but at least you have the heavy-duty battery and starter so hopefully in "normal" use those would last quite a bit longer than the normal counterparts).

I'm still trying to find it, but I seem to remember a thread w/some info when the 3.0T '13s came out where 30 seconds was the magic number. In other words, you'd definitely be saving fuel when stops were 30 sec or longer. There's also a shorter period of time where it's a wash and you can do what you'd like with no difference. Of course these were 3.0T-related so TDI may be different

If your goal is to save fuel, you have to take into account this amount of electrical energy.

Interesting question, OP. I'd also be curious to know if there is any data on this, specifically as it relates to the TDI. Unlike you, I've not gotten used to this feature yet, so I turn it off more often than on, but it would be great to have a better sense of how to most effectively use this feature to maximize whatever benefit there may be. After everything I've read, I'm somehow not convinced that leaving it enabled all the time in all driving situations is the way to go (all things considered).