Switching Engine at longer stops
 

User Manual (2010 X535D) on page 131 (pdf) states that you can save fuel by switching off the engine even if the standing time is as short as 4 seconds. Wouldn't this cause excessive engine wear? Perhaps diesel engines are different. Comments?

Not a diesel issue, same for gasoline engines. It will cause increased starter motor wear, but reduced engine wear. Engines don't tend to wear when they are stopped.

BMW Efficient Dynamics shuts engines off at every stop, and a hybrid shuts off the engine whenever it isn't required.

During cranking, there is sub optimal lubrication so wear is not just limited to the starter. Agreed on hybrids but in that case starting off is assisted by the electric motor and the engine won't have to work very hard. That isn't the case with a non-hybrid gas or diesel engine. You are not only causing more frequent cranking but you are also loading the engine while lubrication is not optimal.

Agreed.

On the singular quesrtion of 'saving fuel' turning off the motor may help. But the overall impact on the environment- from how many more batteries need to be made to increased wear to how much emissions a fresh start generates- is a bit harder to calculate.

Seems like a really dumb statement to put in a manual- especially "4 seconds"

We shut off gas engines any time we will be sitting idle for a short (but pretty well know) period. For example, at a drive in bank teller. We never shut off any engine at traffic lights where having the engine running can be a safety benefit. BUT...

Every time we shut off our 35d engine there is a lot of commotion under the hood that lasts quite awhile -- certainly more than 4 seconds. Until I know for sure what is going on, we have quit shutting off the 35d unless we will be stopped for quite awhile (AKA getting out of it). Besides, the biggest outstanding issue with E70s is the low battery that can leave an E70 disabled. We want to reduce battery load and maximize charging as much as possible.

Funf Dreisig

p.s. I can see the X5's "unfavorable driver" report for owners who drive short trips. "Not only does my owner not spend enough quality time with me, he's a big turn off too!"

I agree that there could be sub-optimal lubrication, but the engine has no load on it when it is being started. My starter won't engage unless the clutch is pushed in. Same concept with an automatic, you need to be in neutral or park. Increased wear is traditionally associated with a cold start, not with the starting of an engine that has been running recently. If you stop for 30 seconds and shut it off, the oil will not all drain down. I noted in my response that the starter will wear more, but only due to increased frequency of use, not due to harder use. I am trying to remember the last time that I had to buy a new starter motor because it was worn out. I think it was in 1976, and even that was only because I was restoring a vehicle.

It would be interesting to know if BMW went to larger starter motors when they introduced auto stop-start on the four cylinders.

We also need to consider that idling is already an sub-optimal condition for an engine, it is in no way benign. Idling engines are not at peak temperatures, and can have incomplete combustion, with a resulting build-up of residues. Avoiding idling is treating your engine right.

I don't buy that starting more frequently is going to wear out a battery, as batteries generally last longer if exercised. If your alternator can not keep the battery charged, then that is another matter, but batteries usually wear out on time (often due to deposition) rather than with use.

Where I live, the city asks you to shut off your engine after 10 seconds, not 4. It is a bylaw infraction at 3 minutes of idling within a 60 minute period. I generally don't shut the engine off unless I am going to be stopped for 30 seconds, and even then not if I am in traffic as it becomes a safety issue.

No more batteries are required. A fresh start will generate more emissions if the engine is cold, but not if it is warm. If the engine is warm, the catalytic converter is up to temperature, and the sensors are warm. Theoretically, the starter motor will wear out sooner with more frequent use. There are also other systems which may go through their cycles, as Funf points out. I can't imagine how many components are part of the boot-up cycle of a 35d.

I think the four seconds is illustrative, and not a driving guide. You can save fuel shutting down if the idle is as short as four seconds, but every situation requires the driver to make an intelligent decision on what the best course of action is. The manual doesn't say to shut down if you are idling for four seconds, it points out the fuel saving simply to counteract the commonly held belief that engines should be left idling. Just my $0.02

There is a well known issue (documented on this forum and others) where the E70 X5 battery does get charged enough and leaves owners with an X5 that does not start. In several cases, the owners have reported that they were told by the dealer that the X5 logs indicated that they were not driving the X5 long enough to fully recharge the battery. They were told to use a trickle charger. Restarting the engine several times in a trip will simply exacerbate this issue.

So until I see/hear of a solution to this E70 electrical issue I will NOT be shutting off my 35d engine unless I'm getting out of the vehicle. The fuel/emissions shutting off the engine might save, will be more than overshadowed by the fuel/emissions of the tow truck :(

Funf Dreisig

I think the 4 second interval suggested by BMW is a bit silly. I switch the engine off at RR crossings or stoplights where the projected stop time is over 30 seconds. I follow the same criteria as EfficientDynamics:

1) Engine must be completely warmed up. We all know cold starts are bad.
2) The battery has a sufficient charge.

With the rest heat feature, shutting off the engine has few drawbacks in cold weather!

Which is exactly why I said that if your alternator can't keep your battery charged, that is a different issue. Starting it more often just won't wear out the battery, as was being suggested. I agree with your plan to not take any chances during the beta development phase of the 35d (generally the 1st year of sales for most new BMW models).

Maybe it is the urea heaters killing the battery on the 35D. Nothing a larger alternator wouldn't fix.