Engineering Explained: How much fuel does idling consume?
 

Second question: How much fuel does starting the engine consume and at what point would you save gas by turning off the air conditioning (and engine)?

I had read it was eight seconds' worth. He cited a study from 2004 that said about 6.7 seconds, so my information was 19% off. However, they surveyed 1,300 viewers who guessed, on average, 3.6 minutes.

3,100% longer.

I had an Ultragauge in my Forester and it said she consumed .25 gallons per minute hour [Thanks Frank Lee!] in neutral and .26 in park. My dead girlfriend broke my Ultragauge and I have not replaced it, so I do not have data on my Hondas, but it must be less, right?

https://www.youtube.com/watch?v=dFImHhNwbJo

Hopefully that's per hour.

I saw that vid before I came here! It was one of his better ones; I especially like the gentle jabs at the shadetree mechanics who form solid opinions without any basis, as Duhmericans are prone to do.

Well explained, and definitely nice to see his humor come through a bit more.

How much fuel used at idle?
 

So, a 1.5L gasoline engine burns at idle as much fuel in 6.4 or 7 seconds to match the amount of gasoline typically spent to charge the battery enough to restart the engine. Years ago seven seconds was asserted in an EM thread, and it has been my rule of thumb ever since.

Wonder what the calcs are on the f250? It uses 1/3 qt an hour idling

Engine idle has to take into account prolonged periods. Above five minutes. After which engine rpm needs to be raised to raise oil pressure to near-normal. That’s where one measures fuel consumption at idle.

The diesel Ford will be closer to 1.0/gl-hr. Which should be the conservative number for planning.

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Ford and BMW are on record at 3 and 5 seconds for their (12v) stop start equipped cars.

Non S/S equipped cars will sometimes go open loop for around 10 seconds after every restart, so you'd have to factor in 10 seconds of open loop acceleration.

According to my OBD scanner, my Fiesta ST with 1,6 liter turbo, idles around 0.4l/h (0.1gal/h), or 0.6l/h with the ac on (0.16gal/hr).

I'm surprised you get almost 9 times more fuel consumption at idle!
Then again, the forester has a 2 to 2.5 liter engine. The larger the engine, the higher the consumption.

Personally I wouldn't bother with start stop technology.
The engine wears out most during startup procedure, according to Shouty Kilmer:

https://www.youtube.com/watch?v=L7524u5w4ZE

Also, a lot of the 3.6 minutes start stop, is from the entire trip.
Many times, cars stop their engines to start them well within 1 minute. Sometimes stopping the engine, only to start it again within seconds, because that's how traffic goes.

I have never ever waited somewhere for 3.6 minutes (save for a railroad crossing with a long train passing at slow speeds, where I do turn off the engine).

The engine wear, starter wear, battery wear...
Not only the items breaking, but just one time charging labor, nullifies the savings you get on fuel.

Thanks for posting this, I probably wouldn't have seen it otherwise!

I also answered a butt-load of comments regarding fuel economy on that video...

I wouldn't call what Shouty's fuel economy discussion in this video "calculations"... it's more like he decided to knock stop/start technology and then just made the numbers fit. I don't like Shouty's videos when he gets into questions like this. He seems mostly to fire from the hip when it comes to anything fuel economy or engineering related. Where I like him is on questions of repair and maintanence.

The Engineering Explained vid is considerably more persuasive as far as calculations.

Also, I am driving a 1998 Civic that I bought in June 2001. It has 260,000 miles and still has the same rods and bearings and piston rings... same 1998 starter. But for the last 90,000 miles I have been stop/starting it maybe 10 times a day.

Just one person's experience... but I think the hardware is a lot more durable that he is claiming.