don't kill (shut off) your motor - a debate!

[DAN]

don't kill you motor. most, all most all, fuel injectors give a little spew of gas to help light off the cat. converter on starts. shut down only if it will be a longer time. not stop lights or down hills

[roflwaffle]

How would a slightly richer mixture when cranking kill the motor?

[DAN]

kill as in shut off.

[roflwaffle]

Yup, you said that. What's wrong with killing the motor for short times?

[brucepick]

I think he's saying that killing the motor has a FE penalty in that when you restart, the injectors spew some fuel to get the cat converter going. So he's suggesting you shouldn't kill it if it's just a few seconds stop.

Dan, if you can give some kind of reference on this that would help. Aside from cold starts where a rich mixture is needed I haven't heard of a fuel squirt being needed or used.

Killing the engine at stops is a pretty standard FE technique for standard trans cars - at least among those very dedicated to FE. I think the general wisdom now is that even a 10-second stop warrants shutting down. I think we're going to need some fact-based references, not just a statement or a figuring-out.

[PaleMelanesian]

I disagree, and so does this:

Click to see the chart like Daox's below.

Any time the engine would be idling more than 10 seconds, it's off. I drive with 2 modes: Pulse, and engine-off glide. No need to use fuel idling the engine when it's not doing any work, especially when sitting still!

There's probably a slight penalty with each restart, but Scangauge doesn't have the precise measurements to show if it's 2 seconds or 10 seconds worth of fuel. I just call it 10 and go from there.

[Daox]

I'd have to 100% agree with PaleMelanesian. Here is my proof:

[NoCO2]

If you are stopped for more then 30 seconds, shutting down a warm engine is actually more fuel efficient then just idling. It doesn't use much gas to startup a warm, fuel injected car. If you have a carborated car it does use more gas to stop and start, but today's modern, fuel injected cars hardly take anything to start up once warm. I'll dig up some references, give me a minute.

http://www.memagazine.org/june07/dep...input_out.html

If you believed the myth that restarting takes more fuel than idling, the myth is busted. Our research showed that a V6 restart takes about the same fuel as 5 seconds of idling. We expect a V8 to save more and a 4-cylinder less.

[PaleMelanesian]

I like to see a controlled test of this. Something like:

- Replace the gas tank with a small (maybe 1 cup) fuel cell.
- Fill up, run idle until it runs out
- fill up again, start, run 10s, stop for 10s, run for 10s...
- repeat for 5sec, 10sec, 15sec ...

[brucepick]

I also thought about a test.
I think it can be done using a ScanGauge.

(read to the end before you say "it won't work because...")

I think you probably have to drive some distance for it to give you any data you can use. If you don't have to drive any distance then the job is even easier than what's below.

With engine warm, reset the trip (segment?) and drive .1 mile.
Now can let it idle for two minutes.
Back in gear and drive .1 mile.
Get the mpg number for that trip.
Read the fuel consumed for the trip.
If it doesn't read out in quarts or liters you can derive it from the mpg because you know the distance.

Reset the trip segment for the next test.

Now repeat the process (drive .1 mile, stop for 2 minutes, then drive another .1 mi) but this time: During the two minutes you let it idle 10 sec, then off ten sec, then idle ten sec, etc.
Get the mpg for that segment and find the amount of fuel used.

Compare with the previous "trip".
Tell us how it turns out!
Probably should be done three times for each method for a more accurate result. Remember to start with a fully warm engine so warmup won't be part of the process.