Shutting engine off at traffic lights with an automatic transmission - Fuel Economy, Hypermiling, EcoModding News and Forum

newhavenpatriot · 10-13-2009, 03:53 AM

My apologies if this is in another thread, but I couldn't find a cut-and-dry answer after searching for a while... With an automatic transmission, is it beneficial to your FE to shut the car off at a red light and then restart it when the light turns green? I understand that there will be greatly increased starter wear, but remanufactured replacement starters only cost $110 for my car, so if the savings in gas over that time would be more than $110 (about 44 gallons of gas as of today's prices of ~$2.50/gal), then it would be worth it.

And if it is worth it, what is the correct procedure for doing this with an automatic? I heard you have to shift into neutral, stop, turn the car off, turn it back on, then shift to drive and wait for the transmission to re-engage before letting off the brake / stepping on the gas. Am I correct here?

Christ · 10-13-2009, 04:00 AM

You're basically correct.

I turn the engine off and engage neutral as soon as I suspect I'll have to stop. Coasting 500 feet won't damage your transmission, the warning is against extended coasting intervals w/ the engine off.

So basically, I hit the bottom of the shallow hill in town, and I can see the red light. It's about 300 feet further from where I can see it. I can tell based on where traffic is moving if I need to shut down or not. At 25MPH, I shut down the engine, coast up to the light, come to a stop. As the guy in front of me moves, I start 'er up and put 'er in gear. By the time the guy in front of me is a comfortable distance away, I can give Cara some pedal, and all's well on the home front.

Many of the townspeople think that my engine doesn't run right because it's always off at that light. They think that it stalls on it's own, because there is no indication that I've just shut it off while coasting.

newhavenpatriot · 10-13-2009, 04:05 AM

I'm sure it would help my FE, but I'm not really comfortable EOCing in my car since it's an automatic.

I was just wondering if shutting off and restarting once I'm already stopped would help at all.

Christ · 10-13-2009, 04:10 AM

If you have to shut off for only a few seconds, no.

Consider the small amount of fuel you use before the PCM enters closed loop mode again, before you get it back into gear, and while you're waiting for the transmission to be actively in gear. You have to defeat that much gas usage to make it worth while, versus the fuel used just idling for a few seconds. You're better off timing the lights, if you can, and preserving momentum as much as possible.

I read somewhere that the cutoff point for one person's car was something like 7.5 seconds of engine-off time to compensate for the fuel used in starting/taking off again. Anything more than 7.5 is black, anything less than 7.5 is red, for that person. I can't say for your car, and it does/will vary with your driving habits.

newhavenpatriot · 10-13-2009, 04:22 AM

Sorry for being unclear... I meant shutting off the engine if there's like a minute-long or 30-second-long light or something. As far as my technique goes, whenever I see a red light, I take my foot off the gas and just coast up to it. I definitely need work on my red light anticipation, though.

Piwoslaw · 10-13-2009, 05:34 AM

If only most traffic lights had LED displays counting down seconds to change...
I've seen that in so different places in Europe, from Spain to Romania, and it is worth gold in my eyes. Unfortunately, Poland hasn't grown up to that yet

But maybe lobbying the local officials will work?

Christ · 10-13-2009, 01:32 PM

Quote:

Originally Posted by newhavenpatriot

Sorry for being unclear... I meant shutting off the engine if there's like a minute-long or 30-second-long light or something. As far as my technique goes, whenever I see a red light, I take my foot off the gas and just coast up to it. I definitely need work on my red light anticipation, though.

That's a good start, though.

After awhile, you'll learn traffic patterns at each light, and be able to anticipate about how long it will be until the light changes. Sometimes, you'll find that it's actually better to use your brakes early on, slowing down enough to allow you to completely preserve the remaining momentum so that you're not starting off from a dead stop at the light.

The less speed you start from, the more energy you waste starting off, in a curved relationship.

IOW - The curve between starting at 0MPH and 10MPH is much steeper than the curve between 10MPH and 20MPH. More energy is wasted (especially in automatics) just getting the hulk of metal to move in the first place.

orange4boy · 10-13-2009, 01:59 PM

I am an unabashed automatic EOCer. I do just what Christ has suggested and it saves a bunch of fuel. More if you go alternatorless. Auto transmissions can be EOC'd safely for short distances. I have EOC'd for over 2KM every day for over a year with no problems (I have an extra vacuum canister for the brakes) It's impossible for the bearings to dry out over short distances with no load.

If you have to coast long distances get an electric pump installed for towing that turns on when you EOC.

Christ · 10-13-2009, 04:44 PM

Quote:

Originally Posted by orange4boy

I am an unabashed automatic EOCer. I do just what Christ has suggested and it saves a bunch of fuel. More if you go alternatorless. Auto transmissions can be EOC'd safely for short distances. I have EOC'd for over 2KM every day for over a year with no problems (I have an extra vacuum canister for the brakes) It's impossible for the bearings to dry out over short distances with no load.

If you have to coast long distances get an electric pump installed for towing that turns on when you EOC.

I don't like to go further than a few hundred feet, but I have no evidence that supports damage for short distance coasting, either.

I've been looking into fish tank pumps for long distance EOC, but I don't think they'll flow enough volume to do anything really... I'm not going to invest too much time into it, either, because I don't plan on having an automagic for too much longer.

I don't even use a vacuum canister for my brakes, I just don't use the brakes if the engine is off, unless I absolutely have to. Often, I'm already going slow enough that I'm not going to run into anyone, but that's a function of learning your vehicle and local traffic patterns.

I do think that after my wife gets out of school, I'm literally going to start EOC the automatic as often as possible, just to see what it really takes to damage the transmission. Of course, that will be after I get something else ready to be put on the road, so that I won't have to go without a vehicle for more than a day or so.

I already coast up to red lights and stop signs at exit ramps. I love coasting off the highway.

newhavenpatriot · 10-13-2009, 05:03 PM

My Yaris has DFCO, so I'm assuming that when I'm coasting up to stop lights, I'm not using any fuel until it gets down to a certain RPM, but I can't be certain. I guess I'll have to buy a ScanGauge and figure out how to use it.