Accidental Transmission discovery (bump started my automatic) - Fuel Economy, Hypermiling, EcoModding News and Forum

apgrok1 · 03-07-2008, 09:43 AM

Lately, I have been throwing the Auto trans in my car in Neutral when approaching a stop light, and if I know it will be a long one, I shut the engine off. Typical hypermiler stuff. I usually will turn the key off for 5 seconds and then turn it right back on. Anything less than 3-5 seconds means the engine has a chance of refiring.
Here's the incredible part, last night I was talking on my cell phone to my wife while driving. As I came up to a light I knew was going to be a long one, I shut the engine off. When I turned the key on five seconds later, the car re-started! I had shut it off in drive and the automatic trans had kept the engine turning! I didn't think that was possible, but I had just bump started my car with a slush box!
On the remainder of the trip home, I performed my discovery again and again and ran through a few other scenarios. It will easily restart after long periods (10-15 seconds) as long as I am above 30 mph. It seems to want to downshift at that point. It seems that the trans will keep turning the Stator as long as the TC housing turns fast enough to keep pressure on the clutch packs. It will also not bumpstart if the engine was shut off in neutral and then put into drive after the engine is off.
It doesn't coast as far (naturally). I have a 2000 Cirrus with a 2.4L and a 41TE (A604) trans.
I am not recomending anyone try this as I have no idea if this damages the trans or not. You should also not do this if you are close behind someone.

Lazarus · 03-07-2008, 10:03 AM

There are varying opinions about coasting in N hurting the transmission but I think everyone is in agreement that coasting engine off in D will hurt it. N is ok for some. Look in the owners manual and see if flat towing is allowed and that will give you an indication of whether N coasting engine off is all right.

NoCO2 · 03-07-2008, 10:06 AM

I noticed this myself...quite by accident as well. I did about the same thing you did, I was coming to a light and just killed the engine without thinking about the shifting and I then clicked the ignition back and it fired again...I was pretty excited as well but I decided since people say that turning the engine off while in D regardless of whether you're moving or not, is a bad idea that I would not do it intentionally. It's definitely worth more looking into though because if it doesn't harm anything it would be useful for sure.

apgrok1 · 03-07-2008, 10:16 AM

I could understand towing in drive would be harmful if you didn't have clutch pressure to start with. In this case, the pressurized clutch packs turn the stator which turns the TC housing which turns the oil pump which keeps the the clutch packs engaged. Also, everything is getting lubricated as well with the oil pump turning.

RH77 · 03-07-2008, 12:14 PM

Quote:

Originally Posted by apgrok1

I could understand towing in drive would be harmful if you didn't have clutch pressure to start with. In this case, the pressurized clutch packs turn the stator which turns the TC housing which turns the oil pump which keeps the the clutch packs engaged. Also, everything is getting lubricated as well with the oil pump turning.

Hmmmm....

What is particularly cold out (or was the fluid cold?)

RH77

trebuchet03 · 03-07-2008, 12:24 PM

Hrmm... Makes sense to me... Bear in mind that you'll be causing a helluva lot of friction material wear as the pressure slowly drops off and you start getting slip on the bands.... I'm saying this because you mentioned you must be going over 30mph. So something tells me the bands are slipping at even higher speeds, but the pressure is high enough that it can still turn the TC...

Something to think about... You're turning the TC with a whole bunch of extra load on it (engine brake).... If your TC is designed for this (that is, keeping the TCC locked for engine braking above XX speed) - no problem. Otherwise, there could be reason for concern.

RH77 · 03-07-2008, 02:00 PM

I just tried it, and it worked -- presumably when the TC is locked. It will downshift and unlock with the engine off; which is when it didn't work.

Next item up for bids on the Price is Right:

Why? I'm trying to think of a situation where this would be applicable (to turn the car off in Drive and key-back on before a stop). Generally on decel, the injectors are cut anyway, or you could downshift to keep the revs up into the fuel-cut range...

RH77

Whoops · 03-07-2008, 02:23 PM

Some valid observations and questions.

On the transmission/bump starting phenomenon, here is the deal. The Automatic transmission's in use, typically have one pump, which runs off of the front main shaft on the transmission. If the engine was running and you leave the car in gear, so the pump continues to turn, then the engine is still turning over, with no fuel being injected. However, once the car speed slows down to where it tries to downshift, the electronics are off, the oil pressure drops and at some point you have the potential for slippage in the transmission.

If, in most cars, with FI, if your in gear, you let of the pedal completely, the ECU will turn off the injectors, so no fuel is flowing. Once your engine RPM gets down to around 1200, or so, the ECU starts firing the injectors again, so their is a smooth transition and the engine is now available to turn the pump on the input shaft of the transmission.

The amount of fuel savings is basically the same, but you haven't placed your transmission in a scenario where it might get a little low on oil pressure. If it gets low on pressure, you might not notice it, right away, but you can be assured you have some degree of accelerated wear on the clutch's and at some point it will fail, sooner, rather than later.

apgrok1 · 03-07-2008, 03:11 PM

I probably won't make this a habit, but I might try this for 50 miles or so to find if FE increases a bunch. However, I beg to differ on everyone's claim that the injectors shut off when you let off the pedal. They still inject a small amount to keep the engine running, even on deceleration. My proof? If they did shut off, the effect would be exactly the same as turning the key off in gear. Your trans would be driving the engine.

elhigh · 03-07-2008, 03:17 PM

I've experienced this myself - engine shutdown with the tranny still in D will keep the engine turning over from tranny drag. I don't think there's any harm to the tranny at all - it's all turning from end to end, so the lubrication is still working. Same for the engine, for that matter. It also keeps your brake vacuum and steering pressures up.

If you live in a really hilly region as I do, a long downhill with the engine off allows you to save brakes and keep the speed down, so I don't see a big issue here.