Automatics and Neutral?

[elhigh]

In neutral, the tranny and engine are each turning at whatever speed they will - the tranny's speed is dictated by the rest of the drivetrain; the engine will hold the speed you determine via the accelerator pedal.

I don't fully understand all of Roflwaffle's above statement - if the engine is being dragged along by the tranny, and the engine is EFI, the injectors won't have anything to do. Little or no fuel will go through the system. If you're looking for a longer coast, though, well then you should shift to neutral. Then your EFI will turn up the wick a little bit to keep the engine turning over. This is as it should be.

Carbureted engines don't coast in gear as well as EFI; the engine tries to become an air decompressor, pulling a vacuum against the back of the throttle plate. EFI doesn't do that.

If you're driving an SUV or an offroader or a Jeep (which is both), you may have a pretty loose Torque Converter in your ride. A looser TC will amplify torque more when there is more slippage between engine and transmission, at the expense of more slippage at full speed. Most TCs have a lockup clutch to prevent that when cruising. Let off the gas, though, and a lot of the lockup clutches immediately release - your engine will drop right down to idle. It feels like neutral, but it really isn't. The TC probably has enough drag in it to pull the engine up off idle a little bit, so the EFI can back way off on the fuel it delivers. Then, as your speed drops to near zero, the TC drags from the other side, and the engine is generating enough power even at idle to continue propelling your car.

Torque converters work great at transmitting power from the engine into the transmission. However, diode-like, they are nearly worthless at transmitting power from the transmission into the engine. That's why you can't roll-start an automatic-equipped car.

NO manufacturer will equip a vehicle that shift out of and back into gear for itself. NONE. This would expose them to tremendous legal risk. Remember Audi in the late 80's, and all that BS with "unintended acceleration?" That was almost the end of Audi, at least in the States.

[DifferentPointofView]

I'm going from my owners manual. It says that it has some sort of thing that seamlessly shifts out of gear and into neutral while coasting, then seamlessly shifts back into gear when power is needed. I might be wrong though, so I'll check the manual again to make sure after I get off from school.

Let off the gas, though, and a lot of the lockup clutches immediately release - your engine will drop right down to idle. It feels like neutral, but it really isn't

Yes, it seems exactly like that almost. but if i'm going down a hill starting out at like 10mph, it will shift to keep at idle speed once it gets over 1000 rpm. I'm usually in 3rd going 30-35. Why does it do this?

[roflwaffle]

Originally Posted by elhigh

I don't fully understand all of Roflwaffle's above statement - if the engine is being dragged along by the tranny, and the engine is EFI, the injectors won't have anything to do. Little or no fuel will go through the system. If you're looking for a longer coast, though, well then you should shift to neutral. Then your EFI will turn up the wick a little bit to keep the engine turning over. This is as it should be.

The reason why accelerating, then coasting in N works, even on slushboxes, is that there is a tremendous difference between load depending on the terrain, so sometimes it's more efficient to run the engine efficiently for a brief period, then decouple the engine/trans and have the vehicle idle along the road. For a given power output over some route at some average speed, as long as the BSFC drops sufficiently during the "pulse", the fuel used to accelerate plus the fuel used to idle will still be lower than the fuel used during cruising if the load is low enough and BSFC is high enough. As a general rule, as the displacement and gearing increase numerically, the average speed at which this becomes more efficient than just cruising increases. This can be exacerbated by grade and wind.

Originally Posted by elhigh

Carbureted engines don't coast in gear as well as EFI; the engine tries to become an air decompressor, pulling a vacuum against the back of the throttle plate. EFI doesn't do that.

Carb'd and EFI engines both behave the same more or less when coasting with the exception of the carb'd engine always giving the amount of fuel needed for idle when coasting while the EFI engine will only start to give the amount of fuel needed for idle when the engine speed is under some amount, usually around 1500rpm if the engine is warm. That being said, there are add-ons for carb'd engines that cut fuel on decel depending on conditions.

Originally Posted by elhigh

Torque converters work great at transmitting power from the engine into the transmission. However, diode-like, they are nearly worthless at transmitting power from the transmission into the engine. That's why you can't roll-start an automatic-equipped car.

Damn, I'm just gonna have to tell my transmission to stop getting the engine involved in downhill descents. I think the reason why automatics are harder to roll start is torque converter stall. The difference between cruising and coasting in terms of engine speed would manifest itself if I try to push start it, so I would need to get it going much faster than a manual to get the engine speed up.

Originally Posted by elhigh

NO manufacturer will equip a vehicle that shift out of and back into gear for itself. NONE. This would expose them to tremendous legal risk. Remember Audi in the late 80's, and all that BS with "unintended acceleration?" That was almost the end of Audi, at least in the States.

Freewheeling transmissions are more popular in Europe right now, but they've been used in the states too, although not nearly as much since gas is way cheaper. The Audi "problem" had nothing to do with the transmission according to this.

Originally Posted by NHTSA

On March 12, 1989, Bradley presented a short update, reporting on NHTSA's findings. He said the study "supported the position of Audi and the other manufacturers," and that investigators "could find no mechanical or electrical failures which would cause sudden unintended acceleration." While the study concluded drivers were mistakenly hitting the accelerator, Bradley noted that it also pointed to possible design problems" the shape, location, and feel of gas and brake pedals" as a contributing factor.

[101Volts]

Originally Posted by Gone4

I've read people arguing both sides of this coin so I wanted to know what everyone here thought or knew. Is it safe to transmission pretty quickly. I've also seen the same arguments for overdrive.

I ask because switching into neutral on the mountains is very very useful sometimes...

Please explain, I'm not sure I fully understand "Is it safe to transmission pretty quickly".

Originally Posted by roflwaffle

N disconnects the engine from part of the transmission, so instead of moving the whole engine/transmission, the wheels only move part of the trans. When you're coasting in gear the above some speed the engine is operating just about as inefficiently as possible.

I might understand what you're saying but I think that could be explained just a bit better; "When you're coasting in gear the above some speed" Leaves room for uncertainty in my mind.

As for my thoughts on it, I say check the manual first to see if the car can be flat-towed over a certain speed. I found out that it isn't worth doing in "Moony" (Its manual says not to do it over 25 MPH) or in "Boxy" (Its manual says not to do it over 35 MPH). However, I will switch to neutral under those speeds at red lights if the conditions are viable.

[user removed]

I've done neutral coasting in every car I have owned for at least the last 7 years. Regular autos, CVTs including an early Insight and an 08 Altima, and now in my 2011 Fiesta, for the last 17k miles in the Fiesta alone.
In some old slushbox designs the transmission will stay in top gear and the torque converter will unlock. My 85 Riviera did this and it took me some time to learn how far it would "coast" when you let off the gas. I loved the car and it averaged 27-28 MPG, but the dang thing put me to sleep if I drove it over 2 hours.
The Fiesta does not go into neutral when you put it there is it is moving. You can feel it downshift as the engaged engine speed drops below 1500 RPM, so at 40 MPH, even in neutral you can feel the transmission "downshift" but the clutch stays disengaged until you shift back into gear.
I have never and will never do engine off coasting in any auto equipped car. Autos require hydraulic pressure to function as well as for lubrication. If you want to fry your transmission and spend a bundle of money then go ahead and shut off the engine while you are going 60 MPH. I know I won't.
The Fiesta coasts down to something like 16 MPH in neutral and still pegs the MPG gauge at 99 MPG. The Insight did 150 MPG down to 16 MPH. The Altima did 60 MPG down to 16 MPH. You can actually calculate your idle fuel consumption using those figures. The Insight went 150 MPG coasting at 16 MPH, os it used 16/150ths of a gallon idling.
All 3 cars would go to normal neutral idle speed when coasting, same RPM as they did in neutral sitting still.

regards
Mech

[sheepdog 44]

Originally Posted by Old Mechanic

I've done neutral coasting in every car I have owned for at least the last 7 years. Regular autos, CVTs including an early Insight...
I have never and will never do engine off coasting in any auto equipped car. Autos require hydraulic pressure to function as well as for lubrication. If you want to fry your transmission and spend a bundle of money then go ahead and shut off the engine while you are going 60 MPH. I know I won't.. The Insight went 150 MPG coasting at 16 MPH, os it used 16/150ths of a gallon idling.

regards
Mech

Please tell me more! At what speeds did you reengage from N to D?

If you switch to imperial units, you'll find that it's doing greater than 150mpg. Going downhill in gear feathering the throttle, it'll do 235mpg at 55mph. Which is consistent with the average mpg i get. Meaning i still leave 10-20mpg on the table if i were to EOC every down hill at infinite mpg.

[user removed]

Speeds as high as 75 MPH with automatics. I have used in gear downhills on my manual VX to keep from being sucked into the rear end of an 18 wheeler coming back from Blacksburg at 85 MPH on a 3 mile 6% grade in the mountains.

Going downhill depends on your speed and whether it is increasing beyond your comfort zone. If your objective is to keep your speed below a certain point than use engine braking (and possibly fuel shut off) to get infinite mileage for your descent.

Where I live there are no downhill grades that steep. Only hills where I would use that technique are an hour west of my home, where grades can be coasted for a mile or more further west.

regards
Mech

[ps2fixer]

I drive a high milage automatic, it is electroncially shifted, so it isn't like the older style autos. Anyway, bought the car with 225k, currently have 260k, been neutral coasting it quite a bit, and quite often the engine is off. Not really sure how much damage that is causing, since typically the auto transmission will go before the engine in the Toyotas. I'm pretty sure the manual says to not to flat tow as well (does for the 92-96 camry, very similar car, don't have a manual for the corolla).

From my standpoint, if you can put your transmission in neutral, and re-engauge gear smoothly, it shouldn't be stressing the transmission much, engine off vs on is your choise, my car was only $650, I can afford to replace it if needed, and transmissions are around $100-200 in my area, I can do the work myself.

[Arragonis]

Originally Posted by 101Volts

Please explain, I'm not sure I fully understand "Is it safe to transmission pretty quickly".

Holy thread recycle Batman - 5 years, wow!

[ps2fixer]

Rofl didn't even notice the OP's post was LONG AGO O.o