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.