Going down moderate hills - what's better for an engine?

[PandaBear]

leaving the engine in neutral or in gear still use fuel, how much it uses depends on the vacuum, rpm, and the emission output.

From what I see in uberdata's forum, engine control takes those parameters into consideration. If you leave it in neutral it uses gas, but less than in gear because the rpm is lower. However, leaving it in neutral is dangerous and can wear your transmission out quicker when you put it back in gear or if you need the power in emergency, you are risking the life of yourself and others by saving say, 0.2mpg during that short period of time. Not a wise move.

Plus, leaving it in gear provide some (not alot) engine braking thus your car won't be as fast.

A transmission is more expensive than brake, and brake is more expensive than fuel. Life of anybody is more expensive than all the above sum up. Remember that.

 

[Bamaro]

I may be wrong but I believe even coasting downhill in gear uses gas. Your plugs are still firing every other revolution. If you put it in neutral and coast, you are using less gas because RPMs drop and there is less wear & tear on the engine.
Here is something interesting about the difference between using the clutch vs. neutral from the Washington Post:

Clutch question: But I've heard lore that keeping the clutch pedal depressed for a long time is bad for the transmission. True or old-wives' tale?

Pat Goss: Old wives tale. Keeping the clutch pedal depressed adds wear to the clutch release bearing, that's bad. But when the clutch pedal is in the up position the input shaft to the transmission, the input bearing in the front of the transmission and various bearings inside the transmission will always be turning and wearing. This is not true when the transmission is in gear but the clutch pedal is held to the floor. It's a trade-off, more wear on the cheap part (release bearing) or more wear on the expensive stuff (transmission bearings etc.).

[ June 14, 2005, 02:01 PM: Message edited by: Bamaro ]

 

[T-Keith]

Also remember that if you you coast to a stop your engine is working for a shorter period of time then if say you drove up until the braking point and then downshifted.

-T

 

[robbobster]

quote:

---

Originally posted by Bamaro:
I may be wrong but I believe even coasting downhill in gear uses gas. Your plugs are still firing every other revolution. If you put it in neutral and coast, you are using less gas because RPMs drop and there is less wear & tear on the engine.

---

On most modern cars, the ECU commands the injectors to shut off when the RPM are up but there is zero load (e.g. going down a hill in gear). So you use zero gas. See previous posts above for more detail.

Leaving it in neutral uses gas because it takes fuel to maintain idle.

 

[XS650]

quote:

---

Originally posted by robbobster:

Leaving it in neutral uses gas because it takes fuel to maintain idle.

---

It depends on the conditions.

You have x amount of KE stored in the 1/2mv^2 of your car.

If slowing in gear with no fuel to the engine eats up your 1/2 mv^2 faster than the wasted fuel/overall efficiency eats your fuel while idling in neutral, then coasting in neutral with the engine idling would use less fuel to cover a given distance.

Something that help's your side of the discussion is that in a lot of cars, when you are coasting above some very slow speed with a manual transmission the idle RPM is elevated a few hundred RPM. They do it to assure adequate power steering and vacuum.

 

[robbobster]

quote:

---

Originally posted by XS650:

quote:

---

Originally posted by robbobster:

Leaving it in neutral uses gas because it takes fuel to maintain idle.

---

It depends on the conditions.

You have x amount of KE stored in the 1/2mv^2 of your car.

If slowing in gear with no fuel to the engine eats up your 1/2 mv^2 faster than the wasted fuel/overall efficiency eats your fuel while idling in neutral, then coasting in neutral with the engine idling would use less fuel to cover a given distance.

Something that help's your side of the discussion is that in a lot of cars, when you are coasting above some very slow speed with a manual transmission the idle RPM is elevated a few hundred RPM. They do it to assure adequate power steering and vacuum.

---

If you are coasting down a hill in gear, and the fuel injectors are off, then you use zero gas. Even with an autobox, the ECU in most cars will command the injectors to turn off until a low-RPM threshhold is reached, or if you hit the throttle.

To maintain idle at any time, even when the car is in neutral, this uses a little gas.

I'm trying to keep it simple to answer the original question, so fuel over distance is not really on my mind - just fuel use - yes or no. But I understand what you are saying.

 

[theguru]

I remember hearing tht it was common for some truckies to do this. It was called using "Angel Gear." You can guess the rest!

 

[MN Driver]

Leaving it in neutral uses gas because it takes fuel to maintain idle.

If you are coasting down a hill in gear, and the fuel injectors are off, then you use zero gas. Even with an autobox, the ECU in most cars will command the injectors to turn off until a low-RPM threshhold is reached, or if you hit the throttle.

With a manual transmission it makes more sense for there to not be any gas use because it is possible with a MT to do that. With an autobox however it is not. Automatic transmissions with an electronically controlled torque convertor unlock the torque converter when you completly let go of the gas. If the injectors turned off there is no way that a fluid coupling from the transmission could overcome the resistence to have the engine spin. By turning the key off in drive, the engine stops as quickly as it would in neutral and there is no noticable drag slowing you down. An automatic in a low gear will lock the torque converter so that you have all the engine braking you need.

I understand he has a manual transmission however, my personal opinion would simply be to just leave it in whatever cruising gear that it is already in and drive. I've never heard of any engine problems from doing this often because I know many poeple who live in the hills of south dakota and colorado who do this on a daily basis to get into town, except with a lower gear, due to a steeper hill. If it is steep then a lower gear is neccessary to not burn up your brakes, but this slight hill of yours is really nothing to worry about, the rpm's aren't even high.

 

[brianl703]

quote:

---

Originally posted by MN Driver:
Automatic transmissions with an electronically controlled torque convertor unlock the torque converter when you completly let go of the gas.

---

Ford vehicles wait until you push the brake pedal to unlock the torque convertor. It's detailed in the shop manual as one of the functions of the oddly-named BOO switch (Brake On/Off).

quote:

---

By turning the key off in drive, the engine stops as quickly as it would in neutral and there is no noticable drag slowing you down.

---

By turning the key off in drive, you've also turned off the power to the powertrain control module and therefore the transmission solenoids, so the torque convertor would definitely be unlocked.

 

[T-Keith]

quote:

---

Originally posted by brianl703:
Ford vehicles wait until you push the brake pedal to unlock the torque convertor. It's detailed in the shop manual as one of the functions of the oddly-named BOO switch (Brake On/Off).

---

Most vehicles have a brake switch that unlocks the TC, but that doesn't mean the converter doesn't unlock at any other time. If you let off the gas at highway speeds, the TC will unlock on almost all automatics, otherwise you would see engine braking.

-T

 

[brianl703]

quote:

---

Originally posted by T-Keith:
If you let off the gas at highway speeds, the TC will unlock on almost all automatics, otherwise you would see engine braking.

---

One thing I noticed..in a Ford vehicle..why doesn't the engine speed drop all the way back to normal idle speed when you take your foot off the gas in a car with an automatic at highway speeds? As I recall, it drops some, but not nearly as much as pushing the clutch in on a manual.

 

[brianl703]

From http://www.diy-efi.org/gmecm/papers/tcc.txt

"j) With decel fuel cutoff in effect the TCC may or may not be
unlocked. The ECM may use an option bit to define whether this
occurs or not."

also

"k) When the vehicle road speed is above a threshold the TCC may
be unlocked. This one is not too common.

Typically it is the opposite. Once the vehicle road speed is above
a set threshold the TCC is locked. No other ECM logic will unlock
the TCC until the vehicle speed drops below this threshold.
Sometimes this threshold has hysteresis."

Also noted in the paper is that DFCO (decel fuel cut off) can cause the TCC to unlock if the ECM is configured to allow it.

Now I must ask..if it's true that if "the injectors turned off there is no way that a fluid coupling from the transmission could overcome the resistence to have the engine spin." then why would anyone in their right mind program the ECM so that DFCO causes the torque convertor to unlock? The engine would stall instantly.


Makes no sense to me.

[ June 15, 2005, 02:26 PM: Message edited by: brianl703 ]

 

[JHZR2]

Manual transmission... going down hills with a manual transmission

JMH

 

[Palut]

quote:

---

Originally posted by JHZR2:
Manual transmission... going down hills with a manual transmission

JMH

---

Just do it the way I describe in the link I posted in this post on June 7th.

 

[Bamaro]

quote:

---

Originally posted by robbobster:

quote:

---

Originally posted by Bamaro:
I may be wrong but I believe even coasting downhill in gear uses gas. Your plugs are still firing every other revolution. If you put it in neutral and coast, you are using less gas because RPMs drop and there is less wear & tear on the engine.

---

On most modern cars, the ECU commands the injectors to shut off when the RPM are up but there is zero load (e.g. going down a hill in gear). So you use zero gas. See previous posts above for more detail.

Leaving it in neutral uses gas because it takes fuel to maintain idle.

---

If that is true then you should be able to shut off the ignition without noticing any change

Anybody care to try it and let us know?

 

[Bamaro]

quote:

---

Originally posted by Bamaro:

quote:

---

Originally posted by robbobster:

quote:

---

Originally posted by Bamaro:
I may be wrong but I believe even coasting downhill in gear uses gas. Your plugs are still firing every other revolution. If you put it in neutral and coast, you are using less gas because RPMs drop and there is less wear & tear on the engine.

---

On most modern cars, the ECU commands the injectors to shut off when the RPM are up but there is zero load (e.g. going down a hill in gear). So you use zero gas. See previous posts above for more detail.

Leaving it in neutral uses gas because it takes fuel to maintain idle.

---

If that is true then you should be able to shut off the ignition without noticing any change

Anybody care to try it and let us know?

---

Hmmmm, nobody brave enough to try it?
If the injectors really cut off, nothing bad will happen.
If they still keep on injecting fuel with no spark because the ignition is off, when the ignition goes back on, boom!

 

[brianl703]

quote:

---

Originally posted by Bamaro:
If they still keep on injecting fuel with no spark because the ignition is off, when the ignition goes back on, boom!

---

On every car I've ever looked at the wiring diagram for, turning the ignition off cuts power to the fuel injectors.

 

[Alan]

I do a lot of coasting in neutral myself.I get much better gas milage this way compared to coasting in gear as evident on my pretty accurate trip computer.

 

[Eddie]

Only a nut case would coast with the engine off. If you do you loose your power brakes and power steering.

 

[robbobster]

Not to mention that your steering wheel will lock up if you turn the key a lil too far...