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Improving gas mileage... coasting in gear uses less gas than coasting in neutral?

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jagec

Lifer
Apr 30, 2004
24,442
4
0
Originally posted by: Zenmervolt
Yeah, 15 minutes at idle. That's almost a whole 1/8 gallon of fuel used! Oh NOES!

Granted, it would make a big difference if everyone did it, but the unfortunate fact is that it's just not enough of an impact at the individual level for most people to bother with it.
dude, it's one thing if we lived at the North Pole where you need the heat...this is SEATTLE. There is no reason to run the engine...NONE. There are even signs TELLING you to shut the engine off.
 

Zenmervolt

Elite member
Oct 22, 2000
24,510
11
81
Originally posted by: jagec
Originally posted by: Zenmervolt
Yeah, 15 minutes at idle. That's almost a whole 1/8 gallon of fuel used! Oh NOES!

Granted, it would make a big difference if everyone did it, but the unfortunate fact is that it's just not enough of an impact at the individual level for most people to bother with it.
dude, it's one thing if we lived at the North Pole where you need the heat...this is SEATTLE. There is no reason to run the engine...NONE. There are even signs TELLING you to shut the engine off.
I'm aware that there's no reason. And I did point out that I shut off my own engine.

But the simple fact is that the cost of 1/8 gallon of gasoline is not sufficient to entice most people to shut off the engine.

Also, if it's right at the beginning of a commute, I'd leave my engine running. I dislike having to shut off my engine unless it has reached normal operating temperature and the coolant has as well. It's hard on the engine to do otherwise. Again though, if the engine's warm already then I don't worry about it and I shut it off.

ZV
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
ZV, you can shut it off safely before it's warmed up, as long as you're going to restart it soon and go ahead and get it hot.
What's not good is shutting it off after it's halfway warmed up, or just after it's to operating temp. The engine has to run awhile at operating temp to evaporate the condensation.
BTW, engine operating temp is measured by the coolant temp, don't know if that was a typo or not.
 

Zenmervolt

Elite member
Oct 22, 2000
24,510
11
81
Originally posted by: Pacfanweb
ZV, you can shut it off safely before it's warmed up, as long as you're going to restart it soon and go ahead and get it hot.
What's not good is shutting it off after it's halfway warmed up, or just after it's to operating temp. The engine has to run awhile at operating temp to evaporate the condensation.
BTW, engine operating temp is measured by the coolant temp, don't know if that was a typo or not.
Not really. The engine is at operating temp right when the thermostat opens and allows the coolant to start flowing through the radiator. That's just a bit before the coolant itself is at normal temperature.

After a while, the engine and coolant are roughly the same temp, but on initial startup the engine reaches normal temperature first.

Of course, that's all a technicality.

ZV
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: Zenmervolt
Originally posted by: Pacfanweb
ZV, you can shut it off safely before it's warmed up, as long as you're going to restart it soon and go ahead and get it hot.
What's not good is shutting it off after it's halfway warmed up, or just after it's to operating temp. The engine has to run awhile at operating temp to evaporate the condensation.
BTW, engine operating temp is measured by the coolant temp, don't know if that was a typo or not.
Not really. The engine is at operating temp right when the thermostat opens and allows the coolant to start flowing through the radiator. That's just a bit before the coolant itself is at normal temperature.

After a while, the engine and coolant are roughly the same temp, but on initial startup the engine reaches normal temperature first.

Of course, that's all a technicality.

ZV
Well technically, that could be right or wrong. Since the coolant is always flowing a bit, it's not like there's hot water in the engine and ice cold coolant in the radiator.
Plus, if it did work the way you said, the t-stat would open, then shut immediately when the cold coolant rushed in.
Put it this way. I doubt you'd see your coolant temp gauge drop when the t-stat opens, if your cooling system is working normally. And if it did, it wouldn't drop much, and wouldn't be dropped for long.

As you said, it's a technicality, and either way you're safe to shut the engine off for 15-20 minutes at a drawbridge or train crossing before it's warmed up completely, again...as long as you're going to restart and drive awhile.
 

conjur

No Lifer
Jun 7, 2001
58,686
2
0
Ok...a thought occurred to me today (rare though that is):

When I cost down a hill with my foot completely off the gas pedal, the exhaust note doesn't really change except for a bit as the speed reduces. If the fuel was shut off, would the noise of the engine and the exhaust change? When I go to neutral, it's all quiet.
 

DivideBYZero

Lifer
May 18, 2001
24,117
2
0
Originally posted by: 5LiterMustang
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill

EIGHTY EIGHT MILES PER HOUR!!!!!
 

spidey07

No Lifer
Aug 4, 2000
65,469
5
76
Originally posted by: Pacfanweb
Originally posted by: Zenmervolt
Originally posted by: Pacfanweb
ZV, you can shut it off safely before it's warmed up, as long as you're going to restart it soon and go ahead and get it hot.
What's not good is shutting it off after it's halfway warmed up, or just after it's to operating temp. The engine has to run awhile at operating temp to evaporate the condensation.
BTW, engine operating temp is measured by the coolant temp, don't know if that was a typo or not.
Not really. The engine is at operating temp right when the thermostat opens and allows the coolant to start flowing through the radiator. That's just a bit before the coolant itself is at normal temperature.

After a while, the engine and coolant are roughly the same temp, but on initial startup the engine reaches normal temperature first.

Of course, that's all a technicality.

ZV
Well technically, that could be right or wrong. Since the coolant is always flowing a bit, it's not like there's hot water in the engine and ice cold coolant in the radiator.
Plus, if it did work the way you said, the t-stat would open, then shut immediately when the cold coolant rushed in.
Put it this way. I doubt you'd see your coolant temp gauge drop when the t-stat opens, if your cooling system is working normally. And if it did, it wouldn't drop much, and wouldn't be dropped for long.

As you said, it's a technicality, and either way you're safe to shut the engine off for 15-20 minutes at a drawbridge or train crossing before it's warmed up completely, again...as long as you're going to restart and drive awhile.
I thought idling for a few minutes acutally uses LESS gas than shutting the motor off. there's a break even point somewhere, but for short durations idling uses less gas.
 

conjur

No Lifer
Jun 7, 2001
58,686
2
0
Originally posted by: spidey07
I thought idling for a few minutes acutally uses LESS gas than shutting the motor off. there's a break even point somewhere, but for short durations idling uses less gas.
I've read more gas is used idling for 60 seconds than is used for startup.

Turning left onto Whittington Pkwy from Hurstbourne Ln. northbound is a prime example, spidey. ;)
 

conjur

No Lifer
Jun 7, 2001
58,686
2
0
Originally posted by: conjur
Ok...a thought occurred to me today (rare though that is):

When I cost down a hill with my foot completely off the gas pedal, the exhaust note doesn't really change except for a bit as the speed reduces. If the fuel was shut off, would the noise of the engine and the exhaust change? When I go to neutral, it's all quiet.
Anyone?
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: conjur
Originally posted by: conjur
Ok...a thought occurred to me today (rare though that is):

When I cost down a hill with my foot completely off the gas pedal, the exhaust note doesn't really change except for a bit as the speed reduces. If the fuel was shut off, would the noise of the engine and the exhaust change? When I go to neutral, it's all quiet.
Anyone?
No, the exhaust note won't change much. It will quiet down in neutral because the engine isn't spinning as fast.
Whether it's getting fuel or not, whether the throttle is open or not, if you're coasting in gear, the engine is still turning more rpm's than it will in neutral, hence the louder exhaust note.
 

axnff

Senior member
Dec 1, 2000
227
0
0
Originally posted by: conjur
But if fuel is shut off, why would there be exhaust? There's nothing to burn.
There shouldn't be a noticeable exhaust note (there might be a slight note, but you probably shouldn't be able to hear it - the engine would still be pumping air pulses down the exhaust).

As far as automagic trannies, the torque converter only locks when the power supplied at a given rpm is sufficient for the load on the engine. Most autos (EVERY one I've ever driven, but I'm sure there are exceptions) will allow the converter to slip to get more revs on the engine to make up for load increases - such as those presented by hills - at the lower-middle rpms. This is usually more efficient than down-shifting. This behavior is most easily noticed with the cruise on in moderately hilly areas.

Also, not all PCMs will drop fuel entirely as soon as you remove your foot - many wait until the revs start climbing past cruising limits before cutting fuel. Cutting off too frequently or readily can cause serious emissions problems - particularly if the catalytic converter drops out of its efficiency range (it requires high heat levels to catalyze NOx). Cars have to be specially engineered to do this (witness the Golf mentioned above).

Finally, if you're worried about startup/operating temp/shutdown issues, you should be most worried about oil temp, not coolant temp. The oil has to be hot enough to vaporize not just water condensation, but acid, too. Motor oil is engineered to be alkaline to take care of some acid, but excessive idling or not getting the oil temperature up on short trips can wear out the base in oil pretty quickly. Once the metal pieces are all warmed up, they should be fine unless you have too much acid in the oil...
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: conjur
But if fuel is shut off, why would there be exhaust? There's nothing to burn.
There is still the same amount of air, and it's being compressed and pushed out the exhaust. Therefore, there is still exhaust noise.
 

OMG1Penguin

Senior member
Jul 25, 2004
659
0
0
Originally posted by: MelikK
Originally posted by: 5LiterMustang
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill
So you are saying that no matter whether it's a flat surface or a 40% incline you are going to go 88mph at 2000rpm??

CC on my car certainly adjusts rpm depending on whether I am going up or downhill.
I don't know about fancy variable transmissions, but on a manual, yes. The only thing that changes is the amount of gas needed to keep it at 2000rpm, therefore 88mph.
 

Vic

Elite Member
Jun 12, 2001
48,700
9,900
136
I don't think 15 minutes at idle would use up even close to an 1/8th gallon of fuel in most cars. Last month, I got stuck for 4+ hours at a CalTrans checkpoint heading up to Shasta on I-5 (due to snow). Despite spending 4+ hours to creep forward stop-and-go for 5 miles, my gas mileage that tank dropped less than 2mpg off my average for the whole trip (or 7%, average was 28mpg in an 01 Impreza RS).
 

psteng19

Diamond Member
Dec 9, 2000
5,953
0
0
Originally posted by: Vic
I don't think 15 minutes at idle would use up even close to an 1/8th gallon of fuel in most cars. Last month, I got stuck for 4+ hours at a CalTrans checkpoint heading up to Shasta on I-5 (due to snow). Despite spending 4+ hours to creep forward stop-and-go for 5 miles, my gas mileage that tank dropped less than 2mpg off my average for the whole trip (or 7%, average was 28mpg in an 01 Impreza RS).
In my experience, small 4 bangers use next to no gas during idle.
 

Vic

Elite Member
Jun 12, 2001
48,700
9,900
136
Originally posted by: psteng19
In my experience, small 4 bangers use next to no gas during idle.
2.5L isn't small for a 4 banger. Relative to an 8 cyl I guess it's small. But yeah, it doesn't seem to take much to keep it ticking over a idle.

And (oh yeah) in this car you can hear the change in exhaust note when it switches to the low-fuel decel cycle. And when out of tune, the switch between modes could be a little abrupt (the throttle position sensor got out of adjustment at one time).
 

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