Driving for better mpg

[compostbrain]

Why is that you use less gas keeping the engine in gear rather than coasting in nuetral when slowing to an anticpated stop or going down hill? This seems counterintuitive to me.

 

[Bob_Fout]

Why is that you use less gas keeping the engine in gear rather than coasting in nuetral when slowing to an anticpated stop or going down hill? This seems counterintuitive to me.

In neutral enough fuel is injected to keep the engine at idle. In gear w/o any accelerator pedal input and above 1200 RPM no fuel is injected.

In neutral the engine is fuel-driven, in the other case, the engine is being turned by the wheels/tranny.

 

[Brock_from_WI]

On the TDI it stops injecting fuel if the engine is moving. Unlike Gassers where it still has to inject some fuel to keep it from stalling.

Although I contend coasting (very little fuel) a 1/2 mile is better then staying on the throttle until 1/4 mile and then decelerating in gear (no fuel).

 

[gdr703]

Why is that you use less gas keeping the engine in gear rather than coasting in nuetral when slowing to an anticpated stop or going down hill? This seems counterintuitive to me.

At low speeds coast in gear,
At high speeds coast in neutral.

Coasting in gear uses no fuel, but you have the engine braking effect, which is less at low speeds, greater as speed increases.
Coasting in neutral uses fuel to idle the engine. Fuel use per distance travelled reduces as speed increases.
Draw the graph and you'll see the two lines cross.

Therefore there is a speed [+/- 45mph?] where it doesnt matter, below that speed you are better off coasting in gear, above that speed you are better off coasting in neutral.

hth.

 

[Muggins]

At low speeds coast in gear,
At high speeds coast in neutral.

Coasting in gear uses no fuel, but you have the engine braking effect, which is less at low speeds, greater as speed increases.
Coasting in neutral uses fuel to idle the engine. Fuel use per distance travelled reduces as speed increases.
Draw the graph and you'll see the two lines cross.

Therefore there is a speed [+/- 45mph?] where it doesnt matter, below that speed you are better off coasting in gear, above that speed you are better off coasting in neutral.

hth.

How far are you going to take your fuel saving fetish? Why don't you just drive a Flintstone car that's foot-powered?

I take great exception to your highly questionable advice of coasting in neutral at any speed, let alone your advice of coasting in neutral at higher speed.

Have you given any thought to the fact that you have greatly reduced the amount of control you have over your vehicle when you've eliminated your ability to accelerate out of harm's way, if need be?

Or are you just trolling for controversy? Draw the graph.

 

[Khal]

Have you given any thought to the fact that you have greatly reduced the amount of control you have over your vehicle when you've eliminated your ability to accelerate out of harm's way, if need be?

Or are you just trolling for controversy? Draw the graph.

I totally agree... It is very dangerous to when you don't have the ability to accelarate out of danger. That is one of the reasons I don't drive for max mpg. I want to have the power when I need it.

 

[MileageDude]

How far are you going to take your fuel saving fetish? Why don't you just drive a Flintstone car that's foot-powered?

Be easy on the dude. Please.

I take great exception to your highly questionable advice of coasting in neutral at any speed, let alone your advice of coasting in neutral at higher speed.

First off, in many States I believe it's illegal to drive a car out of gear while moving. This pertains to the neutral position on your automatic tranny or in neutral phase on a manual gearbox. Coasting with gearbox engaged but clutch pedal depressed does not apply.

I've ridding with people who have this habit. They coast down hills and coast to stop signs. You have a loss of time doing several things when in neutral. For one you lose seconds engaging the clutch and shifting into the proper gear in an emergency, such as over braking and needing to accelerate around an object in your path. For two, G*d help you if you mismatch your gear judging the speed and jam that transmission to oblivion.

But I've ridden with many people who do this constantly. It's a hard habit to break. It annoys me as much as drivers who use their left foot to brake in an automatic transmission. They rest their left foot on the brake pedal and drive around with the rear brake lights lit.

M.D.

 

[rotarykid]

Contrary to what has been said or what is read on a VAg-Com , the engine never stops fueling while the engine is running . Not even in overrun condition . Injector damage would occure . In overrun condition the IQ & ID & Timing are cut to a minimum in overrun condition but never stop fueling completely .

The Vag-Com reading is a calibration of the above listed parameters . And in overrun condition the high rpms in relation to the minimal readings in IQ & ID & Timing creates a value that is unreadable to the Vag-Com . This is why it reads "0" in overrun condition .

There is a minimum amount of fuel per rpm . As rpms increase the amount of fuel injected increases even in overrun condition .

More rpms = more fuel injected allways in all conditions while the engine is running .

So Coasting in "N" does save fuel over coasting in gear . You can save as much as 30 % coasting in "N" of the consumed fuel over coasting gear , I've seen this first hand .

And if you are paying attention which you should always be doing while driving , coasting in gear isn't any safer than coasting in "N" .

In normal driving in a manual we all coast some in "N" while driving in traffic . It isn't possible to drive a stick shift without some coasting out of gear .

And no one is saying coast down a big mountian in "N" , and any law related to coasting is in that context if it exists at all .

 

[PDJetta]

I coast in neutral quite a bit. You can go quite a bit farther coasting in neutral as opposed to coasting in any gear, even 5th.

--Nate

 

[Bob_Fout]

At low speeds coast in gear,
At high speeds coast in neutral.

Coasting in gear uses no fuel, but you have the engine braking effect, which is less at low speeds, greater as speed increases.
Coasting in neutral uses fuel to idle the engine. Fuel use per distance travelled reduces as speed increases.
Draw the graph and you'll see the two lines cross.

Therefore there is a speed [+/- 45mph?] where it doesnt matter, below that speed you are better off coasting in gear, above that speed you are better off coasting in neutral.

hth.

Engine braking...tee hee hee, not with a TDI. We have no throttle plate. This has bee discussed in a previous thread. IIRC all you get you is energy loss due to friction/rotation of the drivetrain, not the engine per se (a small amount of friction though).

As you slow down, downshift to keep the engine above 1200 RPM and you will use zero or near-zero fuel (might as well be zero), less than at idle. Engine load at idle is 13 or 14. Engine load coasting in gear is zero.

Listen to the engine with the clutch pedal engaged at 45 MPH vs. coasting in gear at 45 MPH. In the first case the engine is certainly running, the latter you hardly hear it. The difference is fuel.

 

[Bob_Fout]

Contrary to what has been said or what is read on a VAg-Com , the engine never stops fueling while the engine is running . Not even in overrun condition . Injector damage would occure . In overrun condition the IQ & ID & Timing are cut to a minimum in overrun condition but never stop fueling completely .

The Vag-Com reading is a calibration of the above listed parameters . And in overrun condition the high rpms in relation to the minimal readings in IQ & ID & Timing creates a value that is unreadable to the Vag-Com . This is why it reads "0" in overrun condition .

There is a minimum amount of fuel per rpm . As rpms increase the amount of fuel injected increases even in overrun condition .

More rpms = more fuel injected allways in all conditions while the engine is running .

So Coasting in "N" does save fuel over coasting in gear . You can save as much as 30 % coasting in "N" of the consumed fuel over coasting gear , I've seen this first hand .

And if you are paying attention which you should always be doing while driving , coasting in gear isn't any safer than coasting in "N" .

In normal driving in a manual we all coast some in "N" while driving in traffic . It isn't possible to drive a stick shift without some coasting out of gear .

And no one is saying coast down a big mountian in "N" , and any law related to coasting is in that context if it exists at all .

Please provide data that show you will use more fuel at RPMs above idle (assuming that's what you meant) w/o any accelerator input vs. the engine at idle.

 

[Drivbiwire]

Rotary kid the TDI does infact go to zero fueling during over-run. The control collar is allowing 0 pressure to occur in the pump.

As long as the engine is higher than idle and the requested load is 0, no fuel is injected.

Damage will NOT occur to the injectors, how could they be damaged if there is no combustion? The engine is simply compressing and decompressing air. The fuel system is not making pressure and fuel is still being circulated within the pump on both the VE and PD designs.

IQ values cannot be taken literally simply because they are estimates on actual fueling. This is the same reason why the fuel economy readings are so inaccurate on the TDI motors.

Bosch even states that zero fueling occurs in over-run.

If you are coasting down a long grade leave the car in gear and just let off the peddle to reduce fueling to zero. When pulling off the highway I try to coast in gear as long as possible to drop the turbine temps in the turbo. The engine is simply pumping air through and aids in reducing turbo temps so you can shut down sooner at a pitstop. If you have an ITT probe the temps will drop from 1100-380 in a few seconds. If you can get a 20 second coast on the off-ramp you can shutdown as soon as you pull into the parking spot.

DB

 

[IndigoBlueWagon]

Hey, that's good to know. I thought that coasting into a rest area was the best way to cool the turbo, but in the future I'll coast down in gear.

 

[PDJetta]

In addition to the air rushing through the turbo on engine overrun, the oil pressure is higher, and flow greater, than at idle and that cools the turbo berarings and shaft better too.

--Nate

 

[watercop]

A few words on "powering out of trouble"

Muggins et al:

I often hear the argument that one needs both high power and immediate access to it in order to avoid catastrophe, stay in control of the vehicle, etc. Teenage males have been using this argument to justify a high performance vehicle since the dawn of internal combustion. I don't buy it.

I have 25 years and nearly a million miles of chargeable-accident-free experience and can't think of circumstances where power got me out of a situation other than ones where I foolishly put myself in harms way. Three examples which come to mind are merging onto a limited access roadway, pulling into traffic on a surface street, or crossing a roadway. When I do this and have a near miss I never think to myself, "wow, sure glad I had enough power to avoid a wreck" but rather "you dumbass, what did you do THAT for!" and resolve to go forth and sin no more.

It has been a consistent observation that operators of high powered vehicles (and vessels, my REAL specialty) often tend to operate them more recklessly...because they CAN, they DO.

A few generalizations (tongue partly in cheek):

1) Operators of higher speed vehicles tend to have higher speed accidents.

2) Operators of higher performance vessels experience the same phenomena - they and their vessels or pieces parts thereof tend to be recovered further ashore or from higher branches in shoreline trees, and said pieces parts tend to be both smaller and more widely distributed.

3) Operators of 4-wheel or all-wheel drive vehicles tend to get stuck much deeper in snow or mud much further from hard ground / road than do those with 2 wheel drive (been there, done that - in mud so deep I had to climb out a window)

4) A corollary DBW might be able to expound further upon: Operators of single engine aircraft who experience engine failure promptly and completely focus upon finding a flat spot to set down their now-very-poor-quality glider. Operators of twin-engined aircraft, upon an engine failure think to themselves, "ah-ha, this is why I have two engines!" and promptly use the remaining engine to power themselves to a crash site of their own choosing. I remember reading somewhere that the fatality rate from single engine failures is HIGHER aboard twin engine aircraft than singles.

I love folks and physics both, but some days the two don't get along so well...

 

[IndigoBlueWagon]

Harley riders are known to claim the loud exhausts are there to help others be aware of them: It's a safety feature.

Having said that, one thing that amazes me after years of driving in some of the heaviest traffic in the world (Boston, NYC, LA, SF, etc.) is that people don't consider that they can accelerate out of jam.

I learned to drive in an Austin Healy Sprite, and the only thing that makes you drive more defensively is a motorcycle. Watercop, you're right on target.

 

[Muggins]

Watercopy, I take it you're a fan of coasting in neutral to save fuel.

You say you've rarely needed to power your way out of situations, most times it's because of your own doing. Still, had you been in neutral, it might have resulted in collision.

Finally, as a water vessel operator, do you ever navigate that vessel in neutral while underway? My understanding is that directional control is severely compromised under such circumstances. Similarly in a dry land vehicle.

As in economics, the law of diminishing returns is strongly felt when trying to squeeze a milliltre or two of diesel fuel savings by coasting as a matter of practice.

 

[Khal]

I will agree that riding a motorcycle will make you drive more defensively. I really had an eye opener a couple of times, neither was my fault. I learned to watch and anticipate other people's mistakes.

One day I start slowing down and down shifting for a yellow light (not heavy on the brakes because I have time and room). The cab behind me had another idea to try to run the light. I hear the squeeling tires and a horn. Down shifted already into first (always keep the vehicle in the power band). Twist the wrist and wheelie across the intersection (light had just turned red). I got through the intersection and stopped on the side looking back I saw the cab stopped in the middle of the intersection. I would have been run over at least 20 feet ago.

It is rare when power will get you out of a jam, but I want to be prepared all the time.

When stopped at a light until there is at least one car stopped behind me I leave the car in gear.

 

[durundal]

Watercopy, I take it you're a fan of coasting in neutral to save fuel.

You say you've rarely needed to power your way out of situations, most times it's because of your own doing. Still, had you been in neutral, it might have resulted in collision.

Finally, as a water vessel operator, do you ever navigate that vessel in neutral while underway? My understanding is that directional control is severely compromised under such circumstances. Similarly in a dry land vehicle.

As in economics, the law of diminishing returns is strongly felt when trying to squeeze a milliltre or two of diesel fuel savings by coasting as a matter of practice.

For a vessel on the water the rudder control is a function of the velocity of the fluid moving past the rudder (use Reynolds transport theorum if you want to figure out the force on the rudder for a given fluid at a given velocity), so by cutting the prop for most vessels you go from the velocity of the fluid being moved by the propeller to the velocity of the vessel through the water, so control is lessened. However, the same is not true of cars, where control is a function of friction between the roadway and tire, which generally isn't a strong function of speed. A ship making way at 1 knot won't be able to turn worth a damn, but you can put a car in gear (ride the brake as necessary) at 1 mph and still be able to turn it around within its turning radius.

 

[watercop]

Regarding maneuvering a vessel out of gear there are several issues in play:

Personal watercraft and other jet driven boats characterized by small or no control surface in the water "slip stream" are nearly impossible to maneuver in the absence of power. This causes so many wrecks that "off-throttle steering" is a contributing cause check box on our state vessel crash investigation report.

Inboard vessels characterized by a fixed prop and moveable rudder suffer from a loss of maneuverability when out of gear owing to reduced water flow past the control surface (rudder) but they will still turn if the vessel has way on (motion relative to the water)

Outboard and inboard / outdrive powered vessels have both a control surface (the gearcase) in the slipstream and directed thrust (the prop + gearcase pivot together) This tends to result in best, most intuitive low and high speed maneuvering characteristics.

Another issue is the fact that boats lack brakes, and for a typical power vessel minumum speed at idle in gear is 3 - 5 mph. That is too fast for docking and other close quarters operation so one learns to change in and out of gear fairly rapidly to keep speed down in close quarters. It is a sort of duty cycling technique - if the vessel idles at 5 mph and one needs 2 mph, you are in gear 40% of the time and in neutral 60% of the time. That is an example - no one uses a stopwatch out there, but the switching is fairly fast - every few seconds.

My supervisor was trained at a federal small craft enforcement school in Georgia and swears by a technique where he only does one thing at a time, but in quick sucession - he bumps in gear then out, follows that with "wheel work" and then another throttle cycle during which he does no wheel work. I don't strictly subscribe to that technique but it makes sense and is easier to teach to new boaters.

Short answer: yep, we maneuver vessels in neutral!

 

[MileageDude]

Short answer: yep, we maneuver vessels in neutral!

At 65 mph down the side of a steep grade?

Anywho, my comments on neutral gear postion were taken out of context or misunderstood ...or both. But not by you watercop, perhaps not anyone in this thread directly.

What I've seen on driving habits of other individuals on saving fuel consumption you have seen too. Some of those habit bother me. Some I just became use to. A car or truck out of gear coasting at road speed using gravity to save fuel is and seems dangerous. But I reiterate once more = having your foot on the clutch pedal and disengaging the transimission ...BUT still having the gear shifter in the correct gear matching the speed but clutch pedal to the floor covered ...is acceptable.

If a cop caught you in Colorado on Vail-Loveland-Denver I-70 driving in neutral he'd take your license.

M.D.

 

[Bob_Fout]

At 65 mph down the side of a steep grade?

Anywho, my comments on neutral gear postion were taken out of context or misunderstood ...or both. But not by you watercop, perhaps not anyone in this thread directly.

What I've seen on driving habits of other individuals on saving fuel consumption you have seen too. Some of those habit bother me. Some I just became use to. A car or truck out of gear coasting at road speed using gravity to save fuel is and seems dangerous. But I reiterate once more = having your foot on the clutch pedal and disengaging the transimission ...BUT still having the gear shifter in the correct gear matching the speed but clutch pedal to the floor covered ...is acceptable.

If a cop caught you in Colorado on Vail-Loveland-Denver I-70 driving in neutral he'd take your license.

M.D.

How would he know? X-ray vision?

 

[MileageDude]

How would he know? X-ray vision?

You can tell when someone is coasting in neutral on Vail or Loveland pass, or for that matter just about any major decline. You notice when traveling behind them. It's bright red, a pair of them on the left and right corners of their car and most cases another bright red thing in the centre upper or lower part of their rear window.

Rhode island has the law written, Colorado has the large panel signs over head saying, "Do not take your vehicle out of gear, do not coast."

But heck, what do they know.

M.D.





Using neutral while coasting downhill:

This urban legend is a popular gas saver myth for foothill dwellers and weekend ski trippers. Not only is this ineffective at saving gas, but it is extremely dangerous. There's no reason to cut-off your acceleration control - coasting with your foot off the gas uses the same amount of fuel. And, letting your vehicle drift downhill can generate triple-digit speeds - one reason why coasting downhill in neutral is illegal in many areas. And if you're thinking of the next crazy step - turning off your engine to coast downhill - the cops should cut-up your license.

 

[watercop]

I hope (well, hoped) that it would go without saying that coasting down long steep grades is unsafe and ill-advised. The highest elevation within about 300 miles of my home in any direction is about 300 feet, so I feel I can safely coast anywhere around here.

I very much doubt that brake lights, even ones continuously exhibited on a downgrade, would constitute probable cause for a vehicle stop on a charge of coasting out of gear - consider that automatic transmission equipped vehicles sometimes offer even less engine braking than manuals in top gear. Operators of such vehicles may brake more or less continually on long downgrades.

I am willing to bet that traffic statutes prohibiting coasting exist primarily for and are only applied as a tool to assign fault in a wreck where such coasting was a contributing factor. A similar situation applies to improperly secured loads - you only get the ticket if the mattress falls off your roof.

Heavy trucks are a different matter entirely and demand much more caution on downgrades. A cousin with years of long haul class 8 operation told me a good rule was to descend a given hill in a gear no higher than the one which was used to climb it. I have certainly seen and smelled the smoke from overheated truck brakes on long down grades, as well as trucks hub deep in gravel part way up runaway truck ramps in the Appalachians...

YMMV

 

[MileageDude]

I hope (well, hoped) that it would go without saying
YMMV

Like I've stated earlier, I have nothing against people who drive in neutral to stop signs or whatever... and I have nothing against you personally, matter of fact I respect your opinions on the board greatly, along with 99.9% of the members. As speaking about you in general I respect almost anything you write because it's entirely well thought out.

I'm just not a fan of coasting.

M.D.

 

[greenskeeper]

Interesting to me that nearly every automatic vehicle when driven below 45mph or when the TC is unlocked will coast in neutral when the gas pedal is released...at least in the automatics that I have driven.

I don't care either way it you coast in neutral or stay in gear. TO EACH HIS OWN!

 

[Audi5000TDI]

Harley riders are known to claim the loud exhausts are there to help others be aware of them: It's a safety feature.

Having said that, one thing that amazes me after years of driving in some of the heaviest traffic in the world (Boston, NYC, LA, SF, etc.) is that people don't consider that they can accelerate out of jam.

I learned to drive in an Austin Healy Sprite, and the only thing that makes you drive more defensively is a motorcycle. Watercop, you're right on target.

Let me tell you, those same Harley Riders deserve to be shot for disturbing the peace when they head down the freeway at 3AM in the morning a half a mile away from my home and I can hear them driving by for a timed minute and 20 seconds with their estimated speed at 80.... Nice wake up call and Doppler sound effect. Does that make me right on target too?

Accelerating out of trouble? Laughable unless you have an incredible power to weight ratio. Most cars can decelerate with brakes to safety far faster than they can accelerate, by a factor of what? 3 or 4 to 1?

 

[IndigoBlueWagon]

Accelerating out of trouble? Laughable unless you have an incredible power to weight ratio. Most cars can decelerate with brakes to safety far faster than they can accelerate, by a factor of what? 3 or 4 to 1?

My point was that many people stand on their brakes as their first and only reaction to trouble. Sometimes the trouble is around you or approaching from behind, and you can steer around it and accelerate away from it. This frequently happens when someone darts into my lane going substantially slower than me. I can jam on my brakes, risking that the driver behind me isn't paying attention, or often can move to the lane the other car just came from and smoothly accelerate away from the confusion.

I agree you're not going to power away from a car that's about to hit you.

 

[MileageDude]

Accelerating out of trouble? Laughable unless you have an incredible power to weight ratio. Most cars can decelerate with brakes to safety far faster than they can accelerate, by a factor of what? 3 or 4 to 1?

Every situation has it's variables. Generalizations on methods to justify neutral coasting out of gear vs. being in gear -foot off gas- are and can be skewed.

Not every hill or mountainside is straight down and completely unobstructed of your traveling path/lane.

Right or wrong?

Not every hill or mountain side is single laned, many are double and in rare occassions triple.

Right or wrong?

The scenario that everyone here slams is the acceleration out of harms way theory being in gear. The one's who do not think it's necessary to be in gear have to look at other variables.

If you're traveling down a steep grade and you have multiple lanes and you're coasting down hill there are variables involved if someone ahead slams on their brakes and you must move over to another lane that has traffic. You have to match their speed to merge. Or let’s say if the car passing you on the left decided to cut you off or come across your lane too close? He may have a flat or engine problem and you have to make a quick decision to accelerate over to match the speed in the lane to your left. How about someone riding another car's rear bumper in front of you or the lane next to you while forcing that car to come into your lane? I won't even mention tailgaters or vehicles who follow you too close from behind or pace your speed side by side not allowing you into their lane in case of an emergency, or to seek another path of travel.

Now? What could be in your path of travel without you really noticing it until almost on top of it ...you wonder?

How about tire recap treads?

OH! I just slam on my brakes ...that's all. Car to the left of you, what about traffic behind you? No biggie ...I'm saving fuel out of gear ...I'll just coast into the other lane matching their greater speed and slip behind the dude who's fixated at the tire recap tread in my lane ...and save fuel while I'm at it.

Another scenario, and I realize that this NEVER happens in real life because all hills and mountains are totally straight down and without any obstruction of travel path.

Right?

You are gliding along... care free and in that bend, that turn there is that recap tire tread laying in your path and you have speed ...coasting and you have just a second to accelerate to the other lane squeezing into it ...because you had traffic in that other lane ...just hitting your brakes would just put you in the path of that recap tire tread where as if you were in gear you could have moved over and perhaps avoided it.

But that never happens in real life. And that's why the States that have made coasting out-of-gear illegal should get REAL and change that law because this never happens.

Right or wrong?

All mountains and hills look like the photo in the pictures below. ALL of them. Right or wrong.

Above, he's at a dead stop in front of me, and to my right 3 cars just behind my rear right side one car length behind and that right lane is moving, this one aint. Geez? coast over right?

LOL

 

[durundal]

Every situation has it's variables. Generalizations on methods to justify neutral coasting out of gear vs. being in gear -foot off gas- are and can be skewed.

[...]

Above, he's at a dead stop in front of me, and to my right 3 cars just behind my rear right side one car length behind and that right lane is moving, this one aint. Geez? coast over right?

LOL

I agree with you that there is no good reason to take the car out of gear (see my energy balance thoughts on fuel use in neutral vs. gear on a downhill grade in the other thread on this, http://forums.tdiclub.com/showthread.php?t=122976) but I am a little confused why the vehicle would have less steering control to avoid an accident in neutral (with the engine running for power brakes, steering) vs. in gear.