The Naked Scientists

The Naked Scientists Forum

Author Topic: Does coasting in neutral use more fuel?  (Read 38583 times)

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5337
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Does coasting in neutral use more fuel?
« on: 31/12/2011 17:39:45 »
The DVLA helpfully sent me a guide to how to save money on motoring alongside my fleece - sorry - car tax reminder. (The carbon cost of printing and posting a full-colour leaflet has clearly escaped them).

Anyway, it helpfully contains the following advice under a section called "Be a smarter driver!"

"Stay in gear - Don't coast downhill or towards lights in neutral as this uses more fuel! Stay in gear and ease off the gas gently to reduce fuel flow to the engine to virtually zero."

Maybe I'm missing something, but this sounds like b****cks to me.

When you take your foot off the throttle, the carburettor / injectors / fuel pump on a diesel return to the throttle opening that corresponds to tickover. Therefore closing off the throttle and remaining in gear while coasting will cause engine braking because the car is trying to turn the engine faster than the fuel supply would normally enable it to turn; therefore there is a net braking effect. But surely the fuel flow will by no means be zero, unless there's some very intelligent programming going on in engine management systems on new cars...?

I'd appreciate everyones' opinions...

 


 

Offline Don_1

  • Neilep Level Member
  • ******
  • Posts: 6890
  • Thanked: 7 times
  • A stupid comment for every occasion.
    • View Profile
    • Knight Light Haulage
Re: Does coasting in neutral use more fuel?
« Reply #1 on: 31/12/2011 18:25:59 »
I think you hit the nail on the head with your comment: ".....this sounds like b****cks to me......", Chris. It is plainly obvious to anyone with a brain cell rattling around in their skull, that the DVLA are talking out of their a***s.

If I am doing 2500 revs at 60MPH when I begin to go downhill, if I take my foot off the accelerator, to use the engine brake, and the result is I continue at 2500 revs at 60MPH, then I am using the same amount of fuel. If I depress the clutch, my revs will drop to around 1000RPM but my speed remains at 60MPH (or perhaps a little more). It has to be as plain as the nose on my face, that I must be saving on fuel.

As for reducing the fuel flow to virtually zero, that would cause the engine to cut out.

While I understand that they are trying to disuade motorists from 'freewheeling', I think it would be best if they gave the actual reasons why freewheeling can be dangerous, not take us all for idiots, or show themselves for the idiots they are.

You have reminded me that my tax runs out at the end of January..........
Car tax, road tax, fuel tax, fuel duty, tax on fuel duty, tax on insurance, Dartford Xing, Severn Xing and M6 toll AND tax on them, parking charges, congestion charge, tax on spares and repairs................ AND STILL I CAN'T USE THE B******************Y BUS LANES!!!!


Could this be my last rant of the year, or will I manage to fit in another?
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #2 on: 31/12/2011 19:59:01 »
I would think there are multiple factors including the actual vehicle design.

Carburated vs Injected & engine design.
Coasting to a stop vs Coasting down a hill and continuing at speed.
How long is the hill?
Automatic vs Manual Transmission
Etc.

With your typical carburetor, there is a fixed idle speed adjustment.  So, when you let the throttle off 100%, you will have a fixed amount of fuel bypassing the carburetor per engine stroke.  The faster the engine turns, the more fuel your engine would use.
An injected engine might select fuel to maintain the tick-over, and could theoretically go to NO fuel if the engine was spinning faster than necessary to maintain idle.  And, so one might be able to achieve lower fuel consumption while coasting in gear, using the wheels rather than engine fuel to maintain the engine turning.

That is, of course, if you are braking, for example coasting to a stop.  If you wish to maintain the maximum speed on the downhill, then you might be best to kick it out of gear, so it will not loose speed due to engine braking and require more acceleration.  However, the engine braking might be minimal in the top gear.  Time of the descent might also be an issue in some situations. 

Another option on long, steep hills is to kick it out of gear and shut off the engine.  That is if you don't have to put the %&^*%^&*^#!  thing in park to restart it.  But, you will loose power brakes (after the first application), and power steering.  And you don't want to burn out your brakes on a windy road or very steep hill.

Some automatic transmissions will automatically coast, so your doing it manually will not be of much benefit. 

Of course, if you are stopping with a manual transmission, I would recommend taking it out of gear at some point!!!  As well as downshifting before restarting.

Anyway, I don't think there is any single answer to this.  I will likely depend a lot on your vehicle. 

The newer hybrids and "blue motion technology" will take many of these decisions away from the drivers by automatically optimizing engine management.

 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #3 on: 31/12/2011 21:26:26 »
Agreed. It's a load of complete twaddle!
 
As clifford points out, most automatics have a freewheel clutch just like to one on your bike. (Although it may be disabled to allow the engine to act as a brake in certain situations).
 
Keeping the engine turning consumes energy. There are plenty of sources of friction, and there are pumping losses, so if the vehicle is driving the engine at all, the engine is acting like a brake by convertingthe kinetic energy of the vehicle into heat.
 
The reason you don't want to flip it into neutral (if it has a manual transmission) is because it's dangerous,
 
if the engine stalls you might lose all the power assist to your brakes and steering;
 
if you have to suddenly accelerate it might take too long to get into gear;
 
if you need to jam it into gear to use the engine as an emergency brake, you might not be able to get it into gear, or, if you pick the wrong gear, you might actually break the gearbox, or part of the driveline. If you are really unlucky and you destroy the u-joint at the front of a propshaft, the propshaft can launch you and your vehicle like a polevaulter's pole, or, at least, rip the rear axle off the vehicle;
 
if it's a front wheel drive vehicle you might destroy one of the constant velocity joints which can cause the vehcle to steer rapidly in either direction.
 
(BTW - diesel engines don't have a throttle.)
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8126
  • Thanked: 53 times
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #4 on: 01/01/2012 02:24:57 »
More alleged fuel saving I saw advertised recently ..


http://www.uswitch.com/energy-saving-products/energy-efficient-heating/radiator-insulation-foil#

as if  adding a 0.03 mm metal foil layer to the back of a metal radiator* will improve its performance.

[* despite the name they heat by convection, not radiation]

« Last Edit: 01/01/2012 02:31:52 by RD »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #5 on: 01/01/2012 04:14:18 »
More alleged fuel saving I saw advertised recently ..

as if  adding a 0.03 mm metal foil layer to the back of a metal radiator* will improve its performance.

[* despite the name they heat by convection, not radiation]



It's very artfully written too. If you are not paying attention you might believe it's going to save you 35% on your heating bill.

 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #6 on: 01/01/2012 07:54:13 »
I don't have any steam radiators...
But, it would be simple enough to test.

Take some plain tin (aluminum) foil.
Attach it to the wall, shiny side towards your radiator.

Perhaps behind half of the radiator.

Now, take an IR thermometer, and check the wall temperature behind the covered and uncovered parts of the wall.

The theory is that the foil will reflect the IR emissions back into the room (or the radiator).

The real test would be whether the temperature of the outside wall would be any different behind different radiators in your house.

I'll tell you, a "space blanket" is amazingly warming.
« Last Edit: 01/01/2012 08:26:53 by CliffordK »
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8126
  • Thanked: 53 times
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #7 on: 01/01/2012 09:27:58 »
The stuck-to-the-wall version of the “reflector” foil may lower fuel consumption by accident ...

If the foil trapped some air between itself and the wall, (like bubbles in badly applied wallpaper), it could lower fuel consumption by insulation: air warmed by the back of the radiator would be less able to transmit its heat to the wall because the air bubbles trapped by the foil have a lower thermal conductivity than the unadorned wall.


I'll tell you, a "space blanket" is amazingly warming.

I suspect a black plastic bin bag would be similarly toasty.
« Last Edit: 01/01/2012 09:32:47 by RD »
 

Offline SeanB

  • Neilep Level Member
  • ******
  • Posts: 1118
  • Thanked: 3 times
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #8 on: 01/01/2012 18:02:14 »
Trust me Rd a space blanket is a lot better than a black bin liner to keep you warm on a cold winter night. The only thing most would do with the bag is to get a whole lot of them and other combustibles and burn it for warmth.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #9 on: 01/01/2012 19:53:26 »
Anyway, there is nothing wrong with heating the walls if they are interior walls, or well insulated exterior walls. The walls act as moderators and prevent rapid temperature swings. They also help to maintain a uniform temperature which makes the room feel a lot more comfortable.

There isn't much heat radiated from a radiator, particularly the relatively cool hot water variety that are common in the UK, so you'd be much better to stick a thin sheet of polystyrene insulation on the wall if you really wanted to minimized the heat transfer into your next door neighbor's house.
 

Offline lightspeed301

  • Jr. Member
  • **
  • Posts: 26
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #10 on: 01/01/2012 20:20:39 »
As Betty White said: "It Depends."

Coasting downhill in drive will probably, or at least could, shut down all the injectors and us no fuel. However, the car will experience engine braking. Coasting in neutral eliminates engine braking, but requires fuel injection for idle; and that assumes the engine injectors are off.

I have a long hill and have tried both. In neutral the car speeds up quite a bit, and I suspect that more then compensates for the idle fuel.

There is also a special circumstance where coasting in drive will not shut down the injectors: If the engine is cold. I live on the top of a one mile hill, and am convinced the engine uses less fuel in neutral.

In any event, you are not going to save much fuel anyway. I spent a lot of time experimenting on longish trips. The best I can determine my extra mile experiments MIGHT have gained me one or two tenths on a base of about 27mpg.
 

Offline lightspeed301

  • Jr. Member
  • **
  • Posts: 26
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #11 on: 01/01/2012 20:28:16 »
Geezer

I do not believe any cars have a free wheeling transmission. True, the automatics may deactivate the lock-up torque converter, but I can tell from my tachometer the automatic has not gone freewheeling. I can free wheel by putting the car in neutral.

But I mostly agree with those who say just leave the damned thing in drive. Even EYE have been caught flatfooted when the light turns green after coasting up to a red. Incidentally, the CV joints will be unaffected in automatic cars. I can not even torque brake my '07 MAXX.  It just refuses to move off idle until I let go of the brake pedal. Sad, but true.
 

Offline lightspeed301

  • Jr. Member
  • **
  • Posts: 26
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #12 on: 01/01/2012 20:44:24 »
RD,

There is no way to modify your radiator to improve fuel economy. Remember, the cooling system has thermostat. It will not pass coolant until the temperature gets pretty high. I think most cars today are 195* F. We used to put 180* F thermostats to keep the intake cooler, and the air cooler and at higher density and hence more power.

At the track I was never able to record any difference.  On the otherhand, if you could round up a 215* F thermostat, that might actually increase fuel economy by reducing pumping losses. It makes the intake charge thinner - think of moving towards a vacuum where there would be no pumping losses.

I have done track runs at 40* ambient v. 90* ambient. I gain, perhaps, one or two mph better. I would only do a 215* thermostat for kicks. I like my power, and like positive displacement supercharging at about 1/2 atmosphere. On my Lincoln MK VII it dropped my 1/4 mile time by more then one and a half seconds. And believe it or not, my fuel economy went UP.

At 60 mph I was pushing 30 mpg. That dropped to about 21 mpg at 80 mph, and stayed there no mater how fast I went. And I went FAST back in those days. One hundred and ten was routine.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #13 on: 02/01/2012 01:28:30 »
RD,

There is no way to modify your radiator to improve fuel economy. Remember, the cooling system has thermostat.

Er, you might want to take a look at RD's post again.

There actually is a freewheel in the torque converter of an automatic transmission. The stator element is mounted on a one way clutch that prevents it from rotating while the engine is rotating faster than the output to the transmission. Preventing the stator from rotating enables torque multiplication. When the output reaches almost the same speed as the engine, the fluid forces on the stator reverse and the stator rotates with the impeller and the turbine to minimize unnecessary pumping action. At that point, the torque converter is behaving like a fluid coupling.

When torque is flowing from the transmission to the engine, the stator rotates with the turbine and impeller and there is no speed multiplication to the engine. This explains you can't "bump start" a car with an automatic transmission the way you can with a manual transmission. There is so little torque going from the transmission to the engine that you have to tow an automatic at about 30 mph to get it to start.

 

Offline lightspeed301

  • Jr. Member
  • **
  • Posts: 26
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #14 on: 03/01/2012 21:56:42 »
Geezer

Just today I tested my four speed auto with lockup converter on a steep hill.  At 60 mph in drive and zero throttle the tach showed 2000 rpm. In neutral it showed 1000 rpm.  Clearly it was not free wheeling.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #15 on: 04/01/2012 00:39:18 »
Geezer

Just today I tested my four speed auto with lockup converter on a steep hill.  At 60 mph in drive and zero throttle the tach showed 2000 rpm. In neutral it showed 1000 rpm.  Clearly it was not free wheeling.


If there was no reduction in engine speed when you took your foot off the gas, the lockup clutch was still engaged.

The torque converter will couple some torque from the transmission back to the engine, otherwise it would be impossible to start an automatic by towing it at any speed. It's not a perfect freewheel, but at low speeds, it's pretty close.

When I lift my foot off at 35 mph, the engine drops back almost to idle speed. I'm coasting. That is not what happens with a manual transmission. With a manual transmission there can be no immediate change in the engine speed without an immediate change in road speed.
 

Offline lightspeed301

  • Jr. Member
  • **
  • Posts: 26
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #16 on: 04/01/2012 01:47:32 »
Geez - You wrote: "If there was no reduction in engine speed when you took your foot off the gas, the lockup clutch was still engaged."

I don't think I said there was no reduction when I took my foot off. I only observed what the rpms were after I did that, and what they were when I move the shifter into N. I will take your advice and test for both next time I am out.

However, you, your very own self, said you could push start an automatic at about 35 mph. I am unaware of any automatic that entirely disconnects the drive wheels from the engine at any speed.

With the possible exception of my 2007 CTS which would not creep forward at idle. I came to hate that car for a variety of reasons, including that one. It would produce a driveline snatch every time I started from a dead stop.

 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #17 on: 04/01/2012 03:43:26 »
When your car is stopped in Drive, the impeller is turning a lot faster than the turbine, so the stator is like, static, because of the one way clutch. Therefore, the engine delivers torque to the transmission which is why the car tends to creep. If the engine was stopped and the car was moving slowly, the one way clutch would allow the stator to be like, not static, so the engine would hardly slow down the car at all.   
 
That's why they have a P position on the selector on an automatic. Try parking on a hill in drive without the emergency brake on and you'll see what I mean.   
 
Perhaps what you should try is coasting to a stop from, say, 30 mph in D and in neutral on the same section of road, and see if there is any difference in the distance you travel.   I used to drive a "semi-automatic" VW bug that had a three speed synchromesh transmission, an electrically activtated clutch and a torque converter. I also drove a lot of stick VW's over the years.   
 
Thanks to the torque converter, the semi-automatic provided almost no engine braking at all. Fortunately it had disc brakes, but it chewed through brake pads and rotors like crazy.   
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #18 on: 04/01/2012 06:35:51 »
After more pondering it struck me that I should have pointed out that things would be different if the stator was not supported by a one-way clutch so that it was always static (as its name implies.)

If it was, the torque converter would be symmetrical, or at least a lot more symmetrical (I can't figure out if the angle of the stator blades prevents it from being completely symmetrical or not.)

Anyway, even if it was more symmetrical, it still would not behave like a gearbox. What it would do is act like a gearbox that always tried to increase the torque between the input and the output regardless of the direction.

That means that it would work as expected when the engine was accelerating the vehicle, but when the vehicle was trying to accelerate the engine, the transmission would apply a lot of torque to the engine, by "gearing down", so the engine would be turning much more slowly than the shaft from the transmission.

That's the complete opposite of what's required for engine braking. What you want to do is try to make the vehicle speed up the engine as much as possible.

I've noticed my truck will hold its cruise speed on a serious downgrade using engine braking, but I suspect that only works because the torque converter is completely bypassed with the lock-out clutch so that the whole thing is behaving just like a manual transmission.
 

Offline CZARCAR

  • Hero Member
  • *****
  • Posts: 686
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #19 on: 05/01/2012 18:27:01 »
whats the chance that a pollution control gizmo might be the reason?
 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
Re: Does coasting in neutral use more fuel?
« Reply #20 on: 06/01/2012 13:44:20 »
When you take your foot off the throttle, the carburettor / injectors / fuel pump on a diesel return to the throttle opening that corresponds to tickover.

Just to note, this is not the case for the majority of modern vehicles in all circumstances:  Under some operating conditions a car's engine-management computer can 'decide' to cut fuel off entirely to the injectors.  This should be carefully monitored by the car's ECU to avoid the revs dropping below its stall speed and will only occur if no input for throttle sensor (ie. no pressure from your right foot) is present. On some vehicles light pressure on the brake pedal will indicate to the ECU that the driver wishes to go to this 'no-fuel slow-deceleration' mode.
The easiest situation to imagine where this is advantageous is where a driver is approaching a red traffic light or stationary traffic. If the driver is doing 15mph in 2nd gear, putting the clutch in will allow the car to coast (hardly slowing at all) but at the cost of a small amount of fuel to maintain tick-over.  Alternatively, keeping in gear can allow the car to slow fairly gently (through the additional 'drag' of engine pumping losses) but without a single drop of fuel being burnt.
Apologies if this is repeated - I've not read the whole thread.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #21 on: 07/01/2012 07:03:13 »
Good point PC. So that would mean you might do slightly better if you do stay in gear. Mind you, if you are going to stay in gear, you have to keep changing down if you don't want to stall the engine.

I have a suspicion that most people, if they think are going to come to a complete stop, either disengage the clutch, or stick it in neutral (except during their driving test, of course.)
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #22 on: 07/01/2012 07:41:22 »
I have a suspicion that most people, if they think are going to come to a complete stop, either disengage the clutch, or stick it in neutral (except during their driving test, of course.)

My Electric Vehicle has a clutch and flywheel that will spin quite well without any power.  I find it is actually best to bring the car to a stop in gear, so I can start again from zero, rather than slipping the clutch in first gear.  But, popping it out of gear as one approaches a stoplight is a hard habit to break.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #23 on: 07/01/2012 08:23:18 »
What does the flywheely thing? Is that the motor?

I've never driven an electric vehicle (other than dodgems {bumper cars} of course) but I thought the general idea was to leave it in gear and drive it only with the thingy that controls the motor power.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Does coasting in neutral use more fuel?
« Reply #24 on: 07/01/2012 08:55:43 »
My EV Ranger has a 5 speed manual transmission, connected to the Electric Motor.  With a clutch.

Many people recommend a clutchless setup.  In my vehicle, there seems to be very little motor speed drop when free-wheeling in neutral.  Engaging the clutch while the motor is freewheeling will make the vehicle jump forward.  I'm not sure if it is the weight of the motor armature, or a flywheel, but it would be difficult to shift without using the clutch.

I can accelerate from zero...  but the different gears do actually make a difference.  I generally start in 2nd gear, and work up through the gears. 

I have an ammeter, but not wattmeter, or miles/watt meter. 
I seem to get maximum power at about 200 Amp draw.
If I have it in too high of a gear, I get a lower amp draw, and less power.
If I have it in too low of a gear, I get a lower amp draw, and less power.

So, yes, the transmission does help.

Unfortunately, I am not an electrical engineer, and did not do the initial build on the vehicle.
I think one can vary power somewhat by varying voltage.  So, perhaps one could increase the system voltage from 144V to 288V, and get a wider power range.  Also, some DC motor controllers use pulses of power rather than a true variable voltage, although perhaps they use capacitors to remove ripple, and give true variable voltage.
 

The Naked Scientists Forum

Re: Does coasting in neutral use more fuel?
« Reply #24 on: 07/01/2012 08:55:43 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums