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I always had the habit of shifting to neutral on long declines and using the brakes to slow down as needed. Intuition made me believe I'd be saving fuel as opposed to keeping a gear engaged. However, I noticed that the on-board fuel consumption display shows 0.9 of consumption (I assume this is how much it's using while idle) but by keeping a gear engaged, using it to slow down, the counter goes all the way to 0.

If this reading is plausible, then why is it that keeping the gear engaged saves me fuel as compared to neutral?

Edit: I should add that this is a manual transmission vehicle, in case it makes a difference.

11

As DucatiKiller notes, coasting downhill in neutral is usually considered unsafe, as it increases the chances of overheating your brakes. It is also illegal in many jurisdictions.

Questions about driving techniques are generally off topic here, but you do have a question about how engines work that I think is (maybe marginally) appropriate: why do you get better mileage when coasting in gear?

If you coast in neutral, the only source of energy to keep the engine running is burning fuel to idle it. You consume the same amount of fuel as if you were stopped in neutral. (You don't want to turn off the engine completely, as then you will not have power steering, power brakes, air conditioning, or any other accessories.)

However, if you coast in gear, most modern cars have a feature called deceleration fuel cutoff. (If someone knows of a good link about this, please mention it; I had trouble finding one.) When coasting in gear, the wheels supply plenty of energy to keep the engine running, and in turn runs the car's other systems. So the car can, and does, shut off the fuel supply completely, reducing your fuel consumption to zero. (A side benefit of this is that you get better engine braking.)

So in answer to your question: on a modern car, shifting to neutral on a decline is typically less fuel efficient, in addition to being less safe.

  • I wrote this answer which is exactly in line with what you are talking about. I was going to post about it, but you beat me to the punch. – Pᴀᴜʟsᴛᴇʀ2 Apr 27 '15 at 20:42
  • @Paulster2: Yeah, sorry, after posting I found this had already been answered several times over. – Nate Eldredge Apr 27 '15 at 20:44
  • No worries. I think your answer is specific to the question, where the other answer was not specific to this question, but does answer it. – Pᴀᴜʟsᴛᴇʀ2 Apr 27 '15 at 20:45
  • In general, if one is coasting on a decline the engine will rotate faster than it would in idle with the transmission disengaged. Certainly there is no need for fuel to be fed into the engine if sufficient mechanical energy is available from outside to keep it turning, but that doesn't imply that cars won't needlessly supply fuel to the engine anyway in such cases. Note that it would not be good to merely reduce the amount of fuel; if a given mass of air is being drawn through the engine, the mass of fuel should either have the proper relation to the air mass or be zero. – supercat Apr 28 '15 at 1:07
  • @supercat: My understanding is that, for an engine designed with deceleration fuel cutoff, the car will in fact cut off the flow of fuel in this situation. I am not claiming that this applies to every car, but I believe DFCO has been standard (perhaps even required?) for some time now. Cars without this feature would continue to supply fuel when coasting, of course. – Nate Eldredge Apr 28 '15 at 1:14
6

Over on the linked question, I talked about how engine braking works to reduce fuel consumption at a high level:

Coasting: nothing much. The transmission is effectively disengaged (it's more complicated than that but it's a reasonable approximation). The engine is idling - burning fuel to keep itself spinning.

Engine braking: the transmission is engaged and the whole system's net friction (from the wheels, axles, driveshafts, all the way to the moving pistons) is acting as a brake. The engine is running in a vacuum state (throttle is effectively closed) and the motion of the wheels are helping to keep the engine spinning.

Do either cause undue wear? No. I wouldn't advise engine braking down the hill in a low gear near the redline (as you might run past the rev limiter) but you aren't anywhere near that state.

Does one save more fuel than another? Yes, engine braking is much more efficient. I talk about why that is in this similar question but suffice to say that the free energy provided to the engine from the turning wheels helps a lot. Put your car in the highest gear available to minimize the drivetrain friction on the way down the hill.

There is a nice video on Engineering Explained that discusses the whys and hows of engine braking and efficiency:

Video about efficiency

The tl;dr is:

  1. Proof by existence: you have a numerical measurement that shows that fuel consumption is less when using engine braking.
  2. Passes BS detector: if the wheels can keep the engine turning, why should it use fuel?
  • 1
    I don't think it's quite so clear-cut. Unless the hill is steep enough to maintain speed with the transmission engaged, you're going to have to give it fuel to maintain (or get back up to) the desired speed. There are plenty of situations where coasting can maintain speed but engine braking can't. I suspect you're still better off engine braking than coasting with the engine idling, but if you shut off the engine while coasting, coasting is the clear winner. – R.. Apr 27 '15 at 23:22
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    @R.., you're answering a driving technique question. The OP is asking why the fuel consumption reading is so low. Those are two different things. – Bob Cross Apr 28 '15 at 1:12
  • OK you got me there. I agree my comment isn't relevant as an answer. – R.. Apr 28 '15 at 2:22
2

I understand your primary motive is to save fuel but there are certain risks you encounter when going down a steep grade, especially with corners.

You will prone to 'ride your brakes' if you are required to slow down repeatedly OR are attempting to maintain a certain speed going down the grade. If you are constantly using your brakes in these situations you are increasing your chances of several things occurring.

  1. Warped Rotors - Overheating your rotors could warp them and you will have a vibration in your steering when applying the brakes after this occurs.

  2. Glazed Rotors/Pads - you can glaze your rotors and pads. When your overheat your brakes and they are getting extremely hot the pad will begin it's conversion from a solid to a gas due to excessive heat. These gasses get trapped against the surface of the rotor and pad and fuse in a different from original state and put a very low friction surface on the surface of the rotor and pad. The effect is essentially, you cannot stop. Once pads and rotors are glazed you will frequently need to replace them as well as the brake fluid that may have become too hot and changed a bit chemically as a result.

Riding neutral down grades can be dangerous for those reasons, potential vehicle damage, as well as to your own safety if your brakes were to overheat.

Using your engine to keep you at a particular rate of speed going down the grade is the safest and recommended way to descend a steep grade.

Here is an excellent article on driving tips for descending hills.

  • 1
    While this information is very valid and useful, I'm not sure how it answers the question posted. – Pᴀᴜʟsᴛᴇʀ2 Apr 27 '15 at 20:43
  • Don't worry about it. I won't down vote it and I doubt nobody else will either ... Was just an observation, really. – Pᴀᴜʟsᴛᴇʀ2 Apr 27 '15 at 23:16
1

I believe your fuel consumption indicator will be using a combination of the throttle position sensor, manifold pressure sensor and/or airflow meter to calculate your fuel economy.

While coasting with your foot off the gas, the throttle would be fully closed and the manifold pressure would be negative/vaccuum. The computer would read this as absolute minimum value and therefore say zero fuel consumption. You shouldn't take this as an absolute truth as if there was zero fuel the engine would die/stall.

If you were coasting in neutral you'd have the same throttle position but probably not as much manifold vaccuum/airflow. Therefore it would return a slightly higher value.

  • As an aside: I believe the gains of switching to neutral would be almost negligible. However from a safety perspective i believe engine braking combined with normal braking is much safer and reduces risk of overheating your brakes. – Sir Swears-a-lot Apr 28 '15 at 9:29
  • What you save in fuel you'd spend on replacing brake pads. – Sir Swears-a-lot Apr 28 '15 at 9:30
  • This theory sounds more more plausible than others, e.g. - the difference in the reading is actually a quirk of the method of measuring. – Andyz Smith Apr 28 '15 at 12:00
  • I find that if I coast in neutral in my Subaru with a dial showing Air/Fuel ratio, the AFR shows as 14.7 (avg),while keeping the car in gear and letting off the accelerator moves the AFR over 20.39 (which is the limit of what the rear O2 sensor in my car can read). So, in essence it looks like the car is running richer in neutral (relatively speaking), hence it uses more fuel. – Captain Kenpachi Apr 28 '15 at 12:23
1

It is very important to engage a low gear while coming down a slope or incline. The general thumb rule is that, while coming down an incline, you should use the same gear which you will use to climb that incline. Using a low gear helps in engine braking, thus the brake pads are less burdened, thus less heat is produced and hence lesser wear of the brake pads can be achieved. Also you can use the throttle to control the descend to some extent. Infact in some heavy vehicles, engine braking is a must while coming downhill as prolonged braking from the brake-pads sometimes results in a phenomenon called "Brake Fade" or temporary loss of braking power. Trust me, that is not a good thing to encounter, especially when you are coming downhill with the curves and the traffic ahead. Regarding efficiency, i agree with @Nate Eldredge. You will be saving more while coasting in gear than in neutral.

Happy Motoring:)

0

Most modern fuel-injected cars can idle for over an hour on a gallon of gas, so saving a sip of fuel just doesn't amount to much. When you use your engine to decline down a hill, your ECU recognizes that the engine is operating at an RPM higher than idle, so fuel feed is nearly cut off at the injectors - some automatics completely shut off fuel until RPM drops closer to required idle speed. In a manual transmission car, I'm not sure how the fuel delivery system is handled. In my own cars, I rarely use brakes on a downhill - engaged gears take care of that issue and both of them have "Manual Trans Simulation Mode" which allows the Auto to act like a manual. Pretty cool feature that does in fact slow the car to a near stop in almost all situations. This is how I slow down when leaving the interstate or even going downhill to maintain a certain speed best I can.

  • Perhaps this would need to be rephrased as a question, not as an answer to the OP's question. – ALAN WARD Jul 21 '15 at 17:19
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Fuel consumption is only determined by how wide open the throttle body is, which is only determined by how depressed the gas pedal is. So there is no difference in you case.

0

Your belief is in alignment with how older vehicles(without engine control units) work.
You need to understand "Overrun" condition:A condition in which vehicle drives the engine. Refer Mr Cholmondeley-Warner's answer.

Keep in mind,engine movement must not stop. It should be either driven by fuel or by vehicle momentum.Or else we need to crank again to restart the engine.

When gear is engaged in downhill, vehicle drives the engine.So fuel will be cutoff by control unit.
When shifted to neutral, fuel must be burnt to keep engine running.So fuel supply will be ensured by engine control unit to maintain idle speed of engine.

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