My 2006 Jetta is the same – it’s difficult to get to “—” in neutral (impossible, if the AC is on), but if you take your foot off the gas on a hill, it goes to “—” immediately, indicating that the gas has been shut off. This is also true when just coming to a stop – I see a lot of manual drivers putting their foot on the brake and the clutch at the same time, but if you just assist the engine braking with your actual brakes, you should get better mileage (I’m not starting an argument over downshifting!).
It’s all going to depend on the hill, what speed you want to be at at the bottom, AC on or off, etc. But, when you’re actually on the hill itself, my car will use less (that is, no) gas when you leave it in gear than if you put it in neutral.
Really not trying to be combative here, but I do not see how it would be possible to have any car that runs on fuel only (hybrids and electric excluded) and has an INTERNAL COMBUSTION engine that is RUNNING to not be burning some amount of fuel.
A combustion engine requires fuel to be delivered into the cylinder for ignition and compression to move the pistons.
If the ECU cuts off fuel, then the engine is not active. While I am aware that some newer cars do this at every stop - and that is great - lets clarify that this is the not the norm for most cars. If we are talking about the motor being off then sure no fuel is being used, but I feel that is not the point of question from the OP.
Agreed, except for the “many” and “new” parts. Except for a few of the very earliest systems, virtually every car with electronic fuel injection shuts off fuel supply when coasting in gear. That means pretty much any car sold since the late 80’s or so.
I suppose you can make some academic point about whether the engine is really “running” if it’s just being turned by the car’s inertia, but you’re completely wrong about this not being the norm for most cars. It has been for at least two decades. The driver doesn’t really notice since all the belt driven accessories are still turning and the engine can start producing power again instantly.
It’s a two-way mechanical thing. Engine burns fuel and turns a crank that sends power to the wheels (transmission omitted); and when gravity is pulling the car down the hill the rolling wheels turn the crank in the engine. Imagine being on a bicycle that doesn’t let you coast. Normally, yeah–you gotta expend some energy to work the pedals. But riding such a contraption down hill, well, the pedals work you–your legs will keep moving but you’re expending no energy in the process.
But does this work even with an automatic transmission?
Unless we’re talking about enormous long hills, or very steep ones, or low speed limits, that shift point is often nowhere near the bottom of the hill (and may in fact be right at the top).
Because if you have engine-braked all the way down, your car isn’t suddenly going to make up speed on the last little bit of downhill slope.
Although it has nothing to do with the pros and cons of coasting in neutral as far as MPG goes, it is illegal to do so in several states. I just checked Oregon and California and it is illegal in both.
I wonder if perhaps my scangauge cannot read this properly but when I am going downhill in gear on one stretch of my commute the gauge reads 60 or so MPG, but the second I put it in neutral it jumps to 199 MPG.
That said, I read about the DFCO and it clearly says the ECU cuts fuel using wheel momentum to turn the crank so I am both educated and dumbfounded on the subject
What are the economics of this behavior? That is, if I am spending $250 a month in gas, this is going to save me, what, a couple of dollars? I mean, I don’t spend that much time descending hills. A long-haul trucker, I could understand.
Also, in some states it is illegal to coast. Don’t ask me how they could possibly tell. And I won’t debate the morality or wisdom of it. It’s just a fact.
You should never force a modern car with automatic transmission into neutral. The transmission itself can decide when the neutral is more advantageous and will disconnect the clutch.
It depends on the slope, and whether you’d need to use brakes.
It’s clearly a tradeoff between two factors.
In neutral, you have to burn gas to keep the engine idling
In gear, some engine breaking occurs, but no gas is needed to keep it turning.
Also, the purpose of the brake pedal is to turn kinetic energy into heat, so clearly, using brakes rarely increases fuel economy.
Modern engines do indeed cut fuel delivery, as has been cited in other related threads here on SD. I worked on engine control software at Ford SciLab decades ago, and was interested to learn what happens today. I found the cites in the other threads convincing, though I don’t remember the sources. In any case, I was a pro on this long ago, enough to be able to understand the jargon.
I can’t do the math to find the tipping points between the two factors I listed above, but I’d guess that if you use the highest gear where your speed doesn’t exceed the safe maximum, that would be the most efficient.
If the drivetrain is turning the engine and there’s no demand for power, it need burn no fuel. As soon as there’s demand for power, fuel delivery returns.
I don’t know whether they also cut off spark when fuel delivery is off. My wild guess is no, because the added complexity wouldn’t be justified by the slight increase in spark plug life, and I can’t think of any other significant benefit. The energy use for spark is probable negligible.
I don’t buy this. I don’t see any benefit to running in neutral in this case.
The question would be whether the fuel to idle the engine is more or less than the amount of fuel to replace the kinetic energy lost to the (minimal) engine breaking in the highest gear. While you’re climbing the next hill but before you’ve pressed the gas pedal, you would want to be in the highest gear if engine breaking – assuming you’re going fast.
