Coasting in neutral or with clutch down?

[jime57]

Originally Posted by PaleMelanesian

That's true during that instant, but when you look at the whole trip, the balance tips the other way. I've tested pulse-and-DFCO against pulse-and-neutral idle and against steady speed driving. The results:

Best: pulse and EOC
Good: pulse and neutral idle glide
Ok: steady speed
Worst: pulse and DFCO

You only want to use DFCO if you're needing to lose speed, like coming up to a red light, or down a mountain. Any other time, it's a net loss in mileage. The drag from engine braking overwhelms the gains from no fuel being used.

My honda does DFCO down to 1200 rpm, and below that it restarts the fuel flow. You can feel a surge when this happens.

o.k. your priority list is probably right, for most cars. The thing that bothers me about pulse and EOC is that it involves a lot of extra cycles on the clutch and transmission. Neither were designed for this use. One has to question whether the added fuel economy is worth the extra wear. Not to me, but that's just one opinion. I suppose that for short competition it makes sense, but not for day-in-day-out activity. I still maintain that method 1 has to impose a fairly heavy wear load.

Incidently, EOC=engine off coasting, DFCO= decelleration fuel cut off, FAS= forced auto stop. Many of these acronyms are defined in the glossary link on:

CleanMPG, An authoritative source on fuel economy and hypermiling

[DIMS]

Neutral Shift for Better Fuel Economy - Popular Mechanics

Q: I have a question about fuel economy. If you are driving downhill, do you save gas by putting your transmission in Neutral and coasting, instead of having your vehicle in Drive? I think that you do but my wife seems to disagree. Can you give me the correct answer, so I can tell her that I’m “Mr. Right,” as usual?

A: That depends. The engine isn’t braking the car going downhill if the transmission is in Neutral, so economy would seem to be high. But if you think the engine is still using fuel while coasting downhill in gear, you’re laboring under a misconception. Most fuel-injected cars turn the fuel delivery completely off when you lift your foot from the accelerator. They still burn fuel when idling in Neutral, so do the math. The amount of fuel burned at idle over, say, a couple of miles of coasting downhill is small, but it’s still more than zero. So if you’re driving a modern fuel-injected car, you’re wrong.

Older, carbureted cars would suck gas through the engine while coasting in gear, even if you turned off the ignition switch. In this case you’d be right.

[NeilBlanchard]

Hi,

If you want to carry speed as long as possible, then coasting in neutral is best.

If you want to slow down, before you use the brakes, use the engine to slow the car by downshifting into a lower gear.

[KJSatz]

Yeah, in this case I could disagree with Popular Mechanics's conclusion. Unless there's a stopsign in the near future.

[DIMS]

I'm not sure how far I actually coast in 5th gear to slow down for stop signs and stop lights. I need to figure out my average distance. I also need to figure the time it takes to get close to a stop. I usually leave my car in 5th gear until I get close to a stop. Then the next step would be using neutral method. For me, I think using in gear (5th) will result in the better economy, because just prior to my destination the car goes in closed loop. Putting it in neutral raises the RPMs but using that energy in 5th gear actually propels the car. I think this is one of those cases where different situations require different driving techniques. (one size does not fit all)


In the last 3 months I have driven my bicycle more than my cars.

[roflwaffle]

Originally Posted by DIMS

Neutral Shift for Better Fuel Economy - Popular Mechanics

Q: I have a question about fuel economy. If you are driving downhill, do you save gas by putting your transmission in Neutral and coasting, instead of having your vehicle in Drive? I think that you do but my wife seems to disagree. Can you give me the correct answer, so I can tell her that I’m “Mr. Right,” as usual?

A: That depends. The engine isn’t braking the car going downhill if the transmission is in Neutral, so economy would seem to be high. But if you think the engine is still using fuel while coasting downhill in gear, you’re laboring under a misconception. Most fuel-injected cars turn the fuel delivery completely off when you lift your foot from the accelerator. They still burn fuel when idling in Neutral, so do the math. The amount of fuel burned at idle over, say, a couple of miles of coasting downhill is small, but it’s still more than zero. So if you’re driving a modern fuel-injected car, you’re wrong.

Older, carbureted cars would suck gas through the engine while coasting in gear, even if you turned off the ignition switch. In this case you’d be right.

Who over at Popular Mechanics is writing this stuff? Nothing written so far is right or wrong, so anyone saying it is right or wrong doesn't know what they're talking about or is just guessing. For one, carbs can haz fuel cuts too. Numero dos is that unless the car can stay above the engine speed needed for the fuel cut, it'll more or less idle in gear, and even if the driver accelerates above the fuel cut every time they drop below it then they would have to compare the fuel usage to whatever was used when it idled down the hill. In short, it depends on the car, weather, hill, etc... So the only correct response w/o more information would be "That depends." with all the other erroneous stuff left out.

[Ptero]

RIDING THE CLUTCH
All transmission bearings are cooled by an oil bath that draws heat away and replenishes lubricant. A transmission could care less whether you are in neutral or in a gear. But all throw-out bearings are sealed and self-contained. They have no provision for cooling or lubrication replenishment. Therefore, riding the throw-out bearing for extended periods gets it hot. When the throw-out bearing gets hot, two things happen: 1) the grease thins and 2) the oil seals soften. There is a point where the hot, thin grease can make its way past the soft oil seals and escape. If too much grease is lost over time, the throw-out bearing will seize. When this happens, the outer race of the bearing, which is usually stationary, will spin on the clutch forks. This will wear the clutch fork, causing the clutch to fall out of adjustment and necessitating replacement of the fork. Even worse, the inner race of the throw-out bearing will drag on the release fingers of the pressure plate. Eventually they will wear away or break off, throwing debris into the clutch/flywheel/pressure plate interface, scarring up the flywheel. This requires flywheel removal, resurfacing or replacement.

My experience over many years as both a mechanic and heavy truck driver is NEVER hold the clutch down for longer than it takes to shift - or you will eventually be paying a mechanic all the money you saved (by coasting with the clutch in) to repair your clutch assembly.

COASTING IN NEUTRAL WITH AUTOMATIC TRANSMISSIONS
Never do this. Always "coast" in drive. If you have been doing this in a car that the manual does not specifically say can be "flat towed," you should sell your car.

COASTING IN NEUTRAL WITH MANUAL TRANSMISSIONS
In heavy vehicles, coasting in neutral is extremely dangerous and should never be done. In lightweight vehicles, coasting in neutral has little significance as a control issue and provides the greatest mpg gain possible.

I have reviewed the posts on this thread and would like to point out that it does not take much fuel to idle an engine that is not doing work. If you are coasting, you are receiving a tremendous advantage regardless of whether or not the engine is running. Of course, there is an advantage to having the engine off altogether, but on modern cars this disables other systems like power brakes, power stteering, airbags and brake lights and is therefore not an option. Also, you do not want to pop a dead engine back to life at speed because it is stressful on the components and wears the clutch excessively. It is better to start the engine with the key, rev the engine to match the transmission engagement point and engage. Neophytes can locate this point by very lightly touching the shift lever without engaging the clutch, delicately and slowly revving the engine, and feeling the teeth in the transmission slow down as they approach the mesh point. NEVER engage without tapping the clutch until you get really good at this, for two reasons. One, imperfect matching will wear the brass surfaces of the synchro rings excessively and, two, you can break off a tooth which can destroy the entire transmission.

[Nevyn]

Originally Posted by Ptero

COASTING IN NEUTRAL WITH AUTOMATIC TRANSMISSIONS
Never do this. Always "coast" in drive. Why? Because the pump that lubricates the bearings in most automatic transmissions is driven by the rear wheels, not the engine. If you have been doing this, you should sell your car.

I have to disagree with this. My rear wheels have NOTHING to do with my transmission. 95% of us with autos have FWD, and rear wheels don't do a thing for the transmission.

[Ptero]

Alright, I edited out the references to what drives the lubrication pumps. Unless your car can be "flat-towed", an automatic transmission will not receive proper lubrication and could prematurely fail, so the point still applies.

See the thread Solution for coasting an automatic transmission: external oil pump http://ecomodder.com/forum/showthrea...pump-4149.html

But don't go to the trouble on a transmission that's been coasted extensively. It could already be near failure.

[LeanBurninating]

alright well that confuses me... when you coast in neutral versus drive on an automatic... your wheel speed stays the same or slowly decreases (you are coasting), its your engine speed that drops.

I would see your point if it was the other way around, that your tranny pump was run off your engine. But if your tranny pump is run off the wheels then it shouldnt matter what the engine/tranny is doing, drive or neutral... where am I wrong?