I'm NOT making another gas thread, I just have a simple question.

I've heard recently that while driving, while in gear if you're off the throttle then your injectors are off (thus not using any fuel) and the motor is just being turned over by the car in motion (wheels moving tranny and turning over engine.)

Is this true?

Meaning that if you coast in 5th gear more often rather then, say, shift to neutral, that you use less fuel? Shifting to neutral would require the car to idle on its own (using fuel.)

Just curious, is all.

 

I have no idea, and I've often thought about this (for engine braking). Would a sensor detect a super-lean exhaust and throw a code if no fuel is injected?
Jae

 

During both coasting at idle and cruising the O2 sensors are used to maintain a 14.7:1 air/fuel ratio, or close to it. The demand for air for the engine is higher at 3000 RPM cruising than at idle. So to maintain this ratio, the ECU will compensate with more fuel. So to answer your question, the motor will always comsume less fuel at idle than at any other RPM, regardless of the throttle position.

 

dang, and I thought I was onto something

thanks

 

dang... naw, forget it, i'm still gonna engine brake.
Jae

 

I was under the asumption that in gear, off throttle you will use no gas, plus recycle some of the un burn gas back to the tank.


How could the engine supply gas to maintain that A/F ratio? If it gives gas to do so, then that would cause acceleration wouldnt it? Off throttle and in gear, there is no air going into the engine, and no spray of fuel.

 

Yep yep, this is a very good question. I had always assumed that when you are in gear and off the gas (i.e., the car is pulling the engine) you were using a minumum of fuel, but then a friend and I hooked up a Palm Pilot with Auterra software to my car and we drove around. According to the ECU, instantaneous fuel economy would always be 99 MPG (max reading) when coasting in neutral, whereas it would read only 50 or 60 MPG when coasting in gear. His explanation was the same: even though the engine is being pulled by the car when in gear, if you're at 2,000 RPMs the ECU has to give the engine more fuel to maintain the proper A/F mixture. Nismology's explanation above says it best.

As a result, I pop it into neutral all the time, whether I'm going down a big hill on the highway or just coasting up to a red light.

 

Well I'm glad this thread got started. I thought I was saving a little gas by in gear braking and coasting. Well I'll drop in to neutral a whole lot more now with this new info.

Is it just me or does that sound off that, the engine gives fuel while your coasting, sounds like fuel being given to be burn should cause acceleration but, aparently not.

 

That's impossible. Even at idle or negative load the engine is still on, spark plugs are still firing, combustion is still happening, so fuel is still being injected. If what you said above were true, then the engine would make almost no noise, which we all no to be false.

That's also impossible. Air is going into the engine even at 0% throttle (idle). The engine can't run without air, and it can't run without fuel.

This brings up a question though; does the engine control air flow at all or is that entirely mechanical (driver-operated throttle plate)? I know about the idle-air control valve which allows the ECU to rev the engine higher on cold starts, but is the ECU otherwise at the mercy of the driver's desired throttle (air) position? If so, then when coasting in gear, wouldn't the engine be running really rich, since the throttle is closed but more fuel is apparently being injected?

 

No, fuel is going to the engine at a faster rate since the engine is turning over at a faster rate. It only makes sense that the amount of fuel needed to supply the engine at idle is not sufficient to supply the engine at, say, 2000 RPM, no matter how much load is on the engine. Even with negative load on the engine (car pulling engine), more fuel must be supplied at 2000 RPM or the engine would die and REALLY slow you down fast. Thus the ECU adds enough fuel to compensate, and that fuel is wasted since it's not affecting the motion of the car; the fuel is just going to spin the engine at the higher RPM that the speed dictates.

An analogy would be riding down a hill on a bike and pedaling just fast enough to prevent the ratchet noise from the rear hub, meaning you're applying no postive or negative force to the wheels yet you're using energy to move your legs. Maybe a better analogy would be having someone push you backwards on a bike so the pedals move your feet, so you have to use your muscles just to keep your feet on the pedals or they will fly off, but you're not powering the bike at all. Haha, hope this doesn't confuse anyone more....

 

VQ your right, My mind was taking the adding fuel down another track.

All this in gear braking I've been doing, has been doing the opposite of helping me.

 

My wideband O2 goes off the scale at 20:1 when I coast. This means that the ECU is metering enough fuel for an idle situation even though the engine is pumping more air than it needs to idle. But for idle and cruising it tries to maintain 14.7:1 .

Fuel does go into the engine at a quicker rate, or higher frequency when coasting but that does not mean that the ECU meters more fuel than it would at idle.

 

So which are you saying, that more fuel is used when coasting than at idle, or that the same amount of fuel is used when coasting and at idle?

 

I not only have a wideband but I have a display monitor on my car as well. Now when coming to a stop or slowing down I found out that leaving the car in gear saves fuel. For one my wideband shows leaner than 20:1 and my injector are at 0.6msec (which is practially 0%). However it stays in this condition till around 1200rpm, cause after that the ecu goes into idle fuel trim (1.8-2.4msec). Then again I do have a JWT ecu but I do know the stock ecu was simular.

 

The latter. He's saying that at ZERO load (closed throttle) the same amount of fuel is being metered regardless of RPM. Interesting.......

 

OK, so SR20's and Nismo87SE's posts are compelling enough to reopen the debate.

Can anyone with A/F meters or other electronic equipment installed check to see what kind of instantaneous MPG and/or instantaneous fuel consumption you get when coasting in neutral versus coasting in various gears? I'd like to settle this once and for all. If anyone has time to do a little online research we might be able to find the answer there as well.

 

Nismo87se's explaination is really interesting and makes sense to me. I get 25mpg engine braking in 5th gear.
Jae

 

I am also interested. If anyone could do this it would help put this issue to bed.

 

Engine braking does not conserve as much fuel as neutral coasting in most cases. Because when there is no engine brake load on the drivetrain you can coast much farther, meaning you can go farther on the amount of 'idle' fuel.
I have never been able to make my fuel computer show 99mpg when engine braking, because it simply won't retain enough speed for a long enough period, however I HAVE made it show 99mpg when coasting in neutral.

 

Now that sounds like the answer.

You definitely coast farther in neutral. Unless the engine could use even less fuel than idle when coasting in gear, which is doubtful, idle in neutral will give better gas mileage since the engine is not dragging down the car's momentum.

 

I'm almost 100% positive that the FSM says that when the throttle pos is at 0%, no fuel is injected if the engine speed is above 1500rpm. It wouldn't be a lean condition if no fuel is injected. That's why SR20DE's O2 meter is off the scale, because the A/F mixture is infinite when there is no fuel. If you want to be uber technical, you could say it's not defined (division by zero).

 

I would like to know how an engine can run when no fuel is injected into its cylinders.

EDIT: I did some research and came up with this very interesting thread on nissanmurano.org. Looks like their FSM says the same thing, but you can see the same debate raged in their thread:

http://www.nissanmurano.org/forums/p...013&perpage=22

From another Nissan board, someone says "There are 4 kinds of main fuel cuts: Speed fuel cut, over rev fuel cut, fuel cut in neutral, and deceleration fuel control. They are all controlled by the injectors. And not all Nissans have these 4." http://northwestnissans.com/board/ar...p/t-22828.html

It seems clear that the fuel pump is never shut off and so the fuel pressure remains high. My question is, if the fuel injectors are not being "pulsed", could a small amount of fuel still going into the cylinders (sucked in by the vacuum formed by the opening cylinder)? The bottom line is, there has to be some fuel in the cylinders to combust or else the engine would not be running. Furthermore, I assume that the spark plugs are still firing, so why would they fire if there was nothing to ignite?

 

The kinetic energy of the car/driveline makes the pistons go up and down rather than the gasoline. You could coast with the transmission in gear and turn off your ignition. That would mean that there would be no spark, no fuel, only a little bit of air going in and out of the engine. The engine would still run because the car's motion is keeping the engine running.

 

Obviously, but then why does the engine still make a lot of noise? A lot of the noise of the engine is produced by the combustion, not just the friction of the moving parts.

 

That theory is flawed. I've left my car in gear while coasting and shut the motor off. I have an exhaust system installed so i can clearly hear when the car is running or not. When i leave the car in gear and coast with the car on, there is some exhaust noise. But as soon as i turn the car off i hear nothing, because the car is essentially just pumping air at that point and there's no combustion occuring.

 

We've have two things to prove that there is no gas being injected into the cylinders, the FSM, and the O2 sensor showing a bunch of O2 in the exhaust. What more proof do you need?

Alternator, a/c, and fuel pump will make noise when the ignition is on.

 

You'll still hear intake/exhaust noise if you shut the engine off and coast in gear. Try coasting in gear with the engine off and floor the accelerator, thus opening up the throttle. You'll hear the intake and exhaust make noise even though there's no spark and no fuel. You might get a CEL because all that air flowing through the engine will cool down the heated O2 sensor.

 

Try not to double post like that, it's annoying...

Anyway, my apologies. I just read the FSM and this is what it said:

"Fuel to each cylinder is cut off during deceleration or operation of the engine at excessively high speeds."

You were right...

This was from a 1996 FSM btw...

 

no matter what, if you put the car in N, and coast, you will be saving more gas then just coasting in gear at whatever rpm.

think of it, not only when you are in gear are you using more fuel, BUT, since you are in gear, the car is slowing down a lot faster then if you were in N.

thats my .2 cents.

 

Yeah, I think that coasting in N while the engine is idling is still more fuel efficient than remaining in gear even if no fuel is being used because this slows down the car a lot more. Proof of this is the MPG computer. It calculates gas mileage based on the amt of fuel used over a certain distance. Doesn't get any more complicated than that.

 

Have you read ANYTHING in this thread?? Cripes.

I just posted info from the FSM that said that when coasting in gear there is no fuel going to the injectors. But when you're coasting in neutral the ECU goes to idle fuel trim.

You are using ZERO fuel in gear with 0% throttle. The FSM says so. Besides, the question isn't "Which car will coast farther...the car that's in neutral or the car that's in gear?" The question is "Which car will consume less fuel?" and the answer is the car that's in gear. During deceleration in gear the fuel is cut. While coasting at idle, the ECU goes into open-loop idle fuel trim, which means fuel is being injected.

 

That was mentioned in the Murano thread I posted and I saw that in the 1998 FSM too. But I refuse to believe it means there is NO fuel going into the cylinders when you're coasting in gear for the reasons we've stated. My suspicion is that a tiny amount of fuel is still getting into the cylinders to maintain combustion. The injectors are not injecting but the vaccum in the cylinder could possibly suck a small amount of fuel through the injector since we know that the fuel pump is always on.

 

If the question is which car will consume less fuel then the answer would be, by your definition, "That depends". If coasting for very short distances, the engine brake method would be the answer, but that would be such a tiny difference that it would never et noticed anyway.
But when coasting for longer distances, which I do when I see a light turn red far ahead, the neutral method would be the answer. But even that takes a couple of seconds for the engine to spin down.

But whichever answer is correct and for which situation I think it's all a wash and can't be noticed anyway. The real answer would be "Just find a way to take your foot off the gas". And that is exactly what I have always done anyway. I will exceed the interstate speed limits by a wide margin when I can but I have always failed to understand why anyone would want to hurry up and wait at a stoplight. The very moment I see a red light I put the thing in neutral, granted I have enough inertia to make it.

 

Have you read the thread?? Cripes. His ultimate question is whether it is better for fuel economy to leave it in gear and let it coast with a ton of drag or be in neutral and use a little gas in idling.

In the end, letting it idle in neutral is better because it doesn't put such a large drag on the car. Let's say that you and I are at the top of a hill going 70mph. You leave the car in gear and the hill is steep enough that you maintain your speed going 70mph at the bottom. Me, on the other hand am going let's say 90mph. I can keep coasting for awhile before decelerating to 70mph meanwhile, you're burning a lot more fuel just maintaining your 70mph speed.

In other words, the amount of added kinetic energy you get from letting it idle in neutral is "worth" it.

 

Hmmmmm...that could be a possibility. But i guess to bottom line this thread, coasting in gear uses less fuel than coasting in neutral since the ECU goes into open-loop at idle.

 

[QUOTE=nismology] The question is "Which car will consume less fuel?" and the answer is the car that's in gear.QUOTE]

no, you are wrong.

this means that if you're either in N, or if you are in gear, they both do not use fuel. so, why keep the car in gear to coast, when you can pop it in N? they both use no gas, but you will travel longer just coasting in N.

SO, imagine this, you have the car in gear, and you are coasting.

I have my car in N, and i am coasting.

we are driving right next to each other. you start to fall behind because you are in gear. you have to give it the gas to catch up to me only to keep falling behind.

so I have used no gas, and you used gas to keep up to me. Sooooo.... which car has consumed less gas now... Hmmmm?

 

damn, i waas too slow. I hate being at work

 

Wait...are we all talking about instantaneous fuel comsumption, or fuel consumption over time? Because that's what's causing alot of the confusion in this thread.

 

[QUOTE=silvermax2k2]You use gas when you idle in neutral, not very much though.

 

Well now you need to think a little harder. We are trying to determine which has the best fuel efficiency, overall, in real world circumstances. We are only concerned with deceleration here.

Assuming that an engine being pulled by the car (coasting in gear) uses LESS gas per second than an engine that is idling (coasting in neutral), the following is true:

—If you are coasting up to a light and you're gonna have to brake soon anyway, leave it in gear to help slow down. You will use less gas getting up to the traffic light than if you were coasting in neutral, although you will get there more slowly. But that doesn't matter since it's a red light.

—If you are coasting down a very steep hill, leave it in gear since gravity is more than enough to pull the car and the engine.

—If you are coasting down a less steep hill or a long hill on the highway, put it in neutral since gravity will not be enough to overcome wind resistance and car/engine friction. Even in top gear, you will slow down fairly quickly. You may or may not get better gas mileage down the hill than if you'd left it in gear, but you will maintain speed. If you leave it in gear and slow too much then you have to apply gas anyway.

—So for most situations around town, I'd leave it in gear.

All of the above is based on the assumption above. I don't think we've reached a conclusion yet, despite what the FSM and other say, since we have some experimental evidence that indicates otherwise. We need more evidence before I'll be satisfied.