[RX135]

Without getting into the debate of whether fuel cut off happens or not, I am not sure if this actually means fuel saving.

Let's assume that fuel is actually cut off when you take the foot off the pedal in 5th gear. The vehicle has inertia and it will keep on driving the engine with requiring *any* fuel. But, it also means that the engine is resisting motion of the vehicle. (Remember the compression strokes, where you still compress air without exploding it with fuel now). So, vehicle speed will drop. Now if you want to cruise at constant speed, you'll have to inject fuel again to accelerate and achieve that speed, thus spending more fuel again for that acceleration.

So, question still remains about if this fuel cut off really helps in fuel saving? One way, I feel it can, is to keep the valves partially open during compression stroke in this special case to avoid deceleration. But, not sure if this actually happens or possible that way. Another case would be going downhill, when engine resistance is actually helpful for control and saves fuel too.

[Revvhead]

Originally Posted by ported_head

Umm...That would still be deceleration. Deceleration is not only by way of braking. You take the foot off the throttle in 5th, and speed would still drop, just not as quickly as while braking. But then, I'm sure you already knew that.

you're right ported. i did. however, thats all it says.it says foot off the pedal. doesit say for decelleration only?? it doesnt. it doesnt state decelleration or idling

and i have my doubts whether the baleno engine is tuned to have zero fuel input during decelleration
if thats the case, then it should hold true for decellerating in any gear, and would also kick in in between gear changes while acclerating through the gears...correct?

[swathyd]

Shifting to neutral on steep declines saves fuel. Does it saves even when the engine is not off,as no foot on throttle ?

[RX135]

Originally Posted by swathyd

Shifting to neutral on steep declines saves fuel. Does it saves even when the engine is not off,as no foot on throttle ?

Shifting to neutral will NOT save fuel, if the "Fuel cut-off" is indeed cutting off fuel supply to the engine. Once you shift to neutral, it will use minimal amount of fuel, which is required for engine idling. It will be definitely more than fuel cut-off.

[akroy]

Originally Posted by swathyd

Shifting to neutral on steep declines saves fuel. Does it saves even when the engine is not off,as no foot on throttle ?

Don't do this ever.... From high speed at 5th to Neutral is a very bad practice, there are threads in this forum, you can know why???

Now coming to shutting the engine , boss, never do that in modern mpfi engines, brakes and steering all depend on that...plus your ECU will be gone....basically not only u r risking your life but also u r spending more on repairing...

Abhi

[swathyd]

Originally Posted by akroy

Don't do this ever.... From high speed at 5th to Neutral is a very bad practice, there are threads in this forum, you can know why???

Now coming to shutting the engine , boss, never do that in modern mpfi engines, brakes and steering all depend on that...plus your ECU will be gone....basically not only u r risking your life but also u r spending more on repairing...

Abhi

Give me links to such a thread.
Never i shut engine as i know what happens with brakes.

[ported_head]

Ok, from what little I know. There is a condition known as DFCO (Deceleration Fuel Cut Off), which happens during closed throttle periods from moderate to high engine speeds. Fuel is cut off to prevent excessive emissions and to save fuel. Fuel metering is then resumed at some fixed RPM.

I think, DFCO depends on Engine Coolant temperature, Speed, RPM, TPS value, MAP value and a spark advance threshold. If foot goes off the throttle, spark advance would be reduced at some ramp out rate, so when it reaches a specified threshold spark advance degree, DFCO would be enabled.
Next, would be find a reason to come out off DFCO. The same above values would be referenced, and probably spark ramp up rate till it reaches a specified degree of spark advance due to applying throttle. Also a minimum value of RPM for which DFCO maybe disabled. Some EMS systems are also designed to identify clutch activity which would disable DFCO.

Now at idle if AFR is around 16:1, it is still higher than at DFCO when no fuel is sprayed. But I don't think DFCO condition is enabled at low RPMs.

This last bit, that I read at a Honda forum:
"Try this experiment next time you are out, like I did today; with your car in gear in a decelerating state of speed, hold the braking constant in your approach to the stop,,, AND in gear without depressing your clutch pedal, you will feel the DFSO disengage and go back to normal idle,,, right before you stick your clutch in to save it from choking itself off and dying. You will feel the cross-over to idle if you know your car well enough. It is kind of a slight jerking to idle. You will know what I am saying when you feel it.

So I hope you understood what I was conveying here. To answer more succinctly about whether to place the car into neutral or have in gear; HOLD IT IN GEAR all the way down to the point of cross-over (from DFSO) to idle,,, right before you come to a stop. You will know and you will have time,, (don't worry, it's a Honda,, they are pretty indestructible!) If you want to take it all the way,,, it is a fine line you will find on your own. And that way, you will have been using no fuel and your a/f numbers will remain constant and right where they should be!"

My understanding is very limited. Where are those tooner uncles when you need them?

[mclaren1885]

Originally Posted by Revvhead

its talks about leg off the accelerator in 5th. thats running buddy, not decelleration.

While you are at talking about common sense , does it also tell you that once you take your foot off the throttle your revv's drop and not rise (running you say).. Which equates to deceleration .. If you are not sure , why dont you try it out and give your observations .. Also , this theory is not only based on just googled links , its also from a person who is extensively working with ECU and standalone and I have posted a link to his post too ..

it says foot off the pedal. doesit say for decelleration only?? it doesnt. it doesnt state decelleration or idling

Maybe I do lack common sense (that I didn't quite mention that it amounts to deceleration so that you can understand) but in my subsequent links I have mentioned that DFCO stops once you reach near to idle revvs .. I suggest you do read once carefully before your post .. I am not asking you to accept the theory but it would help if you could put your POV in a nicer way and also stop talking about being slave to elf ..

[Vid6639]

I have a problem in my Alto in which if you back of the gas too fast i.e. like deceleration or you just want to coast to a halt the car gives a jerk.

I did some reading up and spoke to some guys at MUL and came to know of the IAC valve or Idle air control valve. This gets activated to keep the engine running when you back off the gas. the transition from leaving your foot off and the IAC valve kicking in gave a bad jerk.

heres some more info on it.

Idle air control valve overview - Crankshaft Coalition Wiki


ported am confused is the DFCO similar to the IAC valve actuation or is it different. there is a distinct jerk in my car which can be felt when you step off the gas. It clearly feels like something cuting off real sharp.

[Revvhead]

hey ported.
that was a pretty neat explanation, very useful
esp the last bit,(that was the bit that was in english anyway) i think i have felt it, but had no idea what it was, shall look for it now, and see if i can get that.

[v1p3r]

Maybe we ought to define how a car works. There are only three velocity change situations, acceleration, deceleration, and idle. When you take your foot off the accelerator, it's deceleration, or 'decelleration'. That's when the DFCO/DFSO comes into play, until it hits idle or near idle engine speeds.

Originally Posted by mclaren1885

talking about being slave to elf ..

I think it's a fetish some people have. All the salt in the air, Queen of the Arabian Sea and all that jazz...

[Jaggu]

i have noticed the FE indicator jump to above 20 kmpl mark on Crv when i lift off the accelerator and car is moving just on momentum gathered, press the pedal very nominally for acceleration and the figure drops to 10, and while accelerating to overtake and all it will drop to 4 kmpl.

used to be entertaining initially lol

[Revvhead]

yo macca, i have bben through your links, and they are good info sources,
howeverm the best info was on the insight site, and thats a far more advanced car than the baleno, to put it mildly.

secondly, and just to be a little pedantic, adding a link, really doesnt mean that you have gone and explicitly mentioned

as far as my pov goes, its that engines dont run without fuel!!

@v1p3r, you are right, the sea can have an effect, yes i did spel decelrate incorrectly ( i was suspicious abt it from the start), and whats life without a few fetishes.

[mclaren1885]

Originally Posted by revvhead

in fact sam is gonna gonna kill me for this, but Sam has attempted this theory in his younger days, and apparently even his battered old van refused to run on an empty petrol tank.

Infact I remember sam telling me that his van ran on no engine oil .. I am quite sure of this .. And it did run for an extended period of time without seizing against all laws of IC engines that you and me have learnt ..

as far as my pov goes, its that engines dont run without fuel!!

You seem to be confident .. Then why this ??

shall look for it now, and see if i can get that.

secondly, and just to be a little pedantic, adding a link, really doesnt mean that you have gone and explicitly mentioned

Having mentioned this on post number 25 :

Originally Posted by mclaren1885

There is also something known as DFCO (deceleration fuel cut off) .. Try and do some research on it .. I am too lazy to put up the links ..

Its obvious that I had done my homework by reading and asked those who are interested to do the same .. Its like saying , read up on Newton's Third Law which doesn't necessarily require I mention "For ever action there is an equal and opposite reaction" ..

I suggest you stop picking on such a silly issue that I was not clear enough .. Its but obvious that you need to use CS and relate the matter in the links to what I intend to say ..

[Psycho]

DFCO (deceleration fuel cut off) exists on MPFi engines please put on a AFR meter and see. To travel longer in gear while decellerating be in a higher gear.