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DFSO (deceleration fuel shut off) strategies and explanations

37K views 127 replies 28 participants last post by  nyuk98gt 
#1 ·
Hey friends,

I'm looking to find out every thing I can about the DFSO system in my new Focus. I have done some searching on this forum and have found bits and pieces, but perhaps a single thread on the topic can combine all the knowledge this forum holds into one place.

I am trying to create some hypermiling strategies and would like to know the speeds at which DFSO turns on and when it turns off. Also, I like manually turning on the AC button when I am coasting to the light and turning it off when the light turns green. I want to know if having the AC on as I am coasting down to the light will keep the DFSO from activating.

Thanks in advance!
 
#3 ·
Thanks for the data point. And I realize that owning a scangauge is the way to go but not all of us have one of those. I was about to purchase an ultra-gauge until I learned of the battery drain problem.

Perhaps those with scanguages and technical knowledge on the DFSO system could share their knowledge in here so we all benefit. Can a person "feel" when the DFSO is activated?
 
#10 ·
Thanks!

Do you notice the DFSO activating only at particular speeds; for example, only after decelerating from 50mph but not 40mph? Or does it turn on as long as your foot is off the gas and you are over 25mph? Also, have you ever seen it on under 25mph? And have you found the 3 second delay after lifting off the gas to be true in your experience?
 
#5 ·
..........Also, I like manually turning on the AC button when I am coasting to the light and turning it off when the light turns green. I want to know if having the AC on as I am coasting down to the light will keep the DFSO from activating.

Thanks in advance!
Just a heads up, doing this very frequently is actually quite hard on the actuator for the A/C. Whatever money you're trying to save on brakes by doing this, you may end up paying 10x for new A/C parts. [poke] Rather than using cheap brake pads, you're using an expensive climate control component to slow the car down.
 
#6 ·
I think he's trying to save on fuel, not brakes. But this is a very good point. You could never in a million years save enough fuel by activating/deactivating the A/C on decel to make up for the $1500 it will cost to replace the compressor and/or EM clutch.
 
#7 ·
The way I understand this system is the fuel system shuts down any time you're coasting in gear with no accelerator use. Since it's in gear the rolling of the tires keeps the engine cranking.
 
#8 ·
The thing I can't wrap my head around with this is that if there's no fuel being injected then there's no combustion in the cylinders while coasting - wouldn't the engine sound be much different while coasting? Even no engine noise at all or just some kind of mechanical whirring sound?
 
#21 ·
Well, so far I've found that its pretty intermittent on when it happens. For instance, I let off the gas at 30mph and it goes into loop but the next time it doesn't. On one occasion it stayed in loop from 40mph down to 5mph. I should be able to test it above city speeds this weekend but so far its nothing spot on.
 
#34 ·
Very interesting! I know there are several variables the system takes into account before DFSO activates. Is there any chance you hit the breaks on the one occasion at 30mph where it didn't come on? Does it seem to wait 3 seconds to come on after you let off the gas every time?

I would love to know more details about it going from 40 to 5mph!! That is amazing. I always heard it shut off around 25-30mph.

Thank you so much!
 
#27 ·
I suppose you believe that the engineers write it, perhaps? There are technical writing specialists, who work in conjunction with lawyers, to ensure the whole manual is completely fool-proof and lawsuit-proof.

The engineers tell the team how the things work, the design limits, etc... and the technical writers and lawyers go to town with it.

Can you imagine the lawsuits if something described in the manual wasn't clear to the bone and someone got hurt? We're talking about people who sue because there isn't a sign on a steaming cup of coffee indicating it's hot... [facepalm]
 
#31 ·
How cold would your AC low pressure cycle tubing (evap) have to be in order to cause an ice buildup with 90 degree air blowing over it? Freezing tends to happen when the fan itself fails. Which is again, why I suggest that it's more of a safety protocol.

In any case, we're not getting anywhere. At this point, even if I'm dead wrong, it doesn't change the fact that turning the AC on and off every 15 seconds while you're driving isn't going to do it any favors. Call and ask a few shop foremen if they think that's a good idea or bad, if you don't want to hear it from me. It's your car, and I don't care enough to argue it beyond this.
 
#32 ·
Holy smokes Altis, you are so wrong about A/C its not even funny.

There are some A/C systems that continuously run, but those are systems that use a variable displacement compressor. Most compressors used are fixed displacement and are in fact cycled on/off through normal use.

If the A/C compressor didn't cycle on/off, then why would Ford have an A/C system called the "CCOT" or "Clutch cycling orifice tube" setup? Since the orifice tube is a fixed restriction, they need to cycle the A/C compressor on and off so the evaporator outlet doesn't freeze up.

The 2012 Focus uses a TXV system. The TXV is what creates the difference between the high and low side and it opens or closes to allow more or less refrigerant flow through the evaporator. (again, to keep the evap outlet from freezing up). When the temps get too low, it will cut the compressor off until things return to an acceptable point to turn the compressor back on.

Yes, evaporators can freeze up even in 90 degree weather. Place your hand on the evaporator outlet next time your A/C is running. it gets damn cold if the system is operating correctly.

And what the hell is an A/C radiator fan? Are you talking about the engine fan in the front of the car that draws air through the condensor.....or are you talking about the blower motor in the dash? There is no other fan.

Here's a tidbit from the service manual for you to ponder....

"The evaporator discharge air temperature sensor is an input to the BCM and is relayed to the PCM over the HS-CAN. The evaporator discharge air temperature sensor prevents icing of the evaporator core by measuring the temperature of the airflow immediately after the evaporator core. An accurate evaporator temperature is critical for compressor engagement. The PCM uses the temperature measurement to regulate the on and off time of the A/C compressor to maintain the evaporator temperature within an acceptable temperature range.

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM does not allow the A/C clutch to engage."
 
#35 ·
A TXV control valve means the AC doesn't have to be shut off in order to modulate the coolant.

You're right that this is a fixed displacement compressor, which means its either on or off, but that has nothing to do with the fact that it can, and does, run for extended periods without cycling on or off.

The conditions that generally cause the evap to freeze are a fault in the system, most notably the fan that pumps air into the cabin, and thus over the evap... Or certain rare conditions like having the AC on cold, leaving the car off for a short time, then starting it back up... This can cause it, but the system detects it and acts accordingly.

The compressor doesn't need to be constantly cycled to prevent ice buildup. It only deals with it when it needs to. The things you mention are protective elements that are used as needed, but it does not prove that the compressor needs to be continuously cycled between on and off. Usually minor adjustments in the pressure valve is all the adjustment it requires.

The AC has its own radiator, with a fan. Look into how AC units work and you'll see that the high pressure freon must be cooled in order for it to be colder than ambient temperature in the low pressure (evap) circuit. When you're moving, the fan is usually off, but when you stop, you can often hear the fan kick in. The engine fan operates in a similar manner.

OP, it wasn't clear how often you were doing it when I first cautioned... My point was simply that overdoing it isn't good for the compressor. Doing it here and there is unlikely to have any effect while you own the car. I just don't recommend doing it every time you slow down.

I apologize as I did not mean for this thread to get hijacked for a small word of advice that one can consider for themselves.
 
#33 ·
Lots of good discussion here. I want to clear something up; I didn't ever say I was pushing the AC button "every 15 seconds", it's more like half a dozen times during a 15 to 20 minute drive. When the car gets uncomfortably warm, and the fan isn't enough, I press the AC button during deceleration hopefully taking advantage of the DFSO to get some "free" cold air. If things are really bad I will turn it on at cruising speeds as well. What I want to avoid is having it on while accelerating. I always heard that traditional AC systems cycled on and off as part of their normal function so I aSSumed pressing the AC button a few times during a normal trip wouldn't harm anything. I actually thought it might be beneficial because I too have heard of the "cycling to prevent freezing" issue. I figured the AC system is cycling a lot more than I would ever be pressing the button.

Any more opinions on whether or not I am abusing my AC system since we have more participants in this thread?
 
#39 ·
Yes, as the technicians have pointed out, some level of A/C compressor cycling is normal, and has been in many systems for decades. The level of use you are describing shouldn't have an appreciable effect on the life of the clutch. This is just my opinion, you should probably consult a lawyer though.

I would like to give you props for thinking outside the box on this one. What you are doing is very similar to what some hybrids do to assist in recharging batteries, using the vehicle momentum that would otherwise be wasted during braking to accomplish a task. In your case of course to run your A/C compressor.

Hybrid powertrains are optimized to accomplish this task. I am not sure how efficient the DCT is with engine braking, but the main objective would be to stay in gear while slowing down as long as possible. Any neutral condition (i.e. between downshifts or during coasting while slowing to a stop) will result in the engine itself compensating for the extra compressor load, which begins to negate the advantages. This might be better accomplished by downshifting with the toggle switch. Honestly I haven't had my car long enough to really play with the DCT.
 
#41 ·
I'm an engineer and I've had to write a user manual before. Obviously it's checked by others - but I thought it was weird that they would have me (without some type of technical communication degree) write the actual manual itself.

But then again, I'm not surprised. Even when I drunk text I include apostrophes and semicolons.
 
#42 ·
Haha I'm the same way [grinking] Again it depends entirely on the intended audience. With a heavily mass produced car sold to millions of people, it has to be crystal clear so there isn't any misinterpretation to be had.

I agree with the above post that it does make great use of otherwise wasted energy. It's just not designed to be used like that.

As for my original point, which I did not know the frequency of the claimed use, I still maintain that doing it every time you come to a stop is more use than its intended for... But since you're only doing it here and there, I'm sure it will be fine. Personally, I'd reserve the practice for long gradual stops, such as coming off a freeway or stopping from high speeds. Just maybe not for every light in a downtown core ;) as posted above, the revs need to be kept up in order for it to be effective at saving fuel.

I used to do this all the time on my mazda3, but I'll admit I can be a little OCD when it comes to saving a bit of fuel ;) it was automatic, and I would put it in manual shifting so it would go into 4th at 65 kmh instead of 70 in Drive.
 
#45 ·
Just to add a new bit of confusion, and relate this AC back to the OP... I did a few tests in my car on the way home.

I have the manual gearbox, so I can hold the gears right down to idle in order to compression brake.

Now, I did several compression braking runs with the AC on and OFF. I did a bunch where I turned the AC on partway through the compression brake. I also turned it on and off a bunch while driving at a constant speed on a level road, in order to feel the effect of the AC compressor on the compression brake.

Here's the thing... in most cars I've owned (meaning all except the VW which had a variable displacement AC), you can really feel an obvious moment where the AC engages. They all tend to slow down much quicker with the AC on which compression braking.

I had none of that. No indication in any scenario that the AC was on, and only a small sound to indicate the compressor did in fact kick in. The car slowed at the exact same rate whether the AC was on or off. Only acceleration was a bit slower. Cruising at a constant speed and turning it on or off with my foot at the same position (and about 2K RPM), no change in speed at all.

Curiously, I looked into it a bit. Turns out many new, drive-by-wire cars actually add a consistent amount of throttle while the AC is on, in order to negate its effects. For example, if they find that it constantly takes 5% more throttle to make the AC unnoticeable while driving, then they program the ECU to add that 5%. The purpose of this is so the driver doesn't have to feel the AC's effects while driving.

If this is true (which could better be confirmed with the scangauge tool, I believe), then turning the AC on while slowing down actually will make no difference in fuel consumption, as the ECU will be adding that small amount of fuel anyways in order to compensate.

My '04 Corolla and '06 Civic both had a very pronounced effect from the AC being on, especially when engine braking. I didn't get this at all on my Focus.

Could be another aspect to look into. Perhaps it has a clear effect on DFSO.
 
#50 ·
If this is true (which could better be confirmed with the scangauge tool, I believe), then turning the AC on while slowing down actually will make no difference in fuel consumption, as the ECU will be adding that small amount of fuel anyways in order to compensate.
It has already been confirmed by someone with a Scanguage tool that DFSO activates during throttle-off deceleration WITH the AC on.
 
#46 ·
Altis, I was wondering about that hoping someone might chime in.

I know that even some vehicles in the '70s had throttle solenoids that would engage extra throttle position for the carburetor (at idle position) as soon as the compressor was energized. I suspected that might be the case with the new cars also, albiet through programming via the drive-by-wire as you stated.

Something to ponder....
 
#51 ·
I've been playing around with this and monitoring the Scanguage and noted the following:

After achieving at least ~30mph and lifting off the throttle, DFSO engages after a few seconds and the "9999" readout remains until decelerating to ~20-25mph. The readout lag prohibits getting an exact point where DFSO stops.

If you are coasting without DFSO and accelerate (e.g. going down hill), DFSO does NOT engage. I observed this going from about 20mph to 35mph accelerating down hill off throttle.

Hope this helps.
 
#57 ·
Hi, I don't where this theory that 'Many of those cars you can feel the drag of the AC, and it's pretty consistent.. and I've driven for a living for 6 years, in 80 different cars" comes from but any car I have driven in since 1993 with CC has had it's compressor switching on and off. You can hear the clutch clicking, feel it through the throttle pedal and detect a change in the note of the engine as it frequently cycles. The Focus is no different to any other even our 2010 Mazda 2.
Chris
 
#69 ·
I have a few friends that are engineers at Ford. One of them is a mechanical engineer working on engines.

I spoke to him at length about Ford's aggressive deceleration fuel shutoff feature. This feature is a perfect complement to the hypermiling technique of "driving without breaks".

The fuel shutoff feature uses more than half-a-dozen variables to calculate when to shutoff the fuel going into the engine. The main three are: throttle position/ brakes applied, engine RPM, and load (A/C, defrost, read window defrost, all other electric current draw: lights, radio). The other variables are items such as engine temp, vehicle speed.

For example, when coasting down from any high-way speed (foot off the gas pedal), or even city road speeds, the engine computer will shut off the gas to the fuel injectors. When the vehicle nears stopping speed (say at a traffic light) the computer then will turn back on fuel to the engine to maintain a minimum of 600 RPM (the standard idle speed). So do not use the Advanced hypermiling techniques of turning off the engine when coasting down a long hill, or even putting the transmission in neutral to coast. The engine computer shuts off fuel (and turns it back on) way faster and safer, than any human could.

The other feature we talked about was the variable valve timing for the two cam shafts. This is not a switch that has two settings: economy or performance. The engine computer can adjust the timing of both the intake and exhaust valves (independently) to provide a (relatively) infinite range of timings as necessary to respond to throttle position. This provides both economy and performance from the same engine. There is no need to use a high octane fuel to attempt modify valve timing for better gas mileage, with the Twin independent variable camshaft timing (Ti-VCT), the engine computer determines the best valve timing. I asked how I could know if the computer was using maximum economy timing, he said there was no way for me to know, he recommended I use the eco-mode in the display.

The other hypermiling item I noticed is that the Tires: Continental contiprocontact, have a maximum 51 psi. I would guess that filling the tires that full would lower the rolling resistance significantly. (I personally would not fill the tires that high during winter/snow season.) The Focus Super Fuel Economy package uses four items to achieve the extra 2 miles per gallon (highway only): low rolling resistance tires, active grill shutter, rear spoiler, and aerodynamic wheel covers. From what I have read, the low rolling resistance tires probably make up 75% of the 2 MPG improvement.

http://media.ford.com/article_display.cfm?article_id=33942
 
#70 ·
The other hypermiling item I noticed is that the Tires: Continental contiprocontact, have a maximum 51 psi. I would guess that filling the tires that full would lower the rolling resistance significantly. (I personally would not fill the tires that high during winter/snow season.) The Focus Super Fuel Economy package uses four items to achieve the extra 2 miles per gallon (highway only): low rolling resistance tires, active grill shutter, rear spoiler, and aerodynamic wheel covers. From what I have read, the low rolling resistance tires probably make up 75% of the 2 MPG improvement.

http://media.ford.com/article_display.cfm?article_id=33942
At 51 psi, I would imagine that you would lose whatever fuel advantage you gain at that pressure to the cost of replacing the tires early due to wearing the center of the tread prematurely.

The SFE package really only has 3 exclusive items (tires, spoiler, wheel covers) now that the shutters are included on all Focus models after the "Job #2" updates on March 5th.
 
#71 ·
the active grill shutters and the spoiler are available on other trims, so it really is just the tires and wheel covers that are the difference in the SFE.
 
#77 ·
I will be listing one for sale in a few weeks. Don't own any more obdII cars (functioning ones) lol.

BUT about DFCO...... really want to get a good idea? get a wideband. I cant imagine it would be too different but on my 22 year old car, any time the engine is at or near operating temperature, it will cut off fuel as soon as I let off the pedal; load or no load, as long as I have been ON the gas then let off, it will shut off (defaulting the wideband to 22.4:1 afr) It does not do this at idle or cruise as it shouldn't

I'd like you guys to define coasting a bit better with these cars. Can you feel the whole thing slowing down with the engine or does it just coast like it is in neutral?

Wideband has helped me with fuel just a bit.... I have learned not to throw it in neutral and coast but rather keep it in gear to slow down. idle and cruise AFR should be 14.7-15.1. Anything more and it has a chance to burn valves. Under acceleration it can drop to 12 or 13. Deceleration it will default to the max reading which makes it super lean.
 
#78 ·
BUT about DFCO...... really want to get a good idea? get a wideband. I cant imagine it would be too different but on my 22 year old car, any time the engine is at or near operating temperature, it will cut off fuel as soon as I let off the pedal; load or no load, as long as I have been ON the gas then let off, it will shut off (defaulting the wideband to 22.4:1 afr) It does not do this at idle or cruise as it shouldn't
The 2012 has a UEGO (wideband) as the primary sensor.
 
#79 ·
BTW, a scangauge isn't necessary to determine open/closed fuel status.

Put the ICD in test mode and monitor FUEL FLOW.
It is a rolling counter. The counting speed is proportional to the fuel consumption. When the count stops, the injectors are off.

Simple, eh?
 
#81 ·
Just one more data point on the A/C cycling issue...

Lets say it's hot out and you have the A/C on but your stopped and in park. Do you notice that the engine rpm tends to jump between two different rpm's? Now why would that happen if the compressor was always on?

Second point, if DFSO is activated when decelerating from speed with no brake applied but deactivates sometime before you come to a complete stop the idea is that is better for mpg than putting the car in neutral while coasting because the engine needs to burn fuel to keep the engine running in neutral. But, if DFSO deactivates at, say, 20 mph then shifting into neutral at that point would have no impact on mpg because the DFSO had already been deactivated -- right?

Brian
 
#84 · (Edited)
Fuel cut off works anything after 1100 rpm but not below 750 rpm , you must be above .239 load to enter , throttle must be closed for 2 sec to enter and more then 10 sec to enter extended fuel cut , Vehicle speed must be above 35MPH

A/F target for fuel cut off is .899 lambda

All of the above seem to change slightly with ECU codes , your not going to save any fuel by playing in the fuel cut area

Tom
 
#86 ·
Fuel cut off works anything after 1100 rpm but not below 750 rpm , you must be above .239 load to enter , throttle must be closed for 2 sec to enter and more then 10 sec to enter extended fuel cut

A/F target for fuel cut off is .899 lambda

All of the above seem to change slightly with ECU codes , your not going to save any fuel by playing in the fuel cut area

Tom
Since the fuel must be turned on before the vehicle comes to a complete stop at what speed would that be. Some say it's about 25mph but that's probably higher than it needs to be. As you're coming to a stop the transmission, even in first gear, would have to activate the clutch or the engine will die. But, if you activate the clutch at, say 5mph, and the fuel is cutoff the engine will still die so the fuel must be turned back on before the clutch is engaged.


Brian
 
#91 ·
If the fuel cutoff were based on vehicle speed, then how would it be effective in each gear? 5th is at idle around 40 km/h or so, where 3rd would be around 2k rpm... so would the cutoff be enabled or disabled? It only makes sense that it's engine speed based, not vehicle speed... although perhaps there's an upper and lower limit during which it won't activate at all (say, 10 km/h low end and 150 km/h top end, for example).

I noticed the DCT on loaners and fleet cars always keeps the revs up when coming to a stop (so that it's ready to go if you press the gas), until you're stopped. I bet the fuel cutoff is less effective on the auto than manual.
 
#92 ·
Fuel is turned back on by

RPM below 1100
Load at or below .239
Vehicle speed below 12 mph

And there are a few other things , there is changes to the above depending on time , engine temp and many other things , which ever comes first will turn the fuel back ON

Yes VS is part of the fuel cut

Tom
 
#93 ·
Thanks Tom, good info...

It makes sense that when the engines cold the fuel would remain on. The one factor that seems odd is the load at or below .239. Should it be more than .239? OTH, if load is determined from the engine then with fuel cut shouldn't the load be zero? Or does the engine being driven by the wheels constitute the load?

All of that makes sense except the load...

But, what that says is no matter what, when the speed drops below 12mph the fuel is turned back on. 1100 rpm is about what the car would be at in 3rd gear doing 12mph -- in first gear 12mph would be about 2400rpm and 2nd would be about 1500rpm so that suggests the down shift schedule isn't very aggressive.


brian
 
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