Author Topic: Deceleration Fuel Cut Off (Read 1678 times)

Jocko · « **on:** July 08, 2019, 04:05:03 PM »

People keep asking me how I consistently get such good fuel figures. The method I found returns the best results is by using Deceleration Fuel Cut Off (or DFCO as it is usually referred to). This is a system, built into all modern petrol and diesel cars, with a view to cutting emissions. My 2006 Jazz has it and I am sure it has been a function of many Fuel Injected engines before that.
We all know that you can take your foot off the accelerator, while travelling in gear, and the car continues forward, slowing gently. This is when the engine designer makes use of DFCO. Provided the forward momentum is sufficient to keep the engine revs up above a set speed, the fuel injectors are cut off, to prevent unnecessary fuel being burnt. So you are moving forward, in gear and using no fuel. Maximising this situation is what gives great mpg figures.
The easiest way to know when you are in DFCO mode is if you have an instantaneous mpg display on your dashboard. The later Jazz’s with their A & B mpg surely have an instantaneous display. My Mk 1 doesn’t, but I have fitted a ScanGauge E to the OBD II port, and this displays instantaneous mpg for me. If I freewheel, down a hill, with the engine running, it will display 200, 300, 400 mpg, depending on the speed you are traveling, but if you are in gear and utilising DFCO it displays 9,999 mpg. This is like freewheeling, down a hill, with the engine off, except the engine is running, you are in gear, and the car is under complete control.
If I am descending a hill at say 50 mph, foot completely off the accelerator, the car will be in DFCO. If the speed starts to drop to say 45 mph and I apply a slight amount of throttle I will come out of DFCO. I could now be getting 140 mpg, but it is not 9,999 mpg! I prefer to allow the car to travel at 45 mph and continue to get 9,999 mpg than speed up to 50 again. Even with following traffic you are hardly a nuisance.
Below 30 mph 5th gear will not give DFCO, the engine revs are too low, so I will change down to 4th or even 3rd. Sometimes you have to go down to a lower gear to start DFCO but as soon as you are in that mode you can change up and it stays there. It is a function of the hysteresis in the system.
Sometimes I mix DFCO with a bit of coasting in neutral. There is a particular stretch of road I use regularly, a 40 mph zone, a country road restricted after a girl was killed playing “chicken”.
It starts with a slight downhill, then levels out as it approaches a roundabout. I get the benefit of DFCO as I descend the hill in 5th, as the road levels and the speed starts to drop off I plop it into neutral which prevents further slowing, and for the final braking for the roundabout I use engine braking in 3rd, which resumes DFCO mode.
Making most use of DFCO is what starts to push the mpg figures up. Learn how to use it and reap the benefits of it.
Any questions, ask away. Any observations, feel free to voice them.

richardfrost · « **Reply #1 on:** July 09, 2019, 08:57:26 AM »

That's really interesting Jocko. I think, on previous vehicles, and on my Jazz, I have done this naturally, bt not really appreciated or understood what was happening, and therefore not maximised it.

My RAV4 is a hybrid so I am assuming this does not happen in quite the same way on that car, or something different happens, as foot off the gas usually means charging the battery.

Fascinating.

peteo48 · « **Reply #2 on:** July 09, 2019, 09:50:19 AM »

I remember reading on a hypermiling forum about DFCO in connection with a style of driving known as "pulse and glide" and I think this was seen as being especially effective with hybrids but might be wrong there! In essence you accelerated up to a cruising speed and lifted off re-applying power when the speed dropped to minimum level (that wold be decided by the driver) so up to 60 - ease off - down to 50 - reapply power gently - up to 60 and so on.

Not for me but a huge amount to be said for DFCO and absolutely nothing to be said for powering up to road junctions and or lights and then slamming the anchors on. I always lift off the gas now as I approach a junction.

Jocko · « **Reply #3 on:** July 09, 2019, 10:47:42 AM »

Up there with DFCO is anticipation. I am constantly anticipating what other drivers are going to do, what traffic lights are going to change, the things every good driver should do anyway, but with a view to efficiency as well as safe smooth driving.
When I was training to be a bus driver, it was drummed into us that a green traffic light can only change in one direction. You cannot send a bus load of OAPs and their shopping trolleys rattling down the aisle! For fuel efficiency you should treat every green light as about to turn red (unless you saw it change and know you have plenty time). Approach the lights on a closed throttle and ready to stop if required.
I keep up with the speed of traffic but leave a huge gap to the car in front. Nothing worse than being too close, he wants to turn right, he has to stop, you have to stop too. Keeping a good gap maximises the chance you won't need to stop.
Final thing is try to avoid coming to a complete stop if you can, and it is safe to do so. Everyone knows how difficult it is to get a stationary, broken down car, pushed and moving, but once it is going a child can keep it on the move.
All the little things that aid fuel efficiency soon start to add up.