Breakpoint for P&G in N vs. DFCO - Fuelly Forums

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Old 01-06-2012, 02:09 PM   #1
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Breakpoint for P&G in N vs. DFCO

I've been trying to figure out when to use N or just let the electronics cut of the fuel in gear. I realize this will depend on a number of things varying between cars and conditions. I just want to know how you think about this.
Up until now I have always used the dfco out of lazyness I guess or fear of damaging the transmisson (which I dont think it will done right since the transmission is already in the right gear).

One thing about coasting in N is that you maybe need to match the rpm on the engine to the speed. For a short second you are injecting gas in the engine without any real gain in driving the vehicle forward, except saving the transmission. This means there is no point in doing this for short periods of time. A longer slope is needed. And the slope should not be too steep so that you are gaining speed down the hill, possibly getting more wind resistance or speed that makes the planning ahead more difficult.

I believe it also depends on the revs of the enginge at that speed you are traveling at before you let your foot of the throttle. At 100km/h the revs im at is about 2000rpm. The engine is a 2.0L s? the engine brake is not that strong. I do alot of city dfco-coasting at low speeds. Sure coasting in N makes a substantial difference but if the slope is steep enough for the speed to be fairly constant the opportunity to give a bit more throttle using the engine more effectively reducing pump-losses when going up the next hill might be worth it.

At low speeds <56km/h the transmission will downshift to 4:th at about 900rpm. If I'm going just slightly downhill at the same time the speed would be constant and I'm using no fuel at all. Then when going up the consumption is maybe 8-12l/100km. I think this would be better in average than consuming 7L/100km at a constant rate, varying the speed instead or coasting i N on the way down consuming 1.8-3L/km.

Same thing when driving on a winding road with lots of turns and straights. Giving some extra throttle accelerating on the first half of the straights and then decelerating into the turns might give som extra efficiency compared to just going in a straight line at constant speed.

How would you go about testing this? I have a scangauge connected to the car now. I also do alot of flat highway commuting at constant speed but with not so much traffic (mostly lorrys), I'm a bit curious about how much I could gain by skipping the light trottle going more pulse-or-nothing.

Sorry for the long text.
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Old 01-06-2012, 03:05 PM   #2
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Re: Breakpoint for P&G in N vs. DFCO

Regarding the subject line: You can't really compare P&G to DFCO. You can compare neutral coasting to DFCO but neutral coasting is only one component of P&G. One person has posted that he has good results with P&DFCO with an automatic, which while possible, I believe is unlikely to work for most people.

You have to consider where all the energy is going. You turn some fuel into energy. That energy gets wasted in the cooling system, engine (pumping loss, reciprocating loss, internal friction, for each revolution), transmission, tires, etc before what remains gets a chance to get wasted on air resistance. After all the waste is done you have stored some energy in your forward motion as inertia. When you DFCO, you take up some of that inertia to keep your engine turning (pumping loss, reciprocating loss, internal friction, for each revolution). When you neutral coast you use fuel to keep your engine turning - but you turn it fewer revolutions by far.

As a general rule, DFCO is best used when you were already planning to discard that energy through braking. Neutral coasting is the better choice when you plan to maintain speed and expect to need to apply power to the road again later in order to do that.

You can, of course, experiment the usual way with A-B-A-B-A testing.

The Scangauge, reading open/closed loop, should help you get more intimately familiar with your car's DFCO behavior. Some Hondas are known to raise the DFCO floor when accessories (headlights, wipers, air conditioning) are in use, for example. My pickup truck usually has a delay before DFCO engages. My VW often refused to DFCO if I'd recently shifted gears.
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Old 01-06-2012, 04:25 PM   #3
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Re: Breakpoint for P&G in N vs. DFCO

I meant the "glide" in N versus in gear. In gear does give me som glide still since the rpm still is pretty low and the braking effect of the engine is therefore low. I'm new to these expressions so I may not be using them right, but I think you essentially followed me there.

The scangauge shows 0.0L/km (this would be 9999MPG in US units) if throttle is released and lock-up is engaged from high speed in 5-4-3:rd down to about 13mph (or 22km/h) then it goes closed loop again and the torque converter starts to slush-pump the car forward (still reducing the speed some on level ground until it creeps slowly forward in 1:st). If I use the A/C this will happen in 4:th gear instead of 3:rd if the A/C-clutch happens to be engaged at the time. Also if the lock-up decides to disengage (because of too much indecisive use of the gas-pedal) before coasting, it acs as a normal slush-box-automatic reducing the rpm to almost idle instead (this makes the behavior a bit random which I'm not that fond of). Anyway thats what I've noticed behavior wise.

If i set it in N the car will naturally coast longer but not a very big difference because of the low rpm in gear.

So if I understand you correct you would want the car to freewheel mostly when coasting rather than turn the fuel off and breaking some?

What I'm thinking is if you would pulse&coast at say 2500-3000rpm in the pulse and then highest gear in DFCO would this be more inneffective than doing the same but coasting in N at idle if you can manage the coasting to happen downhill and vice versa? Then you would mostly use the engine when you really need to put some more load on it going up hill.

What I would like to see is a car that does these decisions automatically depending on the slope (g-vector sensing?) and accelleration down hill and turns off the engine if the breaking isn't needed. If the indicator is turned on the engine break could be used by default etc. The Honda transmission really does this a little, it will not change the gear to 5:th if the downhill is steep enough and you let off the throttle at the crest after accelerating.
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Old 01-06-2012, 04:41 PM   #4
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Re: Breakpoint for P&G in N vs. DFCO

...If the throttle would fully open when in DFCO when engine breaking is not needed that would be nice too I think. A light press of the breaks (before they even start to engage) could force some more engine breaking etc.
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Old 01-07-2012, 03:20 PM   #5
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Re: Breakpoint for P&G in N vs. DFCO

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Originally Posted by 8$PG View Post
So if I understand you correct you would want the car to freewheel mostly when coasting rather than turn the fuel off and breaking some?
Right. Again, consider the energy that is spent on the additional revolutions of the engine.

I don't call it "coasting" unless the engine is disconnected from the wheels. If it's in gear then it's engine braking, even if there's barely any braking happening.
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Old 01-08-2012, 02:32 PM   #6
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Re: Breakpoint for P&G in N vs. DFCO

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Originally Posted by 8$PG View Post
...If the throttle would fully open when in DFCO when engine breaking is not needed that would be nice too I think. A light press of the breaks (before they even start to engage) could force some more engine breaking etc.
Or shut the valves like some hybrids do.

As pointed out in the OP, there is plenty of variables with car model being the biggest. While tightening economy regulations means newer cars will get more aggressive with DFCO, older automatic vehicles may not shut off the fuel when the foot is off the accelerator. I have to downshift in order for it the activate for example, which means little coasting distance.

THC's guideline of using DFCO when you are planning to be slowing down, and N when you going to maintain an average speed is a good general one, but it is a ymmy case when it comes to the individual practicing it.
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Old 02-01-2012, 04:13 PM   #7
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Re: Breakpoint for P&G in N vs. DFCO

Use CC as often as you can, because you can press decel/coast when you want to coast in gear then accel when you want. I tried putting it in neutral but did not like trying to match rpm to engine when re-engauging. I use these techniques when driving my mother-in-laws 98 Dodge Grand Caravan. I have gotten over 30mpg on that 4800 pound beast. Yesterday i got 32-34 mpg on the freeway on a 40 mile trip driving it. I do other stuff as well that you can discover on your own.
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