Does the 2015 have deceleration fuel cut off?

 

fixed ur title. it should have DFCO

 

Yes, it does.

 

You can see this in action. Take your foot off the gas, and watch your "instantaneous fuel economy" bar graph go to "infinity".

 

All Hondas have had this for a long time. It's why I never understand the "I shift to neutral going downhill" comments. Idling in neutral uses gas. Having gravity spin the engine uses no gas.

 

I'm not very up on this, can you explain a bit further? I have a 6MT, does it matter? Intrigued especially because of Dick W's comment on coasting.

 

Honda programs the FI to figure out when you are coasting/decelerating--i.e., no throttle and rolling fast enough to drag the engine above idle backwards through the gear train. In that state the FI decides fuel is unnecessary so they cut it off. Saves the fuel. Costs driver nothing. Engine still spins, generator and AC and what not still work. It's a true No brainer. They only start using fuel again when the RPMs get so low as to threaten how it's running and needing to use fuel to keep it from just stopping. In my '07 CR-V 4AT this seems to be in the 1,100 to 1,200 RPM range but is tuned to the drive train in question.

You see people assuming here and elsewhere that since the engine is doing 2,200 RPM or whatever in this state that it would use less fuel if they shift to neutral and let it idle at, say, 750 or whatever is the idle speed. But the 750 idling costs gas and the 2,200 just being spun from the kinetic energy of the car costs no fuel, just that kinetic energy.

And no it doesn't matter, principle and theory of operation is the same with the MT or the CVT.

 

Dick W, thanks for the explanation, but let's put it into terms a dummy like me can understand. Let's say I'm cruising the expressway at 3000 rpm/60 mph in sixth gear. I round a curve and now I'm going downhill. Normally I would put in the clutch and/or shift to neutral (and the tach drops to idle at 750 or whatever). But if I'm understanding you correctly, you're saying it's more efficient to take my foot off the gas but leave it in gear, since because of DFCO the engine is using less gas than idle even though the tach is reading 3000. Is that right?

 

Bingo!

If you (or cruise control for that matter) are not pressing the throttle, and the engine is turning more than a few hundred RPMs more than it does at idle, it is using ZERO gasoline. Popping it into neutral and letting the engine idle will, on the other hand, use MORE THAN ZERO. If this were not true, the engine would stop spinning and electric generation via the alternator and AC and so on would stop. As would vacuum for the power brake booster. Bad idea ALL AROUND.

Shifting into neutral in a DFCO car (and I can't imagine that's not all of them for at least the last ten years or so--since they all went to digital fuel injection) "to save gas" is stupid and counter productive in all cases but one: sitting at a stop in a torque converter car. In that case, shifting into neutral will save an infinitesimal amount since the idle will not have to slosh the fluid in the torque converter.

(There may also be a temporary dynamic case for MT shifting--not sure what they program for the case of lift off accelerator, mash clutch, shift, lift off clutch, mash accelerator. They may use just s touch of fuel for the very momentary period while engine is slowing down just for driveability/shift feel considerations. Not so much to over-rev when the clutch goes in but enough for help control the reduction in RPM expected necessary to hit the next gear at the right engine RPM. If they use gas for this, it would probably be an immeasurably small amount.)

These cars are very smart as are (most of) the engineers designing them and, given a problem to optimize, like fuel economy, they'll find all the tricks to do it. DFCO was low fruit once they had "PGM-FI" as Honda called it back then.

 

I should add: if you are going downhill, say a slight grade, and the only way to keep speed is to give it a tiny bit of gas, there might be cases where it would coast in neutral at speed and idle might use slightly less gas than the smidgen it will take to touch the gas pedal slightly to maintain the same speed in gear. I'm betting these cases are rare at best. Again, these are smart machines. Maintaining 60 down that ever so slight grade that it takes a little throttle to do it will take way less gas to maintain X RPM than it will cost to maintain the same X RPM on a flat. Wind effects and other effects (different pavement friction, etc.) all the same.

Watching the instantaneous MPG bar on the cluster can be very instructive.

 

I get it, thanks. Funny, I never heard of DFCO until this thread. And now other things are making sense, for example why my eco-coach goes blue when I'm coasting for more than a second or so and not in gear.

 

Exactly. That second or so is the time it takes the engine RPM to drop to the point where it has to put fuel in to keep the engine from stopping. When you first put it in neutral, the flywheel, etc., have inertia. That inertia is more than offset by compression and friction in the engine and it slows down. When it slows enough, the programming will start injecting fuel and it will idle instead of just stopping.

 

Because of engine braking. Even if there's no fuel.. take energy to spin the engine..

Try it... find a spot and hit it at exactly the same constant speed... with no gas and in neutral.. you'll go way farther in neutral.

 

Obviously. But the engine idling in neutral is *not* free and you will use more gas to get to the place where the compression losses would have stopped the free rolling car in neutral. Most of the other friction and drag losses are the same either way.

Sure, there are scenarios where the idling gas consumption of the engine is offset by how much farther the car will roll in neutral at an acceptable/desirable speed. These cases are not common enough, and most of us are not smart and forward looking enough to be able to mentally calculate the trade to make routine coasting in neutral a winning strategy in all possible driving situations. Now, tell me you put it in neutral and stop the engine and I'll tell you that is a winner gas milage strategy for coasting every time. Dangerous like crazy but…

 

You are absolutely correct robs. I have several sections on my commute where I can coast in neutral (e.g. up to a stop sign) for quite a distance as opposed to being on and off the accelerator using DFCO or waiting until I'm closer to the stop sign to decelerate into DFCO. Uphill, downhill, flats, it doesn't matter, I can neutral coast 4-5 times as far without engine braking.

 

Shifting to neutral means on engine braking, making controlled stops and turns more difficult. It is not only dangerous, but illegal in many jurisdictions.

 

If we are talking about in neutral with the engine idling, I don't buy the dangerous part on a car built in the last 30 years or so. The engine is still making plenty of vacuum for brake boost, the brake reserves and fade margins on modern cars are clearly good enough to not *need* engine braking in any case, and as long as the engine is spinning you have hydraulic power steering. With the EPS in a FIT, you don't even need the engine running as long as you have a battery.

I still think it's a dumb thing to do. But dangerous? Not buying it.

 

Well we are talking about Hondas in general and Fits in particular. All FWD, electric power steering and brakes, vacuum means nothing as you stated, but it is dangerous, to your wallet if nothing else. There's a reason it is called Georgia Overdrive. If you're familiar with Monteagle, TN, throwing a manual shift car into neutral at the top, you could coast all the way to Georgia (providing you could avoid going into a sand trap or completely off the mountain).