Coasting in Neutral vs in Gear DFCO vs no DFCO?
 

So the generally agreed upon coasting hierarchy is E-Off Neutral, E-On Neutral, E-On Gear, Brakes.

Was wondering where would coasting in gear without DFCO would fall, and if anyone else has experimented with this. I'm uncomfortable with shifting in and out of Neutral too often, so I've been trying out adding just enough throttle to prevent DFCO from activating, and get close to idle's fuel rate. I noticed that the ignition timing readout correlates pretty well with when the engine nearing DFCO (48+), and I combine it with the A/F data (18+ = no fuel) to be sure I didn't back off too far.

I have had the opportunity to test it out on a couple of longer drives, the engine is spinning at a higher RPM than neutral so slightly more internal friction to fight, but I can let the RPM steadily drop so the throttle can be restricted a bit more than idle?

I don't really have a way of testing the travel distance difference between neutral and in gear with fuel, but it's definitely less braking force than DFCO.

Weirdly I can feel the steering wheel vibrate a bit at very low throttle openings. FF drive, so whatever the engine is feeling is bouncing through to the steering column, not sure why. Lugging is too low RPM, right?

Sounds about right.

I often clutch coast in the TSX because N to 6th shifting seems like it would put excessive wear on syncros. Some say that's rough on the clutch, but I have no opinion yet.

Lugging and feeling the result is an indication of harsh operation. Avoid that. Engines operate efficiently at higher RPM where lugging isn't necessary... I probably lug more than I should.

For sure, but IIRC Lugging is when you're at too low RPM and demanding more power than it can give, which the ECU for an automatic probably avoids 99% of the time. I'm not sure I've heard of a danger mode as a result of being at mid-RPM and demanding too little power.

Sure, maybe the engine designers didn't really expect someone to be using the very bottom of the engine map all that often, but you'd think they'd make sure everything within the map runs properly at least?

My car has a green light on the dashboard for when it's Okay to short-shift up through the gears.

My car has a white light and a black number for what gear it thinks I need to be in for efficiencies. It's usually wrong.

I'm ... not sure why people are focused on shifting here, when the original topic was about coasting.

Well for me mostly because you were concerned with engine rpm above idle which indicates to me that youre in a gear and maybe lugging engine which was another concern. If you're coasting with the engine disengaged, it won't lug, no major power demands.

Since the just above idle loading practices are rare in typical operation, there is no pressure on designers to remediate odd behaviors with lower gearing and most systems tolerate driveline low rpm slippage to increase rpm and torque

Well, assuming an engine running at extremely low torque probably isn't producing much detrimental combustion characteristics, I was mainly asking whether anyone has experimented with very low throttle coasting.

If not, oh well.

Seems to me that if you are adding the engine drag like dfco but keeping the injectors pumping that you are only one step above pushing the gas pedal.
E-on neutral and E-on gear can flip flop around in different conditions such as coasting distance, incline/decline grade%, etc. Dfco (e-on gear) will sometimes be better than e-on neutral since e-on neutral is still using some gas where dfco uses none.
I'm curious about your aversion to neutral coasting. It's a great invention.

Older vehicle, sometimes the drivetrain lurches a bit when i shift back to D. Don't want to stress out the transmission clutches too much. I reserve Neutral mostly if I know I'm coasting long enough for the speed to drop considerably, thus transmission and engine RPM dropping with it.

A little blip of RPM will take care of the lurch.

I'm on like 50/50 on blipping enough to reduce the lurching, so I figure if I can do something else and get close without bothering with all that hand and foot movement, that'd be worth it. Also, I wonder if blipping brings up the consumption enough to bring it pretty close to whatever I'm getting.

My take on this, from another thread:

https://ecomodder.com/forum/showthre...tml#post690294

As far as coasting without DFCO... all of my vehicles will not coast above idle RPM in gear without DFCO. A minimum amount of throttle needs to be given.

If you can get yours to do this, your guess is as good as mine as to how much fuel the engine burns, but ultimately you're still going to be covering the pumping losses by a combination of loss of momentum, and fuel burned.

EDIT: One additional factor is that, at very low throttle positions, the efficiency of the engine is very low. Partly that's because efficiency is a measure of fuel in vs useful work out, so it's not entirely honest to simply say that low throttle = worst BSFC - many of the losses are simply a function of keeping a vehicle powered and rolling down the road.

That said, the way to maximize fuel economy is, ultimately, to drive with the engine in its peak BSFC zone for the greatest percent time the engine is running, and to turn it off as much as possible otherwise. That, and to drive in such a manner as to have the least energy losses. Things like lower speed (exponential aero losses), and avoiding stops (turning fuel into brake dust).

If you haven't before, take a look at some BSFC charts. Here's one for an Atkinson cycle engine, and a conventional small displacement engine:

https://www.researchgate.net/publica...ine-141-CR.jpg


The top line of the chart is the torque curve of the engine. Being halfway between the top line and bottom of the graph would indicate 50% load at that RPM.

In a vehicle with the 2.0L engine pictured above, the most efficient operating regions would be at ~2500rpm ~65% load, and at ~1900rpm ~50% load. The reason the peak efficiency load is so low, is that these engines pull ignition timing and enrich the air fuel ratio most at low RPM high load, in order to compensate for the extremely high compression ratios. In an engine that has low enough compression that no enrichment or ignition retard is necessary, peak BSFC would likely be right at idle, at WOT. Higher compression moves that peak BSFC island up in the power band, and down in load, which allows for a larger engine to operate more efficiently where it will most typically be operating.

Yeah, I know about BSFC zones. When accelerating or coming out of a glide, I try to keep the engine in like 70-80% load, 1.5K to 2K RPM. It's the gliding part I have several choices to make. I think no-DFCO gliding has slightly slower Decel than idle coasting, somewhat balancing out the 80% higher consumption rate maybe? (0.3 GPH to 0.55)

I imagine it's basically a wash - you're either burning more fuel, or burning more momentum, which requires more fuel later.