My take on this, from another thread:
https://ecomodder.com/forum/showthre...tml#post690294
Quote:
Originally Posted by Ecky
If you need to coast to a stop (say, a stop sign), the best way to reach there is burning zero fuel, so either DFCO or engine-off coasting in neutral would be equal. Coasting in neutral while idling will burn more fuel.
If you're trying to conserve momentum, look at it in terms of cumulative pumping losses. If you DFCO, you have the cumulative pumping losses of the engine turning at whatever your cruising RPM is, being subtracted from your momentum. If you put it in neutral and idle, you have the cumulative pumping losses of idle RPM, but instead of them being subtracted from your momentum, the engine burns a bit of fuel. Shutting the engine off means zero pumping losses. Overall idle is a net win.
If you need to coast downhill and you need to bleed speed (e.g. speeding ticket risk), DFCO bleeds more speed and you burn zero fuel while doing it.
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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:
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.
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