Coasting with a Slushbox

[Erdrick]

I usually coast mine (when the engine is on) in neutral. The gearing in my car is so screwed, that on the same hill I will gain 25 km/hr or maintain the same speed depending on whether I am in drive or neutral. The extra fuel used from idling the engine must be less than maintaining speed after the coast, rather than coasting longer.

[2TonJellyBean]

Okay, so what I am finding is the DFCO is often mapped as a straight line curve to the engine temperature. An example would be 2000 rpm cold and 1100 rpm when the engine is at operating temp.

If it's 1100 rpm for my vehicle and I'm at 45 mph (1400 rpm) coasting to a light, that basically explains why I'm aware of an "after shift" effect as well as the initial quick deceleration.

I get the initial drag from DFCO and then at maybe 1100 rpm DFCO is disabled and then the vehicle coasts a bit better until 1000 rpm when it does the auto downshift. This puts the revs back around 1400 rpm and then there is engine braking again down to perhaps 1100 rpm.

No wonder I like neutral coasting, it's smoother and easier to predict.

Anyway, based on what I'm describing, I'd say that effectively, the DFCO is probably active about half the time if I just let off the gas from 45 MPH. This thing uses very little fuel at idle... 1/3 gallon.

I guess the key question is, if I know of a stop way down the road, I can either stay in gear and wait a bit longer to coast and then cycle through DFCO phases as it goes down through the gears saving perhaps about half the idling gas or I can lift from cruising speed sooner and idle all the way.

I'd love to see a micro fuel measurements of a few different cars and what the net consumption is for the two techniques.

[cfg83]

2TonJellyBean -

Originally Posted by cfg83

...

Ok, I will do that tonight and give you an update.

...

I tested it and it never goes into Open loop. I took my foot off the pedal for engine-decel, and I also hit the brake hard. Always stayed in closed loop.

CarloSW2

[2TonJellyBean]

Carlos, sorry for repeating what I read... I also tried that this morning as well, it always showed closed loop whenever I lifted off.

[omgwtfbyobbq]

Originally Posted by 2TonJellyBean

I'd love to see a micro fuel measurements of a few different cars and what the net consumption is for the two techniques.

Your stop is kind of a unique condition, since ya gotta stop anyway, so I would guess it's better to let the auto downshift since it won't be idling all the time, and ya gotta dump the excess anyway. Otoh, taken in context compared to something like P&G, so long as the driver doesn't have to stop, whether it's effective depends on how much more efficient the engine operates during the pulse compared to at a steady cruise w/ the same average speed, less the gas spent idling on the glide. If there's a big enough difference in engine efficiency, then the savings in gas will outweigh the loss while idling.

[Hockey4mnhs]

I leave mine in gear you can see in my gas log jump is when i started P@ging in N with it on.

[Mr. Pig]

Just to add another example in here, my cavalier with a 3 speed auto does better coasting in neutral rather than leaving it in drive. If I am coasting at 55 in drive, the scan guage reads around 80-85 mpg, then if I bump it into neutral, it jumps to 150-160 mpg. The gearing is really crappy in this car, so putting it in neutral also gets a much better coasting distance. Next commuter car will definitely be a manual.