gas consumption when off of throttle

[RatLabGuy]

Short version: on a modern car with ECU control over efficiency etc - when you're rolling along at high RPM in a manual trans car, and off the throttle - is any gas being put out while you're coasting?

Long version: There's a long slow downhill near here thats like 2 miles long, I find it fun to push in the clutch and just coast and watch the car's MPG's hit 99.9 and think to myself, "I'm using no gas." Which isn't really true bc it idles down to 500 rpm while the clutch is in, and that idle still takes a small flow of juice. I was talking to a coworker and he said, you're better off leaving it in gear and backdrive the motor bc as long as its higher than the idle RPM the ECU will know it doesn't need to apply any gas. Or at least, it will not be told it needs to.

Is that right? I'm not into hypermiling or anything, just idle curiosity (see what I did there) and found it a fun thought experiment.

 

[jeepinmatt]

That's a big argument on the TDI forums. I've had unbelievably good results neutral coasting when combined with conservative driving. Big thing is staying in neutral when stopped. Which isn't the topic of discussion here haha. But I assume the ECU is gonna match the injection rate to the known parameters for a given IAC value, and regardless the difference has to be pointlessly small.

 

[Croatan_Kid]

It's still injecting basically what it would at idle while coasting in neutral. It'll go to 99.99 mpg because of speed/distance traveled vs fuel injected vs engine load. No engine load running 65 mph and idling would definitely net fantastic fuel economy.

If you ever look at a tune file in any of the tuning softwares, there's a whole section for DFCO. Deceleration Fuel Cut Off. It cuts fuel quite a bit, but not entirely. It'll only go in in to DFCO if it's in gear and has load (negative load) on the engine, otherwise, it'd loop in to the idle tables. I'd have to look at the tables in a tune file again, but I'm pretty sure there are also predetermined speeds, RPM, and timing cuts as well. Usually three of more parameters reference each other in a "if this and this, then this" kind of algorithm. Retarding the timing during DFCO is why you'll hear a lot of vehicles popping during deceleration also.


That help any?

 

[Bebop]

Short version: on a modern car with ECU control over efficiency etc - when you're rolling along at high RPM in a manual trans car, and off the throttle - is any gas being put out while you're coasting?

yes

I was told by a calibration engineer at work a long time ago that coasting netted better fuel economy than idling. Never really questioned him.

Shutting the engine off and free-rolling is the way to go. I know I helped develop such a system for an OEM but it was deemed that people would be too freaked out by it and they never implemented it. It netted a significant mileage improvement (around 10/15%) and could be flashed on a lot of cars of the group for close to no money.

 

[jeepinmatt]

Shutting the engine off and free-rolling is the way to go. I know I helped develop such a system for an OEM but it was deemed that people would be too freaked out by it and they never implemented it. It netted a significant mileage improvement (around 10/15%) and could be flashed on a lot of cars of the group for close to no money.

This is why I roll my eyes when I hear people talk about being concerned about the environment and saving the world by saving fuel and blahblahblah. Nobody cares enough to make it happen, and stupidity gets in the way anyway. I want an AWD diesel hybrid with a giant turbo that also cuts off when coasting! And give me a selectable t-case so I can go into full FWD econo mode on the interstate, and full RWD mode for parking lot shenanigans.

 

[Bebop]

This is why I roll my eyes when I hear people talk about being concerned about the environment and saving the world by saving fuel and blahblahblah. Nobody cares enough to make it happen, and stupidity gets in the way anyway. I want an AWD diesel hybrid with a giant turbo that also cuts off when coasting! And give me a selectable t-case so I can go into full FWD econo mode on the interstate, and full RWD mode for parking lot shenanigans.

Here is the order of importance as far as who has a say in implementing a new feature on a vehicle:

1) Legal (if the lawyers say don't do it, you don't do it, and vice versa)
2) Finance (no money, no cars)
3) Marketing (they *know* what the customer wants)
4) Sales (they *know* what they can sell)
5) Coffee machine (the whole place runs on it)
6) Engineers (occasionally, very seldom, the above mentioned people have to listen to them. Only after asking them 5x "are you suuuuure?" when they said "NO" to the question "Can I add extra features to the vehicle and make it cheaper at the same time ? Also I need it done yesterday because we told the management you already agreed to it 30 weeks ago and we forgot to inform you at the time").
7) Interns (hahahahaha, that was funny. Now go get me another coffee !)


* : If a word is in between stars, you can replace it with *make some bullshit up about*

 

[gslander]

yes

I was told by a calibration engineer at work a long time ago that coasting netted better fuel economy than idling. Never really questioned him.

Shutting the engine off and free-rolling is the way to go. I know I helped develop such a system for an OEM but it was deemed that people would be too freaked out by it and they never implemented it. It netted a significant mileage improvement (around 10/15%) and could be flashed on a lot of cars of the group for close to no money.

That would definitely take some getting used to and freak me out if my engine just randomly shuts off at certain times while driving

 

[rodney eppes]

That would definitely take some getting used to and freak me out if my engine just randomly shuts off at certain times while driving

Like the newish engine shut downs when you stop, cranks up when you step on the gas. What will be the expense of replacing that starter [& how long will it last], verses the cost of fuel saved? Anyone here got a Cadillac with the started built Inside the motor? That's Brilliant!

 

[paradisePWoffrd]

Like the newish engine shut downs when you stop, cranks up when you step on the gas. What will be the expense of replacing that starter [& how long will it last], verses the cost of fuel saved? Anyone here got a Cadillac with the started built Inside the motor? That's Brilliant!

It dont believe it uses the starter to restart the engine. I believe it fires an injector and starts it through combustion. Which makes me ask, why even have a starter to begin with?

 

[Ron]

Like the newish engine shut downs when you stop, cranks up when you step on the gas. What will be the expense of replacing that starter [& how long will it last], verses the cost of fuel saved? Anyone here got a Cadillac with the started built Inside the motor? That's Brilliant!

Very very few starters ever wear out because of use. I think I read where the average NMCTF (net mean cycle to failure) of the average consumer auto starter is somehting like 200,000 cranks.
Failure is usually due to electrical corrosion, poor connectivity or improper maintenance (bolts get loose and don’t get caught and tightened )…not caused by mechanical fatigue.

 

[RatLabGuy]

That would definitely take some getting used to and freak me out if my engine just randomly shuts off at certain times while driving

Like the newish engine shut downs when you stop, cranks up when you step on the gas. What will be the expense of replacing that starter [& how long will it last], verses the cost of fuel saved? Anyone here got a Cadillac with the started built Inside the motor? That's Brilliant!

My wife's new mini Cooper has this "feature". I absolutely freaking hate it. I'm sure it saves a little gas, but it drives me crazy that the light turns and I want to go, or even worse I need to get out of the traffic to make a fast left, and there is a pause before I can get up and go.
Thank God there is a nice obvious switch you can just turn it off. Whenever I get in the car that's literally the first thing I do.

 

[RatLabGuy]

It dont believe it uses the starter to restart the engine. I believe it fires an injector and starts it through combustion. Which makes me ask, why even have a starter to begin with?

Totally an off-my-head guess.... in order to do it this way you need to know the exact position of the crank and cam so you know which cylinder to fire, otherwise you'd have crazy knocking. Today's sensors and ECUs are good enough to do that, but we didn't always have that, and now we have the legacy of starters already in place.

 

[Bebop]

Totally an off-my-head guess.... in order to do it this way you need to know the exact position of the crank and cam so you know which cylinder to fire, otherwise you'd have crazy knocking. Today's sensors and ECUs are good enough to do that, but we didn't always have that, and now we have the legacy of starters already in place.

Cam and crank sensors have been fitted in EFI for the past 30 years.

Like the newish engine shut downs when you stop, cranks up when you step on the gas. What will be the expense of replacing that starter [& how long will it last], verses the cost of fuel saved? Anyone here got a Cadillac with the started built Inside the motor? That's Brilliant!

Doesn't use the starter to restart, just puts the transmission back in gear (same as push starting the engine).

Starters are used to start the engine when the vehicle is stationary.
As @Ron mentioned, starter failure rate is very rare.

 

[jeepinmatt]

Clearly some of you guys have never owned a Chrysler product...

 

[RatLabGuy]

Cam and crank sensors have been fitted in EFI for the past 30 years.

yeah but by then everyone was used to using the starter from the previous 40 years. Paradigm changes like that are hard fought.

Doesn't use the starter to restart, just puts the transmission back in gear (same as push starting the engine).

I don't see how that would work on our Mini, which only turns off the ignition when you're idling, stationary, for a few seconds (e.g. stopped at light). Something has to push the crank.

 

[Bebop]

yeah but by then everyone was used to using the starter from the previous 40 years. Paradigm changes like that are hard fought.

I don't see how that would work on our Mini, which only turns off the ignition when you're idling, stationary, for a few seconds (e.g. stopped at light). Something has to push the crank.

Read the words I wrote and it'll all make sense.
I'm talking about the free-roll stop and start (which hasn't been implemented by any manufacturer in series as far as I know) and you're talking about the "standard" stop and start which is pretty much standard on every car these days.