"Jayhawk Jake" (jayhawkjake)
02/28/2014 at 15:43 • Filed to: None
2
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"Jayhawk Jake" (jayhawkjake)
02/28/2014 at 15:43 • Filed to: None | 2
| 44 |
Read this on reddit...not sure if it's true or not
Coasting in gear also increases the fuel efficiency since the wheels turn the engine instead of the engine having to feed fuel in to keep itself running.
Excuse my noobishness, I'm still reasonably new to manuals and I know more about these types of engines:
DrJohannVegas
> Jayhawk Jake
02/28/2014 at 15:45 |
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On fuel injected engines, when the engine is in overrun (so, inertia of the car is "driving" the engine), usually the fuel is cut off.
HammerheadFistpunch
> Jayhawk Jake
02/28/2014 at 15:47 |
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Its...true but not the reasons listed. Coasting in gear means your engine speed is above idle but you aren't added fuel. Fuel injected engines go closed loop in such an event and shut off the injectors so that you aren't using any fuel at all. However, conservation of energy says you aren't being fuel efficient since you are slowing down and you will need all that energy plus the inefficiencies of engines to bring you back up to speed. When slowing to a stop you should do it in gear, or going down a canyon...but do it because of engine braking, that is that you slow down due to the compression of the engine, not because of fuel savings.
Stupidru
> Jayhawk Jake
02/28/2014 at 15:48 |
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Absolutely correct. When you coast in gear at a high enough RPM, the engine cuts fuel. Push the clutch in, and fuel is dumped into the engine to keep it running
Tekamul
> Jayhawk Jake
02/28/2014 at 15:48 |
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In modern fuel injection vehicles, the ECU monitors the throttle, RPM and engine temp. If all of these are within reason (above idle, no throttle, warm engine) the injectors are off completely.
This wasn't possible in carbureted systems, so the traditional approach is to coast in neutral to reduce drag and increase efficiency. But in new vehicles, in gear is more efficient.
vdub_nut: scooter snob
> Jayhawk Jake
02/28/2014 at 15:48 |
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Yes, it is true.
RamblinRover Luxury-Yacht
> Jayhawk Jake
02/28/2014 at 15:48 |
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True... for a diesel, maybe - if you're expecting to need to spool up again shortly, as you likely are coasting in gear. Definitely a more energy efficient way to slow down than the brakes, again for a diesel.
Old-style as engines require a certain amount of fuel/air to run at a given speed, though that's helped a *little* bit in a modern engine. In general, the claim that you'll use less pushing the engine than it takes for the engine to run itself at idle or close to (while coasting) is likely abjectly false.
Edit, I am dumb, forgot about closed-loop. Still doesn't help that much.
CAR_IS_MI
> Jayhawk Jake
02/28/2014 at 15:51 |
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For starters this would be dependent on gearing. but for all intensive purposes essentially any gear WILL slow you down. At what rate is dependent on many things, including gear ratio.
As per turning the engine and using less fuel, this too is vehicle dependent, some will supply fuel based on vacuum with throttle position being the limitation variable. Others cut fuel at 0% throttle (by cut I mean lower to idle fuel levels). in doing so and attempting to keep rpms up by means of force from transmission, you are effective raising the internal temps of the engine.
So to this I say no. If you want to cruise down a hill put it in neutral and let the clutch out. That is unless you like spending $ on frivolous repairs.
BrewskiDoodle
> Jayhawk Jake
02/28/2014 at 15:52 |
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fuel is still going into the engine, just not as much. combustion still needs to take place, but if you leave your foot on the pedal (like maintaining your speed) you will be giving more fuel than letting off the pedal.
McLarry
> HammerheadFistpunch
02/28/2014 at 15:53 |
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This may betray my ignorance, but wouldn't the engine shut off if you weren't using any fuel at all?
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HammerheadFistpunch
> McLarry
02/28/2014 at 15:54 |
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The wheel bone is connected to the trans bone, the trans bone is connected to the shaft bone, the shaft bone is connected to the piston bone. The engine is "off" but the engine keeps spinning and when you put your foot in it, fuel delivery comes right back and you are on your way.
Racescort666
> DrJohannVegas
02/28/2014 at 15:55 |
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It depends on the tune. This fuel map does not:
I spoke to a coworker who has a little more experience than I so and he says that his Cruise has a DFCO (deceleration fuel cutoff) but I don't know how common this is.
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Jayhawk Jake
> McLarry
02/28/2014 at 15:55 |
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No, if you're in gear but not on the throttle it wouldn't need fuel to keep turning, the wheels would keep the engine turning.
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Jayhawk Jake
> CAR_IS_MI
02/28/2014 at 15:57 |
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Cruise down a hill...hah, like I have those.
But seriously, I guess what I'm trying to ask is what's best practice for a modern (MY2013) car when approaching a stop light/stop sign? Right now, when I don't need to hold speed I just put it in neutral and coast. This implies that I should stay in gear, or down shift and slow in a lower gear.
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Jayhawk Jake
> BrewskiDoodle
02/28/2014 at 15:58 |
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In this situation my foot would be off the pedal. The question is when slowing to a stop, is it better to put the car into neutral and coast or leave it in gear and coast. Foot off the gas in both cases.
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McLarry
> HammerheadFistpunch
02/28/2014 at 15:59 |
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That makes sense. Just odd that it still makes internal combustion engine noises if no combustion is actually going on.
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HammerheadFistpunch
> McLarry
02/28/2014 at 16:00 |
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That made sense? man, you must be more tired than me.
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CAR_IS_MI
> Jayhawk Jake
02/28/2014 at 16:01 |
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Two options depending on your comfort level of driving manual.
1. do as you are now, shift to neutral, release clutch, use brakes to stop.
2. Down shift by rev matching RPM to rough estimation of where RPM should be in that lower gear, gears will help slow you down. Note. This is a bit harder on your engine / drive line.
ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
> CAR_IS_MI
02/28/2014 at 16:02 |
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It's "intents and purposes".
Intensive purposes are just, like, really intense, you know what I mean?
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Jayhawk Jake
> CAR_IS_MI
02/28/2014 at 16:03 |
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I'm plenty confident. I rev match often. That being said, I guess it's still not clear what the better method would be. Staying in gear/rev matching should be better for fuel economy, but worse for the engine? Is it noticeably better for fuel economy, or is this one of those 'it's theoretically better but makes little to no difference' situations?
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BrewskiDoodle
> Jayhawk Jake
02/28/2014 at 16:03 |
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Foot off gas but in gear will consume more fuel than neutral coast to a stop.
but on the other hand you will be using your brakes more.
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CAR_IS_MI
> ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
02/28/2014 at 16:03 |
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Yea your right. I was only half paying attention when I wrote this. Still looking at Alfas on my other screen.
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CAR_IS_MI
> Jayhawk Jake
02/28/2014 at 16:04 |
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Nope. N and coasting is best for economy and engine, only thing its harder on is the brakes.
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McLarry
> HammerheadFistpunch
02/28/2014 at 16:06 |
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It's been a long week... :P I'm cool with the principle of the engine being turned by the wheels (kinda like popping the clutch while moving to start the car)... But my question boils down to "why does the engine go vroom if it's 'off'?"
You can tell a Finn but you can't tell him much
> Jayhawk Jake
02/28/2014 at 16:06 |
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It depends on the vehicle. A lot of newer cars have a Deceleration Fuel Cut Off (DFCO) mode in the ECU. Not all cars have that feature though.
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McLarry
> Jayhawk Jake
02/28/2014 at 16:07 |
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I'm good with that...but in this case the engine is 'off' but still turning - why don't you hear it turn off?
davedave1111
> McLarry
02/28/2014 at 16:11 |
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Well, a lot of the noise is from stuff other than exploding gasoline.
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davedave1111
> BrewskiDoodle
02/28/2014 at 16:12 |
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You have that the wrong way around.
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davedave1111
> Jayhawk Jake
02/28/2014 at 16:12 |
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Leave it in gear and coast down.
jariten1781
> Jayhawk Jake
02/28/2014 at 16:16 |
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One thing to keep in mind is that it's better to be in some gear when coming to a stop so that you make evasive maneuvers if you need to. The traditional way this was taught was to stay in your cruising gear until rpms were near idle then clutch in. In modern 5 and six speeds the top gear is too tall for this usually so I tend to down shift to second or third when I'm around 20-25 mph then clutch in when it hits idle rpm. Walking down all the gears is way too much of a pain to do 100% in daily driving but it is the 'right' thing to do safety wise so you're always in the right gear to accelerate if necessary.
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DrJohannVegas
> Racescort666
02/28/2014 at 16:17 |
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I have played around with a few standalone systems, and some of them have DFCO, some don't. (Usually it has a TPS threshold, a MAP threshold, and an RPM threshold, all of which must be met.) From my armchair, I'd suspect most passenger cars do.
But, I'm just conjecturing now.
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Racescort666
> DrJohannVegas
02/28/2014 at 16:26 |
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We were talking about it here. One of the guys that used to work in my building also used to work for GM powertrain. I was about this close to calling him to ask. (fingers held apart) They may only have DFCO active for certain parts of the RPM range. Obviously it can't be active too low or the engine would stall at idle but deliberately having it inactive under certain load conditions would cause the popping we love on overrun.
Also, it's possible that the fuel injectors go to minimum pulse-width rather than turning off completely. This is effectively no fuel but to me it's different from a complete cutout.
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BrewskiDoodle
> davedave1111
02/28/2014 at 16:30 |
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No, i do not.
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davedave1111
> BrewskiDoodle
02/28/2014 at 16:34 |
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Er, you have both bits the wrong way around - I wondered if you hadn't written more when you meant less. You certainly don't need to use your brakes more when you have engine braking.
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BrewskiDoodle
> davedave1111
02/28/2014 at 16:36 |
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No, I assumed coming to a stop coasting vs engine braking.
Even straight coasting, not in gear, you will use LESS gas than leaving it in gear.
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davedave1111
> BrewskiDoodle
02/28/2014 at 16:41 |
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I'm completely confused about what you're trying to say wrt to using the brakes. I think we're agreeing, though, that you'd use the brakes less if the car's in gear, and more if coasting to a stop out of gear.
The second part, I'm told, depends on how new your car is. Anything less than about twenty years old, it's the other way around: leaving it in gear will turn the engine, so you save on the fuel used to idle. Carbureted cars can't cut the fuel completely, so apparently it's better to coast in neutral and save (transmission?) drag.
Of course it also depends on circumstances. If you can coast all the way to your stopping point from further back because you don't have the engine braking, that may be better than staying on the gas longer and then coasting-down in gear.
TheNeonDriver - Now with More BMW!
> jariten1781
02/28/2014 at 17:21 |
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I agree, and this is when I typically practice heal and toe, especially if I am alone. As my car get lousy fuel economy, I use engine braking to my advantage, and slow the car this way. By heal and towing, you are also reducing wear and tear on your clutch, as you are matching engine RPM to transmission RPM.
Petrus
> Jayhawk Jake
02/28/2014 at 17:30 |
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In that kind of engine (turbine ), yes
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Jayhawk Jake
> McLarry
02/28/2014 at 17:39 |
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Turning makes noise
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Jayhawk Jake
> CAR_IS_MI
02/28/2014 at 17:40 |
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But that counters what others are saying about modern engines, that being off throttle and coasting the fuel injectors shut off. If you're in neutral, it needs to feed fuel to keep the engine turning at idle speed
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CAR_IS_MI
> Jayhawk Jake
02/28/2014 at 17:49 |
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Yes, the injectors drop to idle fuel rates at 0% throttle
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McLarry
> Jayhawk Jake
02/28/2014 at 18:23 |
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True, but not 'vroom-vroom' type noise...
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McLarry
> You can tell a Finn but you can't tell him much
02/28/2014 at 18:28 |
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Any empirical way to determine if your car does/does not short of seeking out the ECU documentation?
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You can tell a Finn but you can't tell him much
> McLarry
02/28/2014 at 21:57 |
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Not that I'm aware of. Maybe if you've got a code reader that gives live readings you could pull fuel flow from that. Pretty sure fuel flow is an OBDII output.
JasonStern911
> Jayhawk Jake
03/01/2014 at 02:23 |
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even if somehow it is more fuel efficient, you are putting pointless wear on the engine. so why?