jalibrandi

Had my rx-8 for 2 weeks now. One thing I have found, it is a great coaster. Coming to my house there is a very slight down hill grade for almost a mile. I found if I but it in neutral it maintains speed all the way. Also in other situations in city driving I find myself coasting into traffic lights. I'm sure it helps gas milage but my question is, is there any harm to the car? I wouldn't think so but I am not a mechanic.

shaolin

You're not really harming the car, but that's a very dangerous way to drive. If you do that, you have no control over the car, and it's more of the car driving you, than you driving it. You should always keep the car in gear especially when going downhill. If you start to lose traction, being in neutral will only worsen the situation....actually being in neutral unless you're at a complete stop is a baaaaad habit.

FYI, if you had a truck and were carrying a load, neutral down a hill is asking for trouble...run away truck...

Ellar

I don't know the mechanical ramifications, though I doubt any exist. I know in some places it's actually illegal. No idea why. In some municipalities, it's legal, but it's required that you sing the lyrics to "The Great Space Coaster" theme song, so print these out...

It's the Great Space Coaster. Get on board.
On the Great Space Coaster. We'll explore.

A comet ride of fantasy
To a place where dreams are fast and free
With new friends and new things to see
We'll spin you through the galaxy

On the Great Space Coaster. Oh-oh-oh.
On the Great Space Coaster. Off we go.

Get on board, step inside, soarin' on a magic ride.
Roarin' towards the other side where only rainbows hide.

On the Great Space Coaster (Oh-oh oh)
On the Great Space Coaster (Get on board)
On the Great Space Coaster (Get on board)
On the Great Space Coaster
Off we go!

Rasputin

Coasting in neutral is detrimental to fuel economy, as the engine is in idle mode (and using fuel then) whereas it's in DFCO (Decel Fuel Cut-Off) mode when a gear is engaged and going down a hill.

Fabrice

juikster

Thats why I like tuning in on the forum. To know more about the RX8's internal capabilities.

jalibrandi

Didn't know that, so you actually use more fuel in idle than during decelerating in gear? Sounds counter intuitive but what do I know.

antiver

Thats news to me too.. I always coast with the clutch in, but leave the stick in a gear, so, in emergency, i just have to pop the clutch and off i go, avoiding hazards.

When you leave the car in gear and take your feet off the pedals, i've heard the decel refered to as "engine braking."

Anyone care to enlighten this topic further?

shahram72

As I remember back from my Auto mechanics classes back in high school, detriment to the tranny depends upon wether the fluid in the transmission is driven by the engine or the output shaft. if it is from the engine, then you are not getting much volume while it is idling while the tranny is spinning at driving speeds. Now this teacher was old school, and this may not even apply to modern transmissions. Arent almost all the gears sitting in the oil a little bit? As far as coasting downhil being dangerous. I don't consider it to be if it is a long, fairly straight, gradual slope that does not cause the car to accelerate above safe limits or 45mpg, whichever is lower. but on a twisty downhill, yes it's dangerous. And if you have to use the brakes to slow down, you should be in gear.

Reactionary

It's dangerous regardless because of the possibility of a sudden change in conditions. Think about the cognitive processes involved. Your brain would have enough things to worry about: steering wheel, brake, accelerator. If coasting out of gear, a sudden need for evasive action might require you to get the car into gear and that adds cognitive processes required for clutch and shifter AND the synchronization of both. If you think about it, the brains of stick shift drivers automatically want to go for the shifter and clutch during anything sudden and unexpected. Think about how your brain gets confused when you drive an automatic, trying to go for a shifter and clutch that don't exist. Well, that's sort of the default circuit for a manual driver's brain.

What I'm saying is some people would argue that in an emergency, someone driving a manual wouldn't even worry about the shifter and clutch, only the brakes and steering, but's that not true. During the need for sudden evasive maneuvering, even when coasting, your brain would instantaneously consider going for the shifter and clutch as well as brakes, steering wheel, and even accelerator. If coasting, the increased number of considerations would cause the brain to hesitate before acting, thus increasing the chances for disaster.

Johnnyma45

So you're saying cruising with the tach at 1000 rpm is using more gas than say, decelerating in third with the tach at 3500 rpm? please elaborate.

Reactionary

If he's right, then the engine computer would be running very rich at idle. It has to be something to do with the A/F ratios independent of the rpm, right?

Also, when I'm just parked, when warmed up, my engine idles at around 700 rpm. Why is it that when I'm coasting downhill out of gear, the engine is at around 1000 rpm? Why would it be higher when in motion?

Could it be that the engine computer knows the car is coasting in motion so it keeps the rpm at 1000 in order to prevent the super rich A/F ratio that exists when idling at 700 rpm? If so, then coasting out of gear might not really use more fuel than coasting in gear unless DFCO mode is significantly more efficient.

Johnnyma45

...man, i thought i was saving gas by cruising everywhere. from now on i'm gonna cruise around in 1st until 8000 rpm in traffic...i love the way the rotaries sound like a jet engine when the rpm's lower, but was afraid as to if it was hurting mpg. so, always keep in gear (besides safety reasons) is the consensus?

Rasputin

Yes. Under deceleration, the PCM programs a fuel cut-off (no injection of fuel) to improve fuel economy. Indeed, you don't need injection as the engine is driven by the inertia of the car. It's a well-known feature. You can even detect it with a sCANalyser or similar OBD tool : you can see the AFR going to full scale lean, well beyond lean limit of mixture ignition.

Fabrice

Rasputin

Air fuel ratio at idle is usually set at stoichiometry.

There is a difference because it's mapped that way. The vehicle speed must be inferior to a given figure to trigger the lowest, idle speed. If the vehicle moves, the idle speed is higher. I'll try to find out why. A quicker engine pick up in tip-in might be one reason.

Fabrice

rx8wannahave

Are you sure...really? Gosh...I too thought I was saving more gas since I always seem to pull more city mpg out of the cars I've owned than the EPA listed numbers. My 8 so far gets 19.1+mpg in mainly city...but now I worry.

Please explain a little bit more, I"ll have to experiment with this.

Rasputin

I don't know what more I can say. If you have specific questions, shoot!

Fabrice

Rasputin

After mentionning all this to my wife, she told me to point out one or two things :
- This DCFO strategy has been introduced recently in engine management systems. It was already implemented in 96 when I started working at Ford, but initially not used. Today, I can presume all cars have it.
- My descriptions above are for a car fitted with a manual gearbox. I don't know exactly what happens with an automatic gearbox.
- The DCFO strategy can only come on when the gas pedal is fully released (or almost). If in a downhill drive situation you need to give your car the slighest help not to loose too much speed, you actually getting out of DFCO, and consuming fuel.

Is it clearer now?

Fabrice

grinn253

Do you drive an automatic?

Reactionary

My cars are all manuals. I have driven automatics when driving a rental. Or driving a family member's car. Or a friend's car. Each time, my brain has to make an adjustment. I think most people can relate to this.

Umbra

This sounds fishy to me.

I doubt that, for that to be applicable ALL of the fuel would have to burn perfectly and completely, that is never going to happen under any condition.

I have a hard time taking as fact that the engines uses more fuel at idle than at 3k rpm on a downhill.

As far as having it in neutral I wouldn't do it in a real busy area or in bad weather but saying you have no control over the car in that situation is completely wrong as well, you still have complete control over the stearing and the braking which are probably more important in avoiding an accident than acceleration would be.

Of course I could be wrong....

Rasputin

In an IC engine, combustion is never complete, in any condition. That's why a catalyst is used to complete the oxydation processes on residual HC and CO. To be efficient in both oxydation and NOx reduction, the AFR must be within 3% of stoichiometry.



It's so simple to understand. Under decel conditon, NO FUEL is injected. At idle, fuel is injected to maintain the engine at idle speed. A bit of fuel is more than no fuel at all, isn't it?

Fabrice (engine management calibration engineer, ex-Ford)

RWagz

I think, if I'm interpreting it right, he gave you guys the answer...

In DECEL condition the motor is actually being driven by the by the car, not by internal combustion - I.E. little/no fuel is being consumed. In IDLE mode the engine is basically disengaged from the drivetrain and needs to have some fuel to maintain RPM's. If it had no fuel under this condition it would stop rotating very quickly - how fast does it stop after you turn off your car?

Just my personal interpretation...

Rasputin

... and 100% correct.

Nubo

Short answer: No, there is no harm to coasting.

This is at least the 2nd thread I've seen on this topic and I'm again surprised by some of the claims

-it's illegal....?
-it's inherently extremely dangerous......?

These are both ridiculous statements.

Drive appropriately for the conditions. There are many places where coasting in neutral would be a bad way to drive. There are others where it's perfectly reasonable.

As far as fuel mileage -- maintaining travelling speed while running at idle is pretty darned efficient. And, even if fuel-off decelleration is true, it's going to slow down the car and soon you'll be on the gas again. A long coast in neutral is pretty rare, since the slope has to be enough to maintain speed but not so much that your speed goes too high. On those steeper slopes, you'd want to be in gear anyway, to utilize engine braking to keep the speed down.

GiN

Regarding fuel economy - remember that when you're coasting in gear and have your foot off the gas pedal, the throttle plate is pretty much closed just as how it is during idle. With that said, if the internal fuel mapping of the ECU is dependent upon TPS sensor readings, then the injector pulse and duty cycle during engine brake decel would probably be similar to that used during idle. However, I think ECUs nowadays base their fuel maps on a 3D grid of TPS vs MAP/MAF vs RPM. Therefore, having TPS at 0 with RPM > 1000 pretty much falls off the map, which results in 0 fuel duty.

I do remember reading somewhere in the DMV handbook (or perhaps driver's ed class in high school) that there are laws concerning coasting in neutral on paved roads.