Downhill : neutral or coast in gear

[Hullabaloo]

Not sure if this should go in Lounge or Technical, but since I'm a newbie i thought i would be safer in here. I can't say I know alot about cars so if anyone can help me out with this i'd be very appreciative.

Situation: coasting down a hill.

Question: Which uses less fuel? (for a fuel injected car)

1) car in neutral – most people would agree that you use bare minimal fuel whilst idling.

Or

2) car in gear with no throttle application? (in other words are the injectors still firing upon engaged motor deceleration with no throttle application.)

My opinion is I think that the wheels are connected to the engine and are therefore keeping the engine turning over and preventing it from stalling. And therefore you don’t need to use fuel and the ECU cuts . Where as if you’re idling you need to pump in fuel and ignite it to keep the engine turning and prevent stalling.

A friend has said :
“If you're in neutral, the fuel pump is pumping some minimum amount of fuel into the engine (i.e. the throttle is open some minimum amount). If you're in gear and rolling down the hill without your foot on the accelerator, the throttle is still open the same amount as if you were in neutral. The engine is only spinning faster by virtue of the car's weight in accelerating down hill.”

If this is the case then wouldn’t coasting downhill in gear at say 4000rpm be using 4 times as much fuel as when idling?

Another friend said:
“Research will show the momentum of the wheels keep pushing the engine along. A car only stalls if the engine stops because theres not enough energy to move the pistons in the engine. i.e uphill, not enough accelerator, slow speeds with high gear etc. when you need the power created by the ignition of petrol. if you are going at such high speeds, the energy isn't required. Even tho the fuel pump may be pumping, its to keep the fuel line presurised so fuel is there when you need it and not necessarily going into the engine as it should be controlled by the injectors. may be different in a carby car tho”

Fuel pump keep pumping to keep the lines pressurised but no fuel going anywhere? Wouldn’t pressure keep increasing (unless there is some outlet) if that was the case?

Some people have said that the ecu shuts of the fuel injection when decelerating until the car reaches idle … is this complete rubbish? One person says that :
“I'm looking at my OBD training manals for Hondas, and for MPFI and DPFI systems, it says "if the ECM receives a closed throttle input (under 0.5V), the engine speed is above 1100 RPM, and the engine is warmed, the fuel injectors will be shut off."”

This is in line with what I originally thought, but if this is the case then I thought (to throw a spanner in), won’t the catalyst cool off too quickly due to unheated air flowing through it and therefore emissions standards won’t be reached?

[aaronng]

Coast in gear. It is safer. It still uses some fuel but less than when in neutral.

[destrukshn]

coast in gear.

[JasonGilholme]

Keep it in gear buddy :thumbsup:

[CUL8R]

coast in gear FTW

[solitz]

always coast in gear mate...never in neutral.
you have more control of a car in gear than in neutral

[saxman]

coasting in gear uses less fuel(assuming you're in a high gear... obviously if you're coasting at 7000rpm this doesn't really apply).

As for if the fuel injectors shut off... think about that. The car is idling... meaning that you're getting combustion in the cylinders. If the fuel injectors are shut off, then there is no fuel injecting into the chamber. No fuel = no combustion = no idling.

[JasonGilholme]

coasting in gear uses less fuel(assuming you're in a high gear... obviously if you're coasting at 7000rpm this doesn't really apply).

As for if the fuel injectors shut off... think about that. The car is idling... meaning that you're getting combustion in the cylinders. If the fuel injectors are shut off, then there is no fuel injecting into the chamber. No fuel = no combustion = no idling.

Where as if you are rolling in gear, your driveline is connected to the wheels and this motion keeps the engine spinning. Without the aid of the fuel injectors.

Correct??

[bennjamin]

ALWAYS "coast" in gear.
IF you roll in neutral , you risk loosing control or over heating the brakes in situations.

[Hullabaloo]

Thanks guys. have been coasting in gear because i think it saves fuel and is safer. Will continue to do so for the later reason.

Mainly wanted to know officially which saves fuel. I've tried to do lots of searching and find people giving opinions from both sides (eg cruising uses more fuel or cruising uses less) and haven't found anyone be able to give proof what the engine does: fuel cut off or inject a bit.

Hey saxman, are you're saying that coasting does use fuel and how much you use is related to rpm? even when no throttle is applied? if you're going downhill in gear are you still considered idling?

[marte]

technically when u step on ur accelerator it pumps fuel from ur tank.. if ur coasting and ur not steeping on ur accelerator ur not wasting fuel right?

[bungsai]

another one for in gear.

[bennjamin]

technically when u step on ur accelerator it pumps fuel from ur tank.. if ur coasting and ur not steeping on ur accelerator ur not wasting fuel right?

yes - "at speed" if u coast in gear , the injectors are off and the engine is run by momentum. I cannot say exactly at what load they go on and off - but the reason why you get more k's per tank on highway driving is that there is longer instances of "coasting" etc.

[CRXer]

Ok..let me have another go at this......

i was told a while ago,by string i think,to open up a .bin editor & see that the fuel values for coasting are not zero.....

well,i went one step further & datalogged a coasting event & what do i find...........

coasting= injector opening of 0ms,duty of 0%

this is in keepin with what honda says that they shut off the injectors when rpm over about 1000rpm & closed throttle.

I was told by a few of strings lap dogs (i apologise if it wasnt u,& whoever it was was still tryin to be civil except for his parasites)that i didnt know wtf i was talking about.

Anyway heres a shot of the log showin at the cursor(black dotted line) duty =0,inj = 0

Anyone want to comment?

http://i58.photobucket.com/albums/g268/CRXer/ScreenHunter_1Medium.jpg

[JasonGilholme]

ya hit the nail on da head man.

[SINISTR]

an Idling engine is always said to use more fuel than an engine which is at cruising speed as due to the cars moving inertia its easing load on the engine and allows for less fuel consumption.

Your engine is a power generator, when its idling it needs a certain amount of fuel to basically RUN and turn it over. Same when you're accelerating off the lights or going up a hill, it needs higher revs to make more power and in return it uses more fuel. When cruising at 100km/h in 5th gear for 30 mins its using less fuel than if you idled your car for 15 mins in the garage because the gear ratios actually let the engine do less work, less than idling anyway - so definatelly cruise in 4th/5th down a hill.

[TRU32U]

soooooooooooo is everyone saying if im in 5th and sitting 140kms hr on m4 for example its better to cruise and hold revs at say 4500rpm for 15 mins or so then letting it roll in N??

[e240]

I say if you need to come down this route just to save fuel, take a bus.

[quangsta]

^^^^ lol

interesting.....topic though..

i think coasting in gear maybe more efficient...but more so the fact that its safer being in some sort of control of ur speeds and it will save you money on ur brakes anyway...

[Q_ball]

I say coasting in gear > neutral anytime, not for tite ass purposes, but for safety!!

And yeh, as above, if you really had to decide whether to coast in neutral or gear, for FUEL saving purposes, lol, dont drive! :p

[Vinnie]

i also think its just hilarious that sum1 who has forked out to get a nice car like an ITS is so paranoid about the extremely small amount of difference coasting in/out if gear makes on fuel consumption. If its such a prob u shooda bought urself a 1.3 litre shitebox... i mean ffs jus leave the thing in gear!

[TypeRice]

only roll downhill in neutral if you r in heavy traffic.

[CRXer]

the man asked a simple tech question & wanted to know an answer & he gets the "but what about the children" brigade tellin him the finer aspects of wrappin yourself in cotton wool by always being in gear & the full fuel savings report...........

seriously..if u got nothing to contribute........dont make it obvious u got nothing

[[HIREVZ]]

soooooooooooo is everyone saying if im in 5th and sitting 140kms hr on m4 for example its better to cruise and hold revs at say 4500rpm for 15 mins or so then letting it roll in N??


going 140 on the m4 ur going to have to hold speed at 140 so u would have to press the trottle n keep throttle body open, downhill u dont use throttle therefore throttlebody closed using less fuel

[SINISTR]

the man asked a simple tech question & wanted to know an answer & he gets the "but what about the children" brigade tellin him the finer aspects of wrappin yourself in cotton wool by always being in gear & the full fuel savings report...........

seriously..if u got nothing to contribute........dont make it obvious u got nothing

BAM!!!! :thumbsup:

just one more thing - from memory - cruising between 90-110km/h in 5th is the most efficient speed apparently... on 5spd 1.6-2L cars

[preludacris]

ive also been wondering about this ..

was told , injectors shut off , when coasting , and throttle is closed... so using 0 fuel ..
hope this is true . :)

i used to roll in N alll the time .. haha

[bennjamin]

ive also been wondering about this ..

was told , injectors shut off , when coasting , and throttle is closed... so using 0 fuel ..
hope this is true . :)

i used to roll in N alll the time .. haha


yes - "at speed" if u coast in gear , the injectors are off and the engine is run by momentum. I cannot say exactly at what load they go on and off - but the reason why you get more k's per tank on highway driving is that there is longer instances of "coasting" etc.

Dont roll in N guys ~ its used for the transistion bewteen gears NOT a "gear".

[Hullabaloo]

Thanks to all those with positive and informative replies. I'm just trying learn how engines work. Maybe I should have used the wording "less fuel injected" rather than "save fuel" to prevent people telling me to go catch a bus. Safety comes first so I'll continue to do what I've always done, which is coast in gear, regardless of how much fuel is actually used or not.

I asked the same question to a fellow engineer who knows more about cars than I do and he did tried to do some research for me. He reversed engineered some of the EFI system on a R31 Skyline RB30 engine.

He said that on this ECU when your foot is not on the accelerator the Throttle Position Sensor detects your foot is off and then uses Fuel Cut / Fuel Recover tables to determine what revs to stop injecting (to prevent exhaust backfire) and then when to continue injecting again.

For this ECU there are two sets of tables for neutral and non-neutral injection when TPS is closed (accelerator off). I think from memory the non-neutral may have slightly richer injector settings (especially for autos where load is put on the engine from the torque converter)

He sent me the graphed results for fuel cut and fuel recover. The results are rpm vs against temperature. They show non 0 values, but since he didn't pull the injection amount from the ECU to display it i can't be sure that is the case. From my workplace photobucket and webstorage is blocked so I can't put up his graphs just yet.

After a big more digging I've found out from a mechanic who works for Team Dynamik that some engine management systems do cut fuel completely by means of no injector pulses, where as others tend to pulse the injectors at a low duty cycle. But in EFI cars, all cars have a fuel pump which always runs, a fuel rail, pressure regulator and return line to tank. Fuel is always doing a full loop. The regulator maintains roughly 3 bar on the rail and this is where the injectors get their pressurised fuel from for injection.

[barefootbonzai]

Good stuff, i always wanted to know about this.

[zco]

i honestly didnt know it was mroe dangerous, looks like i learnt something from the noob forum..

first piece of information i have learnt from ozhonda in about a year ?

[TypeRice]

dont they teach u dis shiet when ure on your Ls driving around in ure manual cars..
should get a driving instructor or sumfing
dey would teach you the right way to do it

[kraiye]

sure but u dont get the finer details about how the engine works etc
i always wondered if fuel was injected when coasting :thumbsup:

[zco]

sure but u dont get the finer details about how the engine works etc
i always wondered if fuel was injected when coasting :thumbsup:

exactly, also, some ppl like ME, never went ot manual lessons. learnt at home BY MY SELF and have all my family laugh at me stall. :thumbsup: took me a few hours to get used to..

[string]

Ok..let me have another go at this......

i was told a while ago,by string i think,to open up a .bin editor & see that the fuel values for coasting are not zero.....

well,i went one step further & datalogged a coasting event & what do i find...........

coasting= injector opening of 0ms,duty of 0%

this is in keepin with what honda says that they shut off the injectors when rpm over about 1000rpm & closed throttle.

I was told by a few of strings lap dogs (i apologise if it wasnt u,& whoever it was was still tryin to be civil except for his parasites)that i didnt know wtf i was talking about.

Anyway heres a shot of the log showin at the cursor(black dotted line) duty =0,inj = 0

Anyone want to comment?

Did you do my hill experiment? A video of the test (with sound) would be superb. I tend not to believe what your computer is telling you (and then me). I'd rather you do the hill test and hear your fuel being combusted instead.

[dc2dc2dc2]

in gear ftw!

[JasonGilholme]

Wouldn't you be able to tell with an AF gauge?? Even a narrow band one??

Cause as soon as you step off the throttle the guage should read fully lean because theres no fuel being injected, therefore no fuel in the mixture passing the O2 sensor. yeah??

[aaronng]

Did you do my hill experiment? A video of the test (with sound) would be superb. I tend not to believe what your computer is telling you (and then me). I'd rather you do the hill test and hear your fuel being combusted instead.
It's probably diagnostic information from the OBD II port. How more accurate do you want to be? Hehe.

Some cars can shut off the injectors, while others can't. In his case, his car can.

[CRXer]

Did you do my hill experiment? A video of the test (with sound) would be superb. I tend not to believe what your computer is telling you (and then me). I'd rather you do the hill test and hear your fuel being combusted instead.

Ahh string so it was you,couldnt find that thread for the life of me.

I hope u read me right,that u were the only one tryin to have a civil discussion before the imbociles stepped in.

Anyway,aarong the info is a hondata data log.

I know what your sayin about the engine note string (no i havent tried it,combo of living in peak hour hell & car off the road atm) thats why i asked originally,it confused me also when i found this out & i thought about what sounds the engine would make.

Honda states the info clearly that they shut the injectors off for fuel economy above about 1000rpm(varies slightly between models) & closed throttle.

The datalogs i have confirm this on every single occassion,thats just a 10 sec snip of about & hours worth of logs.

There is definitely no fuel passing thru the injectors,zero firing time & zero duty cycle,thats why i wanted someone to originally explain to me why the engine sounds like its still firing or does it just seem that way.

[JasonGilholme]

The engine is still compressing and exhaling air through the valves and exhaust so there would have to be a sound of somesort. Also, the spinning action of the crank and camshafts would have its own noise through friction.

[string]

The engine is still compressing and exhaling air through the valves and exhaust so there would have to be a sound of somesort. Also, the spinning action of the crank and camshafts would have its own noise through friction.
This is what I was alluding to. Have you ever tried to clutch start a car down a hill? The sound you get from that is completely different to cruising down a hill off throttle. One just sounds like air flapping around, the other sounds like tiny bits of fuel being combusted.

I have no doubts that you datalogs are correct, and that yours (and my) car is supposed to cut injector pulse to 0ms, however, you cannot argue with the different sound, which is where my disbelief arises.