Does a car consume fuel under engine braking?

[91TTZ]

Originally posted by: radioouman
of course it does. Just enough to keep the engine from stalling.

Were you just born yesterday? Have you not read the numerous replies in this thread?

The computer on most fuel injected cars will keep the injectors shut when coasting, as long as the throttle position is zero and the RPMs are a certain amount above idle RPM.

 

[randay]

Every time I try and look up some actual factual information on the intarweb about engine braking, all I get is "Jake brake" and deisel engines. That kinda sucks. I also get more misinformation! On one page a guy actually suggests that by accelerating and coasting in neutral and repeating this over and over you can double your milage. This is a pretty controversial topic with lots of misinformation to be had. Hurray!

 

[MercenaryForHire]

Originally posted by: dave518
I have not read all of this so sorry if anyone has already said this but the only way you wont use any gas at all is to turn the engine off, if you are coasting it is using gas, ure disconected from the drive train if ure in neutral(duh) so the wheels spining on the road cant be turning th engine over. So therefor gas is turning the engine over

Edit: Even if your in gear your engine has got to be getting some fuel, otherwise if you hit the gas, it would just poop out cause it wasnt getting any fuel. How else can you coast without losing speed really quickly, its because it is still getting some fuel to the engine just to keep it alive and breathing and so you can press the gas and move.

WRONG

- Lex Luthor

 

[bluestrobe]

I still wonder how an engine requiring a combustible fuel can be completely cut off from fuel and still idle or even run for a period of time?

 

[spidey07]

Originally posted by: dave518
I have not read all of this so sorry if anyone has already said this but the only way you wont use any gas at all is to turn the engine off, if you are coasting it is using gas, ure disconected from the drive train if ure in neutral(duh) so the wheels spining on the road cant be turning th engine over. So therefor gas is turning the engine over

Edit: Even if your in gear your engine has got to be getting some fuel, otherwise if you hit the gas, it would just poop out cause it wasnt getting any fuel. How else can you coast without losing speed really quickly, its because it is still getting some fuel to the engine just to keep it alive and breathing and so you can press the gas and move.

YACT on ATOT?
yep.

Supreme misinformation abounds?
yep.

ECU CUTS OFF FUEL DELIVERY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[spidey07]

Originally posted by: bluestrobe
I still wonder how an engine requiring a combustible fuel can be completely cut off from fuel and still idle or even run for a period of time?

Does shutting off the fuel prevent the pistons from being turned?

No.

 

[randay]

Here we go, time to end this thread. The answer lies in Deceleration Fuel Cut Off (DFCO) mode.

Text

From page 7.

Deceleration Fuel Cut
During closed throttle deceleration periods from moderate to high engine speeds, fuel delivery is not necessary or desirable. To prevent excessive decel emissions and improve fuel economy, the ECM will not open the injectors under certain decel conditions. The ECM will resume fuel injection at a calculated RPM.

Referring to the graph, fuel cut-off and resumption speeds are variable, depending on coolant temperature, A/C clutch status, and the STA signal. Essentially, when the extra engine loads are present, the ECM will begin fuel injection earlier.

Fuel Tau Cut is a mode employed on some engines during long deceleration time with the throttle valve closed. During these times, excess oxygen would enter the catalytic converter. To prevent this, the ECM will very briefly pulse the injectors.

You can find a lot of supporting evidence for this on google as well.

 

[JulesMaximus]

Originally posted by: funboy42

Originally posted by: letdown427
As was said, modern cars (fuel injected ones) do not use any fuel when coasting in gear.

So anytime your coasting and you want to hit the gas because you come up to a hill you have to restart your car again? Like said before if the engne is running including downhill or coasting it is using gas. The ONLY time its not is when it is shut off.

Granted if coasting going down hill with the clutch engaged it will use the same ammout of fule as it would (or should) as if it was idling but it in no way shuts the fuel source off completly or you would be constantly having to restart your car.

Not also to mention if kept in gear and it shuts the fuel off then the car would chug rally bad for not having that equal momentum or pressure on the pistons. IE: Coast down hill and turn your car off and see what happends. That would be like shutting off the gas and see how fast no real matter the grade your car slows down to a stop.

I feel dumberer evertime I read a post of yours...

 

[BD2003]

Originally posted by: bluestrobe
I still wonder how an engine requiring a combustible fuel can be completely cut off from fuel and still idle or even run for a period of time?

Newtons law. A object in motion will continue in that way until an outside force intervenes.

If it wasn't for wind, tire, and engine resistance, we'd just spend fuel on accelerating, and then let off the gas, and coast forever. The point is, the fuel is needed to spin the engine, and thus spin your wheels. Once it starts spinning, it is going to spin perpetually until something slows it down. That something being the friction in the internal engine parts, the transmission, rolling and wind resistance etc...that takes a while. It's already spinning, you don't need to *add* any more fuel to keep it spinning. Newtons law makes sure of that.

But no one is saying it can idle without fuel, just that it can coast.

Look at it this way. The people saying it cuts off, as I am, are saying it, andthen telling you why. The people saying it uses fuel say it out of opinion and belief, and can only tell you platitudes such as engines always needing fuel. It should be obvious out of those two choices who to take on their word - the ones who can back it up.

 

[Tom]

Maybe it's not so simple..

http://freepatentsonline.com/5119781.html

 

[jagec]

Originally posted by: Tom
Maybe it's not so simple..

http://freepatentsonline.com/5119781.html

First I check the REAL source to make sure it's a legit patent. No offense, but any "free___online.com" site makes me wary

Sure enough, it's a real patent. Then I read the text carefully. Something stands out to me.

A method is described for controlling the injection of fuel in a direct injected, multi-cylinder internal combustion engine, to smooth transients in engine output torque associated with a deceleration fuel cut-off mode of engine operation. This is accomplished by detecting engine operating conditions that call for the initiation of a transition associated with the decelaration fuel cut-off engine operating mode. In response to the detected operating conditions, a transitional period is initiated, during which the injection of fuel into a varying portion of engine cylinders is then interrupted. When the transitional period is associated with entry into the deceleration fuel cut-off mode, the injection of fuel to a progressively increasing number of cylinders is interrupted. When the transitional period is associated with recovery from the deceleration fuel cut-off mode, the injection of fuel to a progressively decreasing number of engine cylinders is interrupted. The rate of entry into the fuel cut-off mode is preferably fixed, while the rate of receovery is determined in accordance with the position of an engine control element for adjusting the amount of torque developed at the engine output. Alternatively, the rate of recovery may be determined by the maximum rate of positional change of the engine control element.

They didn't invent fuel cutoff. They're patenting the idea of cutting off fuel delivery one cylinder at a time, to keep the driver from experiencing the "jerk" that I did in my '80s FI cars when they go to full cutoff. They also resume injection one cylinder at a time, same reason.

 

[Tom]

Yes, I know. My point is, assuming some cars use something like this, then it isn't true that all cars with EFI cut off fuel delivery entirely during engine braking, which a great deal of posts in this thread insists is the case.

 

[Sureshot324]

The engine has a sensor that tells it whether it's above or below idle RPM. If your foot is not on the gas, the engine will get a little bit of fuel if it's at or below idle RPM just to keep it from stalling, but if it is above idle RPM, it's not getting one drop of fuel. So if your engine brake, it's not getting any fuel unless you let it get to a very low rpm.

As to whether it's more efficient to coast in neutral or in gear, I'm not sure. If you leave it in neutral, it's using a bit of gas to keep it idling. If you leave it in gear, you're not using any gas, but the engine is slowing you down quite a bit.

 

[DougK62]

Some do - some don't.

 

[pnad]

Originally posted by: Tom
Yes, I know. My point is, assuming some cars use something like this, then it isn't true that all cars with EFI cut off fuel delivery entirely during engine braking, which a great deal of posts in this thread insists is the case.

The patent listed above is only for direct injected vehicles. Pretty new tech and not many cars out there have it yet. Mostly Audi's and VW's.

I would bet the house that 98% of all EFI equipped cars on the road today allow full fuel cutoff on decel. The 2% being the last few remaining classic EFI cars on the road.

 

[Zenmervolt]

Originally posted by: Tom
Yes, I know. My point is, assuming some cars use something like this, then it isn't true that all cars with EFI cut off fuel delivery entirely during engine braking, which a great deal of posts in this thread insists is the case.

Unless you're buying cars made in a third-world country, all modern FI vehicles have a fuel cut-off under engine braking. It's a "free" way for the auto maker to increase fuel mileage for a vehicle.

There are differing methods for resuming fuel injection when the threshold RPM has been breached, but any modern FI car will cut off fuel under deceleration.

The pattent only addresses the brief (less than one second) transition period from normal fuel flow to fuel cut-off. It actually proves that any car using your patent does have a 100% cut-off mode since the pattent covers only the transition to and from complete fuel cut-off.

ZV

 

[randay]

Originally posted by: Tom
Yes, I know. My point is, assuming some cars use something like this, then it isn't true that all cars with EFI cut off fuel delivery entirely during engine braking, which a great deal of posts in this thread insists is the case.

I can't believe that after all we've been through in this thread, people are still spewing random facts with nothing to back it up. unbelievable!

 

[DivideBYZero]

Is the car still moving? Then it's using fuel. It's physics, duh!

 

[randay]

Originally posted by: DivideBYZero
Is the car still moving? Then it's using fuel. It's physics, duh!

*stab*

 

[jagec]

Originally posted by: DivideBYZero
Is the car still moving? Then it's using fuel. It's physics, duh!

Admit it, you failed physics, didn't you.

 

[d0ofy]

I engine brake pretty often. When I press the clutch when engine braking the RPMs shoot up 1K+. If the fuel is absolutely cut off when engine braking, what causes that sudden increase in RPMs?

 

[DivideBYZero]

Originally posted by: d0ofy
I engine brake pretty often. When I press the clutch when engine braking the RPMs shoot up 1K+. If the fuel is absolutely cut off when engine braking, what causes that sudden increase in RPMs?

Elves, in their Magic Hats.

 

[RichUK]

Originally posted by: d0ofy
I engine brake pretty often. When I press the clutch when engine braking the RPMs shoot up 1K+. If the fuel is absolutely cut off when engine braking, what causes that sudden increase in RPMs?

EDIT: nm i miss read it

 

[d0ofy]

Originally posted by: RichUK

Originally posted by: d0ofy
I engine brake pretty often. When I press the clutch when engine braking the RPMs shoot up 1K+. If the fuel is absolutely cut off when engine braking, what causes that sudden increase in RPMs?

Inertia

EDIT: Also i am pretty sure you cannot engine brake under idle anyway. Not that i have tried.

I have actually done that a few times. Idle on my car is around 750 and I've dipped below 500 before while braking. The moment I pressed the clutch the engine corrected up to above idle then rested at idle.

 

[jagec]

Originally posted by: d0ofy
I engine brake pretty often. When I press the clutch when engine braking the RPMs shoot up 1K+. If the fuel is absolutely cut off when engine braking, what causes that sudden increase in RPMs?

I've never seen a car that behaves as you describe. Are you sure your foot is 100% off the gas, and nothing in your transmission is slipping? When I push the clutch while engine braking, the engine just drops to idle. In every vehicle I've driven. Which is quite a few.