Does a car consume fuel under engine braking?

[geckojohn]

I don't think the car uses fuel when engine braking..

 

[BD2003]

Originally posted by: Staples
I have a 95 Mitsubishi Galant.

I do this all the time. There is a very high highway interchange 1.5 miles before the exit I usually get off nearest my house. I have always shifted the car into neutral as I started going down hill. If I am going 70mph at the begining of the drop, I can coast all the way to the light 2.5 miles away maintaining speeds above 35mph.

You guys are telling me that I am not saving any fuel over just leaving it in 5th gear? I don't think so because the force of the engine weight will slow down the car and I will not be able to get to the stop light without putting throttle to get through the last mile.

Maybe I should turn the car off while I am coasting. That WILL save gas.

That is what we are telling you.

The engine will slow down the car. The higher a gear you are in, the less you will be slowed down. But during this process, you will be using ZERO fuel. Top gear presents very little resistance.

If you coast in neutral, you'll coast that wee bit further, but use fuel *the entire time*.

You have two choices:

Use the stored kinetic energy in your wheels, and add a little in if the hill isnt enough to keep you at speed.

Coast in neutral, and ALL of the fuel being used to keep it at idle will be WASTED, since it isnt being used to move your car.

It is somewhat unintuitive to realize that you will save fuel this way, but any time you potentially *waste* fuel just heating up the air, you are lowering your mpg. If it's any sort of hill worth even considering a hill, you should even accelerate rolling down it in top gear.

 

[BD2003]

Originally posted by: funboy42

Originally posted by: Vic

Originally posted by: DivideBYZero

Originally posted by: Zenmervolt

Originally posted by: DivideBYZero

Originally posted by: mattpegher
Ok I beleive you but why do I lose power steering and power brakes if I run out of gas, when coasting down hill?

No vacuum.

http://en.wikipedia.org/wiki/Manifold_vacuum#Uses_of_manifold_vacuum

If the car is in gear, he loses nothing. If the car is in gear, the engine will still continue to spin (and it will be generating massive vacuum, I'll get a vacuum of 25 millimeters of mercury when coasting with the engine in gear), Since the engine is still spinning, the PS pump will still be turning and he will still have vacuum assist for brakes.

He'll only lose PS and brake assist if he has the car in neutral and the engine stalls.

ZV

Rather than pick the bones I went for the fact that he was using the example you have explained and applying it, needlessly, to a 'rolling down the hill' scenario. I juts couldn't be arsed to pick him up on it.

Pnad, that's for you, too.

Seriously, I'm about the only person in this thread posting decent info and links to basic facts. Please, if you must be jonny-come-lately, at least call out the blatant bollocks in this thread rather than splitting hairs with me.

Sorry, rant.

When an engine dies, the throttle valve closes completely. That's what makes the difference.

Wrong it will never close completly unless your carb or throttle body is defective and even so there are small holes drilled into a throttle body, on some, to allow air still into the engine at an idle. It it woudl closed all the way when you let off how does the air get by it to run at idle. You would be starving the engine for air and it will shut off if your saying they close all the way.

You're right, it doesn't close completely. But that doesn't mean the fuel injectors can't cut off. It just pumps pure air through and through. It doesn't need any more than the slightest bit of air, since doesn't need to mix the stoichiometrio ratio, because there is NO FUEL!

 

[j00fek]

what kind of thread is this?

 

[Alex]

Originally posted by: DonVito

Originally posted by: Demon-Xanth
On a fuel injected engine it can shut down the fuel flow. On a carburated engine, it cannot.

QFT. This is why it's actually more fuel-efficient to leave a fuel-injected car in gear while descending, as opposed to putting it in neutral.

QFT again!
just to kinda really make a point...

 

[natto fire]

An average automobile engine is just an air pump, and there seems to be plenty of people that "think" they know how they operate.

 

[AdamK47]

I actually enjoy engine braking in my GTO at night and watching peoples reactions behind me. With my new exhaust setup it allows for a less restrictive exhaust. At times during engine breaking it causes the DFCO (Direct Fuel Cut Off) to engage. About 50% of the time combustion can partially occure in the exhaust causing flames to shoot from the tips.

 

[JulesMaximus]

Originally posted by: BD2003

Originally posted by: Staples
I have a 95 Mitsubishi Galant.

I do this all the time. There is a very high highway interchange 1.5 miles before the exit I usually get off nearest my house. I have always shifted the car into neutral as I started going down hill. If I am going 70mph at the begining of the drop, I can coast all the way to the light 2.5 miles away maintaining speeds above 35mph.

You guys are telling me that I am not saving any fuel over just leaving it in 5th gear? I don't think so because the force of the engine weight will slow down the car and I will not be able to get to the stop light without putting throttle to get through the last mile.

Maybe I should turn the car off while I am coasting. That WILL save gas.

That is what we are telling you.

The engine will slow down the car. The higher a gear you are in, the less you will be slowed down. But during this process, you will be using ZERO fuel. Top gear presents very little resistance.

If you coast in neutral, you'll coast that wee bit further, but use fuel *the entire time*.

You have two choices:

Use the stored kinetic energy in your wheels, and add a little in if the hill isnt enough to keep you at speed.

Coast in neutral, and ALL of the fuel being used to keep it at idle will be WASTED, since it isnt being used to move your car.

It is somewhat unintuitive to realize that you will save fuel this way, but any time you potentially *waste* fuel just heating up the air, you are lowering your mpg. If it's any sort of hill worth even considering a hill, you should even accelerate rolling down it in top gear.

Even if you hit the gas a bit to keep your speed up you will use very very little gas. Maintaining speed down a slight incline I see my mileage jump up to 50-60mpg...sometimes it will even peg at 99mpg.

You guys are literally arguing over pennies in this thread...:roll:

 

[Scarpozzi]

Engine Braking does 2 things, it slows the car down to take work off the brakes themselves (preserves pads&shoes). This is good if you're hauling a heavy load or a trailor. The second thing is does is a result of saving your brakes...It gives you an advantage by keeping you in control. You're in a lower gear so you can accelerate more quickly if you must...or gives you increased braking power.

The downside is that if you do this too often, or do a hollywood stop (shift to 2nd gear and roll slowly, then pull out) you could be doing damage to your transmission if you aren't in a low enough gear when you pull out. It's important to watch your speed and to keep track of any additional weight past the normal specs of the vehicle before using engine braking techniques to make sure your vehicle can handle it...95% of the time it will be effective.

As for your original question, You will not use up any more gas because the gas itself is controled by the accelerator pedal and the computer for idle. If you're not accelerating, then it is idling....even when the RPMs may spike as a result of the lower gear and declining speed.

 

[pnad]

Originally posted by: funboy42
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.

Modern fuel injected vehicles have this cool computer like thing that controls the engine. Amazing enough, it is 'smart' enough to turn your injectors on and off automatically! Wow. As a matter of fact, all injectors turn on and off rapidly all the time. If the ECU can be programmed precisely enough to open the injectors for a few milliseconds on each intake stroke of a piston, I am pretty confident it can turn them off for coasting.

Any idiot can search for this via google in about .03 seconds
http://www.wikihow.com/Save-Money-on-Gas
See #26

 

[homestarmy]

According to Jeremy Clarkson, on modern engines no fuel is used.

 

[homestarmy]

Originally posted by: Serp86

Originally posted by: Vic

Does a car consume fuel under engine braking?

With modern cars, no. However, modern cars also use very little fuel while idling.

You should never coast in neutral. I'm not going to get into the ridiculous "OMG it's illegal!" garbage, I'm just going to say that it's a stupid and improper way to drive.

This subject has been beat to death on the internet many times in the past.

I live in malta - a very small island with lots of twisty roads and hilly terrain, hence the thread

I never heard of such a law here , however i would think reaction time under neutral coasting would be slightly faster, if you keep the foot on the brake like i do. Under an emergency all you have to do is just push one pedal (brake) as opposed to lifting foot off gas and pushing brake + clutch (not pushing clutch will kill engine + brakes + power steering). Now if you keep the foo of the brake, it can turn into a situation where you push the wrong pedal, but that can also happen when in gear - so whats the point?

Then again my car has never exceeded 90mph due to lack of straight roads so there might be something i'm missing

Leaving it in gear will leave drag (hence engine braking), so you will slow faster. Generally in emergencies, you will not come to a complete stop, so you will not stall.

Plus, in certain emergencies, you will need to give the car gas to get out of a bad situation, easier to take out then to put into gear.

 

[radioouman]

of course it does. Just enough to keep the engine from stalling.

 

[boomdart]

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

Let me get this straight...

For example, I'm cruising down the road in 4th gear at about 3,000rpms.

If I go to fifth, using 2,000rpms for the same speed, am I not saving fuel? I would imagine if the engine is going at 3,000rpms then that's 1,000rpms extra worth of gas to be used.

Are you saying while your foot isn't on the gas pedal the car doesn't inject any fuel?

 

[homestarmy]

This is going to be one of those threads where in the future someone is going to reference when another member says that they know all about cars...

...they will link the post in this thread showing that the person thinks that a fuel injected car uses fuel while engine braking and those of us who know anything about cars will have a good laugh!

 

[DivideBYZero]

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

Hello? Is that God? Can I kill this poster please?

 

[BD2003]

Originally posted by: boomdart

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

Let me get this straight...

For example, I'm cruising down the road in 4th gear at about 3,000rpms.

If I go to fifth, using 2,000rpms for the same speed, am I not saving fuel? I would imagine if the engine is going at 3,000rpms then that's 1,000rpms extra worth of gas to be used.

Are you saying while your foot isn't on the gas pedal the car doesn't inject any fuel?

If you go to higher gear vs. a lower gear while *cruising* (as in, keeping enough throttle to maintain speed), then YES, you are saving fuel by keeping a lower RPM.

But when you are *coasting* (foot completely off the pedal), then it does not matter what gear you are in...you use ZERO fuel.

 

[Zenmervolt]

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.

The injectors re-fire at a prescribed threshold RPM (typically around 1,200 RPM). You do not need to use the key to re-start the engine because the engine is still spinning and also firing the spark plugs, the only thing that stops is the injectors firing (essentially injector duty cycle goes to 0%). Re-introducing fuel at the threshold RPM keeps you from having to turn the key again.

I've coasted downhill with the engine off. Feels exactly the same as when I coast downhill with the engine on. Of course, I've already proven than my car, in fact, does completely shut off fuel flow if the throttle is closed and the RPM is above 1,200 RPM.

ZV

 

[Zenmervolt]

Originally posted by: Toastedlightly
Now, you are assuming this for all cars. All ECUs are not the same. I won't believe ZERO is used, I'll believe less than idle is used until I see the injectors not opening.

I've already proven that the injectors do not fire on my car (Porsche 944, FI system introduced in 1982). RTFT.

ZV

 

[Zenmervolt]

Originally posted by: boomdart
Are you saying while your foot isn't on the gas pedal the car doesn't inject any fuel?

For the billionth time, YES.

ZV

 

[Staples]

Originally posted by: j00fek
what kind of thread is this?

A useful one if the information I am learning is correct.

 

[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.

 

[Eli]

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.

Oh Christ.

Bzzzzzzzt, try again.

Even my carbureted Tercel cuts off the fuel supply during engine braking.

Granted, it's a damn complex and annoying carburetor, but still.

 

[rikadik]

Just read this entire thread, and started to think the point was getting across. Then I read this:

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.

:roll:

 

[Rock Hydra]

Originally posted by: JulesMaximus

No you wouldn't. I drove a stick for 10 years and habitually downshifted when approaching a stop through one or two gears but usually just dropped it into neutral before going into second or first since I was going fairly slowly by that point anyway.

I never coasted any of my manual transmission cars in neutral for any length of time. There is no reason to. If you live in very hilly areas you should always have the car in gear and not ride the brakes.

Yeah, is it not really engine braking if you leave the car in neutral, since the friction of the gears being used to slow down the vehicle? (i.e. using 2nd gear in an automatic to engine brake down a steep incline) Mind you, I don't know much about cars, so correction on this would be nice. (if i'm wrong that is)